Imaging of intracellular rearrangement of photosynthetic proteins in Chlamydomonas cells upon state transition

Oxygenic photosynthesis is conducted by two photoactive units, photosystem I (PSI) and photosystem II (PSII), that utilize light energy to generate the electron flow from water to NADPH. Photosynthetic organisms have developed a mechanism called state transition (ST) to regulate the excitation balance between the two units, since the balance is constantly disturbed by fluctuation in light quality. The traditional ST model assumes shuttling of a light-harvesting complex called LHCII between the two PSs. However, there has been no direct observation of the intracellular rearrangements of LHCII upon ST, which is crucial in order to evaluate the validity of the traditional ST model. Here, the intracellular distributions of the two PSs and LHCII are visualized by using a novel cryogenic optical microscope. The calculated Pearson's correlation coefficient between the relative fluorescence intensity of LHCII and the fluorescence intensity ratio of PSII to PSI provided information about the degree of co-localization of these components. The analysis indicated that the relative emission intensity from LHCII is stronger in the PSII-abundant region than in the PSI-abundant one in both states. On the other hand, a statistical analysis by Welch's test indicated that Pearson's correlation coefficient is significantly higher in state1 than state2, probably reflecting the movement of LHCII from PSII to PSI upon state transition. The study also found an independent cell group in which degree of ST was between those observed for fully converted cells. These cells tended to show lower correlation coefficients than the fully converted ones. This was explained by assuming the existence of free LHCII, which moves to but remains unconnected to PSI in state2.