Dr Marc Hanikenne
email: marc.hanikenne@uliege.be

Sara M Esteves

Steven Fanara

Hatem Rouached

Coordinated homeostasis of essential mineral nutrients: a focus on iron

Keywords: Iron, iron deficiency, iron uptake, nutrient interaction, ion signaling crosstalk, nicotianamine, mugineic acid, root growth

de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Iron (Fe) is an essential element for all living organisms [START_REF] Kobayashi | Iron Uptake, Translocation, and Regulation in Higher Plants[END_REF]. Plants and algae are an important source of Fe entry in the terrestrial and aquatic food web, respectively. However, although it is abundant in the environment, Fe is poorly available to plants in soils [START_REF] Marschner | Marschner's Mineral Nutrition of Higher Plants[END_REF], and hence Fe deficiency is a major issue limiting crop productivity, as well as the quality of agricultural products. Similarly, algae primary productivity is severely impaired by low Fe availability in the open ocean [START_REF] Sunda | Low iron requirement for growth in oceanic phytoplankton[END_REF][START_REF] Strzepek | Photosynthetic architecture differs in coastal and oceanic diatoms[END_REF]. Consequently, Fe deficiency threatens human health. Indeed, according to WHO (http://www.who.int/nutrition/en/), one third of the human population worldwide is affected by Fe deficiency anemia. Therefore, there is an urgent need to better understand how photosynthetic organisms regulate Fe homeostasis.

In plant cells, while Fe can also be found in substantial amount in the nucleolus [START_REF] Roschzttardtz | Plant cell nucleolus as a hot spot for iron[END_REF], up to 80% of cellular Fe is found in chloroplasts in leaves [START_REF] Terry | Function of iron in chloroplasts[END_REF] and Fe represents >60% of micronutrients in plant mitochondria [START_REF] Tan | Alimentation et l'Environnement user on 12 November 2020 34 in interactions with proteins and oxidative stress-induced damage to respiratory function[END_REF], where it is found associated to heme and iron-sulfur clusters or as a cation. Photosynthesis and respiration therefore require adequate Fe supply and are highly sensitive to Fe deficiency [START_REF] Nouet | Chloroplastic and mitochondrial metal homeostasis[END_REF][START_REF] Hanikenne | Ion homeostasis in the Chloroplast[END_REF][START_REF] Vigani | Metal Homeostasis in Plant Mitochondria[END_REF]. Adequate Fe supply is also required for several developmental processes, including for instance root growth, flowering or pollen production (Briat et al., 2015a;Bouain et al., 2019b). Fe deficiency is thus deleterious for the functioning of plant cells.

In contrast, the redox properties that make Fe an essential cofactor of multiple biological processes can be harmful when Fe is delivered to cells in excessive amounts or if its interactions with biological macromolecules are uncontrolled [START_REF] Stohs | Oxidative mechanisms in the toxicity of metal ions[END_REF][START_REF] Marschner | Marschner's Mineral Nutrition of Higher Plants[END_REF]. Therefore, plants possess sophisticated mechanisms, referred to as Fe homeostasis, to adjust, within a range of Fe supply, Fe uptake and distribution to needs throughout their development and upon changes in their environment [START_REF] Kobayashi | Iron Uptake, Translocation, and Regulation in Higher Plants[END_REF][START_REF] Thomine | Iron transport in plants: better be safe than sorry[END_REF]and this special issue).

In addition to Fe, 13 mineral (micro-and macro-)elements are also essential for plant growth and development, whereas 5 others are considered beneficial [START_REF] Marschner | Marschner's Mineral Nutrition of Higher Plants[END_REF]. Plants require an optimal and properly balanced supply of these elements. Nevertheless, in nature, plants can encounter multiple combinations of nutrient excesses and/or limitations. So far, the bulk of experimental studies mainly focused on the responses of plants to variation in the supply of a single element. This enables the detailed dissection of the molecular homeostasis mechanisms for these elements, identifying transporters, chelators, assimilation pathways, as well as a number of (transcriptional) regulators and to a lower extent sensing and signaling mechanisms. More recently, research opened to the investigation of interactions between nutrient homeostatic networks at the molecular level, revealing (i) multiple ways of nutrient inter-dependency: an element may be A c c e p t e d M a n u s c r i p t 4 required for the proper uptake of another, deficiency or excess of one element impacts positively or negatively the uptake another, elements share pathways (e.g. transporters or chelators with broad specificity) or regulatory processes and (ii) hidden responses that are more than the addition of single stress responses (Bouain et al., 2019b). Those networks need to be examined in more detail to better comprehend the challenges faced by plants in their natural environments during their lifetime. This will enable the design of integrated and sustainable approaches to ensure plant production and quality.

Here, we present recent findings on the role of Fe in regulating complex developmental processes in plants, with a focus on the regulation of primary root growth. We then review our current knowledge on the molecular mechanisms orchestrating homeostatic interactions of Fe with other micro-and macronutrients in vascular plants, pinpointing whenever possible commonalities and differences between monocots and dicots. Finally, we also present a brief overview of those interactions in algae in comparison to higher plants, highlighting originalities found in algae (Box 1).

Gaining new insight on how plants regulate Fe homeostasis will likely help to improve crop production on Fe-poor soils and to meet the challenge of global population growth and increasing demand for biofortified food.

Regulation of primary root growth by iron availability

Fe uptake from the soil is mediated by the root system [START_REF] Dubeaux | Getting to the root of plant iron uptake and cell-cell transport: Polarity matters![END_REF]. Root-related processes are major targets of responses to changing Fe availability in the rhizosphere (Fig. 1). Fe deficiency or excess imposes drastic changes on the root system architecture, which varies within and between plant species [START_REF] Gruber | Plasticity of the Arabidopsis root system under nutrient deficiencies[END_REF]. This supports the potential for discovering key genes and signaling pathways by screening for phenotypic diversity in Fe stress responses across plant ecotypes and accessions, using quantitative genetic approaches [START_REF] Dubeaux | Getting to the root of plant iron uptake and cell-cell transport: Polarity matters![END_REF][START_REF] Rouached | System-level understanding of plant mineral nutrition in the big data era[END_REF]. Recently, genome-wide association studies (GWAS) were employed in the discovery of genes underpinning the root architecture in response to either Fe limitation or excess, in the model plant Arabidopsis thaliana (Arabidopsis). FRO2 (FERRIC REDUCTASE OXIDASE 2) was indeed identified as a gene responsible for the regulation of primary root growth (PRG) in response to Fe deficiency [START_REF] Satbhai | Natural allelic variation of FRO2 modulates Arabidopsis root growth under iron deficiency[END_REF]. PRG variation in Fe-deficient conditions is explained by sequence variation of the FRO2 locus that causes differential gene expression, as well as ferricchelate reductase activity [START_REF] Satbhai | Natural allelic variation of FRO2 modulates Arabidopsis root growth under iron deficiency[END_REF]. The same GWAS approaches were used more recently to identify additional genes involved in the control of root growth rate in response to Fe deficiency in Arabidopsis. Several candidate genes were identified, although their functional A c c e p t e d M a n u s c r i p t 5 validation and the discovery of causal polymorphisms await future study (Bouain et al., 2019a).

GWAS has also been employed to identify loci associated with the natural variation of root growth in response to Fe toxicity. In this context, a causal gene named S-NITROSOGLUTATHIONE-REDUCTASE (GSNOR) was identified and characterized as essential in determining root growth in high Fe conditions [START_REF] Li | GSNOR provides plant tolerance to iron toxicity via preventing iron-dependent nitrosative and oxidative cytotoxicity[END_REF]. Plants with a non-functional GSNOR gene display higher Fe tolerance than plants with functional alleles. This mechanism appears to be related to the activities of a gaseous molecule, nitric oxide (NO). GSNOR likely plays a central role in NO metabolism and regulates the plant's ability to respond to cellular stress and damage [START_REF] Li | GSNOR provides plant tolerance to iron toxicity via preventing iron-dependent nitrosative and oxidative cytotoxicity[END_REF]. It has been proposed that in response to high Fe concentrations, the NO levels increase, impairing the tolerance of the plant roots [START_REF] Li | GSNOR provides plant tolerance to iron toxicity via preventing iron-dependent nitrosative and oxidative cytotoxicity[END_REF]. This mechanism is likely conserved between monocots and dicots, but it remains to be confirmed if this mechanism can be generalized to all higher plants. GSNOR and NO signaling contribute to salt stress response in the green alga Chlamydomonas, but have not been tied to Fe homeostasis [START_REF] Chen | The role of nitric oxide signalling in response to salt stress in Chlamydomonas reinhardtii[END_REF].

Iron and micronutrient interactions: root uptake

The interactions of Fe homeostasis with other micronutrients [e.g. zinc (Zn), manganese (Mn)] or toxic and non-essential metal cations [e.g. cadmium (Cd)] are strongly dependent on the Fe uptake mechanisms, which can come in two different flavors in plants (Fig. 2) and are even more diversified in algae (Box 1). In dicot plants such as Arabidopsis exists a reduction-based strategy (or Strategy I)

where Fe 2+ is taken-up by cells, whereas chelation and uptake of chelated Fe 3+ (Strategy II) is found in graminaceous monocots, i.e. grasses [START_REF] Marschner | Strategies of plants for acquisition of iron[END_REF]. Both strategies however share many molecular actors (see below and [START_REF] Grillet | Iron acquisition strategies in land plants: not so different after all[END_REF]. Unchelated Fe 3+ is also taken up directly in many algae (Box 1) (Blaby-Haas and Merchant, 2017). Both strategies of Fe 2+ or Fe 3+ uptake coexist in several species, rice or Chlamydomonas for instance (see below and Box 1) (Blaby-Haas and Merchant, 2017;[START_REF] Kobayashi | Iron transport and its regulation in plants[END_REF]. Divalent metal cations (Zn 2+ , Mn 2+ , Cd 2+ ) will therefore have a higher impact on Fe homeostasis in plants using the reduction strategy and Fe 2+ uptake. For instance, Zn excess in Arabidopsis results in shoot chlorosis, reduced shoot Fe accumulation and more generally in the induction of a secondary Fe deficiency. These Zn toxicity symptoms result from competition for uptake as well as for Fe binding sites in proteins (e.g. involved in respiration and photosynthesis), and are alleviated by elevated Fe supply [START_REF] Fukao | iTRAQ analysis reveals mechanisms of growth defects due to excess zinc in Arabidopsis[END_REF][START_REF] Shanmugam | Differential expression and regulation of iron-regulated metal transporters in Arabidopsis halleri and Arabidopsis thaliana--the role in zinc tolerance[END_REF][START_REF] Zargar | Quantitative proteomics of Arabidopsis shoot microsomal proteins reveals a cross-talk between excess Downloaded from[END_REF][START_REF] Lešková | Heavy Metals Induce Iron Deficiency Responses at Different Hierarchic and Regulatory Levels[END_REF]. Conversely, Fe deficiency results in increased shoot accumulation of Zn, but also Mn and cobalt (Co) [START_REF] Baxter | The leaf ionome as a multivariable system to detect a plant's physiological status[END_REF]. Such interactions were also documented in multiple dicot species in addition to Arabidopsis [START_REF] Foy | The Physiology of Metal Toxicity in Plants[END_REF][START_REF] Broadley | Zinc in plants[END_REF], for instance Brassica rapa [START_REF] Li | Expression profiling reveals functionally redundant multiple-copy genes related to zinc, iron and cadmium responses in Brassica rapa[END_REF] or Poplar [START_REF] Ariani | RNA sequencing of Populus x canadensis roots identifies key molecular mechanisms underlying physiological adaption to excess zinc[END_REF].

