Oxidation of P700 Induces Alternative Electron Flow in Photosystem I in Wheat Leaves

Abstract

Oxygen (O2)-evolving photosynthetic organisms oxidize the reaction center chlorophyll, P700, in photosystem I (PSI) to suppress the production of reactive oxygen species. The oxidation of P700 is accompanied by alternative electron flow in PSI (AEF-I), which is not required for photosynthetic linear electron flow (LEF). To characterize AEF-I, we compared the redox reactions of P700 and ferredoxin (Fd) during the induction of carbon dioxide (CO2) assimilation in wheat leaves, using dark-interval relaxation kinetics analysis. Switching on an actinic light (1000 μmol photons m−2 s−1) at ambient CO2 partial pressure of 40 Pa and ambient O2 partial pressure of 21 kPa gradually oxidized P700 (P700+) and enhanced the reduction rate of P700+ (vP700) and oxidation rate of reduced Fd (vFd). The vFd showed a positive linear relationship with an apparent photosynthetic quantum yield of PSII (Y[II]) originating at point zero; the redox turnover of Fd is regulated by LEF via CO2 assimilation and photorespiration. The vP700 also showed a positive linear relationship with Y(II), but the intercept was positive, not zero. That is, the electron flux in PSI included the electron flux in AEF-I in addition to that in LEF. This indicates that the oxidation of P700 induces AEF-I. We propose a possible mechanism underlying AEF-I and its physiological role in the mitigation of oxidative damage.

Photorespiration Enhances Acidification of the Thylakoid Lumen, Reduces the Plastoquinone Pool, and Contributes to the Oxidation of P700 at a Lower Partial Pressure of CO2 in Wheat Leaves

“…Under the two pO 2 conditions, vH + showed a positive linear relationship with Jf, with an origin of zero. These results agree with those of Avenson et al [26] Kadota et al [38] reported that the electron flux in ferredoxin (Fd)-dependent cyclic electron flow (CEF) activity is negligible compared to the electron flux in LEF under high light intensity conditions [38]. Therefore, it is implied that in a steady state, vH + is equal to the rate of H + accumulation in the thylakoid lumen driven by LEF.…”

“…Furthermore, we recently found that the LEF rate and Fd oxidation rate have a similar relationship to that between the LEF rate and Jg [38]. These results show that ferredoxin (Fd)-dependent CEF [40][41][42][43][44][45][46] is negligibly small; that is, photosynthetic LEF is responsible for the majority of ∆pH formation [38]. The induction mechanism of ∆pH formation across the thylakoid membranes is shown in the following manner: ∆pH formation is observed as an ECS signal increase [26,47].…”

Molecular Mechanism of Oxidation of P700 and Suppression of ROS Production in Photosystem I in Response to Electron-Sink Limitations in C3 Plants

“…On the other hand, a decrease in Jg does not induce the reduction of Fd [61]. For example, lowering the intercellular partial pressure of CO 2 decreases the photosynthesis rate, and Jg decreases; however, the reduced level of Fd either does not change or decrease.…”

“…These results indicate that photosynthetic linear electron flow is the sole driver of both photosynthesis and photorespiration. In other words, Jf = Jg 1Recently, a positive linear relationship with an origin of zero between the reduced Fd oxidation rate (vFd) and the photosynthetic linear electron flow rate (J f ) was found (Equation 2 [61]). These results indicate that electron flow via Fd is driven by both photosynthesis and photorespiration.…”

“…The pmf is the driving force to synthesize ATP by ATP synthase using ADP and Pi, and the pmf-dissipation rate (vH + ) is expressed as gH + × pmf. As a robust model, Jf = vFd = Jg [61]; vFd is the oxidation rate of reduced Fd; kJf × Jf = gH + × pmf [64,65]. vH + = gH + difference in H + concentration (∆pH) between the stroma and the lumen, i.e., the proton motive force (pmf).…”