Characterization of the apx2 mutant in Chlamydomonas

One of the most common reactive oxygen species (ROS) produced in eukaryotic organisms during photosynthesis is hydrogen peroxide (H2O2) [1]. To evaluate the impact of H2O2-scavenging enzymes on algal fitness, we selected a mutant deficient in ascorbate peroxidase 2 (APX2) in the green microalga Chlamydomonas reinhardtii. Ascorbate peroxidase (APX) uses ascorbate and H2O2 as substrates to form monodehydroascorbate (MDHA) and H2O. The enzyme, encoded by eight isoforms in Arabidopsis thaliana, has been already well-described in plants as one of the main actors involved in the Mehler reaction in which electrons obtained from H2O oxidation in photosystem II (PSII) are linearly transported to the acceptor side of photosystem I (PSI) where they are used to completely reduce the superoxide (O2-) to H2O [2]. In Chlamydomonas, four APX isoforms are present and APX2 is predicted to be targeted to the thylakoid membrane of the chloroplast using Target P [3]. In this study, we focus on the importance of APX2 in the Mehler reaction and during linear electron flow. To assess these two functionalities of APX2, we evaluated photosynthetic parameters under control light and high light in cells grown phototrophically. Therefore, we compared the quantum yield of PSII (φPSII) by monitoring in vivo chlorophyll a fluorescence, and the photooxidation and the re-reduction rate of the PSI of wild type (WT) and apx2 mutant cells. We found that the effects of the loss of APX2 are mainly seen in cells grown in control light. The apx2 mutant cells present a decreased rate of the relative electron transport (rETR) in phototrophic control light growth condition and the PSII efficiency resembles the one of WT under high light or H2O2 stress. We also observed a higher PSI photooxidation rate for the apx2 mutant compared to WT in cells grown in control light, suggesting an electron donor side limitation at the level of the PSI. Further, the lower re-reduction rate of the PSI in apx2 mutant compared to the WT after a short pulse of saturating light supports our observations. All in all, the APX2 enzyme seems to exert its function under unstressful physiological condition and could be involved in the early redox poise of photosynthesis.

Funded by FNRS-FWO EOS Project 30829584

[1] Asada, Plant Physiol. 141, 391 – 396 (2006)
[2] Dietz et al., J. Exp. Bot. 53, 1321 – 1329 (2002)
[3] Emanuelsson et al., J. Mol. Biol. 300, 4, 1005 – 106 (2000)