Abstract
We elucidated the inactivation mechanism of ascorbate (Asc) peroxidase (APX) in chloroplasts. In the presence of nigericin, illumination of intact chloroplasts decreased electron flux in PSII (Je(PSII)), with the uptake of O2. APX was simultaneously inactivated. These results show that inactivation of APX suppressed the consumption of electron from PSII for the regeneration of Asc. On the other hand, in the absence of nigericin, both the decrease in Je(PSII) and O2 uptake were suppressed. This indicates that non-photochemical quenching and cyclic electron flow within PSII protect WWC activity by preventing H2O2 production in its cycle. Furthermore, we found that APX reaction rate in chloroplasts depended on H2O2 production rate and that Asc regeneration did not limit H2O2 scavenging. These results indicate that even in the presence of Asc, H2O2 produced in WWC decomposes APX intermediates, that is, functioning of WWC is dangerous for APX.
