Doctorate thesis of Montpellier University

Friday, December 18, 2020

at 2 PM –Campus de La Gaillarde- BPMP library room without audience in videoconference.

 Doctorale school : GAIA – Biodiversité, Agriculture, Alimentation, Environnement, Terre, Eau
Spéciality : BIDAP – Biologie, Interactions, Diversité Adaptative des Plantes
Montpellier university

Team: Integration of nutrient signaling pathways

Jury:

Summary:

Nitrate (NO3-) is an essential mineral ion for plant growth and is the main source of nitrogen (N). In A. thaliana, NRT2.1, NRT2.4 and NRT2.5 are the three main transporters of the high affinity transport system (HATS) for the uptake of NO3- by roots. Among them, NRT2.1 is the major player in HATS. NRT2.1 is transcriptionally controlled by all the regulations that affect the transport of NO3- : i) induction by NO3-, ii) repression by products resulting from the assimilation of NO3- and iii) induction by light and sugars. In addition to these transcriptional regulations, recent approaches indicate that NRT2.1 is also subject to post-translational regulations. Concerning the other members of the NRT2s family, NRT2.4 and NRT2.5 have been characterized as transporters with very high affinity for NO3-. Their expression is induced by N deficiency, while light and sugars also induce NRT2.4. Despite the functional importance of these transporters for plant growth, the mechanisms involved in their regulation are still poorly understood, in particular for the regulation by light and sugars as well as post-translational regulations of NRT2.1. In this context, the objectives of my thesis were: i) to characterize the light/sugar signaling pathway involved in the regulation of NRT2.1 and NRT2.4, (ii) to characterize the role of new regulators of N signaling pathways and C identified in my team and (iii) to participate in the study of post-translational regulations of the NRT2.1 protein. One of the major results of my thesis work concerns the regulation of NRT2s by light and sugars. The work previously carried out by the team had made it possible to determine that sugar signaling involves the oxidative pathway of pentose phosphates (OPPP). The results that I obtained allowed: (i) to identify that G6PD, the first enzyme of the OPPP pathway, seems to be at the origin of the sugar signal and (ii) to hypothesize that NADPH produced by this enzyme as well as redox signaling are involved in this regulation. In parallel, I characterized the role of BHLH093 transcription factor, previously identified in the team. I demonstrated its role in the regulation of NRT2.4 by light and sugars and highlighted the existence of another signaling pathway that is probably independent of the OPPP. A second important part of my thesis work concerned the study of the regulation of NRT2.4 and NRT2.5 transporters by N deficiency. This allowed me to demonstrate: (i) that NO3- is the signal involved in the repression of these two transporters in non-N-deficient plants and (ii) that this regulation depends on the signaling pathway linked to NRT1.1/NPF6.3. In addition, I was able to place known regulators of the response to high N into the signaling pathway that regulates NRT2.4 and NRT2.5 in response to repression by NO3-. Finally, I could confirm the essential role of the phosphorylation of the Ser501 residue, located in the C-terminal part, to repress the activity of NRT2.1 under repressive conditions, by studying its post-translational regulation. The hypothesis of a cleavage of the C-terminal part of NRT2.1 also led me to initiate an approach aiming to determine if the peptide released after cleavage and which contains the phosphorylated Ser501 site, could have a role in signaling NO3-.