Isolation and characterization of nitrate reductase-deficient mutants of Arabidopsis thaliana

Chlorate resistant mutants of Arabidopsis thaliana were isolated, of which 10 exhibited a lowered nitrate reductase activity and 51 were chlorate-resistant because of an impaired uptake of chlorate. The 51 mutants of this type are all affected in the same gene. The mutants with a lowered nitrate reductase activity fall into 7 different complementation groups. Three of these mutants grow poorly on media with nitrate as the sole nitrogen source, while the others apparently can reduce sufficient nitrate to bring about growth. In all cases a low nitrate reductase activity coincides with an enhanced nitrite reductase activity. After sucrose gradient centrifugation of wildtype extracts nitrate reductase is found at the 8S position, whereas cytochrome-c reductase is found both at 4 and 8S positions. It is suggested that the functional nitrate reductase is a complex consisting of 4S subunits showing cytochrome-c reductase activity and a Mo-bearing cofactor. All mutants except B25 are capable of assembling the 4S subunits into complexes which for most mutants have a lower S value and exhibit a lower nitrate reductase activity than the wildtype complexes. Since the mutants B25 and B73 exhibit a low xanthine dehydrogenase activity, the Mo-bearing cofactor is probably less available in these mutants than in the wildtype. B73 appears to be the only mutant which is partly repaired by excessive Mo. The possible role of several genes is discussed.