A c c e p t e d M a n u s c r i p t 6 In Arabidopsis, upon acidification of the soil [START_REF] Santi | Dissecting iron deficiency-induced proton extrusion in Arabidopsis roots[END_REF] and reduction of Fe 3+ into Fe 2+ by FRO2 [START_REF] Robinson | A ferric-chelate reductase for iron uptake from soils[END_REF], Fe 2+ is taken up in root epidermal cells by IRT1 (IRON-REGULATED TRANSPORTER 1), a transporter of the ZIP (ZINC-REGULATED AND IRON-REGULATED-like PROTEINS) family (Fig. 2A). IRT1 is a high affinity transporter but has a low specificity: in addition to Fe 2+ , it transports a range of other divalent metal cations, including Zn 2+ , Mn 2+ , Co 2+ , Ni 2+ and Cd 2+ [START_REF] Eide | A novel iron-regulated metal transporter from plants identified by functional expression in yeast[END_REF][START_REF] Vert | IRT1, an Arabidopsis transporter essential for iron uptake from the soil and for plant growth[END_REF][START_REF] Nishida | AtIRT1, the primary iron uptake transporter in the root, mediates excess nickel accumulation in Arabidopsis thaliana[END_REF]. IRT1 is a major contributor to the interaction of Fe homeostasis with other metal cations, in such a way that back-up systems are implemented in the Fe deficiency response to accommodate the IRT1-mediated non-specific uptake of non-Fe metal cations. Hence, up-regulation of HMA3 (HEAVY METAL ATPASE 3, [START_REF] Morel | AtHMA3, a P1B-ATPase allowing Cd/Zn/Co/Pb vacuolar storage in Arabidopsis[END_REF], MTP3 (METAL TOLERANCE PROTEIN 3, [START_REF] Arrivault | The Arabidopsis metal tolerance protein AtMTP3 maintains metal homeostasis by mediating Zn exclusion from the shoot under Fe deficiency and Zn oversupply[END_REF], MTP8 [START_REF] Eroglu | The Vacuolar Manganese Transporter MTP8 Determines Tolerance to Iron Deficiency-Induced Chlorosis in Arabidopsis[END_REF] and FPN2 (FERROPORTIN2, [START_REF] Schaaf | AtIREG2 encodes a tonoplast transport protein involved in iron-dependent nickel detoxification in Arabidopsis thaliana roots[END_REF]Morrissey et al., 2009) upon Fe deficiency ensures increased vacuolar storage capacity of, respectively, Zn/Cd, Zn, Mn and Co/Ni in root vacuoles, thus effectively limiting their toxicity in roots and their root-to-shoot translocation, whereas IRT2 ensures buffering of Fe itself in intracellular vesicles [START_REF] Vert | Arabidopsis IRT2 cooperates with the high-affinity iron uptake system to maintain iron homeostasis in root epidermal cells[END_REF]. This back-up system is coordinated by the complex action of the master Fe deficiency regulator FIT (FER-like IRON-DEFICIENCY-INDUCED TRANSCRIPTION FACTOR) and a multi-layer combination of additional transcription factors [e.g. more than 14 bHLH (BASIC HELIX-LOOP-Helix) proteins, 2 MYB (MYELOBLASTOSIS)] and regulators [e.g. E3 Ubiquitin

Ligases BTS (BRUTUS) and BTS-like targeting proteins for proteasome degradation], as well as hormonal control (e.g. by jasmonic acid, ethylene or gibberellins) and NO signaling. These processes have been extensively detailed in recent reviews (see for instance [START_REF] Brumbarova | Molecular mechanisms governing Arabidopsis iron uptake[END_REF][START_REF] Buet | Nitric oxide and plant iron homeostasis[END_REF][START_REF] Rodríguez-Celma | Hemerythrin E3 Ubiquitin Ligases as Negative Regulators of Iron Homeostasis in Plants[END_REF][START_REF] Kobayashi | Iron transport and its regulation in plants[END_REF]Wu and Ling, 2019 and this special issue). Recently, FBP (FIT-BINDING PROTEIN) was shown to control FIT activity, and to play an important role in fine-tuning Fe and Zn interactions [START_REF] Chen | A FIT-binding protein is involved in modulating iron and zinc homeostasis in Arabidopsis[END_REF]. In addition, the IRT1 protein itself is also subjected to a complex regulation by not only Fe but also by non-Fe metals (Zn, Mn, Co), at the transcriptional and post-transcriptional levels [START_REF] Connolly | Expression of the IRT1 metal transporter is controlled by metals at the levels of transcript and protein accumulation[END_REF][START_REF] Fukao | iTRAQ analysis reveals mechanisms of growth defects due to excess zinc in Arabidopsis[END_REF][START_REF] Shanmugam | Differential expression and regulation of iron-regulated metal transporters in Arabidopsis halleri and Arabidopsis thaliana--the role in zinc tolerance[END_REF]. Internalization of IRT1 from the plasma membrane upon ubiquitination followed by its recycling to the plasma membrane or its targeting for degradation in vacuoles is regulated by non-Fe metal (Zn, Mn, Co) availability to ensure proper balance between Fe uptake and the hazardous effects of excessive uptake of other divalent metals [START_REF] Barberon | Monoubiquitin-dependent endocytosis of the iron-regulated transporter 1 (IRT1) transporter controls iron uptake in plants[END_REF][START_REF] Barberon | Polarization of IRON-REGULATED TRANSPORTER 1 (IRT1) to the plant-soil interface plays crucial role in metal homeostasis[END_REF][START_REF] Dubeaux | Metal Sensing by the IRT1 Transporter-Receptor Orchestrates Its Own Degradation and Plant Metal Nutrition[END_REF].

Evidence is accumulating that the broad specificity of IRT1 plays also a role in metal uptake in socalled metal hyperaccumulator plants [START_REF] Krämer | Metal hyperaccumulation in plants[END_REF][START_REF] Hanikenne | Metal hyperaccumulation and hypertolerance: a model for plant evolutionary genomics[END_REF][START_REF] Merlot | Physiology and Molecular Biology of Trace Element Hyperaccumulation[END_REF] and that IRT1 may be a major determinant of the intraspecific variation of the trait. Indeed, variation of IRT1 gene expression or function is linked to (i) variation in Zn and/or Cd shoot [START_REF] Corso | Contrasting cadmium resistance strategies in two metallicolous populations of Arabidopsis halleri[END_REF][START_REF] Schvartzman | Adaptation to high zinc depends on distinct mechanisms in metallicolous populations of Arabidopsis halleri[END_REF] and Noccaea caerulescens [START_REF] Halimaa | Transcriptional effects of cadmium on iron homeostasis differ in calamine accessions of Noccaea caerulescens[END_REF]; and (ii) Ni shoot accumulation in Ni-hyperaccumulating populations of N. caerulescens (Halimaa et al., 2014a,b) and in Senecio coronatus [START_REF] Meier | Comparative RNA-seq analysis of nickel hyperaccumulating and non-accumulating populations of Senecio coronatus (Asteraceae)[END_REF]. More generally, exposure to high metal levels and metal hyperaccumulation itself interferes with Fe homeostasis [START_REF] Talke | Zinc-dependent global transcriptional control, transcriptional deregulation, and higher gene copy number for genes in metal homeostasis of the hyperaccumulator Arabidopsis halleri[END_REF][START_REF] Krämer | Transition metal transport[END_REF][START_REF] Lin | Arabidopsis IRT3 is a zinc-regulated and plasma membrane localized zinc/iron transporter[END_REF][START_REF] Willems | Quantitative trait loci analysis of mineral element concentrations in an Arabidopsis halleri x Arabidopsis lyrata petraea F2 progeny grown on cadmium-contaminated soil[END_REF][START_REF] Sinclair | The zinc homeostasis network of land plants[END_REF][START_REF] Charlier | Zinc triggers a complex transcriptional and post-transcriptional regulation of the metal homeostasis gene FRD3 in Arabidopsis relatives[END_REF], and maintaining Fe homeostasis is thus key in hyperaccumulator species. This is achieved, in at least some populations or species, by inducing an Fe deficiency response, which mobilizes specific parts of the multi-layer regulation pathway found in Arabidopsis [START_REF] Shanmugam | Differential expression and regulation of iron-regulated metal transporters in Arabidopsis halleri and Arabidopsis thaliana--the role in zinc tolerance[END_REF][START_REF] Corso | Contrasting cadmium resistance strategies in two metallicolous populations of Arabidopsis halleri[END_REF][START_REF] Schvartzman | Adaptation to high zinc depends on distinct mechanisms in metallicolous populations of Arabidopsis halleri[END_REF].

If Fe deficiency results in Zn, Co and Cd accumulation in Arabidopsis shoots (Col-0 accession), it has the opposite effect on molybdenum (Mo) shoot accumulation [START_REF] Baxter | The leaf ionome as a multivariable system to detect a plant's physiological status[END_REF]. Indeed, the acidification of the medium associated to the Fe uptake strategy I limits Mo bioavailability [START_REF] Marschner | Marschner's Mineral Nutrition of Higher Plants[END_REF]. In addition, several micro-RNAs initially identified as major players on Cu homeostasis [START_REF] Pilon | The copper microRNAs[END_REF] were recently shown to also be linked to Fe homeostasis, by controlling the oxidative stress response and oxidative metabolic processes [START_REF] Carrió-Seguí | The Altered Expression of microRNA408 Influences the Arabidopsis Response to Iron Deficiency[END_REF][START_REF] Waters | Rosette iron deficiency transcript and microRNA profiling reveals links between copper and iron homeostasis in Arabidopsis thaliana[END_REF].

In grasses, Fe acquisition involves the secretion of mugineic acid family (MA) phytosiderophores, high affinity Fe 3+ chelators able to solubilize Fe from the soil (Fig. 2B) [START_REF] Marschner | Strategies of plants for acquisition of iron[END_REF][START_REF] Kobayashi | Iron Uptake, Translocation, and Regulation in Higher Plants[END_REF]. Upon synthesis from S-adenosyl-L-methionine (SAM), with nicotianamine (NA, see below) as intermediate, MA are secreted into the soil by TOM1 (TRANSPORTER OF MUGINEIC ACID FAMILY PHYTOSIDEROPHORES 1) [START_REF] Nozoye | Phytosiderophore efflux transporters are crucial for iron acquisition in graminaceous plants[END_REF]. MA-Fe(III) complexes are then taken-up by YS1/YSL transporters (YELLOW-STRIPE 1/YS-Like) [START_REF] Curie | Maize yellow stripe1 encodes a membrane protein directly involved in Fe(III) uptake[END_REF][START_REF] Inoue | Rice OsYSL15 is an iron-regulated iron (III) -deoxymugineic acid transporter expressed in the roots and is essential for iron uptake in early growth of the seedlings[END_REF]. This strategy is particularly advantageous on calcareous soils where high pH reduces strongly Fe availability [START_REF] Marschner | Strategies of plants for acquisition of iron[END_REF]. Many commonalities have been found between the regulation of the Fe deficiency response in dicots and grasses, including the involvement of bHLH transcription factors and E3 Ubiquitin Ligases, but with also some differences, including the key role of IDEF1 and IDEF2 (IRON DEFICIENCY-RESPONSIVE ELEMENT BINDING FACTOR) transcription factors in transcriptional regulation and Fe status sensing in rice (for detailed reviews, see Kobayashi andNishizawa, 2012, 2015;[START_REF] Kobayashi | Iron transport and its regulation in plants[END_REF].

