Salinity-induced changes in the PSII/LHCII phosphorylation and organization of the photosynthetic protein complexes...

Halophytes have been widely used to investigate plant salt tolerance, but the mechanisms regulating photosynthesis under salinity are still poorly understood. Here, the effect of 10-day NaCl irrigation on the phosphorylation status of photosystem II (PSII), a light-harvesting complex of PSII (LHCII) and the organization of protein complexes in thylakoids of the halophyte Mesembryanthemum crystallinum L. (common ice plant) was investigated. In salt-acclimated plants, increased phosphorylation of LHCB1, LHCB2 and D1 proteins was observed under dark and light conditions. This was accompanied by reduced capability to perform state transitions in response to different light quality, as inferred from changes in the steady-state chlorophyll a fluorescence. Low-temperature chlorophyll fluorescence emission spectra revealed decreased PSII fluorescence in the dark and light in salt-acclimated plants, with a significantly smaller decrease of PSI fluorescence in the dark than in well-watered controls. These data indicate, that salinity reduces nocturnal LHCII disconnection from PSI, resulting in a permanent State II. The decrease in the functional PSII antenna in salt-acclimated plants was confirmed by the parameters of the fast kinetics of chlorophyll a fluorescence. Furthermore, blue native polyacrylamide gel electrophoresis revealed salinity-induced partial disassembly of PSII supercomplexes. Our results show that changes in the PSII/LHCII phosphorylation levels and the organization of thylakoid protein complexes play a role in acclimation to salinity in this halophytic species.