Evaluating the Oxidation Rate of Reduced Ferredoxin in Arabidopsis thaliana Independent of Photosynthetic Linear Electron Flow: Plausible Activity of Ferredoxin-Dependent Cyclic Electron Flow around Photosystem I

Abstract

The activity of ferredoxin (Fd)-dependent cyclic electron flow (Fd-CEF) around photosystem I (PSI) was determined in intact leaves of Arabidopsis thaliana. The oxidation rate of Fd reduced by PSI (vFd) and photosynthetic linear electron flow activity are simultaneously measured under actinic light illumination. The vFd showed a curved response to the photosynthetic linear electron flow activity. In the lower range of photosynthetic linear flow activity with plastoquinone (PQ) in a highly reduced state, vFd clearly showed a linear relationship with photosynthetic linear electron flow activity. On the other hand, vFd increased sharply when photosynthetic linear electron flow activity became saturated with oxidized PQ as the net CO2 assimilation rate increased. That is, under higher photosynthesis conditions, we observed excess vFd resulting in electron flow over photosynthetic linear electron flow. The situation in which excess vFd was observed was consistent with the previous Fd-CEF model. Thus, excess vFd could be attributed to the in vivo activity of Fd-CEF. Furthermore, the excess vFd was also observed in NAD(P)H dehydrogenase-deficient mutants localized in the thylakoid membrane. The physiological significance of the excessive vFd was discussed.

Enhanced Reduction of Ferredoxin in PGR5-Deficient Mutant of Arabidopsis thaliana Stimulated Ferredoxin-Dependent Cyclic Electron Flow around Photosystem I

“…The oxidation rate of the reduced Fd displayed a nonlinear relationship with Y(II) in both WT and pgr5 hope1 ( Figure 5 A). The increase in vFd deviated from the rise in Y(II) in both cases, indicating the presence of excessive vFd unrelated to photosynthetic linear electron flow—the electron flux in Fd-CEF ( Figure 5 A) [ 24 ]. Furthermore, we observed a deviation in the relationship between vFd and qP in both WT and pgr5 hope1 ( Figure 5 B).…”

“…In this study, we monitored the redox reaction of Fd concurrently with chlorophyll fluorescence, P700 + and PC + absorbance changes, and net CO 2 assimilation using intact leaves of Arabidopsis thaliana . Successful measurement of the electron flux in Fd-CEF in Arabidopsis thaliana had been previously achieved [ 24 ]. The oxidation rate of reduced Fd, independent of the photosynthetic linear electron flow—termed the extra oxidation rate of Fd—was designated as the electron flux in Fd-CEF, denoted as vCEF.…”

“…The regulation of vCEF was found to be linked to the reduction–oxidation state of PQ. Specifically, as PQ became oxidized with an increase in CO 2 assimilation, vCEF exhibited a corresponding increase [ 24 ]. These observed characteristics align with the established model of cyclic electron flow [ 3 ].…”

“…These observed characteristics align with the established model of cyclic electron flow [ 3 ]. Importantly, our inability to detect the extra Fd oxidation reaction [ 24 ] might be attributed to the suppressed in vivo occurrence of this reaction under conditions of limited photosynthesis. In instances where PQ was highly reduced and the apparent quantum yield of PSII was low, the extra Fd oxidation reaction was inhibited.…”

“…Fd-CEF necessitates Fd as the electron donor to effectively reduce oxidized PQ [ 3 ]. Typically, the redox state of Fd remains constant in response to decreases in both the net CO 2 assimilation rate and the photosynthetic linear electron flow under constant actinic light intensity [ 19 , 24 , 25 , 26 ]. Our recent findings revealed an augmented reduction of Fd in a PGR5-less mutant ( pgr5 hope1 ) of Arabidopsis thaliana due to the inhibited oxidation of P700 in PSI [ 19 ].…”