Rice (Oryza sativa) has been the most used model to uncover metal homeostasis mechanisms in grasses. However, in contrast to other graminaceous plants, it uses a Fe uptake strategy, which possibly evolved prior to rice domestication [START_REF] Wairich | The Combined Strategy for iron uptake is not exclusive to domesticated rice (Oryza sativa)[END_REF], combining elements of both reduction and chelation strategies and this may have biased our view on how Fe homeostasis takes A c c e p t e d M a n u s c r i p t 8 place in grasses. Indeed, OsIRT1 and OsIRT2, two proteins related to the Arabidopsis IRT1 protein [START_REF] Evens | The role of ZIP transporters and group F bZIP transcription factors in the Zn-deficiency response of wheat (Triticum aestivum)[END_REF], are involved in Fe 2+ uptake, in the absence of both acidification by H + -ATPase and ferric-chelate reductase activity. The two corresponding genes are highly induced upon Fe deficiency. In addition to the chelation strategy, direct uptake of Fe 2+ comes handy in paddy fields, which have low oxygen and high Fe 2+ levels [START_REF] Ishimaru | Rice plants take up iron as an Fe 3+ -phytosiderophore and as Fe 2+[END_REF][START_REF] Cheng | Mutation in nicotianamine aminotransferase stimulated the Fe(II) acquisition system and led to iron accumulation in rice[END_REF]. It possibly comes at a cost of specificity as overexpression of OsIRT1 in rice not only results in increased accumulation of Fe, but also of Zn and Cd in root and shoot tissues, and in higher sensitivity to high Zn and Cd exposure [START_REF] Lee | Over-expression of OsIRT1 leads to increased iron and zinc accumulations in rice[END_REF]. OsIRT1 and OsIRT2 were suggested to contribute to higher Cd accumulation upon Fe deficiency [START_REF] Nakanishi | Iron deficiency enhances cadmium uptake and translocation mediated by the Fe 2+ transporters OsIRT1 and OsIRT2 in rice[END_REF]. In contrast to its Arabidopsis homolog, however, the OsIRT1 transcript level is not induced by Zn excess, suggesting a distinct impact of Zn in the two species [START_REF] Ishimaru | Synthesis of nicotianamine and deoxymugineic acid is regulated by OsIRO2 in Zn excess rice plants[END_REF]. The combined strategy of Fe uptake is limited to rice and a few relatives [START_REF] Ishimaru | Synthesis of nicotianamine and deoxymugineic acid is regulated by OsIRO2 in Zn excess rice plants[END_REF][START_REF] Wairich | The Combined Strategy for iron uptake is not exclusive to domesticated rice (Oryza sativa)[END_REF]. Although its genome contains an IRT1 homolog [START_REF] Li | Identification and characterization of the zinc-regulated transporters, iron-regulated transporter-like protein (ZIP) gene family in maize[END_REF][START_REF] Evens | The role of ZIP transporters and group F bZIP transcription factors in the Zn-deficiency response of wheat (Triticum aestivum)[END_REF] and genes putatively encoding an H + -ATPase and a ferricchelate reductase [START_REF] Li | Is there a strategy I iron uptake mechanism in maize?[END_REF], maize (Zea mays) is indeed strictly dependent on MA-Fe(III) uptake for Fe acquisition and survival, whereas rice plants unable to synthetize MAs are healthy when supplied with Fe 2+ [START_REF] Cheng | Mutation in nicotianamine aminotransferase stimulated the Fe(II) acquisition system and led to iron accumulation in rice[END_REF]. An IRT1 homolog is also found in barley [START_REF] Evens | The role of ZIP transporters and group F bZIP transcription factors in the Zn-deficiency response of wheat (Triticum aestivum)[END_REF], but is important for Mn uptake, and not Fe [START_REF] Pedas | Manganese efficiency in barley: identification and characterization of the metal ion transporter HvIRT1[END_REF].

Grasses are more sensitive to Zn toxicity than dicots, especially on calcareous soils. This observation can be linked to the Fe deficiency-induced production of phytosiderophores by roots that also strongly solubilizes Zn [START_REF] Chaney | Zinc Phytotoxicity[END_REF]. MAs were indeed shown to contribute to chelation and uptake of non-Fe metals, including Zn upon Zn deficiency in the form of MA-Zn(II), in barley and maize [START_REF] Von Wirén | Roots of Iron-Efficient Maize also Absorb Phytosiderophore-Chelated Zinc[END_REF][START_REF] Suzuki | Biosynthesis and secretion of mugineic acid family phytosiderophores in zinc-deficient barley[END_REF]. In rice, secretion of MAs upon Zn deficiency is low compared to other grasses [START_REF] Suzuki | Deoxymugineic acid increases Zn translocation in Zn-deficient rice plants[END_REF]), but appears to be sufficient to account for Zn uptake in these conditions [START_REF] Arnold | Evidence for the mechanisms of zinc uptake by rice using isotope fractionation[END_REF][START_REF] Ptashnyk | Enhanced zinc uptake by rice through phytosiderophore secretion: a modelling study[END_REF]Markovid et al., 2017). MAs are also involved in Zn translocation to shoot tissues in rice [START_REF] Suzuki | Deoxymugineic acid increases Zn translocation in Zn-deficient rice plants[END_REF]. In many cases, overexpression of elements of the MA synthesis, secretion, uptake or translocation pathways results in an increased Fe but also Zn content in rice grains [START_REF] Bashir | The road to micronutrient biofortification of rice: progress and prospects[END_REF].

Iron and micronutrient interactions: intracellular and long distance trafficking

In addition to uptake, Fe and micronutrient intracellular trafficking and long distance distribution within the plant is mediated by common metal chelators and (chelated) metal transporters (Fig. 2).

For instance, the ZIP transporter IRT3 is a plasma-membrane located Zn and Fe transporter in Arabidopsis, and was therefore suggested to contribute to cellular uptake of Fe and Zn [START_REF] Lin | Arabidopsis IRT3 is a zinc-regulated and plasma membrane localized zinc/iron transporter[END_REF]. The IRT3 gene is induced by Zn deficiency [START_REF] Talke | Zinc-dependent global transcriptional control, transcriptional deregulation, and higher gene copy number for genes in metal homeostasis of the hyperaccumulator Arabidopsis halleri[END_REF] but not by Fe deficiency [START_REF] Yang | Transcriptional profiling of the Arabidopsis iron deficiency response reveals conserved transition metal homeostasis networks[END_REF], and its expression level, together with that of ZIP4, has been linked to variation of Zn A c c e p t e d M a n u s c r i p t 9 deficiency tolerance among Arabidopsis accessions [START_REF] Campos | Natural variation in Arabidopsis thaliana reveals shoot ionome, biomass, and gene expression changes as biomarkers for zinc deficiency tolerance[END_REF]. IRT3 is also associated with a quantitative trait locus for Fe accumulation in shoots of A. halleri, suggesting that it may contribute to maintenance of Fe homeostasis in the Zn/Cd hyperaccumulator [START_REF] Willems | Quantitative trait loci analysis of mineral element concentrations in an Arabidopsis halleri x Arabidopsis lyrata petraea F2 progeny grown on cadmium-contaminated soil[END_REF].

Several other Arabidopsis ZIP transporters were shown to transport Fe and Zn (ZIP3) or to be regulated by both the Fe and Zn status of the plant (ZIP10, ZIP11, ZIP12) [START_REF] Shanmugam | Control of Zn uptake in Arabidopsis halleri: a balance between Zn and Fe[END_REF], but as for most ZIP transporters in plants, their individual contributions to metal homeostasis remain largely unknown [START_REF] Ricachenevsky | Got to hide your Zn away: Molecular control of Zn accumulation and biotechnological applications[END_REF].

Transporters of the NRAMP (NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN) family are also tying Fe to other microelements, such as Mn and Zn. Indeed, the Arabidopsis NRAMP1 was shown to transport Mn and Fe in yeast [START_REF] Thomine | Cadmium and iron transport by members of a plant metal transporter family in Arabidopsis with homology to Nramp genes[END_REF][START_REF] Curie | Alimentation et l'Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 23 thaliana in iron transport[END_REF], and to be the main Mn uptake system in plant roots [START_REF] Cailliatte | High-affinity manganese uptake by the metal transporter NRAMP1 is essential for Arabidopsis growth in low manganese conditions[END_REF]Bian et al., 2018). NRAMP1 also contributes to Fe uptake together with IRT1 [START_REF] Castaings | The high-affinity metal Transporters NRAMP1 and IRT1 Team up to Take up Iron under Sufficient Metal Provision[END_REF]. As IRT1 (see above), NRAMP1 undergoes intracellular trafficking, involving a phosphatidylinositol 3-phosphate-binding protein, with a localization on endomembranes or the plasma membrane, possibly enabling the fine-tuning of Fe, and Mn, uptake [START_REF] Agorio | Phosphatidylinositol 3-phosphate-binding protein AtPH1 controls the localization of the metal transporter NRAMP1 in Arabidopsis[END_REF]. Higher expression of NRAMP1 has been associated with higher shoot Cd accumulation among rice (Takahashi et al., 2011a,b) or N. caerulescens [START_REF] Milner | Root and shoot transcriptome analysis of two ecotypes of Noccaea caerulescens uncovers the role of NcNramp1 in Cd hyperaccumulation[END_REF] accessions. In rice and possibly other monocots, NRAMP5, primarily a Mn transporter, is a major determinant of Cd accumulation in grains (reviewed in [START_REF] Clemens | Metal ligands in micronutrient acquisition and homeostasis[END_REF].

NRAMP3 and NRAMP4 are two additional Fe and Mn transporters, localized in the vacuolar membrane in Arabidopsis. Together, they are essential for Fe mobilization from vacuoles during seed germination [START_REF] Lanquar | Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 is essential for seed germination on low iron[END_REF], and for providing sufficient Mn for the photosynthetic apparatus in mature leaves [START_REF] Lanquar | Export of vacuolar manganese by AtNRAMP3 and AtNRAMP4 is required for optimal photosynthesis and growth under manganese deficiency[END_REF]. Both proteins are furthermore contributing to Zn and Cd tolerance by mediating appropriate Fe and Mn supply to chloroplasts, thus maintaining the photosynthetic function when exposed to Zn excess or Cd [START_REF] Molins | Mutants impaired in vacuolar metal mobilization identify chloroplasts as a target for cadmium hypersensitivity in Arabidopsis thaliana[END_REF]. NRAMP3 is overexpressed in A. halleri and N. caerulescens, suggesting a contribution to Zn/Cd hypertolerance or hyperaccumulation [START_REF] Talke | Zinc-dependent global transcriptional control, transcriptional deregulation, and higher gene copy number for genes in metal homeostasis of the hyperaccumulator Arabidopsis halleri[END_REF][START_REF] Van De Mortel | Large expression differences in genes for iron and zinc homeostasis, stress response, and lignin biosynthesis distinguish roots of Arabidopsis thaliana and the related metal hyperaccumulator Thlaspi caerulescens[END_REF][START_REF] Oomen | Functional characterization of NRAMP3 and NRAMP4 from the metal hyperaccumulator Thlaspi caerulescens[END_REF]. The NRAMP3 gene is further associated with a Zn tolerance QTL for photosynthetic yield in a F2 progeny of an intraspecific cross in A. halleri [START_REF] Karam | Genetic architecture of a plant adaptive trait: QTL mapping of intraspecific variation for tolerance to metal pollution in Arabidopsis halleri[END_REF]. The counterpart of NRAMP3 and NRAMP4 in Arabidopsis is VIT1, which is key for Fe vacuolar storage in seeds (Kim et al., 2006). In rice, two VIT1 homologs (OsVIT1 and OsVIT2) are vacuolar transporters of Fe and Zn, and are responsible for vacuolar storage of these metals in flag leaf blade and sheath, and consequently controlling the transfer of both elements to grains (Zhang et al., 2012).

