Abstract
Photosynthesis in plants involves photosystem-I (PSI) and -II (PSII), both of which are converting light energy into biochemical energy. To keep optimal photosynthetic rate under fluctuating light conditions, the excitation energy of those photosystems are balanced via migration of light-harvesting complexes II (LHCII) (state transitions). It has been proposed that phosphorylation of LHCIIs are essential for their reassociation with PSI in state 2. However, no direct evidence that phosphorylation is indispensable for the reassociation of LHCII with PSI have been provided. Here, we show that the PSI excitation energy level increases under state 2-promoting conditions in a LHCII kinase mutant of the green alga Chlamydomonas reinhardtii, while its PSII excitation energy level unchanged. A formation of PSI-LHCI/II supercomplex was also observed in the same mutant. Moreover, the LHCIIs in PSI-LHCI/II supercomplex were not phosphorylated. Hence, we concluded that state 2 complexes (PSI-LHCI/II) could be formed without LHCII-phosphorylation, and therefore phosphorylation of the LHCIIs were only required for their detachment from PSII.
