Electron flow accompanying the ascorbate peroxidase cycle in maize mesophyll chloroplasts and its cooperation with linear electron flow to NADP + and cyclic electron flow in thylakoid membrane energization

Abstract

Potential roles for cyclic and pseudocyclic electron flow in C4 plants are to provide ATP for the C4 cycle and, under excess light, to down-regulate PS II activity through membrane energization. Intact mesophyll chloroplasts of maize were used to evaluate forms of electron transport including the Mehler peroxidase reaction (linear electron flow to O2, formation of H2O2 which is reduced by ascorbate, and linear flow linked to reduction of oxidized ascorbate). Addition of H2O2 to isolated chloroplasts in the light in the presence of an uncoupler induced Photosystem (PS) II activity, as determined from increases in photochemical quenching of chlorophyll fluorescence (qp) and the quantum yield of PS II. H2O2 also induced dissipation of energy by thylakoid membrane energization and non-photochemical fluorescence quenching (qn), which was inhibited by addition of an uncoupler. These effects of H2O2 on qp and qn were inhibited by addition of KCN, an inhibitor of ascorbate peroxidase. The results suggest that H2O2 is reduced via ascorbate, and that the oxidized ascorbate is then reduced by linear electron flow contributing to photochemistry and thylakoid membrane energization. Evidence for function of pseudocyclic electron flow via the Mehler peroxidase reaction was obtained with only oxygen as an electron acceptor, as well as in the presence of oxaloacetate a natural electron acceptor in C4 photosynthesis. KCN decreased qp and PS II yield in the absence and presence of oxaloacetate and, in the former case, it severely reduced q_n. KCN also decreased ΔpH formation across the thylakoid membrane based on its decrease in the light-induced quenching of 9-aminoacridine fluorescence, particularly in the absence of oxaloacetate. Antimycin A, an inhibitor of cyclic electron flow, also diminished ΔpH formation. These results provide evidence for shared energization of thylakoid membranes by the Mehler peroxidase reaction, cyclic electron flow, and linear electron flow linked to the C4 pathway.