||Chlamydomonas reinhardtii, as a single-celled alga, are used to study the response of algae to high light illumination. This study elucidates the role of NADPH oxidase in the regulation of C. reinhardtii cells in adapting to high light illumination.|
The effect of different high light intensity (750 μmol m-2 s-1 and 1,500 μmol m-2 s-1) on physiological response of C. reinhardtii cells was determined. The transfer of C. reinhardtii cells grown under 50 μmol m-2 s-1 conditions to 750 μmol m-2 s-1 condition resulted in an inhibition of Fv/Fm and Fv’/Fm’, followed by a recovery, while cells showed normal growth ability without oxidative damage (lipid peroxidation, MDA). But, the Fv/Fm and Fv’/Fm’ of algal cells was inhibited by illumination at 1,500 μmol m-2 s-1 with severe oxidative damage and cell death. Accordingly, the role of NADPH oxidase and H2O2 on the modulation of high light adaptation of C. reinhardtii cells was studied under 750 μmol m-2 s-1 conditions.
The treatment with DMTU, a H2O2 scavenger, causes a significant oxidative damage and finally leads to cell death, indicating that H2O2 plays a role in the adaptation of C. reinhardtii cells to high intensity illumination. The application of an inhibitor of NADPH oxidase, DPI, also exhibited a similar result. It indicates that NADPH oxidase is associated with the high light adaptation in C. reinhardtii cells. An exposure to 750 μmol m-2 s-1 illumination induced an increase in the activity of glutathione reductase (GR) and the contents of total glutathione (GSH) to remove reactive oxygen species (ROS). Although total ascorbate (AsA) content did not increase after exposure to 750 μmol m-2 s-1 illumination, the ratios of reduced AsA/oxidized AsA and the expression of CrMDAR gene were increased. These response could be inhibited by DPI treatment, but we also found that DPI treatment up-regulated the expression of several antioxidant genes, including APX gene (CrAPX1), AsA synthetic enzyme of VTC2 (CrVTC2), and DHAR (CrDHAR). It is still not clear the effect of DPI on the up-regulation of CrAPX1, CrVTC2, and CrDHAR. The expression of gene of defender against apoptotic death (DAD1) was stimulated upon exposure to 750 μmol m-2 s-1 illumination but inhibited by DPI treatment. DPI treatment also induced nitric oxide (NO) production and the treatment with cPTIO (a NO scavenger) reversed DPI effect and prevented cell death. The results of the present study demonstrated that NADPH oxidase and H2O2 modulates the antioxidant defense system and represses the cell death by inhibiting NO production and cell death process in C. reinhardtii in adaption to high light.