[en] One of the most common reactive oxygen species (ROS) produced in eukaryotic organisms during photosynthesis is hydrogen peroxide (H2O2) [1] derived from superoxide dismutation and scavenged by Ascorbate peroxidase (APX) in the water-water cycle [1,2]. To evaluate the impact of enzymatic H2O2-scavenging on the photosynthetic performance, we selected an insertional mutant of C. reinhardtii from the CLIP library deficient in Ascorbate peroxidase 2 (APX2), predicted to be targeted to the chloroplast. We compared the quantum yield of PSII (φPSII) and oxygen evolution, the photo-oxidation and the re-reduction rate of the PSI of the wild type (WT) and the apx2 mutant cells. We found that the effects of the APX2 loss are mainly seen when cells grown in low light are exposed to high light. apx2 cells presented a lower φPSII in phototrophic growth condition which resembles the one of WT under high light or H2O2 stress. This result is also confirmed under fluctuating light condition, in which after some days the growth of apx2 cells was impacted, as also observed in other apx2 mutants from the CLIP library.
The apx2 mutant was also characterized by a higher PSI photo-oxidation rate and a decreased cyclic electron flow in cells grown in low light, suggesting an electron donor side limitation at the level of the PSI. Furthermore, a lower re-reduction rate of the PSI after a short pulse of saturating light supports our observations of donor side limitation. All in all, the APX2 enzyme seems to exert its function in low light acclimated cells and could be involved in the early redox poise of photosynthesis. This might prevent PSI high photo-oxidation and its damage during light transition, but also reduce the oxidative pressure at the acceptor side level, supporting the idea that APX2 is important during the fast transition from low to high light.
Funded by FNRS-FWO EOS Project 30829584
[1] Asada (2006) Production and scavenging of reactive oxygen species in chloroplasts and their functions. Plant Physiol. 141, 391 – 396
[2] Dietz et al. (2002) The function of the chloroplast 2‐cysteine peroxiredoxin in peroxide detoxification and its regulation. J. Exp. Bot. 53, 1321 – 1329
