Photosystem II is vulnerable to various abiotic stresses such as strong visible light and heat. Under both stresses, the damage seems to be triggered by reactive oxygen species, and the most critical damage occurs in the reaction center-binding D1 protein. Recent progress has been made in identifying the protease involved in the degradation of the photo- or heat-damaged D1 protein, the ATP-dependent metalloprotease FtsH. Another important result has been the discovery that the damaged D1 protein aggregates with nearby polypeptides such as the D2 protein and the antenna chlorophyll-binding protein CP43. The degradation and aggregation of the D1 protein occur simultaneously, but the relationship between the two is not known. We suggest that phosphorylation and dephosphorylation of the D1 protein, as well as the binding of the extrinsic PsbO protein to Photosystem II, play regulatory roles in directing the damaged D1 protein to the two alternative pathways.

Moderate heat stress (40 degrees C, 30 min) on spinach thylakoids induced cleavage of the D1 protein, producing an N-terminal 23-kDa fragment, a C-terminal 9-kDa fragment, and aggregation of the D1 protein. A homologue of Arabidopsis FtsH2 protease, which is responsible for degradation of the damaged D1 protein, was abundant in the stroma thylakoids. Two processes occurred in the thylakoids in response to heat stress: dephosphorylation of the D1 protein in the stroma thylakoids, and aggregation of the phosphorylated D1 protein in the grana. Heat stress also induced the release of the extrinsic PsbO, P and Q proteins from Photosystem II, which affected D1 degradation and aggregation significantly. The cleavage and aggregation of the D1 protein appear to be two alternative processes influenced by protein phosphorylation/dephosphorylation, distribution of FtsH, and intactness of the thylakoids.

Moderate heat stress (40°C for 30 min) on spinach thylakoid membranes induced cleavage of the reaction center-binding D1 protein of photosystem II, aggregation of the D1 protein with the neighboring polypeptides D2 and CP43, and release of three extrinsic proteins, PsbO, -P, and -Q. These heat-induced events were suppressed under anaerobic conditions or by the addition of sodium ascorbate, a general scavenger of reactive oxygen species. In accordance with this, singlet oxygen and hydroxyl radicals were detected in spinach photosystem II membranes incubated at 40°C for 30 min with electron paramagnetic resonance spin-trapping spectroscopy. The moderate heat stress also induced significant lipid peroxidation under aerobic conditions. We suggest that the reactive oxygen species are generated by heat-induced inactivation of a water-oxidizing manganese complex and through lipid peroxidation. Although occurring in the dark, the damages caused by the moderate heat stress to photosystem II are quite similar to those induced by excessive illumination where reactive oxygen species are involved. Photosystem II (PS II)3 in higher plants is a multisubunit complex composed of more than 25 proteins and the associated cofactors. Excitation energy captured by the chlorophylls and carotenoids in the light-harvesting chlorophyll protein complexes of PS II is finally transferred to P680, the reaction center chlorophyll of PS II, where charge separation takes place. In particular, PS II performs oxidation of water and reduction of plastoquinone molecules via chlorophyll-mediated photochemical reactions. Although it plays such an important role in the primary photochemical reaction of photosynthesis, PS II is vulnerable to various environmental stresses such as excessive visible light and high temperature.When irradiated with excessive visible light, the D1 protein is oxidatively damaged, and electron transport is inhibited. This process is referred to as photoinhibition of PS II (1-4). The photo-damaged D1 protein is subsequently degraded by specific proteases (5), and the repair of PS II is accomplished by the integration of a newly synthesized D1 protein to the PS II complex (6). Photoinhibition of PS II is caused by either the socalled acceptor-side or donor-side mechanism or both (4, 7). The acceptor-side photoinhibition takes place when the acceptor side of PS II is over-reduced by excessive illumination and the double-reduced Q A molecule is released from its binding site. Reversed electron flow from the primary electron acceptor pheophytin to P680 in the absence of Q A generates the triplet state P680, which reacts with molecular oxygen to form singlet oxygen ( 1 O 2 ). The 1 O 2 eventually damages the nearby polypeptide, the D1 protein. Alternatively, oxygen molecules may be reduced at the acceptor side of PS II to produce superoxide anion radicals (O 2 . ), which are turned into hydrogen peroxide (H 2 O 2 ) and finally hydroxyl radical (HO ⅐ ) through the Fenton reaction (8). It is claimed, however, that the generation of...