Complex regulation of Photosystem II protein phosphorylation via redox state of chloroplast

Abstract

Phosphorylation of photosystem II (PSII) core and light harvesting II complex (LHCII) proteins is regulated in vivo by complex redox systems involving reduction of plastoquinone and cytochrome b6f complex as well as thiol redox state in chloroplasts. This redox regulation is universal in all tested plant species including a model plant, Arabidopsis thaliana. CP43 protein was highly phosphorylated in darkness in several plant species, but some phosphorylated D1 and D2 proteins may also be detected in dark-adapted plants. The shift of leaves from darkness to very dim light best induced dephosphorylation of PSII core phosphoproteins, while further increase in irradiance up to the growth level significantly increased the number of PSII centres with phosphorylated core proteins. Both in vitro and in vivo experiments indicated that the kinase(s) for PSII core phosphoproteins is regulated via reduction of plastoquinone pool that strongly depends on metabolic state of plant leaf. Redox regulation of LHCII protein phosphorylation differs from that of PSII core proteins. The LHCII kinase is activated via reduction of plastoquinone and the cytochrome b6f complex. High redox state of chloroplast, on the other hand, inactivates the phosphorylation of LHCII, disclosing a second regulatory mechanism for LHCII phosphorylation. This novel regulation of LHCII phosphorylation is likely to be mediated by the ferredoxin-thioredoxin system. A cooperative model for the function of these two regulation mechanisms that determine the phosphorylation level of the LHCII proteins in vivo will be presented.