Atau Tanaka
email: atau.tanaka@mshparisnord.fr

Anne Sèdes
email: anne.sedes@mshparisnord.fr

Alain Bonardi
email: alain.bonardi@mshparisnord.fr

Stephen Whitmarsh
email: stephen.whitmarsh@mshparisnord.fr

David Fierro
email: david.fierro@mshparisnord.fr

Francesco Di Maggio
email: francesco.dimaggio@mshparisnord.fr

Brain-Body Digital Musical Instrument Work-in-Progress

Keywords: Electroencephalogram, electromyogram, digital musical instrument (DMI), brain-computer interface (BCI)

The Brain/Body Digital Musical Instrument (BBDMI) will create a prototype for a digital musical instrument system that uses physiological signals from the human body: from the brain and muscles. The instrument system will be validated in a range of musical settings from concerts to the conservatoire, with a diverse range of musicians. The project brings together an interdisciplinary team of researchers, designers, musicians and engineers that will mutually inform each other through a mixed methods approach. The consortium represents the spectrum of research: from lowlevel technical development to innovative user-centered design, and the integration of state-of-the art methods integrating neuroscience and musical practice.

Introduction

The use of physiological signals from the human brain and body in human-machine interaction has come of age, constituting the research field of physiological computing. Advances in microelectronics have enabled low-cost consumer brain-computer interfaces (BCIs) and muscle interfaces for gaming and virtual reality applications. Musicians have been at the forefront of the development of digital musical instruments (DMIs) that adopt physiological interfaces, but their efforts can be isolated from mainstream scientific and musical communities, limiting how knowledge and practice inform wider art, music and science interests [START_REF] Donnarumma | Biotechnological Performance Practice[END_REF][START_REF]Guide to Brain-Computer Music Interfacing[END_REF]. BBDMI seeks to address this gap.

In 1934, the neurophysiologist and Nobel laureate E. D. Adrian, together with B. H. C. Matthews, first sonified the human EEG, discovered years earlier by Hans Berger (1929), into audio signals. In 1965, the American composer Alvin Lucier, together with engineer Edwards Dewan, created the first musical work for live brain activity and percussion instruments. With the advent of electronic synthesizers in the 1970s, experimental musicians such as David Rosenboom used EEG to explore musical compositions [START_REF] Rosenboom | Biofeedback and the arts: results of early experiments[END_REF]. Well known performance artists Laurie Anderson and Pamela Z have used the Bodysynth system in the 1990s to integrate electromyogram (EMG) interaction into their multimedia stage performances [START_REF] Mason | Pamela Z Sculpts Musical Abstraction From The Sounds (And Sights) Of Daily Life[END_REF]. Yoichi Nagahima has used EMG to extend the performance of Japanese traditional music and interactive media art [START_REF] Nagashima | Bio-sensing systems and biofeedback systems for interactive media arts[END_REF].

The Project

The Brain/Body Digital Musical Instrument (BBDMI) project will create a digital musical instrument system using physiological signals from the human body -the electromyogram (EMG) from muscle contraction and the electroencephalogram (EEG) from brain activity. It combines musical organology with biomedical technologies in a modular and accessible manner to make a system that will be useful for electronic artists, musicians, and educators. Its development methodology is based on iterative feedback between creative practice, design and research activities. The instrument system will be validated in a range of musical settings from concerts with a diverse range of musicians.

The consortium is led by la Maison de Sciences de l'Homme Paris Nord (MSH), with as industry partner the company, Soixante Circuits, and as partners, Paris Brain Institute (ICM), and the Centre de Recherche Informatique et Création Musicale (CICM/MUSIDANSE) Université Paris 8. The consortium represents the spectrum of research: from low-level technical development to innovative usercentered design, and the integration of state-of-the-art methods integrating neuroscience and musical practice.

Project Objectives

The objectives of the BBDMI project are threefold, to:

Technical: EEG and EMG

Electroencephalography (EEG) is the recording of the electrical activity of the brain from the scalp, measured in microvolts (µV). Brain oscillations in the alpha frequency (~10Hz) are remarkably stable over time and across different testing situations, promoting its usefulness in brain-computer interfaces (BCIs) [START_REF] Pfurtscheller | Future prospects of ERD/ERS in the context of brain-computer interface (BCI) developments[END_REF]. Alpha oscillations reflect one of several attentional mechanisms in the brain and its amplitude correlates with our subjective experience of attention [START_REF] Stephen Whitmarsh | Metacognitive awareness of covert somatosensory attention corresponds to contralateral alpha power[END_REF]. Electromyography (EMG) is the recording of muscle activity and correlates with the strength of muscle contraction, even in the absence of overt movement. Saponas has demonstrated that EMG can find practical use in HCI applications [START_REF] Scott Saponas | Making Muscle-computer Interfaces More Practical[END_REF]. We have shown that EMG can be associated with a user's sense of physical effort [START_REF] Baptiste Caramiaux | Understanding Gesture Expressivity through Muscle Sensing[END_REF].

Interface systems based on either brain or muscle signals have limitations when treated separately. Brain-based systems suffer from small signal amplitudes and low accuracy, interference and noise, and the variability between the users' brains that require adapting to individual signal features. Muscle-based interfaces suffer from subject fatigue and spasticity, especially in users that have less motor control (but who may benefit most). By combining EEG and EMG, their individual strengths become complementary: while muscle activity can be fast and accurate, EEG signals can indicate fluctuations of attention; EMG can provide an index of physical effort, while EEG can provide an index of cognitive effort. We describe this hybridization of EEG and EMG by adopting the term, ExG.

EAVI ExG Hardware

We have developed a biosignal hardware prototype, the EAVI ExG that is designed from the ground up as a musical instrument device. It is based on the Texas Instruments ADS129x, a single chip solution for biosignal amplification and digitization. This front end is paired with a STM32F427 Cortex-M4 microcontroller.The board communicates withover Bluetooth LE 4.2 and also USB where it registers with the host as a class compliant audio and MIDI device.

Embedded signal processing is implemented using the OWL framework [START_REF] Webster | The OWL Programmable Stage Effects Pedal: Revising the Concept of the Onstage Computer for Live Music Performance[END_REF] and combines biosignal and audio signal processing into a single DSP pipeline. The OWL platform offers tools to develop "patches" in C++, or using FAUST [START_REF] Orlarey | FAUST : an Efficient Functional Approach to DSP Programming[END_REF], or Pure Data [START_REF] Miller | Pure Data[END_REF] music programming environments.

Software

We have adopted a multi-strand software development strategy. One goal is to see how much signal processing can be run in the OWL firmware directly on the EAVI board, making it a standalone biosignal music synthesizer. For heavier tasks, functionality is divided between the EAVI and software running on a host computer. In this scenario, the EAVI handles biosignal pre-processing and feature extraction. By using FAUST, we have a multi-platform software development pipeline where sound synthesis can be prototyped on a range of music host environments (including Max, VST), then be embedded in the EAVI, microcontroller power permitting.

On the computer side, the multi-platform strategy includes, in addition to FAUST, Python and Cycling'74 Max. This includes, for example porting earlier work on EEG sonification developed in Python 1 .

We propose a modular architecture that allows users to flexibly interchange bespoke sound and modulation stages for creating multimodal instruments with a range of different devices. Modules deal with both lists of numeric data and multichannel audio signals, depending on the phase of data processing and the specifics of the acquisition device. The library consists of the following module types: input, control processing, feature extraction, machine learning, sound synthesis, utilities and output. The architecture follows a node-based paradigm, nesting multiple streams of data into single connectors to simplify patching for the end-user. We combine this with a novel graphic user-interface (GUI) design where the node blocks can expose their control parameters for direct on-screen manipulation.

In FAUST we have implemented a range of signal processing tasks, including utilities such as an EMG/EEG signal simulation and an instrumental noise generator. We have also developed a new powerful live audio granulator that can be instantiated in multichannel and be controlled by neural network-based regression models.

Multichannel output from synthesis modules are treated by spatial processing using abclib2 [START_REF] Goutmann | Approaching Spatial Audio Processing by Means of Decorrelation and Ring Modulation in Ambisonics-Sound 2[END_REF] and HOA [START_REF] Sèdes | The HOA library, review and prospects[END_REF] in a way where spatialization is not a separate, post-production step, but where immersive elements like ambisonic decorrelation become a core part of the synthesis itself. We call this spatial synthesis [START_REF] Sèdes | Approche musicale de la décorrélation microtemporelle dans la bibliothèque HOA[END_REF].

Users and Events

BBDMI events so far have consisted of four public workshops, three user studies, and two concerts.

Workshops

The first workshop took place on 19-20 January 2022 and served as a project kick-off event. It brought together research team members, members of the advisory board, and project partners. The workshop program consisted of sessions on technical overviews, summaries of existing artistic practices, and demos. The technical program included overviews of existing EEG systems, of sound synthesis strategies using FAUST and the OWL framework, and industrial design.

