Nitrate is an essential element for plant nutrition and its application as fertilizer in agriculture is a common practice aiming at ensuring satisfactory development and yield. In the last decades, it appeared however that besides its nutritional role, nitrate is sensed by plants as a signal molecule involved in short term signaling (range of minutes), named primary nitrate response and a long term signaling role as evidenced by its effect on root architecture via local and systemic signaling pathways or its involvement in plant-microorganism interaction. Our aims are: (i) deciphering the nitrate signaling pathway that leads to a reduction of primary root growth in Medicago truncatula and (ii) studying the crosstalk between signaling pathways triggered by nitrate on one hand and by rhizospheric microorganism on the other.

■ Nitrate modulates root growth via ROS-dependent signaling

Nitrate is not only a nutrient but also a signal involved in the regulation of primary root growth. At early stage, we previously showed in M. truncatula that nitrate inhibits primary root growth (via down-regulation of cell elongation) and that the nitrate transporter MtNPF6.8 acts as a nitrate sensor (Pellizzaro et al., 2014). In the project IoNIS, we further showed that root growth inhibition is mediated by the mitigation of reactive oxygen species (ROS) in the primary root tip (Zang et al., 2020). It was notably shown that the hydroxyl radical (^•OH) involved in cellular elongation is down-regulated by nitrate and this is due to change in cell wall peroxidase activity (project PHC PAVLE SAVIC, collaboration with Vidovic lab, University of Belgrade). To uncover novel aspects of the root response to nitrate, we have initiated transcriptomic and proteomic studies in the primary root tip using both the wild type and npf6.8 mutant genotypes (collaboration with Bi-Defi and PAPPSO). Interestingly, we found that many genes responding to nitrate encompass genes involved in response to oxidative stress. Candidate genes for peroxidase of class III orchestrating the changes in ^•OH levels were identified. In parallel, to determine whether a change in nitrate sensitivity has an impact on nitrate assimilation, we performed a metabolomic study using the same genotypes (ECLENUS project). These later results are currently under analysis.

■ The role of nitrate signaling in plant-microorganism interactions (collaboration with LEVA-ESA)

We have studied, up to now, plant-microorganism interaction by focusing on the potential signaling role of nitrogen-rich compounds exudated by legumes roots and consequences on microbial composition in the rhizosphere. Bacteria present a positive chemotaxis towards exudated organic nitrogenous compounds, such as amino acids, which are indeed the main nitrogen compounds released into the soil. We have also shown an interest in water stress on the basis that in cropping systems of Northern Europe, Legumes are particularly sensitive to this stress, which alters symbiotic N2 fixation, yield and nitrogen rhizodeposition. We have shown that in pea, the pattern of root-exudated compounds changes not only with plant maturity but also under water stress, promoting beneficial rhizospheric microbial communities, which in turn helps plants acclimation to water stress (Bobille et al., 2016; Bobille et al., 2019 and IMAGINE project).

Our interest is extended to the involvement of nitrate in plant-microorganism interactions. This is a long-lasting item that was mainly considered from a nutritional angle where nitrate was seen either as a source of nutrients for both the host and the microorganism or as a precursor of defense molecules in case of pathogens. More recently the idea of a signaling role of nitrate in plant-microorganism interactions either when considering plant health or plant interaction with beneficial microbes emerged and is supported by an increasing number of evidence. Strategies of plants to interact with microorganisms are increasingly studied and known to be organized in a cascade of molecular events that involve receptors and signaling molecules like salicylic acid, or hormones such as ethylene and jasmonic acid that trigger the expression of specific genes. Our question is how nitrate signaling intervenes in this well-organized signaling machinery? Nitrate interference is probably complex; thus, it is essential to obtain a better understanding of the crosstalk between signaling pathways triggered by nitrate on one hand and by the rhizospheric microorganism on the other. This would be a solid strategy to bring reliable elements to answer our question.