IMANA

Nitrogen (N) is an essential macronutrient for all living organisms, since it is a component of proteins, nucleic acids, and many secondary metabolites. Plants and fungi are the only multicellular organisms able to assimilate inorganic N. Mineral N, mainly in the form of nitrate, is generally limiting for plant growth and productivity because of spatial and temporal fluctuations of availability in the soil, which hamper sustained acquisition by the root system. Plants have developed adaptive responses allowing them to cope with these limitations and maintain growth despite changes in external N availability. In particular, it is now well established that plants are able to sense external nitrate availability, and hence that nitrate acts as a signal molecule that locally regulates many aspects of plant intake, metabolism and development. In addition a systemic signalling pathway informs the roots of the overall nutrient status of the plant. This systemic signalling is responsible for a finely tuned adjustment of root nitrate uptake by the whole nutrient demand and essential for the plant to control nutrient homeostasis. Despite the in-depth knowledge on N metabolism, only recently molecular players involved in these regulatory circuits have been discovered. A previous collaboration between two partners of this proposal revealed the key role of the transcription factor NLP7 in early nitrate signalling. Interestingly NLP7 activity is tightly regulated by an original mechanism of nuclear retention in the presence of nitrate Taken together, these considerations highlight the critical need to better understand how nitrate signalling and the systemic feedback signalling are integrated, as both govern the response of a plant to N availability. Taking advantage of tools we already have at our disposal for the study of NLPs and the master role of NLP7 in nitrate signalling, we now propose to determine whether NLP7 is a key integrator of both signalling pathways (none has been identified so far). The objective of this project is thus to develop a strategy to go beyond current knowledge of the role of NLP7 and other NLPs to a system-wide and cell type-specific levels in order to understand the whole-plant response to N and, in the long term, to apply this knowledge to improve N use efficiency (NUE) of crops. With this goal, we will advance the understanding of the central role of NLPs by addressing four questions and integrating all results using a modelling approach aiming at visualizing the whole NLP-dependent N regulatory genomic network in different cell types: 1) Is NLP7 an integrator of both local nitrate signalling and systemic N feedback signalling? 2) Does the NLP7-dependent response to N differ between different root cell types? 3) Is tissue-specific regulation by NLP7 associated with changes in chromatin states? 4) Are other NLP proteins involved in N signalling? Two of the partners involved in this project are pioneers in the identification and study of regulatory proteins involved in N signalling. In addition, the three partners have a long experience in the functional analyses of Arabidopsis genes both at the cellular and whole plant level. The IJPB laboratory (Partner 1) is specialized in the early response to nitrate, the BPMP laboratory (Partner 3) is an expert on N satiety signalling, whereas Partner 2 is specialized in the study of chromatin modifications at the genome scale (IBENS) and has recently participated together with partner 1 in the identification of NLP7 as a key regulator of early nitrate signalling. In addition Partner 3 has developed techniques allowing a rapid identification in protoplasts of transcription factor targets and is an expert in system-wide modelling approaches. This collaborative project builds on the complementary expertise brought by the three partners and should lead to an integrated view of the nitrogen signalling and of the transcriptional network orchestrated by NLP7.