The second workshop took place on 23 March. It was presented by BBDMI team members, with an audience of university students from Paris and London. Invited experts included: a computational artist/performer, and a medical doctor working in motor rehabilitation. The program was organized in the form of three ateliers: 1) Introduction to machine learning, 2) Comparison of the commercial Myo system and EAVI for EMG, and 3) Live demonstration of EEG sound mapping with volunteers from the audience.

The third and fourth workshops were public events. One was as part of the Journées Européen du Patrimoine (European Days of Patrimony) on 17 September, and the other the CNRS 3 open labs during the Fêtes des Sciences (Festival of Science) on 7 October. In both cases, members of the public visited the sound studio at the MSH Paris Nord, a facility with an 8 channel, 7 order ambisonic sound system. Groups of ~15 people took part in each session. After an introduction to the project, we asked for volunteers to try out our platform. Two volunteers were connected to EMG, and one volunteer a commercial EEG headset (Mentalab Explore4 ). A sequence of exercises of increasing complexity and interactivity then ensued:

• Sonification of raw EMG • Modulation of pitch by EEG alpha activity • Direct modulation of subtractive synthesis and sound spatialization • Mixed modulation using a neural network to combine all volunteers into a single controller • The addition of a granular synthesizer • Live audio input in the style of mixed music We learned to give the audience members sufficient time to explore the sonic landscape and their connection between sound and muscle or brain activity.

User Studies

We carried out three weeklong user studies adopting a common protocol with diverse musicians: one, an autistic guitar student, a virtuoso cellist (Fig. 1), a guitarist who performs electro-acoustic music. Each study consisted of:

• Introduction to EEG and EMG and musical applications • Presentation of the hardware including electrodes for EMG, and electrode cap for EEG • Sound synthesis techniques in studio The empirical work consisted of trying different electrode positions to capture gestures made while playing their instrument. We wanted to study each musician's existing instrumental practice, with pieces they were playing, and musical projects they were working on.

The musical gesture captured by EMG allowed subtle interactions with spatialized granular synthesis. The 3 Centre national de la recherche scientifique affordance of EMG captured the intention of the musician as they prepared a gesture or musical phrase. To our surprise, we observed a focus on the slowness of gesture, where subtle gestures mixed anticipation and listening in the corporeal act. This led to the idea that it could be written or transcribed as a kind of musical choreography. They were all, despite their differing levels of expertise, able to adopt and incorporate this interaction in their playing technique.

While the EMG worked well with gestural articulation and transitory movement, the EEG functioned at another time scale, one that is much slower and dilated in time. This took the musician outside of a control paradigm, but nonetheless kept them in a live musical interaction.

Concerts

Tanaka and Whitmarsh gave a network performance during ICLI 2022 (June 20-23) as an EEG/EMG duo. Tanaka performed EMG in London with his audio streamed Whitmarsh in Paris using JackTrip. At the Paris site, the audio was processed through an analogue modular synthesizer. At the same time, EMG features were sent over the UDP as Open Sound Control message and patched to the modular synthesizer. Whitmarsh recorded his EEG in realtime with the Mentalab Explore, using the EEGsynth to extract EEG features (oscillatory band power), sent as control signals to the modular synthesizer. Part of the patch in turn modulated Tanaka's audio stream. The resultant mix was streamed to ICLI in Lisbon over Zoom.

We presented a concert on 8 December at the MSH where BBDMI team member Tanaka performed with the brutpop artist Cicanoise (Robin Dussurget), both using the EAVI EMG. Dussurget is an autistic musician and has lived with motor control issues of the lower body since birth and is in a wheelchair. He performs noise music with modular synthesizers. Cicanoise and Tanaka performed a duo using software modules developed (above) by Di Maggio and Fierro. Cicanoise controlled his analogue synth oscillators using concentrated tension, making fists and jabs, while Tanaka played an EMG homage to the Theremin followed by a spatial-granular deconstruction of rock tracks sampled from vinyl. The concert took in the context of the Soigner en Temps de Crise (Healing in Times of Crisis) colloquium at the MSH Paris Nord. Insights drawn from the rehearsals and the performance enter the user study cycle by informing continuing collaboration and subsequent system design.

JIM workshop/concert

We will present BBDMI next on 24 May at the ISEA2023 affiliated event, the Jounées d'Informatique Musicale (JIM) conference. The workshop will include:

• Presentation of the EAVI ExG prototype • Presentation of the BBDMI software suite • Experimentation with modular electronic music and mixed music with acoustic instrumentalists We will present the next BBDMI concert, this time including EEG with partipation from other team members.

Conclusion and Future work

This paper summarizes the first-year activity for the BBDMI project as work-in-progress and announces the next workshop and concert in the context of JIM2023. The project continues for two and a half more years, into 2025. Planned activities for Year 2 of the project include:

• Release of the software system as open source • Study of dry electrode technology • Ergonomic design of electrode harnesses • Hardware design strategy in view of supply chain issues • Integration into modular synthesis racks A technical study of EMG is ongoing, with a review of surface electrode technologies. This will lead to design studies with partner Soixante Circuits on ergonomic designs that will allow electrode placement not just on the arms but on different parts of the body including the upper arms, shoulder, legs, and back. Our initial user study results will guide these design studies. They will enable the project to accommodate non-normative bodies and uses.

We will build upon the user studies to roll out a series with students from the St-Denis Conservatory and the University of Paris 8 during their introductory courses and ateliers on electronic and mixed music.

Finally, the user base will grow as we roll out the hardware to a group of beta testers. These include Jose Corredera, for hospital trials in EMG driven musical sonification of motor rehabilitation exercises. Goldsmiths PhD student Lucy Strauss will use the EAVI EMG to monitor viola bowing position and apply music movement exercises including the Tuttle technique to the design and development of an interactive music partner. Finally, Electronic musician and PhD student Fabi Pereira will present a first experiment in an embodied EMG based electronic music system in a lecture/performance in Lisbon in January 2023. We will continue working with users in pairings of composer and performer, including young music students and in contexts of neurodiversity.

Figure 1 .

 1 Figure 1. Cellist Guilherme Carvalho with the EAVI board by his write, and with EMG electrodes on the upper arms and shoulder.

https://github.com/eegsynth/eegsynth www.EEGsynth.org

https://github.com/alainbonardi/abclib

https://mentalab.com/

Acknowledgements

The project has been supported by funding from the Agence Nationale de la Recherche (ANR CE38-21). We recognise expertise brought to the project by synthesizer designer Martin Klang of Rebel Technologies for EAVI hardware design (Horizon2020 ERC 789825), and external advisor Robert Oostenveld. We would like to thank participants Laurie Pompidour, Guilherme Carvalho, Amèlia Mazarico.


Valentin Chaput
email: valentin.chaput@supagro.fr

Antoine Martin
email: antoine.martin@supagro.fr

Laurence Lejay
email: laurence.lejay@inra.fr

Redox metabolism: the hidden player in C and N signaling?

Keywords: ROS, redox metabolism, nitrate uptake, carbon signaling, nitrogen signaling, carbon/nitrogen interaction

Introduction

Nitrogen (N) metabolism in plants is tightly regulated by N and carbon (C) signaling. These signaling pathways target many genes of N transport and metabolism, and account (i) for the modulation of N use efficiency by both changes in external N availability, and variations of the organic N demand of the plant for growth [START_REF] Crawford | Molecular and physiological aspects of nitrate uptake in plants[END_REF][START_REF] Gojon | Root uptake regulation: a central process for NPS homeostasis in plants[END_REF] and (ii) for the balance between photosynthate supply with N uptake and assimilation to ensure the production of amino acids [START_REF] Vincentz | Regulation of nitrate and nitrite reductase expression in Nicotiana plumbaginifolia leaves by nitrogen and carbon metabolites[END_REF][START_REF] Oliveira | Carbon and amino acids reciprocally modulate the expression of glutamine synthetase in Arabidopsis[END_REF][START_REF] Stitt | Steps towards an integrated view of nitrogen metabolism[END_REF][START_REF] Lejay | Oxidative pentose phosphate pathway-dependent sugar sensing as a mechanism for regulation of root ion transporters by photosynthesis[END_REF]. This interplay is nicely illustrated in the case of NO 3 reductase (NR), the first step of the NO 3 -assimilation pathway, which catalyses the reduction of NO 3 -to NO 2 -. NR is considered as a controlling step in NO 3 -assimilation and is a complex enzyme that is regulated by a number of different processes. Its expression is induced by NO 3 and repressed by amino acids [START_REF] Vincentz | Regulation of nitrate and nitrite reductase expression in Nicotiana plumbaginifolia leaves by nitrogen and carbon metabolites[END_REF], and the inductive action of NO 3 -is completely abolished during C starvation [START_REF] Klein | Regulation of nitrate reductase expression in leaves by nitrate and nitrogen metabolism is completely overridden when sugars fall below a critical level[END_REF][START_REF] Matt | Decreased Rubisco activity leads to dramatic changes of nitrate metabolism, amino acid metabolism and the levels of phenylpropanoids and nicotine in tobacco antisense RBCS transformants[END_REF]. The potential complexity underlying this scenario is illustrated by the fact that this enzyme is also subject to a complex regulation at the level of translation, protein degradation and protein phosphorylation [START_REF] Lillo | Signalling cascades integrating light-enhanced nitrate metabolism[END_REF].

