Photosynthetic Performance and Antioxidant Metabolism in a Paraquat-Resistant Mutant of Chlamydomonas reinhardtii L.

A paraquat-resistant mutant was isolated from photomixotrophic cultures of Chlamydomonas reinhardtii, which showed a factor of resistance of 76.5 in light and 107 in dark compared to the WT cultures. When treated with eightfold higher concentrations of paraquat, this paraquat-resistant mutant showed inhibition of photosynthetic oxygen evolution and Fv/Fo ratios similar to those of WT. Exclusion of paraquat from the target site could be responsible for the expression of this resistance. The constitutive levels of antioxidant enzymes were similar in WT and paraquat-resistant cultures. However, the levels differed when the cultures were treated with paraquat. The paraquat-resistant mutant showed similar increases in the levels of antioxidant enzymes when grown in light or dark, which could possibly be attributed to the cytosolic or mitochondrial isoforms of these enzymes. Expression of resistance may occur due to exclusion of paraquat and/or detoxifica tion of paraquatgenerated active oxygen species by the antioxidant enzymes.