Figure 3: Mechanism of redox sensing in chloroplast. In presence of excess photochemical energy (EPE), plastoquinone became predominantly reduced (PQred), resulting in the activation of LHCII kinase via structural changes around Rieske centre (2Fe–2S) protein of cytochrome bf complex. The kinase in turn phosphorylates LHCII and PSII, resulting in migration of LHCII away from PSII and subsequently reducing light absorption by PSII. Oxidation of PQred reverses structural changes of cyt bf and leads to kinase inactivation. Phosphatase mediated dephosphorylation of mobile LHCII leads to reassociation of PSII and further increase in light energy by PSII. Alternatively, redox state of PQ controls adjustment of stoichiometry of PSI and PSII by transcriptional regulation of chloroplastic genes that encode apoprotein PSI (psA and psB proteins) and PSII (D1 proteins) reaction centers. ROS produced under EPE utilization may cause physical separation of PSII from LHCII by degrading D1 proteins, thus reducing light-energy absorption.