Title : NADPH oxidase-mediated signaling pathway mediates the high light activation of ascorbate-glutathione cycle (AGC) in Chlamydomonas reinhardtii against photo-oxidative stress
The ascorbate-glutathione cycle (AGC) is essential for Chlamydomonas reinhardtii cells to cope with high light stress. NADPH oxidase acts as key hub for the regulation of stress tolerance in plants. High intensity illumination (HL, 1,200 mmol m-2 s-1) did not impact Chlamydomonas growth while the application of diphenyleneiodonium (DPI), a NADPH oxidase inhibitor, in the concentration of 5 or 10 mM inhibited cell growth along with an increase of reactive oxygen species (ROS, O2.-, H2O2, and 1O2) and lipid peroxidation. The RNA-seq analysis and enzyme activity assay reveal that HL triggered an increase in CAT, APX, DHAR and GR enzyme activity and FeSOD (FSD1), APX1, DHAR1, and GSHR1 transcript abundances. But, the increase in the transcript abundances of APX1 and GSHR1 by HL was enhanced in the presence of DPI. The induction in the activity of enzymes in AGC can be inhibited in the presence of 10 mM DPI, accompanied with increased ROS production. However, the increase in the transcript abundances of FSD1, APX1, DHAR1, and GSHR1 under HL condition can be enhanced in the presence of DPI. It reflects that NADPH oxidase-mediated signaling pathway modulates the induction of AGC in the ways other than mRNA level. In conclusion, the present findings demonstrate that NADPH oxidase modulates the upregulation of ascorbate-glutathione cycle for the acclimation of Chlamydomonas cells to photo-oxidative stress.