Both inter-and intra-cellular movements of Fe and other metals (e.g. Zn, Mn) are facilitated by the production of common chelating compounds such as the organic acid citrate and NA, with which A c c e p t e d M a n u s c r i p t 10 these metals form soluble complexes (Morrissey and Guerinot, 2009;[START_REF] Sinclair | The zinc homeostasis network of land plants[END_REF][START_REF] Flis | Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 24 in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection[END_REF]. In grasses, MA phytosiderophores are additionally contributing to these processes [START_REF] Suzuki | Deoxymugineic acid increases Zn translocation in Zn-deficient rice plants[END_REF][START_REF] Kobayashi | Iron Uptake, Translocation, and Regulation in Higher Plants[END_REF]. The production and subcellular distribution of these metal chelator molecules are affected by metal availability. It is indeed well known that Fe deficiency and Zn excess both result in increased concentration of citrate in roots (i.e. in the xylem sap) [START_REF] López-Millán | Effects of iron deficiency on the composition of the leaf apoplastic fluid and xylem sap in sugar beet. Implications for iron and carbon transport[END_REF][START_REF] Sarret | Forms of zinc accumulated in the hyperaccumulator Arabidopsis halleri[END_REF]. Citrate has an optimum chelating capacity and forms more stable complexes at acidic pH (ranging from 5.5 to 6), a pH similar to that of the xylem sap [START_REF] López-Millán | Effects of iron deficiency on the composition of the leaf apoplastic fluid and xylem sap in sugar beet. Implications for iron and carbon transport[END_REF][START_REF] Ryan | Function and mechanism of organic anion exudation from plant roots[END_REF][START_REF] Yoneyama | Route and Regulation of Zinc, Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters, Metal Speciation, Grain Cd Reduction and Zn and Fe Biofortification[END_REF]. At this pH range, Fe is transferred from cytoplasmic NA to citrate upon loading in the xylem sap as [FeCitrateOH] -1 and [FeCitrate 2 ] -3 complexes [START_REF] López-Millán | Effects of iron deficiency on the composition of the leaf apoplastic fluid and xylem sap in sugar beet. Implications for iron and carbon transport[END_REF][START_REF] Ryan | Function and mechanism of organic anion exudation from plant roots[END_REF][START_REF] Yoneyama | Route and Regulation of Zinc, Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters, Metal Speciation, Grain Cd Reduction and Zn and Fe Biofortification[END_REF].

Similarly, Zn-citrate complexes are also largely present in the xylem sap [START_REF] Cornu | Contrasting effects of nicotianamine synthase knockdown on zinc and nickel tolerance and accumulation in the zinc/cadmium hyperaccumulator Arabidopsis halleri[END_REF][START_REF] Flis | Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 24 in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection[END_REF]. Citrate loading into the xylem results from the activity of the citrate effluxer FRD3 (FERRIC REDUCTASE DEFECTIVE 3), a member of the MATE (MULTIDRUG AND TOXIC EFFLUX) family [START_REF] Durrett | The FRD3-mediated efflux of citrate into the root vasculature is necessary for efficient iron translocation[END_REF], whose expression in Arabidopsis is not only induced upon Fe deficiency but also upon Zn excess in roots and Zn deficiency in shoots [START_REF] Pineau | Natural variation at the FRD3 MATE transporter locus reveals cross-talk between Fe homeostasis and Zn tolerance in Arabidopsis thaliana[END_REF][START_REF] Charlier | Zinc triggers a complex transcriptional and post-transcriptional regulation of the metal homeostasis gene FRD3 in Arabidopsis relatives[END_REF]. frd3 mutant plants are small and chlorotic, and display constitutive overexpression of the strategy I Fe uptake machinery (see above) as well accumulation of Fe, Mn and Zn in tissues [START_REF] Delhaize | A metal-accumulator mutant of Arabidopsis thaliana[END_REF][START_REF] Rogers | FRD3, a member of the multidrug and toxin efflux family, controls iron deficiency responses in Arabidopsis[END_REF][START_REF] Green | FRD3 controls iron localization in Arabidopsis[END_REF][START_REF] Durrett | The FRD3-mediated efflux of citrate into the root vasculature is necessary for efficient iron translocation[END_REF]. In addition to severe Fe deficiency in shoot tissues, the growth defect of the frd3 mutant results from Mn toxicity as well as Fe accumulation-induced reactive oxygen species production and biotic stress response [START_REF] Scheepers | Intertwined metal homeostasis, oxidative and biotic stress responses in the Arabidopsis frd3 mutant[END_REF]. Zn excess partially restores growth of the mutant, by abolishing Mn accumulation in shoot and Fe accumulation in the cell-wall in roots [START_REF] Scheepers | Intertwined metal homeostasis, oxidative and biotic stress responses in the Arabidopsis frd3 mutant[END_REF]. This not only depicts the crosstalk between Fe and Zn homeostasis but also the role of FRD3 as an essential regulator of this interplay [START_REF] Pineau | Natural variation at the FRD3 MATE transporter locus reveals cross-talk between Fe homeostasis and Zn tolerance in Arabidopsis thaliana[END_REF][START_REF] Scheepers | Intertwined metal homeostasis, oxidative and biotic stress responses in the Arabidopsis frd3 mutant[END_REF]. An homolog of FRD3 has been characterized in rice (OsFRDL1, FRD-Like 1) and shares a similar function in citrate efflux to the root xylem for Fe translocation to the shoots and in Fe distribution in panicles [START_REF] Inoue | A rice FRD3-like (OsFRDL1) gene is expressed in the cells involved in long-distance transport[END_REF][START_REF] Yokosho | OsFRDL1 is a citrate transporter required for efficient translocation of iron in rice[END_REF][START_REF] Yokosho | OsFRDL1 expressed in nodes is required for distribution of iron to grains in rice[END_REF][START_REF] Yoneyama | Route and Regulation of Zinc, Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters, Metal Speciation, Grain Cd Reduction and Zn and Fe Biofortification[END_REF]. However, osfrdl1 mutants do not display severe growth phenotypes as an Arabidopsis frd3 mutant [START_REF] Yokosho | OsFRDL1 is a citrate transporter required for efficient translocation of iron in rice[END_REF], suggesting that chelators, in addition to citrate, are important for Fe translocation to shoot in rice. Hence, PEZ1 (PHENOLICS EFFLUX ZERO 1), a MATE transporter, is responsible for phenolic compound release into the xylem in rice roots contributing to Fe mobilization and long distance Fe translocation [START_REF] Ishimaru | A rice phenolic efflux transporter is essential for solubilizing precipitated apoplasmic iron in the plant stele[END_REF]. TOM2 [START_REF] Nozoye | The Phytosiderophore Efflux Transporter TOM2 Is Involved in Metal Transport in Rice[END_REF] and ENA1 (EFFLUX TRANSPORTER OF NA 1) [START_REF] Nozoye | Characterization of the Nicotianamine Exporter ENA1 in Rice[END_REF] are additionally required for efflux of DMA and NA into the apoplasm, respectively, and also contribute to Fe mobilization in this compartment.

A c c e p t e d M a n u s c r i p t 11 NA has a key function in the control of metal cellular levels in both monocots and dicots. The formation and the stability of the NA-metal complexes are optimal at pH above 6.5, suggesting that NA serves as a chelator predominately enriched within cells and phloem sap where it contributes to compartmentalization of metals and long distance transport [START_REF] Von Wirén | Nicotianamine chelates both Fe III and Fe II . Implications for metal transport in plants[END_REF]. NA is synthesized from SAM by NA SYNTHASE (NAS) enzymes [START_REF] Pianelli | Nicotianamine over-accumulation confers resistance to nickel in Arabidopsis thaliana[END_REF]Haydon and Cobbett, 2007a). The Arabidopsis genome contains four NAS genes (AtNAS1-4), whose expression is upregulated upon Fe or Zn deficiency, as well as Zn excess [START_REF] Colangelo | The essential basic helix-loop-helix protein FIT1 is required for the iron deficiency response[END_REF][START_REF] Van De Mortel | Large expression differences in genes for iron and zinc homeostasis, stress response, and lignin biosynthesis distinguish roots of Arabidopsis thaliana and the related metal hyperaccumulator Thlaspi caerulescens[END_REF][START_REF] Klatte | The analysis of Arabidopsis nicotianamine synthase mutants reveals functions for nicotianamine in seed iron loading and iron deficiency responses[END_REF][START_REF] Chen | A FIT-binding protein is involved in modulating iron and zinc homeostasis in Arabidopsis[END_REF]. nas4x-1 and nas4x-2 quadruple mutants are sensitive to Fe deficiency but not Zn deficiency and are unable to properly distribute Fe and Zn to flowers due to the drastic reduction in NA levels, thereby resulting in sterility [START_REF] Klatte | The analysis of Arabidopsis nicotianamine synthase mutants reveals functions for nicotianamine in seed iron loading and iron deficiency responses[END_REF][START_REF] Schuler | Transcriptome analysis by GeneTrail revealed regulation of functional categories in response to alterations of iron homeostasis in Arabidopsis thaliana[END_REF][START_REF] Schuler | Nicotianamine functions in the Phloem-based transport of iron to sink organs, in pollen development and pollen tube growth in Arabidopsis[END_REF]. NA levels are a key element controlling radial movements of metals in the roots, either favoring vacuolar storage or translocation to the shoots, two processes where Fe and Zn compete as well [START_REF] Arrivault | The Arabidopsis metal tolerance protein AtMTP3 maintains metal homeostasis by mediating Zn exclusion from the shoot under Fe deficiency and Zn oversupply[END_REF][START_REF] Chen | A FIT-binding protein is involved in modulating iron and zinc homeostasis in Arabidopsis[END_REF][START_REF] Haydon | Vacuolar nicotianamine has critical and distinct roles under iron deficiency and for zinc sequestration in Arabidopsis[END_REF]. NA overaccumulation in A. thaliana results in increased sensitivity to Fe deficiency, but Ni resistance, as well as decreased levels of Fe in roots resulting from enhanced translocation of Fe to shoots, and Zn elevation in both roots and shoots [START_REF] Pianelli | Nicotianamine over-accumulation confers resistance to nickel in Arabidopsis thaliana[END_REF][START_REF] Kim | Increased nicotianamine biosynthesis confers enhanced tolerance of high levels of metals, in particular nickel, to plants[END_REF][START_REF] Cassin | Increased sensitivity to iron deficiency in Arabidopsis thaliana overaccumulating nicotianamine[END_REF], Chen et al., 2018). It was also shown that an elevated production of NA is a key factor for Zn hyperaccumulation in A. halleri [START_REF] Weber | Comparative microarray analysis of Arabidopsis thaliana and Arabidopsis halleri roots identifies nicotianamine synthase, a ZIP transporter and other genes as potential metal hyperaccumulation factors[END_REF][START_REF] Uraguchi | Elevated root nicotianamine concentrations are critical for Zn hyperaccumulation across diverse edaphic environments[END_REF].

