Plants switch photosystem at high temperature to protect photosystem II

Plants are often exposed to temperatures of around 40 °C. These temperatures can cause serious damage to photosystems, yet plants can survive with minimum damage. Here, we show that plants switch photosystem to protect photosystem II (PSII) at 40 °C. Using wheat and Arabidopsis seedlings, we investigated the mechanisms of heat-derived damage in the dark and avoidance of damage in the light. Heat treatment at 40 °C in the dark caused serious damage to PSII: the maximum quantum yield of PSII (Fv/Fm) and oxygen evolution rapidly decreased. The damage was due to the degradation of the D1 protein (shown by immunochemical analysis) and the disturbance of energy transfer in PSII core chlorophyll-binding proteins CP43 and CP47 (shown by time-resolved fluorescence measurement). The damage to PSII might be attained to enhanced introduction of electrons from the reducing power of the stroma into thylakoid membranes, causing subsequent electron backflow to PSII. Plants treated at 40 °C in the light avoided PSII damage and showed preferential excitation of photosystem I (PSI), phosphorylation and migration of light-harvesting complex II (LHCII), which indicate state transition of the photosystem to enhance thermal dispersion and light-driven cyclic electron flow around PSI. These results suggest that heat damage to PSII is probably due to a backflow of reducing power from the stroma to PSII, and that light causes a state transition of photosystem, driving cyclic electron flow and thus protecting PSII from damage.