Rice plants with a high protochlorophyllide accumulation show oxidative stress in low light that mimics water stress

This report describes the possible involvement of singlet oxygen (1O2) as the causative prooxidant in water stress i n plants. Rice seedlings were loaded with 5-aminolevulinic acid (ALA), incubated in the dark, exposed to light, and then subjected to measurements of oxidative and antioxidative responses of cells to such treatment. The responses were very similar to those observed earlier in seedlings under water stress conditions. It was particularly noteworthy that both ALA treatment and water deficit treatment gave rise to virtually the same pattern of antioxidative enzymatic responses in low light. That is, the activities of enzymes which directly remove superoxide and H2O2 were not enhanced to any significant extent, whereas, the last three enzymes of the ascorbate-glutathione cycle, whose collective function is to regenerate ascorbic acid (AA) from its oxidized forms, showed markedly increased activities. Because ALA feeding resulted in a high accumulation of protochlorophyllide P-627, a potent photodynamic generator of 1O2, the results suggest not only the intermediacy of 1O2 in stress-developing processes in photosynthetic cells encountering water-limited environments, but also the crucial roles of AA, in combination with a-tocopherol action, and the AA-regenerating enzyme system in defense against the detrimental effects of 1O2 overproduction in chloroplast membranes.