Low concentrations of NaHSO3 enhance NAD(P)H dehydrogenase-dependent cyclic photophosphorylation and alleviate the oxidative damage to improve photosynthesis in tobacco

Bisulfite at low concentrations (L-NaHSO₃) increases cyclic electron transport around photosystem I (PSI) and photosynthesis. However, little is known regarding the detailed contribution of cyclic electron transport to the promoted photosynthesis by L-NaHSO₃. In the present work, we used tobacco mutant defective in ndhC-ndhK-ndhJ (??ndhCKJ) to investigate the role of NAD(P)H dehydrogenase (NDH)-dependent cyclic electron transport around PSI in an increase in photosynthesis by L-NaHSO₃. After the treatment of tobacco leaves with L-NaHSO₃ (10 ??mol L^?1), the NDH-dependent cyclic electron transport, monitored by a transient post-illumination increase in Chl fluorescence and the amount of NDH, was notably up-regulated in wild type (WT). The NDH-dependent cyclic electron transport was severely impaired in ??ndhCKJ and was not significantly affected by treatment with L-NaHSO₃. Accordingly, the NDH-dependent transthylakoid membrane proton gradient (??pH), as reflected by the slow phase of millisecond-delayed light emission (ms-DLE), was increased by L-NaHSO₃ in WT, but not in ??ndhCKJ; the enhancement of cyclic photophosphorylation (PSP) activity by L-NaHSO₃ was more obvious in WT than ??ndhCKJ. The accumulation of both superoxide and hydrogen peroxide was reduced in WT when subjected to L-NaHSO₃ treatment, but not in ??ndhCKJ. Furthermore, the increase of photosynthetic O₂ evolution rate by L-NaHSO₃ was more significant in WT than in ??ndhCKJ. We therefore conclude that L-NaHSO₃ alleviates the photo-oxidative damage by the enhancement of NDH-dependent cyclic PSP, thereby improving photosynthesis.