Reactive oxygen species modulate the differential expression of methionine sulfoxide reductase genes in Chlamydomonas reinhardtii under high light illumination.

Illumination of Chlamydomonas reinhardtii cells at 1000 (high light, HL) or 3000 (very high light, VHL) µmol photons m−2 s−1 intensity increased superoxide anion radical (O2•–) and hydrogen peroxide (H2O2) production, and VHL illumination also increased the singlet oxygen (1O2) level. HL and VHL illumination decreased methionine sulfoxide reductase A4 (CrMSRA4) transcript levels but increased CrMSRA3, CrMSRA5 and CrMSRB2.1 transcripts levels. CrMSRB2.2 transcript levels increased only under VHL conditions. The role of reactive oxygen species (ROS) on CrMSR expression was studied using ROS scavengers and generators. Treatment with dimethylthiourea (DMTU), a H2O2 scavenger, suppressed HL- and VHL-induced CrMSRA3, CrMSRA5 and CrMSRB2.1 expression, whereas H2O2 treatment stimulated the expression of these genes under 50 µmol photons m−2 s−1 conditions (low light, LL). Treatment with diphenylamine (DPA), a 1O2 quencher, reduced VHL-induced CrMSRA3, CrMSRA5 and CrMSRB2.2 expression and deuterium oxide, which delays 1O2 decay, enhanced these gene expression, whereas treatment with 1O2 (rose bengal, methylene blue and neutral red) or O2•– (menadione and methyl viologen) generators under LL conditions induced their expression. DPA treatment inhibited the VHL-induced decrease in CrMSRA4 expression, but other ROS scavengers and ROS generators did not affect its expression under LL or HL conditions. These results demonstrate that the differential expression of CrMSRs under HL illumination can be attributed to different types of ROS. H2O2, O2•– and 1O2 modulate CrMSRA3 and CrMSRA5 expression, whereas H2O2 and O2•– regulate CrMSRB2.1 and CrMSRB2.2 expression, respectively. 1O2 mediates the decrease of CrMSRA4 expression by VHL illumination, but ROS do not modulate its decrease under HL conditions.