Study on the effects of temperature stress on plants and the acclimation mechanism

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High-temperature-induced inhibition of oxygen evolution was due to release of the Mn-stabilizing 33 kDa protein from the photosystem (PS) II complex. We found that semipermeability of thylakoid membranes is also lost at temperatures at which the oxygen evolution activity is inhibited. Decreases in the fluidity of isolated thylakoids by the addition of cholesterol also decreased the thermo-sensitivity of oxygen evolution reaction. Thermo-stabilities of the semipermeability and oxygen evolution changed in parallel in wheat and Arabidopsis grown at 15, 25, and 35 ℃. These results suggest that changes in the ion composition of thylakoid lumen cause release of the 33 kDa protein form the PSII complex.We also found that cyclic electron flow around PSI is accelerated by high temperatures. Experiments using various inhibitors and an ndhB-less mutant of tobacco showed that not NAD(P)H dehydrogenase but a flavin enzyme is related to this phenomenon.Another important result was obtained using Synechocystis sp. PCC6803, a cyanobacterium, the CP47 protein of which is His-tagged. Oxygen-evolving PSII core complexes were easily isolated and purified using this mutant. The isolated thylakoid membranes from the 25 ℃-grown cells (25 ℃-thylakoids) showed less thermo-stability than those obtained from the 35 ℃-grown cells (25 ℃ -thylakoids). On the other hand, the core complexes isolated from the 25 ℃-grown cells showed the same thermostability as did the 35 ℃-grown cells. Incorporation of the PSII core complexes into liposomes made from the lipids of the 25 ℃-thylakoids or of the 35 ℃-thylakoids showed that the thermostability of the PSII reaction center activity is determined by the characteristics of the thylakoid lipids.