Comparison of plastoquinone reduction, LHCII phosphorylation and state transitions

Abstract

When plants are exposed to sudden changes in the quality or quantity of light they respond by redistributing the mobile light-harvesting antenna (LHCII) between PSII and PSI. These so-called state transitions are controlled by the redox state of plastoquinone and known to correlate with the phosphorylation of LHCII. However, in vivo studies have shown that phosphorylation of LHCII is maximal at low light and decreases at higher light; contrary to what one would expect (Rintamaki et al., 1997). We therefore examined the correlation between the redox state of plastoquinone, phosphorylation of LHCII, and state transitions as determined by fluorescence measurements. Arabidopsis thaliana plants lacking either subunit PSI-H or PSI-N and pumpkin were used in the study. When PSI-H less plants are exposed to state 2 light, the plastoquinone pool becomes over-reduced and the level of phosphorylated LHCII is higher than in the wild type, but the LHCII-P remains bound to PSII. Therefore, phosphorylation of LHCII is not sufficient for its dissociation from PSII. Under increasing light intensity the redox level of plastoquinone is almost identical in the wild type and the PSI-H less plants, but the level of LHCII-P is 2-4 times higher in the PSI-H less plants. Moreover, the amount of LHCII-P decreases at higher light although the plastoquinone is significantly more reduced. Under the high light conditions state transitions are still pronounced in spite of the lack of LHCII phosphorylation. Alternative mechanisms for state transitions are discussed.