Ascorbate in thylakoid lumen functions as an alternative electron donor to photosystem II and photosystem I.

The roles of ascorbate (Asc) in the thylakoid lumen to support photosynthetic electron transport are investigated. Asc can be photooxidized in photosystem (PS) II and PSI. When the water oxidase complex (WOC) was inactivated by acidic pH or by UV-B, Asc was photooxidized in PSII with Asc replacing water as the electron donor. An apparent Km was 2.5mM. At 20mM Asc, the electron transport rate reached 50 micromol NADP+ -reduced mgChl-1h-1. Asc was not oxidized by the PSII reaction center complex having an active WOC, and hence was suggested to function as an emergency donor only when WOC is inactivated. In the presence of 3-(3,4-dichlorophenyl)-1,1'-dimethylurea, Asc was photooxidized by PSI, with a lower affinity and an electron transport rate of 70 micromol NADP+ -reduced mgChl-1h-1 in 50mM Asc. Thus, Asc can support a PSI-mediated electron flow at a reasonably high rate at Asc concentrations in the physiologically relevant range. During the photooxidation of Asc by PSI, we observed the production of the monodehydroascorbate radical (MDA) that was unscavenged by the exogenously added MDA reductase. Reduction of P700+ by Asc was not affected by the inactivation of plastocyanin with cyanide. These results indicated that Asc was univalently oxidized on the lumenal side of PSI, directly by P700+. The electron transport Asc --> PSI --> NADP+ did not form a proton gradient across the thylakoid membrane, as determined by 9-aminoacridine fluorescence. Based on these results, we propose that the Asc-dependent cyclic electron flow around PSI, and that through both PSI and PSII can operate when the linear electron transport is partially impaired.