Doctorate thesis

Monday, December 22, 2014

Molecular and electrophysiological analysis of the diversity of transport systems involved in root K+ uptake in rice

Meriem Daly
BPMP, “Ion Channels” team

Jury :
Nathalie LEONHARDT, CR CEA, Cadarache, Rapporteur
Jean-Marie FRACHISSE, CR CNRS ISV, CNRS, Gif-sur-Yvette, Rapporteur
Michel DESARMENIEN, DR CNRS, IGF CNRS, Montpellier, Examinateur
Karima SAMIR, Prof habilité, faculté des sciences Ben M’sik Faculté des Sciences Ben M’sik, Examinateur
Anne-Aliénor VERY, CR CNRS, BPMP Montpellier SupAgro, CoDirectrice de thèse
Said MAHBOUB, Prof Faculté des Sciences Ben M’sik ,Faculté des Sciences Ben M’sik, CoDirecteur de thèse

Abstract :
Soil K+ availability and efficient K+ uptake by the root are crucial for plant growth. K+ channels from the Shaker family and transporters from the Kup/HAK family are present at the plasma membrane of root periphery cells and contribute to rapid K+ uptake even from diluted soil solutions. In Arabidopsis, genes coding for the Shaker channel AKT1 and the H+-K+ symporter AtHAK5 are highly expressed in root peripheral tissues and play key roles in K+ absorption from the soil. Another Shaker gene, AtKC1, shares with AKT1 the same expression pattern in roots. The Shaker subunit encoded by AtKC1 is silent when present alone, but is able to form functional heteromeric channels in association with AKT1 subunit. In rice, OsAKT1 is the counterpart of AKT1, and OsK4.1 and OsK4.3 are the closest homologues of AtKC1. This work aimed at functionally characterizing the three rice Shaker subunits, in order to identify similarities and differences with their Arabidopsis counterparts. Co-expression in Xenopus oocytes of OsAKT1 with its activating partners OsCIPK23 and OsCBL1 led to exogenous inwardly rectifying K+ channels functionally similar to AKT1 although activating at much more negative potentials. The OsK4.1 and OsK4.3 subunits, on the other hand, were found to be functionally very different from AtKC1, mediating inward currents in Xenopus oocytes. The three rice Shaker channels OsAKT1, OsK4.1 and OsK4.3 differed in several functional properties (voltage gating, selectivity to NH4+ and Rb+, affinity, sensitivity to pH, …), suggesting differences in their role in planta. Finally, my project targeted also transporters from the HKT family specific of monocotyledonous plants, working as Na+-K+ symporters when expressed in heterologous systems. I produced rice transgenic lines expressing in the same cultivar and under the control of the promoter of one of these HKT genes, OsHKT2;1, three HKT variants of such symporters differing in their K+ and Na+ permeabilities (and originating from different rice cultivars). A rice line expressing the GUS reporter gene under control of the OsHKT2;1 promoter was also produced. These lines were produced as tools to analyse the expression pattern of OsHKT2;1 in the root and to examine the role of such Na+-K+ symporters in K+ absorption in rice.