During the past decade, these regulatory mechanisms have been mainly studied at the level of root NO 3 -transporters and they depend on both local and long-range signaling pathways. On one hand, NO 3 -acts as a localized positive signal and relates to what is called the NO 3 primary response (NPR). It is characterised by a rapid induction of root NO 3 -uptake and is associated with an increase in the expression of the root NO 3 -transporters NRT2.1 and NPF6.3 shortly after NO 3 -treatment [START_REF] Medici | The primary nitrate response: a multifaceted signalling pathway[END_REF][START_REF] Zhao | The Arabidopsis NLP7 gene regulates nitrate signaling via NRT1.1-dependent pathway in the presence of ammonium[END_REF]. On the other hand, the metabolites resulting from NO 3 -assimilation in leaves are involved in feedback repression that down-regulates root NO 3 -uptake and the NO 3 -transporters NRT2.1, NRT2.2, NRT2.4 and NRT2.5 under N satiety conditions [START_REF] Lejay | Molecular and functional regulation of two NO3-uptake systems by N-and C-status of Arabidopsis plants[END_REF][START_REF] Zhuo | Regulation of a putative high-affinity nitrate transporter (Nrt2;1At) in roots of Arabidopsis thaliana[END_REF][START_REF] Kiba | The Arabidopsis nitrate transporter NRT2.4 plays a double role in roots and shoots of nitrogen-starved plants[END_REF][START_REF] Lezhneva | The Arabidopsis nitrate transporter NRT2.5 plays a role in nitrate acquisition and remobilization in nitrogen-starved plants[END_REF]. This repression is relieved when plants experience N limitation. For NRT2.1, split-root experiments have demonstrated that this regulation relies on systemic signaling pathways [START_REF] Gansel | Differential regulation of the NO3-and NH4+ transporter genes AtNrt2.1 and AtAmt1.1 in Arabidopsis: relation with long-distance and local controls by N status of the plant[END_REF]. Since C skeletons are essential for the incorporation of inorganic N into amino acids, the regulation of root NO 3 -uptake is linked to the production of sugar through photosynthesis in the leaves [START_REF] Delhon | Diurnal regulation of NO3-uptake in soybean plants IV. Dependence on current photosynthesis and sugar availability to the roots[END_REF][START_REF] Lejay | Molecular and functional regulation of two NO3-uptake systems by N-and C-status of Arabidopsis plants[END_REF][START_REF] Lejay | Regulation of root ion transporters by photosynthesis: functional importance and relation with hexokinase[END_REF]. It is characterized by a diurnal rhythm of NO 3 -uptake, with a peak during the light period and a minimum in the dark. But the dark-dependent decrease of NO 3 -A c c e p t e d M a n u s c r i p t 4 uptake can be reversed by addition of sucrose. Moreover, NO 3 -uptake is enhanced by the CO 2 level. At the molecular level, the regulation of root NO 3 -uptake by light and sugars has been correlated with changes in the expression of NRT2.1, NRT2.4 and NPF6.3 [START_REF] Lejay | Oxidative pentose phosphate pathway-dependent sugar sensing as a mechanism for regulation of root ion transporters by photosynthesis[END_REF]. Altogether, these regulations enable plants to activate genes involved in N-assimilation when C skeletons are abundant and internal levels of organic N are low, or to inhibit N assimilation when levels of photosynthate are low or internal levels of organic N are high.

These data emphasize the importance of C/N ratio signaling in the regulation of N metabolism.

Concerning the molecular components of these signaling mechanisms, major breakthroughs have been made on local NO 3 -sensing and components of the long distance signaling of N and C status have also been found during the past few years. Interestingly, for both C and N long distance signaling, it is starting to reveal a possible close relationship with ROS and the oxidative status of the plant. Generally, the implication of ROS in cellular lifecycle can typically be divided in two different ways. First, ROS can accumulate to such levels that they generate a highly imbalanced redox status, leading to oxidative stress due to molecular and cellular damages [START_REF] Schieber | ROS function in redox signaling and oxidative stress[END_REF]. In another way, a more recent view of ROS homeostasis supports a major role for ROS in a large number of signaling pathways [START_REF] Noctor | ROS-related redox regulation and signaling in plants[END_REF]. However, these two aspects of ROS homeostasis, both leading to functional regulation of biological processes, may not be that easy to distinguish, especially in the case of the interaction with N metabolism or signaling pathways.

In this review, we highlight recent discoveries concerning the regulatory mechanism involved in long distance signaling for the regulation of N metabolism by C and N in connection with redox-related mechanisms. We then discuss how the oxidative status of the plant could be an important player to explain C/N signaling interactions and how it could help to explain plants' adaptation to extreme environment.

Regulation of N metabolism by C signaling

Sugar sensing pathways for the regulation of N metabolism

Sugars produced from plant photosynthesis are important signals controlling many aspects of plant metabolism and development [START_REF] Rolland | SUGAR SENSING AND SIGNALING IN PLANTS: Conserved and Novel Mechanisms[END_REF][START_REF] Eveland | Sugars, signalling, and plant development[END_REF].

Although sucrose is the main sugar for systemic transport from source to sink in plants [START_REF] Zhang | The involvement of hexokinase in the coordinated regulation of glucose and gibberellin on cell wall invertase and sucrose synthesis in grape berry[END_REF], many of the sugar responses observed in plants are channeled through invertases or sucrose synthases [START_REF] Barratt | Normal growth of Arabidopsis requires cytosolic invertase but not sucrose synthase[END_REF][START_REF] Ruan | Sucrose metabolism: gateway to diverse carbon use and sugar signaling[END_REF] to generate glucose and (i) An hexokinase (HXK) dependent pathway, in which the regulation of gene expression by glucose requires HXK signaling function independently of its enzymatic activity [START_REF] Moore | Role of the Arabidopsis glucose sensor HXK1 in nutrient, light, and hormonal signaling[END_REF][START_REF] Cho | Structure, expression, and functional analysis of the hexokinase gene family in rice (Oryza sativa L.)[END_REF][START_REF] Granot | Substantial roles of hexokinase and fructokinase in the effects of sugars on plant physiology and development[END_REF]. Indeed, HXKs were the first demonstrated intracellular glucose sensors in plants and there is much available evidence that plant hexokinase-dependent glucose metabolism can be separated from hexokinase-dependent glucose signaling. For example, overexpressing yeast HXK2 in Arabidopsis causes a dominant negative effect by elevating HXK catalytic activity but reducing glucose repression of gene expression [START_REF] Jang | Hexokinase as a sugar sensor in higher plants[END_REF]. It shows that HXK is a dual-function enzyme with a distinct regulatory function not interchangeable between plants and yeast.

(ii) An HXK-independent pathway, in which glucose induction and repression of genes is observed independently of glucose metabolism, i.e. independently of sense and antisense overexpression of hexokinase [START_REF] Price | Global transcription profiling reveals multiple sugar signal transduction mechanisms in Arabidopsis[END_REF][START_REF] Rolland | SUGAR SENSING AND SIGNALING IN PLANTS: Conserved and Novel Mechanisms[END_REF]. This pathway could be linked to RGS1 (regulator of G-protein signaling), a seven-transmembrane domain protein on the plasma membrane, which has been proposed to play a critical role as external glucose sensor in plants [START_REF] Grigston | D-Glucose sensing by a plasma membrane regulator of G signaling protein, AtRGS1[END_REF][START_REF] Urano | Endocytosis of the seven-transmembrane RGS1 protein activates G-protein-coupled signalling in Arabidopsis[END_REF].

(iii) A glycolysis-dependent pathway, that is this time dependent on the enzymatic activity of HXK, but not on its signaling function. It could involve an unknown metabolite(s)

downstream HXK in the glycolytic pathway [START_REF] Xiao | The role of hexokinase in plant sugar signal transduction and growth and development[END_REF].

