Coupled Regulation of Cyclic Electron Flow Around PSI with Photosynthesis — Its Contribution to Non-photochemical Quenching Evidenced with Transplastomic Tobacco Plants Over-Expressing Ferredoxin in Chloroplasts

Abstract

We tested the hypothesis that ferredoxin (Fd) limits the activity of cyclic electron flow around PSI (CEF-PSI) in vivo and the relief of its limitation causes non-photochemical quenching (NPQ) of Chl fluorescence to be enhanced. To test this hypothesis, we made a transplastomic plant expressing Fd, originated from Arabidopsis thaliana, in chloroplasts of tobacco leaves. In transplastomic plants, minimum yield of Chl fluorescence (Fo) was higher than that in wild-type plants. Its increase in Fo was suppressed to the level of wild-type plants on the illumination of farred light to leaves of transplastomic plants. These results implied the enhanced donation of electrons by Fd-quinone oxidoreductase (FQR) from stroma in chloroplasts to plastoquinone in transplastomic plants. That is, it is assumed that transplastomic plants had a higher activity of CEF-PSI more than wild-type plants did. In fact, the activity of CEF-PSI in transplastomic plants became higher at the limited activity of photosynthetic linear electron flow in chloroplasts than wild-type plants. Furthermore, NPQ of Chl fluorescence was enhanced in transplastomic plants, compared to wild-type plants. On the other hand, pool size of xanthophylls-cycle pigments and the amount of PsbS protein were almost the same each other between them. These results support our hypothesis strongly. Strengthening NPQ of Chl fluorescence by the enhancement of CEF-PSI activity can be a strategy for making plants to have tolerance against abiotic stress, where light utilization-efficiency as observed in Φ(PSII) is low.