Defect in chloroplastic NAD(P)H dehydrogenase complex resulted in stromal over-reduction after exposure to strong light

The cyclic electron transport around PSI by NAD(P)H dehydrogenase complex (NDH) in tobacco chloroplast has been demonstrated in several laboratories, using chloroplastic transformants in which ndh genes were insertionally inactivated. In this report, to clarify physiological relevance of NDH, effects of photo-oxidative stress on the NDH-defective leaves were investigated. After a brief exposure to saturating light, photo-oxidizable P700 was remarkably lowered in the NDH-defective transformants. Since the lowered P700+ signal was recovered by the incubation with methyl violegen, it was ascribed to P700 charge recombination induced by acceptor limitation. This finding suggested that over-reduction of stromal redox pool took place in the NDH-defective transformants. The stromal over-reduction not only lowered quantum yield of PSI, but also induced reduction of the plastoquinone pool, which resulted in lower quantum yield of PSII and photoinhibition of PSII. These results suggest that cyclic electron transport around PSI protects photosynthetic electron transport chain from over-reduction under strong light. Plants in early developmental stage were more susceptible to the stromal over-reduction and consequent photoinhibiton than old plants. Causal relation of defect in NDH and stromal over-reduction will be discussed