In addition to glucose, ample evidence indicates that sucrose itself is also perceived as a sugar signal, which cannot be substituted by glucose. The best-known example is the sucrosespecific down-regulation of bvSUT1 (Beta vulgaris Sucrose Transporter 1), encoding the phloem-located proton-sucrose symporter, which is neither elicited by hexoses nor affected by mannoheptulose, a hexokinase inhibitor [START_REF] Vaughn | Sucrose-mediated transcriptional regulation of sucrose symporter activity in the phloem[END_REF]. However, the mechanisms of sucrose perception remains unknown.

Concerning the regulation of N metabolism, specific investigations of sugar sensing have been conducted at the level of root NO 3 -transporters in Arabidopsis using NRT2.1 as a model gene. It revealed that the regulation of root NO 3 -transporters by light and sugar was not related to HXK-sensing activity, but was dependent on a glycolysis-dependent pathway downstream HXK, as described above [START_REF] Lejay | Regulation of root ion transporters by photosynthesis: functional importance and relation with hexokinase[END_REF]. To gain further insight on this sugar-signaling pathway, NRT2.1 and NPF6.3 were used as model genes to look for possible correlations between the level of C metabolites downstream of the HXK step in glycolysis [START_REF] Lejay | Oxidative pentose phosphate pathway-dependent sugar sensing as a mechanism for regulation of root ion transporters by photosynthesis[END_REF]. It showed that upregulation of the NO 3 -transporters NRT2.1 and NPF6.3 is related to the concentration of glucose 6-phosphate (G6P) in the roots, a metabolite involved in the upper part of glycolysis.

G6P is located at the cross road between three metabolic pathways, Glycolysis, Oxidative Pentose Pathway (OPPP) and Trehalose metabolism (Fig. 1). Trehalose 6-phosphate (T6P) levels has emerged as an essential signal metabolite in plants, linking growth and development to C availability [START_REF] Lunn | Trehalose metabolism in plants[END_REF]. However, the addition of trehalose in the nutrient solution is not able to mimic the inductive effect of sucrose in the dark, while 6aminonicotinamide (6-AN), an inhibitor of the OPPP has a strong inhibitory effect on NPF6.3 and NRT2.1 expression [START_REF] Lejay | Oxidative pentose phosphate pathway-dependent sugar sensing as a mechanism for regulation of root ion transporters by photosynthesis[END_REF]. These results strongly suggested that T6P is not involved and that sustained C flow through OPPP is required for sugar induction of both NRT genes. This result was confirmed by de Jong et al. (2014) and the role of OPPP in C signaling has then been extended to the regulation of N metabolism. Indeed, the use of a knock-down mutant for PGL3, the plastidic-localized OPPP enzyme 6-phosphogluconolactonase 3, revealed that NR and NiR genes expression are no more induced by sugar in this mutant compared to wild-type plants [START_REF] Bussell | Requirement for the plastidial oxidative pentose phosphate pathway for nitrate assimilation in Arabidopsis[END_REF]. Interestingly, the link in roots was found between OPPP and the regulation by sugars of transporters involved in N or S acquisition, two metabolisms requiring the reducing power produced by the OPPP for nitrite reductase, GOGAT and the assimilation of SO 4 2-into Cys [START_REF] Oji | Nitrite reduction in barley-root plastids: dependence on NADPH coupled with glucose-6-phoshate and 6-phosphogluconate dehydrogenases, and possible involvement of an electron carrier and a diaphorase[END_REF][START_REF] Bowsher | Nitrite reduction and carbohydrate metabolism in plastids purified from roots of Pisum sativum L[END_REF][START_REF] Bowsher | Reductant for glutamate synthase is generated by the oxidative pentose phosphate pathway in non-photosynthetic root plastids[END_REF][START_REF] Neuhaus | Nonphotosynthetic Metabolism in Plastids[END_REF][START_REF] Yonekura-Sakakibara | Analysis of reductant supply systems for ferredoxin-dependent sulfite reductase in photosynthetic and nonphotosynthetic organs of maize[END_REF][START_REF] Kopriva | Control of sulphate assimilation and glutathione synthesis: interaction with N and C metabolism[END_REF]. As a consequence, OPPP and N or S assimilation are tightly coordinated processes. In particular, genes encoding OPPP enzymes are among those most affected by NO 3 -signaling in Arabidopsis roots [START_REF] Wang | Genomic analysis of a nutrient response in Arabidopsis reveals diverse expression patterns and novel metabolic and potential regulatory genes induced by nitrate[END_REF][START_REF] Wang | Microarray analysis of the nitrate response in Arabidopsis roots and shoots reveals over 1,000 rapidly responding genes and new linkages to glucose, trehalose-6-phosphate, iron, and sulfate metabolism[END_REF] and NH 4 + can induce an isoform of G6P dehydrogenase in barley (Hordeum vulgare) roots [START_REF] Esposito | Ammonium induction of a novel isoform of glucose-6P dehydrogenase in barley roots[END_REF]. Altogether, these results support the existence of a common OPPPdependent sugar signaling mechanism for regulation of N and S metabolism, which would coordinate the availability of all three amino acid components (C, N and S) for adequate amino acid synthesis.

Candidate signaling mechanisms linked to the Oxydative Pentose Phosphate Pathway

The OPPP is an ubiquitous pathway present in all eukaryotes and most bacteria [START_REF] Kruger | The oxidative pentose phosphate pathway: structure and organisation[END_REF]. It divides into an oxidative and non-oxidative branch. The oxidative branch is highly active in most eukaryotes and converts G6P into carbon dioxide, ribulose 5phosphate and as a byproduct NADP + is reduced to NADPH. The non-oxidative branch A c c e p t e d M a n u s c r i p t 7 metabolizes the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3phosphate as well as sedoheptulose sugars, yielding ribose 5-phosphate for the synthesis of nucleic acids and sugar phosphate precursors for the synthesis of amino acids [START_REF] Stincone | The return of metabolism: biochemistry and physiology of the pentose phosphate pathway[END_REF] (Fig. 2). Whereas the oxidative branch is considered unidirectional, the non-oxidative branch can supply glycolysis with intermediates derived from ribose 5-phosphate and vice versa, depending on the biochemical demand. The initial reactions of the OPPP are carried out by glucose-6P dehydrogenase (G6PDH) producing 6-phospho-glucono lactone, which is then hydrolyzed by a 6-phosphogluconolactonase (6PGL) to 6-phosphogluconic acid. Then, the phosphogluconic acid dehydrogenase (6PGDH) detaches a CO 2 molecule, reducing a NADP + to NADPH, thus forming ribose-5P [START_REF] Esposito | Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants[END_REF]. In plants, the OPPP subcellular localization suggests a complex network of coordination of C pathways in cells. Although the cytosolic OPPP cycle represents the major part of the measured activity, the existence of a complete OPPP confined in the plastidial compartment has been widely demonstrated [START_REF] Esposito | Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants[END_REF].

Concerning the signaling mechanism linked to the OPPP three hypothesis can be made: (i) one of the C metabolites generated through the OPPP could play the role of a signal molecule;

