Ultrastructure of Chlamydomonas reinhardtii following exposure to paraquat: comparison of wild type and a paraquat-resistant mutant

A mutant (NL-51) of the unicellular green alga Chlamydomonas reinhardtii 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, indicating 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 of one of the enzymes capable of detoxifying active oxygens. Ultrastructural examination of mutant and wild-type cells exposed to paraquat revealed that the mutant cells were 3 to 4 times more resistant, but both strains showed the same sequence of deterioration. Damage was first manifested as swelling of the mitochondria and dilation of the perinuclear space. This was followed by disintegration of the nuclear matrix and the chloroplast thylakoids. Key words: Chlamydomonas reinhardtii, methionine resistance, paraquat, paraquat-resistant mutant, ultrastructure.

Heterogeneity in endogenous amino acid pools of Chlamydomonas reinhardtii and C. eugametos

Levels of endogenous amino acid pools were measured for two species of the unicellular green alga Chlamydomonas. In C. reinhardtii, opposite mating types, designated + and −, of strain 137c were examined. The soluble pool of glutamic acid was most abundant in strain 137c−. In strain 137c+ similar levels of this amino acid were present, and high levels of cystine were observed. A soluble pool for the amino acid methionine was not detected in either strain. In opposite mating types of C. eugametos (strain Nos. 9 and 10), a soluble pool of alanine was most abundant, whereas neither cystine nor methionine were detected. When C. reinhardtii strain 137c+ was grown in the dark, cystine was not detectable, and proline markedly increased. Similarly, growth of strain 137c+ on nitrogen-deficient medium supplemented with a limited amount of arginine as a nitrogen source resulted in an apparent disappearance of pools of cystine. No apparent difference in the growth patterns of strains 137c+ and 137c− was observed. Tetrad analysis indicates that the high cystine content found in strain 137c+ is neither a characteristic of mating type nor under control of a single Mendelian gene. Key words: Chlamydomonas reinhardtii, Chlamydomonas eugametos, amino acid pool, cystine.