Mechanism for Relaxation of Photooxidative Stress in Plants.

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Under environmental conditions where plants are exposed to a bright sunlight that cannot be utilized, excess photons generate active oxygens, which cause photooxidative damages to plants. For the protection from such damages, plants have the following two relaxation systems.1. Suppression of the production of active oxygensCyclic electron flow suppresses the production of active oxygens through the down-requlation of photosystem II,but, the mediator between the photoreductants (either ferredoxin or NADPH) had not been identified. In the cyanobacterium Synechocystis 6803, a NADPH-specific pyridine nucleotide dehydrogenase (NDH) mediates an electron transfer from NADPH to the thylakoid-plasto-quinone (PQ), which was established by analysis of chlorophyll fluorescence and redox reaction of P700 using the NDH-less mutant cells. Further, in maize thylakoids, LED-array spectrophotometer allows to detect a cytochrome b-559 with a low redox potential and its participatation in the ferredoxin-mediated cyclic electron flow around photosystem I.Kinetic analysis of O_2^--production in thylakoids indicates also the O_2^--mediated cyclic electron flow around photosystem I.2.Scavenging of active oxygens in chloroplastsMicrocomparteantalization of the scavenging enzymes of active oxygens in chloroplasts was established by localization of the participating enzymes by fractionation of the organella and by immunoelectronmicrosccopy. It was found that the scavenging is proceeded by the thylakoidal and stromal systems. Further, molecular properties of ascorbate peroxidase and monodehydroascorbate reductase were characterized using the purifed enzymes and the overproducing enzymes in Escherichia coli.