Doctorate thesis of National Institute of Further Education in Agricultural Science

Saturday, September 6, 2014
Faculté des sciences de Sfax

Study of HKT-type K+ and / or Na+ transporters involved in salt stress tolerance in rice and wheat

Siwar Ben Amar
BPMP, “Ion Channels” team

Jury :
Hervé SENTENAC, Directeur de Recherche, INRA, CoDirecteur de thèse
Khaled MASMOUDI, Professeur, International Center for Biosaline Agriculture (ICBA), CoDirecteur de thèse
Jean-Marie FRACHISSE, Chargé de Recherche, CNRS, Institut des Sciences du Végétal, Rapporteur
Chedly ABDELLY, Professeur, Centre de Biotechnologie de Borj Cédria, Rapporteur
Ferjani FERJANI BEN ABDALLAH, Rapporteur
Ahmed MLIKI, Rapporteur

Abstract :
The understanding of mechanisms responsible for Na+ and K+ transport and accumulation constitutes a major objective for improvement of plant tolerance to salt stress. Some Na+ and/or K+ transporters from the HKT family correspond to major QTL of salt stress resistance, in particular in cereals. In a first part of this thesis, we have studied three HKT transporters: TdHKT1;4-1 and TdHKT1;4-2 from durum wheat and TmHKT1;4-A2 from Triticum monococcum. The functional characterization of these three transporters by electrophysiology in an heterologous system (Xenopus oocytes) shows that these systems behave as typical Na+-selective ‘uniporters’ from the HKT subfamily 1. However, these three systems distinguish themselves in terms of macroscopic conductance and affinity for Na+. On the other hand, the analysis of transgenic plants expressing the promoters of TdHKT1;4-1 and TdHKT1;4-2 fused to the GUS reporter gene reveals a dominant expression in vascular tissues, which corresponds to a specific trait of HKT subfamily 1 genes. This localization suggests their involvement in the control of root to shoot Na+ distribution by desalination of the ascending xylem sap and/or Na+ recirculation from shoots to roots via phloem sap loading. In a second part of the thesis, we studied the structure-function relationship of two rice HKT transporters contrasting in their ion permeability: OsHKT1 is permeable to both Na+ and K+ while OsHKT8 is permeable to Na+ only. The approach was based on electrophysiological analysis of HKT pore mutants obtained by site-directed mutagenesis. Results demonstrate that a glycine residue in the pore domains is not sufficient for induction of K+ permeability, since the replacement of the serine by a glycine in OsHKT8 does not allow K+ permeability in this latter system. However, our results suggest that the residue (serine or glycine) at this position could have an impact on the HKT transporter conductance, especially in presence of K+ in subfamily 2