(ii) an enzyme of the OPPP could generate a signal like HXK in glycolysis and (iii) the reducing power produced by the OPPP could be involved in redox regulation of N metabolism via, for example, a NADPH-dependent signaling pathway. The results obtained by de Jong et al. (2014), showing that OPPP metabolism is required for Glc-mediated NRT2.1 expression and can be rescued by OPPP-derived amino acid precursor metabolites, such as pyruvate, in an HXK mutant, suggest that C-metabolites or an enzyme of the OPPP could be involved. However, more recently, NRT2.1 expression has been shown to be regulated by the redox status of the plant suggesting that the production of reducing power by OPPP could be involved in the C signaling pathway [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF]. This mechanism could ensure the coordination of root NO 3 -uptake with the production of reducing equivalents required for assimilating N into amino acids. Interestingly, the tight relationship between OPPP and N metabolism in plants has been found mainly at the level of G6PDH. Indeed, the promoter sequences of NiR and G6PDH all present the same NIT-2 motif, a N metabolism regulating factor [START_REF] Neuhaus | Nonphotosynthetic Metabolism in Plastids[END_REF], while GOGAT activity in roots is clearly supported by plastidic G6PDH, which is able to satisfy the increased request of reducing power upon ammonium assimilation [START_REF] Bowsher | Reductant for glutamate synthase is generated by the oxidative pentose phosphate pathway in non-photosynthetic root plastids[END_REF][START_REF] Esposito | Ammonium induction of a novel isoform of glucose-6P dehydrogenase in barley roots[END_REF][START_REF] Esposito | Glutamate synthesis in barley roots: the role of the plastidic glucose-6-phosphate dehydrogenase[END_REF][START_REF] Esposito | Glutamate synthase activities and protein changes in relation to nitrogen nutrition in barley: the dependence on different plastidic glucose-6P dehydrogenase isoforms[END_REF]. In the OPPP, G6PDH is considered as the rate-limiting enzyme and it determines the level of NADPH by controlling the metabolism of glucose via OPPP. The [START_REF] Kruger | The pentose phosphate pathway is a metabolic redox sensor and regulates transcription during the antioxidant response[END_REF][START_REF] Stincone | The return of metabolism: biochemistry and physiology of the pentose phosphate pathway[END_REF]. In plants, the possible physiological and biochemical roles played by OPPP and namely G6PDH have been demonstrated upon cold stress [START_REF] Van Heerden | Dark chilling increases glucose-6-phosphate dehydrogenase activity in soybean leaves[END_REF][START_REF] Lin | Functional analysis of PsG6PDH, a cytosolic glucose-6-phosphate dehydrogenase gene from Populus suaveolens, and its contribution to cold tolerance improvement in tobacco plants[END_REF], heat [START_REF] Gong | Involvement og G6PDH in heat stress tolerance in thecalli from Przewalskia tangutica and Nicotiana tabacum[END_REF], metal pollution [START_REF] Xu | Mediation of cadmium-induced oxidative damage and glucose-6-phosphate dehydrogenase expression through glutathione depletion[END_REF][START_REF] Devi | Cadmium induced changes in carbohydrate status and enzymes of carbohydrate metabolism, glycolysis and pentose phosphate pathway in pea[END_REF][START_REF] Pérez-Chaca | Cadmium induces two waves of reactive oxygen species in Glycine max (L.) roots[END_REF][START_REF] Corpas | Activation of NADPH-recycling systems in leaves and roots of Arabidopsis thaliana under arsenic-induced stress conditions is accelerated by knock-out of Nudix hydrolase 19 (AtNUDX19) gene[END_REF], drought [START_REF] Liu | Glucose-6-phosphate dehydrogenase plays a pivotal role in tolerance to drought stress in soybean roots[END_REF][START_REF] Landi | Glucose-6phosphate dehydrogenase plays a central role in the response of tomato (Solanum lycopersicum) plants to short and long-term drought[END_REF] and salinity [START_REF] Cardi | The effects of salt stress cause a diversion of basal metabolism in barley roots: possible different roles for glucose-6-phosphate dehydrogenase isoforms[END_REF]. In yeast, it has been shown that oxidative treatments induce OPPP enzymes on transcript and protein levels [START_REF] Godon | The H2O2 stimulon in Saccharomyces cerevisiae[END_REF][START_REF] Shenton | Protein S-thiolation targets glycolysis and protein synthesis in response to oxidative stress in the yeast Saccharomyces cerevisiae[END_REF][START_REF] Chechik | Activity motifs reveal principles of timing in transcriptional control of the yeast metabolic network[END_REF]. The benefits of this metabolic transition are explained by the production of NADPH in the OPPP, the primary redox-cofactor for glutathione synthesis and for enzymes of the antioxidative machinery [START_REF] Juhnke | Mutants that show increased sensitivity to hydrogen peroxide reveal an important role for the pentose phosphate pathway in protection of yeast against oxidative stress[END_REF][START_REF] Pollak | The power to reduce: pyridine nucleotides--small molecules with a multitude of functions[END_REF][START_REF] Grant | Metabolic reconfiguration is a regulated response to oxidative stress[END_REF]. Cells with an increased influx to the OPPP have a higher NADPH/NADP + ratio and are more resistant to oxidants [START_REF] Ralser | Dynamic rerouting of the carbohydrate flux is key to counteracting oxidative stress[END_REF]. On the opposite, deletion of G6PDH causes hypersensitivity to multiple oxidants in various organisms [START_REF] Juhnke | Mutants that show increased sensitivity to hydrogen peroxide reveal an important role for the pentose phosphate pathway in protection of yeast against oxidative stress[END_REF][START_REF] Ho | Enhanced oxidative stress and accelerated cellular senescence in glucose-6-phosphate dehydrogenase (G6PD)-deficient human fibroblasts[END_REF].

Concerning the implication of the OPPP in the regulation of root NO 3 -uptake, the impact of 6-AN on NPF6.3 and NRT2.1 regulation could indicate a role of G6PDH and thus of the redox status of the plant in this regulatory mechanism [START_REF] Lejay | Oxidative pentose phosphate pathway-dependent sugar sensing as a mechanism for regulation of root ion transporters by photosynthesis[END_REF]. Indeed, 6-AN is an inhibitor of both G6PDH and 6PGDH, the two steps producing NADPH in the OPPP [START_REF] Kohler | Inhibition of NADP dependent oxidoreductases by the 6-aminonicotinamide analogue of NADP[END_REF][START_REF] Rawat | Glucose-6-phosphate dehydrogenase and NADPH redox regulates cardiac myocyte L-type calcium channel activity and myocardial contractile function[END_REF]. For the regulation of root NO 3 -transporters by C, the implication of the redox status of the plant is also supported by the recent discovery of the role of the transcription factor Elongated Hypocotyl 5 (HY5) in the regulation of NRT2.1 expression. HY5 is a shoot-to-root phloem-mobile signal and it mediates the regulation of root growth and NO 3 -uptake by light [START_REF] Chen | Shoot-to-Root Mobile Transcription Factor HY5 Coordinates Plant Carbon and Nitrogen Acquisition[END_REF]. In the shoot, HY5 promotes C assimilation and translocation, whereas in the root it mediates the activation of NRT2.1 expression, supporting the fact that it coordinates plant nutrition and growth in response to fluctuating light environments. Furthermore, HY5 regulates the sucrose metabolism and sucrose movement into phloem cells for shoot-to-root translocation by increasing the expression levels of SWEET11 and SWEET12, two genes encoding sucrose efflux transporters required for sucrose phloem loading [START_REF] Chen | Sucrose efflux mediated by SWEET proteins as a key step for phloem transport[END_REF][START_REF] Chen | Shoot-to-Root Mobile Transcription Factor HY5 Coordinates Plant Carbon and Nitrogen Acquisition[END_REF]. The signaling pathways involving HY5 remain largely unknown but it has been shown that HY5 can directly bind the G-box element present in the promoters of ROS-responsive genes to regulate [START_REF] Chen | Antagonistic basic helix-loophelix/bZIP transcription factors form transcriptional modules that integrate light and reactive oxygen species signaling in Arabidopsis[END_REF]. It revealed that HY5 can mediate crosstalk between light and ROS-signaling and thus could be involved in a cross-talk between sugar and the redox status of the plant for the regulation of NRT2.1 by C through OPPP. However, the regulatory mechanisms involving the OPPP could be more complex.

Indeed, Bussel et al. (2013) found that the induction of N assimilation genes by sucrose is affected when the step after G6PDH is mutated in the knock-down mutant 6pgl3.

Furthermore, G6PDH activity was found higher in 6pgl3 compared to the WT and no significant differences were detected in the NADPH/NADP + ratio [START_REF] Xiong | A c c e p M a n u s c r i p t 22 oxidative section of the plastidic pentose phosphate pathway in plant growth and development[END_REF].

Altogether, these results suggest that for N assimilation genes, the OPPP-derived signal is independent of G6PDH and of the production of reducing power. Unfortunately, the conditions used by Bussel et al. (2013) did not allow to determine the impact of 6pgl3 mutation on the induction of root NO 3 -transporters by sugars. However, it cannot be excluded that several signaling pathways linked to OPPP co-exist in plants. Indeed, as demonstrated in yeast, the deletion of a nonoxidative PPP enzyme decreased oxidant sensitivity in a yeast strain that cannot use the PPP for NADP + reduction. Vice versa, increased flow into the nonoxidative PPP via the expression of a foreign enzyme, which generates the PPP intermediate sedoheptulose-7P from the non-PPP metabolite sedo-heptulose, increased yeast's H 2 O 2 tolerance [START_REF] Kruger | The pentose phosphate pathway is a metabolic redox sensor and regulates transcription during the antioxidant response[END_REF]. Altogether, these results support the existence of a second, NADPH-independent function of the PPP in the stress response.

Regulation of N metabolism and signaling by redox-related mechanisms

N metabolism in plants is central and limiting for growth, biomass accumulation and development. It is therefore connected to many facets of primary and secondary metabolisms in plants and it is finely tuned by the N demand of the whole organism [START_REF] Ruffel | Signal interactions in the regulation of root nitrate uptake[END_REF].

Over the past few years some components of the systemic N signaling pathway involved in the regulation of root NO 3 -transporters have started to be identified and are related to redox signaling mechanisms.