Numerous proteins are involved in both the partitioning and long distance transport of NA-metal complexes. Two major facilitator superfamily transporters, ZIF1 and ZIF2 (ZINC-INDUCED FACILITATOR), localize to the tonoplast where they control the vacuolar storage of NA-chelated metals (i.e. Fe 2+ , Fe 3+ , Zn 2+ ) in Arabidopsis [START_REF] Beneš | Metal complex formation by nicotianamine, a Downloaded from[END_REF][START_REF] Von Wirén | Nicotianamine chelates both Fe III and Fe II . Implications for metal transport in plants[END_REF]. ZIF1 plays a critical role in Zn detoxification under Fe deficiency (Haydon and Cobbett, 2007b), and omission of Zn from the media partially suppresses the growth defect of the Fe-deficient zif1-3 mutant [START_REF] Haydon | Vacuolar nicotianamine has critical and distinct roles under iron deficiency and for zinc sequestration in Arabidopsis[END_REF]. ZIF1 expression is induced upon Zn excess (Haydon and Cobbett, 2007b) or Fe limitation [START_REF] Haydon | Vacuolar nicotianamine has critical and distinct roles under iron deficiency and for zinc sequestration in Arabidopsis[END_REF]. Increased vacuolar partitioning of NA through ZIF1 overexpression leads to vacuolar Zn sequestration in roots and ultimately to decreased Zn shoot levels as a result of the impairment of both Zn symplastic mobility and xylem loading [START_REF] Haydon | Vacuolar nicotianamine has critical and distinct roles under iron deficiency and for zinc sequestration in Arabidopsis[END_REF]. ZIF1 overexpressing plants also display reduced leaf cell-to-cell mobility of Fe, leading to constitutive symptoms of Fe deficiency [START_REF] Haydon | Vacuolar nicotianamine has critical and distinct roles under iron deficiency and for zinc sequestration in Arabidopsis[END_REF]. Higher expression of ZIF2 upon Zn excess leads to increased Zn tolerance thanks to favored root vacuolar Zn sequestration and prevention of NA-Zn translocation to shoots [START_REF] Remy | Intron retention in the 5'UTR of the novel ZIF2 transporter enhances translation to promote zinc tolerance in arabidopsis[END_REF]. In contrast to non-accumulator species that achieve root vacuolar sequestration of excessive metals when exposed to increased amounts of chelating [START_REF] Deinlein | Elevated nicotianamine levels in Arabidopsis halleri roots play a key role in zinc hyperaccumulation[END_REF][START_REF] Cornu | Contrasting effects of nicotianamine synthase knockdown on zinc and nickel tolerance and accumulation in the zinc/cadmium hyperaccumulator Arabidopsis halleri[END_REF].

Involved in the transport of both NA-and MA-metal complexes in monocots, the YSL function is restricted to the mobilization of NA-chelated metals (NA-Fe, -Zn, -Ni or -Cu) in dicots [START_REF] Schaaf | ZmYS1 functions as a protoncoupled symporter for phytosiderophore-and nicotianamine-chelated metals[END_REF][START_REF] Waters | Mutations in Arabidopsis yellow stripe-like1 and yellow stripe-like3 reveal their roles in metal ion homeostasis and loading of metal ions in seeds[END_REF][START_REF] Sinclair | The zinc homeostasis network of land plants[END_REF]. In Arabidopsis, the 8 YSL genes display differential patterns of regulation Fe, Zn and Cu deficiency and/or excess [START_REF] Didonato | Arabidopsis Yellow Stripe-Like2 (YSL2): a metal-regulated gene encoding a plasma membrane transporter of nicotianamine-metal complexes[END_REF][START_REF] Schaaf | A putative function for the Arabidopsis Fe-Phytosiderophore transporter homolog AtYSL2 in Fe and Zn homeostasis[END_REF][START_REF] Jean | A loss-of-function mutation in AtYSL1 reveals its role in iron and nicotianamine seed loading[END_REF][START_REF] Waters | Mutations in Arabidopsis yellow stripe-like1 and yellow stripe-like3 reveal their roles in metal ion homeostasis and loading of metal ions in seeds[END_REF][START_REF] Conte | Arabidopsis thaliana Yellow Stripe1-Like4 and Yellow Stripe1-Like6 localize to internal cellular membranes and are involved in metal ion homeostasis[END_REF]. Single (ysl1) or double mutant (ysl1ysl3 and ysl4ysl6) plants mainly display alterations of NA and metal concentrations in tissues, characterized by increased levels of Zn, Mn and Cu, but reduced amount of Fe, in leaves, as well as a subsequent deficiency of Fe, Zn and Cu in seeds, leading to a low fertility (Le [START_REF] Jean | A loss-of-function mutation in AtYSL1 reveals its role in iron and nicotianamine seed loading[END_REF][START_REF] Waters | Mutations in Arabidopsis yellow stripe-like1 and yellow stripe-like3 reveal their roles in metal ion homeostasis and loading of metal ions in seeds[END_REF][START_REF] Chu | Successful reproduction requires the function of Arabidopsis Yellow Stripe-Like1 and Yellow Stripe-Like3 metalnicotianamine transporters in both vegetative and reproductive structures[END_REF][START_REF] Conte | Arabidopsis thaliana Yellow Stripe1-Like4 and Yellow Stripe1-Like6 localize to internal cellular membranes and are involved in metal ion homeostasis[END_REF].

In addition to citrate and NA, glutathione (GSH) is known to contribute to the crosstalk between Zn and Fe homeostasis in Arabidopsis. GSH is an antioxidant contributing to the ascorbateglutathione cycle that plays a role in Fe-mediated Zn tolerance. Indeed, a loss-of-function mutation in GSH1, which encodes a γ-glutamylcysteine synthetase involved in the biosynthesis of GSH, leads to increased sensitivity to Zn excess in the concomitant presence of additional Fe [START_REF] Shanmugam | ZINC TOLERANCE INDUCED BY IRON 1 reveals the importance of glutathione in the cross-homeostasis between zinc and iron in Arabidopsis thaliana[END_REF]. GSH also links Fe and Mo homeostasis. Indeed, the ATM3 (ABC TRANSPORTER OF THE MITOCHONDRIA 3) exports GSH-polysulfide out of the mitochondria and is required for the maturation of Fe-S cytosolic enzymes and for molybdenum cofactor (Moco) biosynthesis [START_REF] Schaedler | A Conserved Mitochondrial ATP-binding Cassette Transporter Exports Glutathione Polysulfide for Cytosolic Metal Cofactor Assembly[END_REF][START_REF] Teschner | A novel role for Arabidopsis mitochondrial ABC transporter ATM3 in molybdenum cofactor biosynthesis[END_REF]. atm3 mutants are moreover sensitive to Cd (Hanikenne et al., 2005b;Kim et al., 2006).

Iron and sulfur homeostasis interaction

Evidence at the molecular and morphological levels in support of the interplay between Fe and sulfur (S) homeostasis is constantly growing. It has been shown that despite the presence of sufficient S in the medium, Fe deficiency conditions lead to a S deficiency response at the molecular level in durum wheat (Triticum turgidum L.; [START_REF] Ciaffi | Transcriptional and physiological changes in the S assimilation pathway due to single or combined S and Fe deprivation in durum wheat (Triticum durum L.) seedlings[END_REF]. Grasses (Strategy II plants) respond to Fe deficiency by secreting phytosiderophores into the rhizosphere, which are required for chelation as well as Fe uptake (see above). Interestingly, the release of these molecules is modulated by S availability: S deficiency reduces their release, whereas sulfate resupply enhances it in barley (Hordeum vulgare L.) [START_REF] Astolfi | Supply of sulphur to S-deficient young barley seedlings restores their capability to cope with iron shortage[END_REF][START_REF] Ciaffi | Transcriptional and physiological changes in the S assimilation pathway due to single or combined S and Fe deprivation in durum wheat (Triticum durum L.) seedlings[END_REF]. On the other hand, Fe nutrition appears to affect S transport in plants, with Fe starvation increasing the expression of sulfate transporters, which enhances S uptake from the soil and influences S remobilization from the vacuole [START_REF] Schuler | Transcriptome analysis by GeneTrail revealed regulation of functional categories in response to alterations of iron homeostasis in Arabidopsis thaliana[END_REF][START_REF] Couturier | The iron-sulfur cluster assembly machineries in plants: current knowledge and open questions[END_REF][START_REF] Paolacci | Iron deprivation results in a rapid but not sustained increase of the expression of genes involved in iron metabolism and sulfate uptake in tomato (Solanum lycopersicum L.) seedlings[END_REF][START_REF] Kaur | Integrative analysis of hexaploid wheat roots identifies signature components during iron starvation[END_REF]. Fe also affects the S distribution between roots and shoots. In leaves, Fe and S are known to interact in the building of A c c e p t e d M a n u s c r i p t 13 Fe-S clusters, which are a major sink for Fe and are known to be essential for photosynthetic electron transfer, chlorophyll metabolism, respiration, and many cellular enzymatic reactions [START_REF] Couturier | The iron-sulfur cluster assembly machineries in plants: current knowledge and open questions[END_REF][START_REF] Kaur | Integrative analysis of hexaploid wheat roots identifies signature components during iron starvation[END_REF]. This suggests a tight coordination between the metabolisms of these two nutrients in support of vital plant biological processes. However, the mechanisms regulating the integrated homeostasis of these two elements remains to be deciphered.

While Fe limitation strongly reduces the total S content in both shoots and roots of tomato (Solanum lycopersicum) plants (Zuchi et al., 2015), S limitation causes a decrease in the Fe concentration in leaves. For instance, tomatoes grown under -S conditions display a reduction in Fe concentrations in comparison to plants grown under S-sufficient conditions. This low Fe accumulation could be explained by the reduction in Fe uptake capacity associated with a decrease in the expression and activity of the Fe uptake machinery. In dicots such as Arabidopsis, S deficiency strongly represses the expression of the high affinity Fe transporter IRT1 [START_REF] Forieri | Toward new perspectives on the interaction of iron and sulfur metabolism in plants[END_REF]. Interestingly, when challenged with both S and Fe starvation, the expression of IRT1 is induced but to a lower extent compared to Fe deficiency conditions, which can be explained by a decreased requirement of Fe for Fe-S cluster synthesis under -S [START_REF] Forieri | Toward new perspectives on the interaction of iron and sulfur metabolism in plants[END_REF]. Finally, in tomato plants, the activity of Fe transporters is reduced suggesting that S deficiency prevents the typical responses to Fe deficiency (Zuchi et al., 2009).

