Stress priming using X-irradiation or H2O2 treatment enhances growth and metabolite production in Chlamydomonas reinhardtii cells

Ionizing radiation, in addition to its genotoxic effects, induces reactive oxygen species (ROS) that can cause cellular damage or act as signaling molecules in cells. Typically, plants or algae that are exposed to mild primary stress exhibit resistance to subsequent severe stress through metabolic remodeling, which is referred to as stress priming. In contrast, this study explored a different stress priming effect in which ionizing radiation and ROS induced growth stimulation and metabolic activation in Chlamydomonas reinhardtii under normal growth conditions. To further examine this counterintuitive role of stress priming, the genotoxicity and cytotoxicity of stress were minimized by employing indirect or transient treatment. The C. reinhardtii cells were treated for 1 h with Xirradiated or H2O2-containing tris-acetate-phosphate (TAP) medium for indirect X-irradiation or transient H2O2 treatment, and then cultured to compare growth and metabolic activity. Indirect X-irradiation at 6 or 50 Gy or transient H2O2 treatment at 30 or 110 mu M did not induce changes in DNA damage or cell death but did significantly increase the growth and chlorophyll, protein, starch, and lipid contents in a dose- or concentrationdependent manner. Such growth stimulation was inhibited by treatment with the ROS scavenger ascorbic acid and enhanced by treatment with sodium bicarbonate, an additional carbon source. Furthermore, the expression of the cell cycle regulatory CrCDKA1 gene was strongly induced by chromatin modification after transient H2O2 treatment. These results suggest that indirect irradiation with X-rays or transient treatment with H2O2 within a specific dose or concentration range can induce growth stimulation by enhancing the metabolic activity in C. reinhardtii. These effects may be partially explained by chromatin modification and metabolic remodeling through ROS-mediated signaling associated with stress priming and concomitant stress memory.