A forward genetic screen identified HIGH NITROGEN INSENSITIVE 9 (HNI9), a chromatin modifier, as a factor involved in the repression of NRT2.1 expression and NO 3 uptake under high N provision [START_REF] Widiez | High nitrogen insensitive 9 (HNI9)-mediated systemic repression of root NO3-uptake is associated with changes in histone methylation[END_REF]. Indeed, NRT2.1 expression is partly derepressed in a hni9 mutant background under high N provision. Interestingly, it has been now demonstrated that HNI9, in cooperation with the transcription factor ELONGATED HYPOCOTYL 5 (HY5), positively regulates the expression of a set of detoxification genes A c c e p t e d M a n u s c r i p t 10 induced under high N, which suggests that a high N environment leads to an increase in ROS content. Indeed, ROS strongly accumulate in hni9 and hy5 mutants due to the mis-expression of detoxification genes (Fig. 3A) [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF]. Further experiments using ROS treatments or ROS scavenging molecules showed that NRT2.1 level of expression is in fact correlated with a higher level of ROS [START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF].

It is known for decades that N deprivation leads to the accumulation of ROS, especially in the roots [START_REF] Shin | Reactive oxygen species and root hairs in Arabidopsis root response to nitrogen, phosphorus and potassium deficiency[END_REF][START_REF] Kong | Increasing in ROS levels and callose deposition in peduncle vascular bundles of wheat (Triticum aestivum L.) grown under nitrogen[END_REF]. Indeed, starvation in most of the major nutrients induces an elevation in the production of H 2 O 2 , but the ROS accumulation might be the highest in the case of plants that are deprived of N [START_REF] Shin | Reactive oxygen species and root hairs in Arabidopsis root response to nitrogen, phosphorus and potassium deficiency[END_REF]. The accumulation of ROS in the roots of N-deprived plants occurs mainly in epidermis and cortical cells, which are interestingly the site of N uptake and N root assimilation, and is visible from the elongation zone to more mature zones of the root. The origin and the function of this accumulation of ROS under N starvation is not clearly understood, although it may be tempting to make a link between the role of N in growth and development, and the fact that ROS can mediate signaling events to regulate growth and development, especially in roots [START_REF] Foreman | Reactive oxygen species produced by NADPH oxidase regulate plant cell growth[END_REF]. More recently, several reports have linked ROS accumulation, as well as effectors of ROS metabolism, with downstream signaling events of the N starvation response. Indeed, treatment of Arabidopsis N-starved plants with ROS scavenging molecules inhibits the induction of NRT2.4 and NRT2.5, which are molecular markers of the N starvation response in plants, and essential actors of plant survival under N starvation [START_REF] Safi | HRS1/HHHOs GARP transcription factors and reactive oxygen species are regulators of Arabidopsis nitrogen starvation response[END_REF]. ROS accumulation during N starvation response in plants might be, at least partly, under the control of the HHO family of transcription factors, which induce the expression of a set of genes related to ROS homeostasis under N satiety. Another study revealed that shootderived polypeptides named CEPD1 and CEPD2 act as descending long-distance mobile signals for the regulation of NRT2.1 [START_REF] Ohkubo | Shoot-toroot mobile polypeptides involved in systemic regulation of nitrogen acquisition[END_REF]. They upregulate the expression of the NO 3 -transporter gene NRT2.1 in roots specifically when NO 3 -is present in the rhizosphere and Arabidopsis plants deficient in this pathway show impaired systemic N acquisition response along with N-deficiency symptoms. Interestingly, the genes coding for CEPD1 and CEPD2 are assigned to the plant-specific class III glutaredoxins (Grxs) family, which are involved in redox regulation [START_REF] Rouhier | Genome-wide analysis of plant glutaredoxin systems[END_REF]. Grxs are small oxidoreductase enzymes that can reduce disulfite and Cys-glutathione bonds within target proteins, and are thus potential actors in detoxification pathways due to their redox buffering capacities [START_REF] Rouhier | The role of glutathione in photosynthetic organisms: emerging functions for glutaredoxins and glutathionylation[END_REF]. The Grx isoforms belonging to class III are specific to land plants and from genetic analyses, it appears that class III Grxs isoforms, also named CC-type Grxs or [START_REF] Meyer | Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance[END_REF][START_REF] Couturier | Cysteine-based redox regulation and signaling in plants[END_REF]. Several class III GRXs are transcriptionally regulated by NO 3 variations, and several members of this family of 21 homologues emerged as important factors of the plant responses to NO 3 -variation. AtGRXS3/4/5/8 expression is induced in response to NO 3 -resupply after starvation, and their induction leads to the repression of primary root growth (PRG), which corresponds to one of the developmental events, aiming at reshaping root system architecture under NO 3 --sufficient soil environment [START_REF] Patterson | Nitrate-Regulated Glutaredoxins Control Arabidopsis Primary Root Growth[END_REF]. In addition, expression of others AtGRXs, among them CEPD2 (also known as AtGRXC13 and ROXY9), is up-regulated by NO 3 -starvation [START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF]. Interestingly, the functions of CC-type GRXs have been related to their ability to interact with TGA transcription factors, likely regulating their redox states. In NO 3 -metabolism, a functional connection between AtGRXS3/5/5/8 (ROXY11 to ROXY15) with TGA1 and TGA4 seems likely to regulate primary root growth in response to NO 3 - [START_REF] Patterson | Nitrate-Regulated Glutaredoxins Control Arabidopsis Primary Root Growth[END_REF].

Furthermore, since TGA1 and TGA4 have been involved in the regulation of NRT2.1 in response to NO 3 -, TGAs and Grxs could be a module involved in both the regulation of root NO 3 -transporters and root development in response to N availability [START_REF] Alvarez | Systems approach identifies TGA1 and TGA4 transcription factors as important regulatory components of the nitrate response of Arabidopsis thaliana roots[END_REF].

Collectively, these studies shed light on Grxs as small oxidoreductase enzymes acting in different facets of N signaling. Although a formal role involving their redox buffering properties is still required, it supports a close interaction between the plant redox properties and the regulation of growth and development through N availability (Fig. 3B).

Therefore, the association between ROS and N metabolism appears under different physiological condition, reinforcing the idea that N metabolism and ROS homeostasis are intrinsically linked, especially for the regulation of NO 3 -uptake genes. A correlation between ROS levels and NO 3 -assimilation has also been made, as ROS levels increase in an Arabidopsis nia1 nia2 mutant showing a complete absence of NO 3 -reductase activity [START_REF] Pan | Nitrate Reductase-Mediated Nitric Oxide Regulates the Leaf Shape in Arabidopsis by Mediating the Homeostasis of Reactive Oxygen Species[END_REF]. Altogether, these observations tend to conclude that ROS homeostasis is just as much affected by N metabolism that the latter is modulated by ROS levels or signaling.

Indeed, N starvation and N excess both leads to ROS accumulation, and normal physiological conditions for cellular ROS levels take place in between, in intermediate nutritional conditions. This consideration is in fact not restricted to plants, as in animals limited or excessive nutrition generate abnormal ROS accumulation, which can be considered as pathologic in certain situations [START_REF] Gorlach | Reactive oxygen species, nutrition, hypoxia and diseases: Problems solved?[END_REF].

A c c e p t e d M a n u s c r i p t 12

ROS signaling at the nexus of C/N regulations

In plants, C and N metabolisms need to be tightly coordinated for C-N-containing metabolite synthesis at the biochemical level and for long distance signaling for the C/N balance. Indeed, the process of N assimilation, especially when NO 3 -is its main source, is energetically costly and requires large amount of ATP and C-skeletons produced from the C metabolic processes [START_REF] Foyer | Respiration and nitrogen assimilation: targeting mitochondria-associated metabolism as a means to enhance nitrogen use efficiency[END_REF]. On the other hand, large amounts of N are essential for plant photosynthetic machinery and for CO 2 assimilation. If the existence of interactions between C

and N signaling has been well described for the regulation of gene expression, the molecular mechanisms involved remain mostly unknown [START_REF] Palenchar | Genome-wide patterns of carbon and nitrogen regulation of gene expression validate the combined carbon and nitrogen (CN)-signaling hypothesis in plants[END_REF][START_REF] Gutierrez | Qualitative network models and genome-wide expression data define carbon/nitrogenresponsive molecular machines in Arabidopsis[END_REF].

