Reversible phosphorylation of LHCII proteins in rye leaves — redox control and physiological significance

Abstract

The photosystem II (PSII) proteins D1, D2, CP43, the 9 kDa psbH gene product as well as the light harvesting chlorophyll a h binding complex (LHCII) of higher plants undergo reversible light-dependent phosphorylation of the N-terminal threonine residue on the stromal side of the thylakoid membrane (1, 2). Additionally, a chill-induced light-dependent phosphorylation of CP29 has been reported (3). Regulation of the kinases responsible for PSII protein phosphorylation has been suggested to be mediated via the redox state of thylakoid plastoquinone pool and the cyt b6/f complex (4). Moreover, we have recently reported that the thiol redox state of the chloroplasts exerts an additional control over LEICII phosphorylation (5). The regulatory mechanisms and the physiological signifigance of PSII protein phosphorylation have not yet been fully resolved. Not only higher plants, but also lower plants with LHCII antenna show reversible phosphorylation of both LHCII and the PSII core proteins D2 and CP43, but not of the D1 protein (6). Light-intensity-dependent regulation of LHCII protein phosphorylation- in, vivo was similar in lower plants and in higher plants with maximal phosphorylation of LHCII at low light and nearly complete dephosphorylation under high irradiance (6). To get further insights into the physiological significance of thylakoid protein phosphorylation, we explored phosphorylation of PSII proteins in vivo in winter rye under different environmental conditions.