ULTRASTRUCTURE OF CHLAMYDOMONAS-REINHARDTII FOLLOWING EXPOSURE TO PARAQUAT - COMPARISON OF WILD-TYPE AND A PARAQUAT-RESISTANT MUTANT

A mutant (NL-5 1) of the unicellular green alga Chlamydomonas reinhard tii Dangeard isolated from a wild-type strain (137c+) was shown to be resistant to the bipyridilium herbicide paraquat at the concentration at which growth of the wild type was inhibited. Tetrad analysis from a cross between the mutant and the wild type showed 2:2 segregation, in dicating that the resistance is under control of a single gene. Cross- resistance of the mutant to methionine and to methionine combined with riboflavin suggested that the resistance is due to increased levels o f one of the enzymes capable of detoxifying active oxygens. Ultrastruc tural examination of mutant and wild-type cells exposed to paraquat re vealed that the mutant cells were 3 to 4 times more resistant, but bot h strains showed the same sequence of deterioration. Damage was first manifested as swelling of the mitochondria and dilation of the perinuc lear space. This was followed by disintegration of the nuclear matrix and the chloroplast thylakoids.