Interestingly, the recent advances on long distance signaling for C and N presented above bring more and more evidence that modulation of the redox status of the plant might be an important signaling mechanisms for the coordination between C and N assimilation. Indeed, all the elements presented are involved in redox signaling and HY5, beyond its role in C and redox signaling, has been found to regulate many N related genes in a N concentration dependent manner [START_REF] Huang | HY5 regulates nitrite reductase 1 (NIR1) and ammonium transporter1;2 (AMT1;2) in Arabidopsis seedlings[END_REF]. These recent results build on several reports indicating that the redox status plays an important role in the integration of C and N metabolism. For example, studies of the Nicotiana sylvestris CMSII mutant, which lacks complex I functionality, demonstrated that the adjustment of respiratory pathways was associated with alterations of foliar CN balance, as well as with increases in NAD + and NADH levels. This indicates that NADH availability is a critical factor influencing NO 3 assimilation and in coordinating ammonia assimilation with the production of C skeletons [START_REF] Dutilleul | Mitochondriadriven changes in leaf NAD status exert a crucial influence on the control of nitrate assimilation and the integration of carbon and nitrogen metabolism[END_REF]. Another study, wherein plants were subjected to different levels of N and varying atmospheric CO 2 concentrations, revealed that NAD + and NADH levels were highly integrated with those of glutamate, glutamine, asparagine and arginine in N metabolism, suggesting a potential link between nutritional status and NAD-dependent signaling [START_REF] Hager | Conditional modulation of NAD levels and metabolite profiles in Nicotiana sylvestris by mitochondrial electron transport and carbon/nitrogen supply[END_REF].

There is a longstanding recognition that in addition to being an intrinsic part of metabolism, redox status plays an active role in metabolic regulation [START_REF] Geigenberger | Metabolic control of redox and redox control of metabolism in plants[END_REF].

However, despite intense research activities, the exact mechanism underlying the signaling and its specificity is still unclear. ROS have received great attention and accumulating evidences suggest that H 2 O 2 is an active signaling molecule and that its accumulation through redox sensing leads to variety of cellular responses [START_REF] Bhattacharjee | Reactive oxygen species and oxidative burst: roles in stress, senescence and signal transduction in plants[END_REF][START_REF] Del Rio | Reactive oxygen species and reactive nitrogen species in peroxisomes. Production, scavenging, and role in cell signaling[END_REF][START_REF] Halliwell | Reactive species and antioxidants. Redox biology is a fundamental theme of aerobic life[END_REF] A c c e p t e d M a n u s c r i p t 13 sensitive regulation of its level is therefore important for its signaling function and detoxification [START_REF] Apel | Reactive oxygen species: metabolism, oxidative stress, and signal transduction[END_REF]. The ascorbate (Asc)/glutathione (GSH) cycle forms the main H 2 O 2 -detoxification system in plants and it uses NADPH, produced in the roots by the OPPP, to regenerate the oxidized form of GSH [START_REF] Noctor | ASCORBATE AND GLUTATHIONE: Keeping Active Oxygen Under Control[END_REF]. Interestingly, the increased level of H 2 O 2 has been recently associated with the induction of NRT2.1 in response to high N in the hni9 mutant [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF]. Furthermore, treatments with Asc cause a decrease in NRT2.1 expression, while H 2 O 2 leads to an increase in NRT2.1 transcript level, like what is observed in the Asc-deficient mutant, vtc2 [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF].

Altogether, these results could explain a potential link between the NADPH produced by OPPP in the roots, which is regulated by both C and N level [START_REF] Esposito | Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants[END_REF] and the H 2 O 2detoxification system for the regulation of NRT2.1 by C/N signaling and ROS production.

Unfortunately, this potential "sensing" mechanism cannot explain the specificity of the regulations linked to C and N signaling. Indeed, GSH and Asc are considered to be unspecific reducer molecules because of their small molecular masses and the Asc/GSH cycle is induced by all kind of stresses affecting ROS level. However, plants have also complex enzymatic antioxidant systems composed of thioredoxins (TRX) and glutaredoxins (GRX), known under the name of redoxins (RX) [START_REF] Serrato | Plastid thioredoxins: a "one-for-all" redox-signaling system in plants[END_REF]. On the contrary to GSH and Asc, surface topology of RX allows specific target interactions. The major function of RX is to reduce disulfide bridges in target proteins so that its own active site becomes oxidized to a disulfide.

For a new catalytic cycle, oxidized RXs are then reduced in a reaction catalyzed by NADPHdependent thioredoxin reductases (NTR) for TRX or NADPH-dependent gluthatione reductases (GR) for GRX [START_REF] Meyer | Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance[END_REF]. In non-green tissue, these reactions depend on the NADPH produced by the OPPP. TRX and GRX are able to reduce different types of oxidized thiols and generally use distinct reduction mechanisms. However, although certain targets are highly specific, TRX and GRX targets overlap, thereby enabling the development of interaction networks and strong buffering capacity. In Arabidopsis, the number of TRX, TRX-like proteins or proteins with TRX-domains have risen to 44 members [START_REF] Meyer | Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance[END_REF], many of them without any assigned function. TRX are classified, according to their subcellular location and sequence similarity, into 15 subgroups [START_REF] Meyer | Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance[END_REF]. The classification of vascular plant GRX is more difficult to establish than that of the TRX. In Arabidopsis, there are 31 GRX genes that can be classified into three distinct subgroups based on the amino acid sequences at their active sites: the CPYC, CGFS and CC-type GRX classes [START_REF] Meyer | Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance[END_REF]. Out of the 31 GRXs in Arabidopsis, 21 GRXs belong to the CC-type, now named ROXY, which is specific for land plants, while the other two GRX classes, A c c e p t e d M a n u s c r i p t 14 CPYC and CGFS, are common to eukaryotes and prokaryotes [START_REF] Rouhier | Genome-wide analysis of plant glutaredoxin systems[END_REF][START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF]. Although the roles of GRXs have been associated mainly with oxidative stress, there is growing evidence that GRXs and especially CC-type GRXs also play important roles in cell signaling and development. Interestingly, several CC-type glutaredoxins (ROXY) have already been involved in plant response and signaling under NO 3 -starvation as described above [START_REF] Ohkubo | Shoot-toroot mobile polypeptides involved in systemic regulation of nitrogen acquisition[END_REF][START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF]. Furthermore, H 2 O 2 treatments have been shown to differentially regulate the expression of the ROXYs suggesting also the involvement of ROS in response to NO 3 -deficiency, in agreement to what was found for NRT2.1 regulation [START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF][START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF]. Interestingly, TRXs are also involved in the regulation of C allocations by C signaling through the redox regulation of AGPase, a key enzyme for starch synthesis. Indeed, the redox regulation of AGPase is mediated, in roots, by the bifunctional protein NADP-thioredoxin reductase C (NTRC) [START_REF] Michalska | NTRC links built-in thioredoxin to light and sucrose in regulating starch synthesis in chloroplasts and amyloplasts[END_REF]. NTRC is a bimodular enzyme directly catalyzing the transfer of reducing groups from NADPH to TRX and from TRX to target proteins. Based on the biochemical properties of NTRC, including its high affinity for NADPH produced from sugars by the OPPP and the presence of a TRX domain at the C terminus, it has been proposed that NTRC in roots act as a redox switch able to convert reducing power in the form of NADPH into a redox signal through the thiol groups of its TRX domain [START_REF] Kirchsteiger | NADPH thioredoxin reductase C is localized in plastids of photosynthetic and nonphotosynthetic tissues and is involved in lateral root formation in Arabidopsis[END_REF]. Thus NTRC may constitute a direct pathway for redox homeostasis in heterotrophic plastids and is a very interesting candidate to explain the link between C/N signaling, the level of ROS and NADPH produced by the OPPP for the regulation of root NO 3 -transporters. There is now a need for focused genetic studies to dissect the role of the different members of the thioredoxin and glutaredoxin gene families in the regulation of CN metabolism. In particular, the identification of specific targets of NTRC and the different TRXs or root plastids will help to establish the metabolic processes that depend on these enzymes. It is also interesting to note that transcription factors, such as HY5, can induce different genes depending on the signal leading to ROS accumulation. Indeed, HY5 has been involved in the control of ROS homeostasis in response to light and cold treatments but the set of genes induced by HY5 to balance ROS levels seems to differ from one stimulus to another [START_REF] Catala | Integration of low temperature and light signaling during cold acclimation response in Arabidopsis[END_REF]Chen et al., 2013;[START_REF] Chai | LSD1 and HY5 antagonistically regulate red light induced-programmed cell death in Arabidopsis[END_REF]. Furthermore, [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF] found that the genes involved in the ROS detoxification network in response to high N was dependent on HY5 but independent on the presence of a HY5 binding site in their promoter. It suggests that HY5 may also function as an indirect regulator of other transcription factors, which could specify the response associated to the level of ROS according to the environmental signals. This will require genetic and metabolomics approaches as well as the application of techniques to analyze redox states at the subcellular level.

The role of redox metabolism in the regulation of N metabolism has far reaching implications in the context of climate change. Indeed, future crop plants will need to be more performant under increasingly frequent and severe growth conditions, which will likely impact the redox status of the plants. Furthermore, the use of fertilizer along with water and pesticide will be increasingly constrained. The oxidative phase transforms glucose into pentose and produces two molecules of NADPH.

The non-oxidative phase transforms pentose into C metabolites able to feed-back glycolysis.

