Ferredoxin limits Cyclic Electron Flow around PSI (CEF-PSI) in Higher Plants-Stimulation of CEF-PSI enhances Non-photochemical quenching of Chl Fluorescence in Transplastomic Tobacco-

Abstract

We tested the hypothesis that ferredoxin (Fd) limits the activity of CEF-PSI in vivo and that the relief of this limitation promotes the non-photochemical quenching (NPQ) of Chl fluorescence. In transplastomic tobacco expressing Fd from Arabidopsis in its chloroplasts, the minimum yield of Chl fluorescence (Fo) was higher than in the wild type. Fo was suppressed to the wild-type level upon illumination with far-red light, implying that the transfer of electrons by Fd-quinone oxidoreductase from the chloroplast stroma to plastoquinone was enhanced in transplastomic plants. The activity of CEF-PSI became higher in transplastomic than in wild-type plants under conditions limiting photosynthetic linear electron flow. Similarly, NPQ of Chl fluorescence was enhanced in transplastomic plants. On the other hand, pool sizes of the pigments of the xanthophyll cycle and the amounts of PsbS protein were the same in all plants. All these results supported the hypothesis strongly.