Beyond ion transport activity, Fe nutrition profoundly impacts S assimilation in both graminaceous and non-graminaceous plants [START_REF] Forieri | Toward new perspectives on the interaction of iron and sulfur metabolism in plants[END_REF]Briat et al., 2015b;Bouain et al., 2019b). For instance, in Arabidopsis, gene expression profiling has revealed that a number of genes involved in the S assimilation pathway are co-expressed with Fe-deficient genes in response to Fe deficiency [START_REF] Schuler | Transcriptome analysis by GeneTrail revealed regulation of functional categories in response to alterations of iron homeostasis in Arabidopsis thaliana[END_REF], suggesting that Fe availability partially controls the S assimilation-related genes. This control is visible at the transcriptional, posttranscriptional, and protein activity levels.

Altogether, these findings provide physiological and molecular evidence for a crosstalk between the Fe and S pathways in different plant species, with marked species specificities. These findings also reveal the presence of many transcriptional and posttranscriptional regulatory mechanisms that plants can use to adapt to Fe fluctuation by modulating their S homeostasis, and vice versa. Further investigations are now needed to decipher the molecular details of these mechanisms.

A c c e p t e d M a n u s c r i p t 14

Iron and phosphate homeostasis interaction

The interaction between Fe and phosphate (Pi) is well-recognized in soil as well as in plants [START_REF] Müller | Genome-wide analysis of the Arabidopsis leaf transcriptome reveals interaction of phosphate and sugar metabolism[END_REF][START_REF] Thibaud | Dissection of local and systemic transcriptional responses to phosphate starvation in Arabidopsis[END_REF]Briat et al., 2015b;Bouain et al., 2019b). Indeed, the complexation of Fe by Pi in the soil leads to the formation of precipitates, which decreases the availability of these two elements for plants (Briat et al., 2020). While low Pi availability in soil leads to the Fe overaccumulation in plants, the converse is also true [START_REF] Misson | A genome-wide transcriptional analysis using Arabidopsis thaliana Affymetrix gene chips determined plant responses to phosphate deprivation[END_REF][START_REF] Hirsch | Phosphate deficiency promotes modification of iron distribution in Arabidopsis plants[END_REF][START_REF] Müller | Genome-wide analysis of the Arabidopsis leaf transcriptome reveals interaction of phosphate and sugar metabolism[END_REF][START_REF] Thibaud | Dissection of local and systemic transcriptional responses to phosphate starvation in Arabidopsis[END_REF]. Indeed, plants have evolved a coordinated gene expression network in order to respond jointly to element deficiencies and/or excesses. For instance, gene expression profiling of Pi-deficient plants revealed an increase in abundance of transcripts of both Pi starvation-related and Fe excess-responsive genes [START_REF] Misson | A genome-wide transcriptional analysis using Arabidopsis thaliana Affymetrix gene chips determined plant responses to phosphate deprivation[END_REF][START_REF] Müller | Genome-wide analysis of the Arabidopsis leaf transcriptome reveals interaction of phosphate and sugar metabolism[END_REF][START_REF] Thibaud | Dissection of local and systemic transcriptional responses to phosphate starvation in Arabidopsis[END_REF]. Such examples can be seen in the remarkable induction of expression of AtFER1-4 mRNAs, which encode the Fe storage protein ferritin, accurately reflecting the increase in available Fe under Pi starvation conditions [START_REF] Hirsch | Phosphate deficiency promotes modification of iron distribution in Arabidopsis plants[END_REF]. Interestingly, AtFER1 higher expression appears to be mediated by PHOSPHATE RESPONSE 1 (PHR1) and PHR1-LIKE 1 (PHL1), key regulators of the phosphate starvation response (PSR) in plants, thus revealing a role for these transcriptions factors in the overall control of Fe homeostasis that is integrated with the Pi status of the plant (Bournier et al., 2013;Bouain et al., 2019b) (see also Box 1). Decades of research have helped to understand how plants respond to Pi deficiency, mainly in the roots, leading to the identification of a mechanism that is tightly linked to variation in Fe accumulation. Key genes involved in this process include the cell wall-targeted FERROOXIDASE (LPR1), a P5-type ATPase (PDR2), as well as the SENSITIVE TO PROTON RHIZOTOXICITY (STOP1) transcription factor and its target ALUMINUM ACTIVATED MALATE TRANSPORTER 1 (ALMT1) (Fig. 1). PDR2 controls LPR1 activity, which mediates Fe accumulation in the root tip. STOP1 upregulates ALMT1, which modulates exudation of malate [START_REF] Müller | Genome-wide analysis of the Arabidopsis leaf transcriptome reveals interaction of phosphate and sugar metabolism[END_REF][START_REF] Thibaud | Dissection of local and systemic transcriptional responses to phosphate starvation in Arabidopsis[END_REF][START_REF] Balzergue | Low phosphate activates STOP1-ALMT1 to rapidly inhibit root cell elongation[END_REF][START_REF] Mora-Macías | Malate-dependent Fe accumulation is a critical checkpoint in the root developmental response to low phosphate[END_REF], consequently promoting Fe deposition in the root. Thus these genes could be considered as Feresponsive genes, and they form two modules that act in concert to provoke Fe overaccumulation in roots, which is likely a primary cause of PRG inhibition under Pi deficiencies [START_REF] Müller | Genome-wide analysis of the Arabidopsis leaf transcriptome reveals interaction of phosphate and sugar metabolism[END_REF][START_REF] Ward | The effect of iron on the primary root elongation of Arabidopsis during phosphate deficiency[END_REF][START_REF] Ticconi | ERresident proteins PDR2 and LPR1 mediate the developmental response of root meristems to phosphate availability[END_REF][START_REF] Thibaud | Dissection of local and systemic transcriptional responses to phosphate starvation in Arabidopsis[END_REF]Briat et al., 2015b;Bouain et al., 2019b). This is consistent with the fact that simultaneous Fe and Pi deficiencies restore PRG in Arabidopsis. This observation reveals that plants respond to the combined Fe and Pi stress in a manner distinct from individual Fe or Pi deficiencies (Bouain et al., 2019b). Key genes associated with this phenotype were recently identified and include VARIANT IN METHYLATION 1, FORMIN-LIKE-PROTEIN-6 and VOLTAGE-DEPENDENT ANION-SELECTIVE CHANNEL PROTEIN 3 (Fig. 1) (Bouain et al., 2019b).

Additional candidate genes to regulate root growth under co-occurring Fe and P deficiency stress A c c e p t e d M a n u s c r i p t 15 have been identified (AT1G51310; AT2G36200; AT4G09940; AT5G16580) and await functional validation (Bouain et al., 2019b). It is worth mentioning that a recent study showed, unexpectedly, that if Pi deficiency stimulates early root growth rate of most accessions, Fe deficiency reduces it (Bouain et al., 2019a). The combination of both Pi and Fe deficiency leads to a suppression of the growth inhibition exerted by Fe deficiency alone. Surprisingly, the Arabidopsis accession Columbia (Col-0) is not representative of the species under Pi deficiency (Bouain et al., 2019a), and therefore used inappropriately as reference for these stresses in the literature. Both discoveries go against the main stream of thinking and will have an impact on future research aiming at understanding plant response to nutrient deficiency. Finally, it is clear that Fe content plays not only a nutritional role in plants but also a central "signaling" role in controlling different aspects of plant growth under other nutrient deficiencies. For example, recent work has revealed a role for Fe in regulating the number of lateral roots in response to potassium deficiency via a complex mechanism involving auxin and changes in DNA methylation [START_REF] Shahzad | Cryptic variation in RNA-directed DNA-methylation controls lateral root development when auxin signalling is perturbed[END_REF]. In short, Arabidopsis roots exhibit increased numbers of lateral roots in response to this stress, and this response is dependent on the Fe status [START_REF] Shahzad | Cryptic variation in RNA-directed DNA-methylation controls lateral root development when auxin signalling is perturbed[END_REF].

Once taken up by the roots, Fe can interact with Pi, leading to a reduction in their translocation to the shoots [START_REF] Cumbus | The influence of phosphorus, zinc and manganese on absorption and translocation of iron in watercress[END_REF][START_REF] Mathan | Influence of iron on the uptake of phosphorus by maize[END_REF]. In shoots, these nutrients are stored in vacuoles or distributed among organelles such as the chloroplast. It is well known that photosynthesis is severely affected by either Fe or Pi deficiency [START_REF] Chen | Iron Nutrition of Plants in Calcareous Soils[END_REF][START_REF] Carstensen | The Impacts of Phosphorus Deficiency on the Photosynthetic Electron Transport Chain[END_REF]. For instance, leaf Fe content correlates well with photosynthetic capacity [START_REF] Terry | Function of iron in chloroplasts[END_REF][START_REF] Shikanai | PAA1, a P-type ATPase of Arabidopsis, functions in copper transport in chloroplasts[END_REF] and Fe deficiency causes chlorosis. Intriguingly, even if the leaf Fe level is sufficient, leaves with high Pi content still show chlorosis [START_REF] Dekock | Active Iron in Plant Leaves[END_REF]. This provides further support for the interplay between Fe and Pi homeostasis in shoots. Remarkably, plants grown under a combined limitation of Fe and Pi do not show any chlorotic phenotype [START_REF] Saenchai | The Involvement of OsPHO1;1 in the Regulation of Iron Transport Through Integration of Phosphate and Zinc Deficiency Signaling[END_REF][START_REF] Shi | Iron Retention in Root Hemicelluloses Causes Genotypic Variability in the Tolerance to Iron Deficiency-Induced Chlorosis in Maize[END_REF]. Despite the importance of this phenomenon in agronomy, the characterization of Fe and Pi interactions involved in regulating photosynthesis is still in its infancy, and more research is needed to understand the complexity of how the various pathways involved are integrated. Systems genetic approaches can be used to screen for the phenotypic diversity of Fe and Pi stress responses across plant ecotypes and accessions, and would help to better understand how plants integrate nutrient deficiency signals to control photosynthesis [START_REF] Rouached | System-level understanding of plant mineral nutrition in the big data era[END_REF]. At the end of the plant life cycle, Fe is accumulated in the seed vacuoles, where it is mainly chelated and stored via its close interaction with Pi ions from phytate [START_REF] Lanquar | Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 is essential for seed germination on low iron[END_REF]. Phytate content is one of the key factors in Fe remobilization during germination and seedling establishment. It has been shown that transgenic plants overexpressing [START_REF] Belgaroui | Over-expression of the bacterial phytase US417 in Arabidopsis reduces the concentration of phytic acid and reveals its involvement in the regulation of sulfate and phosphate homeostasis and signaling[END_REF]. From a dietary point of view for humans, phytate is limiting Fe, and also Zn, absorption in the intestinal tract, therefore contributing to mineral deficiencies [START_REF] Gibson | Implications of phytate in plant-based foods for iron and zinc bioavailability, setting dietary requirements, and formulating programs and policies[END_REF]. Finally, whereas it is known that Fe and Pi interact throughout the plant life cycle, our understanding of how this interaction, and the underlying molecular basis, varies between stages of plant growth and development remains limited.

