Abstract
The molecular entity responsible for catalyzing ferredoxin (Fd)-dependent cyclic electron flow around photosystem I (Fd-CEF) remains unidentified. To reveal the in vivo molecular mechanism of Fd-CEF, evaluating ferredoxin reduction-oxidation kinetics proves to be a reliable indicator of Fd-CEF activity. Recent research has demonstrated that the expression of Fd-CEF activity is contingent upon the oxidation of plastoquinone. Moreover, chloroplast NAD(P)H dehydrogenase does not catalyze Fd-CEF in Arabidopsis thaliana. In this study, we analyzed the impact of reduced Fd on Fd-CEF activity by comparing wild-type and pgr5-deficient mutants (pgr5(hope1)). PGR5 has been proposed as the mediator of Fd-CEF, and pgr5(hope1) exhibited a comparable CO(2) assimilation rate and the same reduction-oxidation level of PQ as the wild type. However, P700 oxidation was suppressed with highly reduced Fd in pgr5(hope1), unlike in the wild type. As anticipated, the Fd-CEF activity was enhanced in pgr5(hope1) compared to the wild type, and its activity further increased with the oxidation of PQ due to the elevated CO(2) assimilation rate. This in vivo research clearly demonstrates that the expression of Fd-CEF activity requires not only reduced Fd but also oxidized PQ. Importantly, PGR5 was found to not catalyze Fd-CEF, challenging previous assumptions about its role in this process.