The activity of G6PDH is regulated by the ratio between NADPH and NADP + , which depends on the level of ROS. The blue arrow and red line represent positive and negative control, respectively. Abbreviations are as follow: G6P, glucose-6-phosphate, G6PDH, glucose-6-phospahte dehydrogenase, 6PGL, 6-phosphogluconolactonase, 6PGDH, 6phosphogluconate dehydrogenase. stimulates the accumulation of ROS, which is involved in the induction of NRT2.1 expression.

However, in the same time the increase of ROS accumulation and/or NH 4 NO 3 induce the expression of HY5 as well as its possible interaction with HNI9, which are both involved in the induction of the expression of detoxification genes. Thus, collectively, these regulations maintain NRT2.1 expression at a low level in response to high N [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF]. (B)

Low N conditions also induce ROS accumulation. It results in the induction of the activity of specific glutaredoxins (GRXS or GRXC) involved in the regulation of primary root growth (PRG) [START_REF] Patterson | Nitrate-Regulated Glutaredoxins Control Arabidopsis Primary Root Growth[END_REF] and of NRT2.1 expression [START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF]. This regulatory mechanisms could also be involved in the regulation of NRT2.

  . It then triggers signal transduction via direct perception by sensors or indirect signaling by energy and metabolite sensors. In plants, at least three different glucose signaling pathways have been documented:

  The recent developments we discussed in this review show that the redox status clearly plays a central role in the regulation of N metabolism and especially of root NO 3 -transporters in response to C and N signaling. These results offer new hypotheses for a comprehensive understanding of the regulatory mechanisms associated to C/N interactions. Indeed, as summarized in Fig. 4, the level of ROS could be a way to integrate C/N level and regulate root NO 3 -transporters through either (i) the activation of the OPPP and/or the production of NADPH, which could activate a signaling pathway linked to NTRC or (ii) the direct activation, by other yet unknown mechanisms of transcription factors such as HY5 and/or GRXs such as the CEPDs. However, further work is now required to resolve the network of redox signaling linked to CN balance in order to get a comprehensive understanding of both the control of redox by C and N and conversely, the redox control of N metabolism. A major question also concerned the targets of candidates GRXs and TRXs for C and N signaling.

Fig. 1 .

 1 Figure Legends

Fig. 2 .

 2 Fig. 2. Schematic representation of the pentose phosphate pathway (PPP). The oxidative phase of PPP is represented in blue and the non-oxidative phase of PPP is represented in gray.

Fig. 3 .

 3 Fig. 3. Schematic representation of possible signaling pathways for the regulation of root NRT2 transporters and root growth by N and ROS. (A) High N conditions (10mM NH 4 NO 3 )

  4 and NRT2.5 by ROS accumulation in response to low N conditions. Blue arrows and red lines represent positive and negative control, respectively. Plain lines represent regulatory mechanisms, which have already been described, while dashed lines are hypothesis based on the analysis of published data.

Fig. 4 .

 4 Fig. 4. Model for the regulation of root NO 3 -transporters by C/N interactions through the level of ROS. Plain lines represent regulatory mechanisms, which have already been described, while dashed lines are hypothesis based on the analysis of published data. Abbreviations are as follow: OPPP, oxydative pentose phopshate pathway, NTRC, NADPH thioredoxin reductase C, Trxs, thioredoxins, CC-Grxs, CC-glutaredoxins, TFs, transcription factors.

Fig. 1 .Fig. 1 .Fig. 2 .

 112 Fig. 1. Schematic representation of the fate of G6P in the OPPP, glycolysis and trehalose metabolism. Abbreviations are as follow: TPS, trehalose-6-phosphate synthase, TPP, trehalose-6phosphate phosphatase, TRE1, Trehalase, G6PDH, Glucose-6-phosphate dehydrogenase, 6PGDH, 6phosphogluconate dehydrogenase, F6P, Fructose-6-phosphate, F-1-6-P, Fructose-1-6-biphosphate, G3P, Glyceraldehyde-3-phosphate, DHAP, Dihydroxyacetone phosphate, 3-PGA, 3phosphoglycerate, T6P, Trehalose-6-phosphate.

  Downloaded from https://academic.oup.com/jxb/advance-article-abstract/doi/10.1093/jxb/eraa078/5736809 by guest on 24 February 2020

  Downloaded from https://academic.oup.com/jxb/advance-article-abstract/doi/10.1093/jxb/eraa078/5736809 by guest on 24 February 2020 NADPH-producing function of the OPPP is critical to maintain redox balance under stress situations. As such, it has been suggested that this pathway represent a true metabolic sensor during response to oxidative stress and it has been implicated in several human diseases such as neurodegeneration (Alzheimer's disease) and cancer

	importance of the
	M a n u s c r i p t
	A c c e p t e d
	8

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. However, if H 2 O 2 accumulates at a high level it becomes toxic and a

  Downloaded from https://academic.oup.com/jxb/advance-article-abstract/doi/10.1093/jxb/eraa078/5736809 by guest on 24 February 2020

  In this context, plants adapted to extreme environments such as deserts become an interesting tool. Indeed, despite extreme water deficit, nutrient poor soils and high light radiation diverse plant species are able to grow in those environments. The identification of the genes and mechanisms that allow these "extremophiles" to grow could help us to understand how redox metabolism has adapted and would allow new strategies for

	the	selection	of	future	crops.	Downloaded from https://academic.oup.com/jxb/advance-article-abstract/doi/10.1093/jxb/eraa078/5736809 by guest on 24 February 2020

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transporters and root growth by N and ROS. (A) High N conditions (10mM NH 4 NO 3 ) stimulates the accumulation of ROS, which is involved in the induction of NRT2.1 expression. However, in the same time the increase of ROS accumulation and/or NH 4 NO 3 induce the expression of HY5 as well as its possible interaction with HNI9, which are both involved in the induction of the expression of detoxification genes. Thus, collectively, these regulations maintain NRT2.1 expression at a low level in response to high N [START_REF] Bellegarde | The Chromatin Factor HNI9 and ELONGATED HYPOCOTYL 5 Maintain ROS Homeostasis under High Nitrogen Provision[END_REF]. (B) Low N conditions also induce ROS accumulation. It results in the induction of the activity of specific glutaredoxins (GRXS or GRXC) involved in the regulation of primary root growth (PRG) [START_REF] Patterson | Nitrate-Regulated Glutaredoxins Control Arabidopsis Primary Root Growth[END_REF] and of NRT2.1 expression [START_REF] Jung | CC-type glutaredoxins mediate plant response and signaling under nitrate starvation in Arabidopsis[END_REF]. This regulatory mechanisms could also be involved in the regulation of NRT2.4 and NRT2.5 by ROS accumulation in response to low N conditions. Blue arrows and red lines represent positive and negative control, respectively. Plain lines represent regulatory mechanisms, which have already been described, while dashed lines are hypothesis based on the analysis of published data.


Aurélien Pertuzon

Carole Barthélémy

Emeline Comby

Aurélien Pertuzon1

Carole Barthélémy2

Emeline Comby3

Energy and ecological transitions in the Rhône River in the context of climate change: converging or diverging transitions ?

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

INTRODUCTION

• Press analysis with IRaMuTeQ, a lexicometry software • Study of the documentation from different institutions • Exploratory and semi-structured interviews with different local stakeholder

MATERIALS AND METHODS

The impact of climate change on the river and its ecosystems

The Agence de l'eau Rhône Méditerranée Corse predicts a drop in flows on the river by 2050 that will have an impact on :

• Increased operating constraints for nuclear power plants

• A reduction in the theoretical summer output of run-of-river hydroelectric installations.

• The aquatic biodiversity of the Rhône will be weakened.

 They recommand to "renaturalise" the river

RESEARCH QUESTIONS Ecological restoration of the river

Since the 90s It has undergone ecological restoration in several sectors :

• Increasing instream flows

• Restoration of the lônes

• Partial dismantling of built-up margins

• Restoring fish migration routes

STUDY OF THE PRESS COVERAGE OF THE RHÔNE TRANSITIONS : FROM GLOBAL TO LOCAL?

In our study, we use the concept of transition as "the passage between two supposedly stable states, by focusing on the one hand on institutional legacies and, on the other, on the actors who drive and support 'transitions'" (Aykut, Evrard, 2017). How the consequences of climate change will impact on the development of the transition project on the Rhône ? Will the environmental gains made be affected by the new energy challenges or not ? How can the concept of transition be used to discuss and take account of these potentially contradictory issues?

THE RHÔNE, A LOW-CARBON ENERGY SOURCE ECOLOGICAL RESTORATION IN PROGRESS ON THE RIVER CONCLUSION AND OPENING

The initial results of our study show a cross-section of transition issues raised by the press in the Rhône transition. The consequences of infrastructures on ecosystems are covered to a larger extent by the local press, and a survey of local stakeholders is planned.