Biphasic regulation of superoxide radical levels in Mn-depleted and photoactivated photosystem II

In the past decades, it has become clear that superoxide radical (O2 ·−) can be generated from photosystem II (PSII) during photosynthesis. Depending on the extent of its accumulation, O2 ·− plays an important role in plant physiology and pathology. The photoinhibition/repair cycle is a typical process in PSII which is mainly responsible for the survival of plants under the photoinihibition condition. It is therefore of significant importance to determine O2 ·− production in this cycle, and then explore how O2 ·− is controlled by PSII within a normal physiological level. With this in mind, we herein investigate the variation of the O2 ·− levels in PSII under Mn-depleted and photoactivated conditions mimicking the photoinhibition/repair cycle in vitro. The effect of intrinsic SOD-like component on the O2 ·− levels was also studied. Results show that PSII has the ability to regulate the O2 ·− levels in these two processes by simultaneously modulating the O2 ·− generation activity and intrinsic SOD-like activity. This finding could shed new lights on the photoprotective property of PSII against O2 ·− and other reactive oxygen species.