Conclusion and perspectives

Fe deficiency or excess impacts plant metabolic functions, resulting in major physiological disorders that can adversely affect growth and development. We have discussed here that Fe homeostatic mechanisms are furthermore engaged in a wide range of interactions with other nutrients. The reductionist view of ion homeostasis regulation, in which each ion's level is controlled by its own mechanisms and signaling pathways, therefore needs to be revised (Bouain et al., 2019b). This is even more clear when we consider the mitigated success rate of research efforts focused on improving Fe content in plants [START_REF] Shahzad | Combating mineral malnutrition through iron and zinc biofortification of cereals[END_REF]. Investigating the precise molecular mechanisms coordinating Fe response pathways that shape plant developmental programs (i.e. root and shoot growth) in response to single or multiple nutrient deficiencies is topical. In this context, the existence of a tight coordination between the homeostasis of Fe and other mineral nutrients offers the potential to identify new mechanisms for Fe uptake and transport. A good example is the recent discovery of a new route for Pi transport and accumulation by the activation of a Zn deficiency signaling pathway (Bouain et al., 2014;[START_REF] Khan | Coordination between zinc and phosphate homeostasis involves the transcription factor PHR1, the phosphate exporter PHO1, and its homologue PHO1;H3 in Arabidopsis[END_REF][START_REF] Pal | TransDetect Identifies a New Regulatory Module Controlling Phosphate Accumulation[END_REF][START_REF] Kisko | LPCAT1 controls phosphate homeostasis in a zinc-dependent manner[END_REF]. Similar to the effect of Zn deficiency on Pi accumulation, it has been shown that Fe accumulation strongly increases in response to combined nitrogen (N) and Pi deficiencies, as compared to the small increase observed under either single Pi or N starvation stress [START_REF] Kellermeier | Analysis of the Root System Architecture of Arabidopsis Provides a Quantitative Readout of Crosstalk between Nutritional Signals[END_REF]. Moreover, other micronutrient deficiencies (Zn or Mn for instance) often cause increased Fe uptake (and vice versa), likely because micronutrients share transport and chelation systems with lax specificities, and mechanisms ensuring the tight coordination of micronutrient homeostasis are only starting to be uncovered. In addition to proteins involved in Fe uptake, transport, and storage, genetic manipulation of mechanisms recently shown to play a role in Fe dynamics in plants is of equal importance. For example, autophagy, i.e. the recycling of cytoplasmic components via encapsulation in vesicles and subsequent degradation in the vacuole (recently reviewed in [START_REF] Chen | Autophagy and Nutrients Management in Plants[END_REF], influences Fe (and also N, Zn, Mn) remobilization to seeds during leaf senescence [START_REF] Pottier | Autophagy as a possible mechanism for micronutrient remobilization from leaves to seeds[END_REF][START_REF] Pottier | Autophagy is essential for optimal translocation of iron to seeds in Arabidopsis[END_REF][START_REF] Shinozaki | Autophagy Increases Zinc Bioavailability to Avoid Light-Mediated Reactive Oxygen Species Production under Zinc Deficiency[END_REF]. Hence, genetic inactivation of AuTophaGy 5 (ATG5) results in a 30% reduction of Fe, as well as a 10-40% reduction of N, in Arabidopsis seeds (Pottier et al. This response involves the LPR1 (cell wall-targeted FERROOXIDASE), PDR2 (P5-type ATPase), STOP1 (SENSITIVE TO PROTON RHIZOTOXICITY), encoding a transcription factor, and ALMT1 (ALUMINUM ACTIVATED MALATE TRANSPORTER 1) genes. PRG is similar to Fe sufficiency upon simultaneous P (-P) and Fe (-Fe) deficiencies and this response may be controlled by the AT1G51310, AT2G36200, AT4G09940 and AT5G16580 candidate genes (Bouain et al., 2019a). PRG under -P and Fe excess (++Fe) remains to be studied and is shown by a question mark (?). [START_REF] Flis | Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 24 in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection[END_REF]. (B) Rice. In rice, as in grasses, iron uptake from the soil relies on a reduction strategy (Strategy II). Rice is however an exception among grasses as it also uses part of the Strategy I.

The mechanisms shared by all grasses for iron uptake are represented in red, whereas the function of IRT1 specific to rice is in orange. Note that the biosynthesis of MAs from SAM may take place in rough endoplasmic reticulum-derived vesicles, which are not represented here [START_REF] Kobayashi | Iron transport and its regulation in plants[END_REF]. Red: uptake processes taking place at the root epidermis, including processes related to the IRT1 lack of specificity in Arabidopsis (A); Purple: intracellular and long distance trafficking processes; Green: chelator and chelator transporter-related processes. Vacuoles and vesicles are represented by green and yellow bubbles, respectively. The intracellular green arrow represents symplastic mouvements of metal chelates. Note that phenolics secretion by roots into the rhizosphere is not represented (Kobayashi et al., 2019). Most of these processes are induced by iron deficiency under the control of complex and multi-layer regulatory mechanisms (see text). Hypothetical processes are labelled with a question mark (?). AHA2: ARABIDOPSIS H + -ATPASE 2; Cit: Citrate [START_REF] Falciatore | Perception of environmental signals by a marine diatom[END_REF]. Inside the cells, Ca interacts with Fe storage in vacuole-like acidocalcisomes in several algal species, including Chlamydomonas and the red alga Cyanidioschyzon merolae (Blaby-Haas and Merchant, 2014). In addition to Fe, these dense acidic organelles, made mostly of Ca and polyphosphate complexes, constitute the main storage compartment for divalent cations such as Zn, Mn and Cd [START_REF] Docampo | Acidocalcisomes ? conserved from bacteria to man[END_REF][START_REF] Penen | Pools of cadmium in Chlamydomonas reinhardtii revealed by chemical imaging and XAS spectroscopy[END_REF][START_REF] Tsednee | Manganese co-localizes with calcium and phosphorus in Chlamydomonas acidocalcisomes and is mobilized in manganese-deficient conditions[END_REF].

Although N availability is key for biomass production, most photosynthetic organisms cannot fix N from the massive reservoir that is atmospheric N 2 (Boyd and Peters, 2013). In soil, legumes can however form a symbiosis with a diazotrophic bacteria in order to fix N 2 [START_REF] Mus | Symbiotic Nitrogen Fixation and the Challenges to Its Extension to Nonlegumes[END_REF]. Hence, the plant Medicago truncatula provides Fe to the symbiont, ensuring nitrogenase activity in the symbiont [START_REF] González-Guerrero | Fixating on metals: new insights into the role of metals in nodulation and symbiotic nitrogen fixation[END_REF][START_REF] Escudero | Medicago truncatula Ferroportin2 mediates iron import into nodule symbiosomes[END_REF]. Algae can use different sources of N and their Fe requirement varies in function of the N source [START_REF] Raven | The iron and molybdenum use efficiencies of plant growth with different energy, carbon and nitrogen sources[END_REF]. Indeed, diazotrophic cyanobacteria have a higher Fe requirement than algae growing on nitrate, which is superior to those growing on ammonia.

Ammonia can be readily used from the media, but nitrate reduction and N 2 fixation require Fe as cofactor and source of reductive power [START_REF] Schoffman | Iron-Nutrient Interactions within Phytoplankton[END_REF].

Finally, P and Fe also interact both in fresh and marine water (Benitez-Nelson, 2000;[START_REF] Qiu | Effect of complex iron on the phosphorus absorption by two freshwater algae[END_REF]. In freshwater, both Fe concentration and speciation have a significant impact on P uptake in Scenedesmus quadricauda [START_REF] Qiu | Effect of complex iron on the phosphorus absorption by two freshwater algae[END_REF]. This contrasts with very acidic lakes, where pH is usually below 4 rendering Fe more bioavailable. There, by forming less bioavailable Fe-P complexes, Fe promotes P deficiency, even when P concentration is not limiting, possibly contributing to low primary productivity in Fe-rich acidic lakes [START_REF] Spijkerman | Decreased phosphorus incorporation explains the negative effect of high iron concentrations in the green microalga Chlamydomonas acidophila[END_REF]. Algae species found in these habitats, such as Chlamydomonas acidophila, can tolerate approximately twenty times higher amount of Fe compared to neutrophile species, such as Chlamydomonas [START_REF] Spijkerman | Decreased phosphorus incorporation explains the negative effect of high iron concentrations in the green microalga Chlamydomonas acidophila[END_REF].

  of the Zn/Cd hyperaccumulators Arabidopsis halleri

  Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020

  Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 12 compounds, hyperaccumulator species tend to favor the mobility of NA-chelated metal (i.e. Zn) towards the xylem

  Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 16 bacterial phytase, an enzyme that degrades phytate, remobilize Fe faster during germination than wild type plants

  ,

A

  et al., 2019). Taken together, it is clear that designing research strategies to examine how Fe accumulation in plants is concomitantly influenced by the crosstalk of N, Pi and other micronutrient signals will help identifying new players involved in Fe uptake and transport. Future research efforts need to focus on the molecular basis of the coordinated Fe, micro-and macronutrient homeostatic mechanisms, which will enable integrated approaches to improve Fe accumulation in crops. Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020 CR. 2000. The biogeochemical cycling of phosphorus in marine systems. Earth-Science Reviews 51, 109-135. Bian B, Kageshima S, Yano K, Fujiwara T, Kamiya T. 2018. Screening Arabidopsis thaliana mutants for low sensitivity to manganese identifies novel alleles of NRAMP1 and PGSIP6. Journal of Experimental Botany 69, 1795-1803. Blaby-Haas CE, Merchant SS. 2012. The ins and outs of algal metal transport. Biochimica Biophysica Acta 1823, 1531-1552. Blaby-Haas CE, Merchant SS. 2014. Lysosome-related organelles as mediators of metal homeostasis. The Journal of Biological Chemistry 289, 28129-28136. Blaby-Haas CE, Merchant SS. 2017. Regulating cellular trace metal economy in algae. Current Opinion in Plant Biology 39, 88-96. Bouain N, Korte A, Satbhai SB, Nam H-I, Rhee SY, Busch W, Rouached H. 2019a. Systems genomics approaches provide new insights into Arabidopsis thaliana root growth regulation under combinatorial mineral nutrient limitation. PLoS Genetics 15, e1008392. Bouain N, Krouk G, Lacombe B, Rouached H. 2019b. Getting to the Root of Plant Mineral Nutrition: Combinatorial Nutrient Stresses Reveal Emergent Properties. Trends in Plant Science 24, 542-552.Bouain N, Shahzad Z, Rouached A, Khan GA, Berthomieu P, Abdelly C, Poirier Y, Rouached H. 2014. Phosphate and zinc transport and signalling in plants: toward a better understanding of their homeostasis interaction. Journal of Experimental Botany 65, 5725-5741. Bournier M, Tissot N, Mari S, Boucherez J, Lacombe E, Briat J-F, Gaymard F. 2013. Arabidopsis ferritin 1 (AtFer1) gene regulation by the phosphate starvation response 1 (AtPHR1) transcription factor reveals a direct molecular link between iron and phosphate homeostasis. The Journal of Biological Chemistry 288, 22670-22680. Boyd ES, Peters JW. 2013. New insights into the evolutionary history of biological nitrogen fixation. Frontiers in Microbiology 4, 201. Boyd PW, Jickells T, Law CS, Blain S, Boyle EA, Buesseler KO, Coale KH, Cullen JJ, de Baar HJW, Follows M, Harvey M, Lancelot C, Levasseur M, Owens NPJ, Pollard R, Rivkin RB, Sarmiento J, Schoemann V, Smetacek V, Takeda S, Tsuda A, Turner S, Watson AJ. 2007. Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions. Science 315, 612-617. Briat J-F, Dubos C, Gaymard F. 2015a. Iron nutrition, biomass production, and plant product quality. Trends in Plant Science 20, 33-40. Briat J-F, Gojon A, Plassard C, Rouached H, Lemaire G. 2020. Reappraisal of the central role of soil nutrient availability in nutrient management in light of recent advances in plant nutrition at crop and molecular levels. European Journal of Agronomy 116, 126069.

