Doctorate thesis of Montpellier SupAgro

Wednesday, December 5, 2018
à 14h – Amphithéâtre 208 Campus La Gaillarde


Dialogue moléculaire dans l’établissement de la symbiose rhizobienne : recherche de canaux ioniques participant aux signaux électriques et calciques dans le poil absorbant de Medicago truncatula

Julien Thouin
BPMP, “Electrophysiology of plant mineral nutrition and root symbioses” team


Jury :
M. Hervé Sentenac, Biochimie & Physiologie Moléculaire des Plantes, Directeur de thèse
M. Christian Mazars, Laboratoire de Recherche en Sciences Végétales UMR5546 UPS/CNRS, Rapporteur
M. Jean-Marie Frachisse, Institut de Biologie Intégrative de la Cellule (I2BC), Rapporteur
M. Nicolas Pauly, Laboratoire des Interactions Plantes Micro-organismes (LIPM), Examinateur
M. Renaud Brouquisse, Institut Sophia Agrobiotech, Examinateur
Mme Valérie Hocher, UMR Laboratoire des Symbioses Tropicales et Méditerranéennes (LSTM), Examinateur


Abstract :
The first events detectable after Nod factor perception by legume root hairs include electric and calcium signals: the hair membrane undergoes a rapid electrical depolarization, caused by an influx of Ca2+, an inhibition of H+ ATPases and an efflux of anions, followed by a repolarization due to an efflux of K+. Transduction of the Nod signal then involves oscillations of the perinuclear calcium concentration, which initiates the reprogramming of gene expression involved in development of symbiotic nodules. The general objective of my work is to contribute to the understanding of these processes by identifying membrane transport systems responsible for the underlying ion fluxes, in the model symbiotic couple Medicago truncatula-Sinorhizobium meliloti. I am interested in K+ and/or Na+ channels or transporters that could be involved in cell membrane repolarization or allow Ca2+ influx into the cytosol from the outer solution or internal stocks. Targeted transport systems belong to the family of potassium channels named Shaker, to the family of K+ and /or Na+ transporters named HKT, or to the family of channels named CNGC, which may constitute permeable conductances to Ca2+. The implemented approach associates analyses of the root hair transcriptome to identify candidate genes, electrophysiological approaches in heterologous system (Xenopus oocyte) or in planta (membrane potential recordings), plant transformation and gene expression studies (Q-RT-PCR, GUS reporter genes), obtaining and phenotyping mutant plants (from the Noble Foundation) in terms of plant growth, production of infection threads, production of nodules and symbiotic efficiency). I have demonstrated that the outwardly rectifying Shaker potassium channel, MtSKOR, plays an important role in membrane repolarization during the initial electrical signaling process. Its inactivation can affect plant growth, nodulation capacity, and symbiotic efficiency, possibly in a manner dependent on environmental conditions, but not the root ability to form infection threads. I have been involved (through electrophysiological analyzes in oocytes) in a research project that has identified 3 Ca2+ permeable channels, present on the nuclear membrane and playing a major role in the formation of perinuclear calcium oscillations and the establishment of the symbiotic interaction (Charpentier et al., 2016). I identified a channel, named CNGC1b, that is suggested by, a first series of experiments to play a role in plant nodulation capacity. Finally, I functionally characterized (by expression in oocytes) M. truncatula HKT transporters, which are shown to be permeable to Na+.

Key words: Legumes, symbiosis, S. meliloti, M. truncatula, membrane depolarization, potassium channel, HKT, CNGC