Figure 1 .

 1 Figure legends Figure 1. Primary root growth in Arabidopsis depends on iron and phosphate availability. (A) Under phosphate sufficiency (+P), iron deficiency (-Fe) or excess (++Fe) inhibit primary root growth (PRG) compared to Fe sufficiency (+Fe). The PRG responses to -Fe and ++Fe involve the FRO2 (FERRIC REDUCTASE OXIDASE 2) and GSNOR (S-NITROSOGLUTATHIONE-REDUCTASE) genes, respectively. (B) Simultaneous P deficiency (-P) and Fe sufficiency (+Fe) inhibits PRG compared to Fe sufficiency (+Fe).

Figure 2 .

 2 Figure 2. Iron and micronutrient uptake and trafficking interactions in Arabidopsis and rice roots. Only the interface with the rhizosphere (left) and the xylem (right) is detailed. (A) Arabidopsis. In dicots, iron uptake from the soil relies on a reduction strategy (Strategy I). The localization of IRT1 and NRAMP1 was shown to be dynamic, between plasma membrane and endo-membranes (see text). Metal ligands in the xylem, with their relative abundance represented by different font size, are based on data in Pisum sativum[START_REF] Flis | Environnement user on 12 November 2020 A c c e p t e d M a n u s c r i p t 24 in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection[END_REF]. (B) Rice. In rice, as in grasses, iron uptake from the soil relies on a reduction

  ; ENA1: EFFLUX TRANSPORTER OF NA 1; FRD(L): FERRIC REDUCTASE DEFECTIVE(-LIKE); FPN: FERROPORTIN; FRO2: FERRIC REDUCTASE-OXIDASE 2; IRT: IRON-REGULATED TRANSPORTER; HMA3: HEAVY METAL ATPASE 3; MA: MUGINEIC ACID FAMILY Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020

  Ca) signaling was shown to be crucial for the response to changes of environmental Fe concentration in the diatom Phaeodactylum tricornutum

  Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020

  Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020

Downloaded from https://academic.oup.com/jxb/advance-article/doi/10.1093/jxb/eraa483/5974327 by INRAE Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement user on 12 November 2020

Acknowledgements

Research in the author's laboratories is funded by the "Fonds de la Recherche Scientifique-FNRS" (MIS-F.4511.16, CDR J.0009.17 and PDR-T0120.18 to MH), the University of Liège (ARC GreenMagic to MH) and the "Agence Nationale de la Recherche (ANR)" (ANR-19-CE13-0007 to HR). The Michigan State University also supports HR. MH is Senior Research Associate of the F.R.S.-FNRS and SF was doctoral fellow of the FRIA.

Author Contribution

MH and HR conceptualized the manuscript. MH, HR, SE and SF wrote the draft manuscript. MH, HR and SF made the Figures. MH and HR reviewed and edited the manuscript.

Box1. Fe and nutrient interplay in eukaryotic algae

Land plants frequently come to mind when talking about photosynthetic organisms and the heterogeneous group known as algae is often overlooked. Algae is an umbrella term used to describe a polyphyletic group of (mostly aquatic and photosynthetic) eukaryotes [START_REF] Prasanna | Evolutionary relationships among cyanobacteria, algae and plants: Revisited in the light of Darwinism[END_REF][START_REF] Brodie | The Algal Revolution[END_REF] that acquired photosynthesis through primary or secondary endosymbiosis [START_REF] Keeling | The number, speed, and impact of plastid endosymbioses in eukaryotic evolution[END_REF][START_REF] Brodie | The Algal Revolution[END_REF]. Algae produce ~50% of oxygen on earth and provide multiple ecosystem and economic services to humans [START_REF] Brodie | The Algal Revolution[END_REF]. Their diverse evolutionary origins and diverse habitats, characterized by different Fe availability give ample room for originality and innovation in Fe homeostasis and its interactions with other nutrients. In this box, we will highlight a few examples of the peculiarities found in algae.

Fe is known to limit algae growth in marine environments [START_REF] Sunda | Feedback Interactions between Trace Metal Nutrients and Phytoplankton in the Ocean[END_REF][START_REF] Smetacek | Deep carbon export from a Southern Ocean iron-fertilized diatom bloom[END_REF][START_REF] Brodie | The Algal Revolution[END_REF] as well as in freshwater ecosystems [START_REF] Liu | Effect of limitation of iron and manganese on microalgae growth in fresh water[END_REF]. Fe is a particularly limiting nutrient in open ocean, the so-called high-nutrient, low-carbon areas (Boyd et al., 2007;[START_REF] Morrissey | Iron Utilization in Marine Cyanobacteria and Eukaryotic Algae[END_REF], leading to adaptation of the photosynthetic machinery in algae inhabiting those areas [START_REF] Strzepek | Photosynthetic architecture differs in coastal and oceanic diatoms[END_REF][START_REF] Cardol | An original adaptation of photosynthesis in the marine green alga Ostreococcus[END_REF]. Fe uptake pathways are very diversified in algae. In Chlamydomonas, Fe 3+ is first reduced by ferric chelate reductases, then high affinity Fe uptake is mediated by a multicopper ferroxidase that reoxidizes Fe 2+ to Fe 3+ and an Fe permease that transports Fe 3+ into the cell, as it occurs in yeast and mammals. This re-oxidation step, and subsequent uptake of Fe 3+ , confers a high metal specificity to this high-affinity Fe uptake system (La [START_REF] Fontaine | Copper-dependent iron assimilation pathway in the model photosynthetic eukaryote Chlamydomonas reinhardtii[END_REF][START_REF] Herbik | The involvement of a multicopper oxidase in iron uptake by the green algae Chlamydomonas reinhardtii[END_REF][START_REF] Merchant | Between a rock and a hard place: trace element nutrition in Chlamydomonas[END_REF]. However, two ZIP homolog proteins, named IRT1 and IRT2, are also induced by Fe deficiency in Chlamydomonas and expected to form a parallel pathway of Fe 2+ transport [START_REF] Hanikenne | Transition Metal Nutrition[END_REF][START_REF] Urzica | Systems and trans-system level analysis identifies conserved iron deficiency responses in the plant lineage[END_REF]. Cd exposure results in the strong induction of both uptake pathways, which enables to limit the impact of Cd on Fe homeostasis but comes at the cost of higher Cd uptake (Thiriet-Rupert and Hanikenne, unpublished). This response is in part controlled by [START_REF] Wykoff | Psr1, a nuclear localized protein that regulates phosphorus metabolism in Chlamydomonas[END_REF][START_REF] Rubio | A conserved MYB transcription factor involved in phosphate starvation signaling both in vascular plants and in unicellular algae[END_REF][START_REF] Moseley | Genome-based approaches to understanding phosphorus deprivation responses and PSR1 control in Chlamydomonas reinhardtii[END_REF] (see main text). This indicates that, as in monocots, Fe homeostasis is impacted by other metal cations also in organisms relying partly on Fe 3+ uptake.

Several other algal species also re-oxidize Fe 2+ into Fe 3+ and, in addition, possess, as in animals, transferrin-like proteins responsible for Fe 3+ uptake (e.g. the halophyte alga Dunaliella salina and the sea weed Ulva mutabilis) [START_REF] Paz | A multicopper ferroxidase involved in iron binding to transferrins in Dunaliella salina plasma membranes[END_REF]Blaby-Haas and Merchant, 2017;[START_REF] De Clerck | Insights into the Evolution of Multicellularity from the Sea Lettuce Genome[END_REF]. As plants that export/secrete siderophores or phenolic compounds to mobilize and/or scavenge Fe 3+ in the soil, algae also employ diverse strategies to capture Fe from their environment: using extracellular Fechelating proteins, such as the secreted FEA1 and FEA2 (Fe-Assimilation) in Chlamydomonas or the plasma-membrane anchored ISIP2a (Fe Starvation-Inducible Protein) in Phaeodactylum tricornutum [START_REF] Allen | FEA1, FEA2, and FRE1, encoding two homologous secreted proteins and a candidate ferrireductase, are expressed coordinately with FOX1 and FTR1 in iron-deficient Chlamydomonas reinhardtii[END_REF][START_REF] Morrissey | A novel protein, ubiquitous in marine phytoplankton, concentrates iron at the cell surface and facilitates uptake[END_REF]Blaby-Haas and Merchant, 2017); or relying on bacterial siderophores, as suggested in U. mutabilis and other species [START_REF] Hopkinson | The role of siderophores in iron acquisition by photosynthetic marine microorganisms[END_REF][START_REF] De Clerck | Insights into the Evolution of Multicellularity from the Sea Lettuce Genome[END_REF]. FEAs and ISIP2a genes are induced by Fe deficiency (but not by deficiencies of other metals) [START_REF] Allen | FEA1, FEA2, and FRE1, encoding two homologous secreted proteins and a candidate ferrireductase, are expressed coordinately with FOX1 and FTR1 in iron-deficient Chlamydomonas reinhardtii[END_REF][START_REF] Morrissey | A novel protein, ubiquitous in marine phytoplankton, concentrates iron at the cell surface and facilitates uptake[END_REF], and ISIP2a was actually shown to bind Fe [START_REF] Morrissey | A novel protein, ubiquitous in marine phytoplankton, concentrates iron at the cell surface and facilitates uptake[END_REF]. Whether these Fe scavenging strategies are influenced by interactions with other nutrients has not been examined in details.

Copper (Cu) and Zn also play important roles in Fe homeostasis in algae. In several species, such as Chlamydomonas, Thalassiosira oceanica and D. salina, multicopper oxidases (MCO) take part in high affinity Fe uptake [START_REF] Maldonado | Copper-dependent iron transport in coastal and oceanic diatoms[END_REF][START_REF] Paz | A multicopper ferroxidase involved in iron binding to transferrins in Dunaliella salina plasma membranes[END_REF]Blaby-Haas and Merchant, 2017). In contrast, in the marine alga Ostreococcus tauri, where Fe uptake is dependent on an IRT1-like protein, Zn, and not Cu, appears to be required for proper Fe-uptake and its regulation [START_REF] Lelandais | Ostreococcus tauri is a new model green alga for studying iron metabolism in eukaryotic phytoplankton[END_REF].

An interaction between Zn and the regulation of IRT1 also occurs in Chlamydomonas where an induction of IRT1 takes place under Zn-deprivation (Blaby-Haas and Merchant, 2017). In addition, Cu-based plastocyanin and Fe-based cytochrome c 6 are interchangeable in the photosynthetic electron transfer chain in green algae such as Chlamydomonas. Upon Cu starvation, plastocyanin can be replaced by cytochrome c 6 as part of a Cu-saving program [START_REF] Howe | The novel cytochrome c6 of chloroplasts: a case of evolutionary bricolage[END_REF][START_REF] Merchant | Trace Metal Utilization in Chloroplasts[END_REF]. An extreme example of this is the red alga Cyanidioschizon merolae, which inhabits acidic Fe-rich low-Cu hot springs and completely lacks plactocyanin (Hanikenne et al., 2005a). It is also lacking MCOs, but possesses multiple Fe permeases (Blaby-Haas and Merchant, 2012;Hanikenne et al., 2005a).