Inheritance of Nitrate Reductase Activity

-Since the F1 hybrid (B14 X Oh43) had been shown to have a higher (heterotic) level of nitrate reductase activity than either inbred parent (B14 or Oh43), studies were undertaken to determine the mode of inheritance. Standard methods for determining MNendelian inheritance were used to study segregation for level of nitrate reductase activity of individual plants. The genetic material used was the inbreds B14 and Oh43, F1, F1 backcrossed to both parents, F2, F3, and F4 generations of the cross B14 X Oh43. The plant material was grown in the field and in growth chambers. It was shown that the maize inbreds B14 and Oh43 differ at two loci that control the level of nitrate reductase activity. Each inbred is homozygous for a dominant or partially dominant allele at one locus and homozygous recessive at a second locus. The locus at which B14 carries a dominant allele carries the recessive allele in Oh43. Oh43 has both a higher in vivo rate of synthesis of nitrate reductase and higher in vivo and in vitro loss of enzyme activity (decay) than B14. Thus, the rates of both enzyme synthesis and decay are factors governing the level of nitrate reductase activity in corn. The data suggest that the heterotic level of nitrate reductase activity in the F1 hybrid is the result of inheritance of qualities that gives it "intermediate" rates of enzyme synthesis and decay. Introduction.-Single-locus control of enzymatic activity has recently been shown for each of several enzymes. However, the mechanism of control appears to be quite different for different enzymes. Feinstein et al.' and Courtright2 proposed allelic regulator loci controlling the level of enzyme synthesis. Rechcigl and Heston3 demonstrated that in vivo enzyme turnover rate was regulating the level of catalase activity and that some external factor regulated by a pair of alleles at a single locus was controlling the in vivo turnover rate. In addition many mutant enzymes can be distinguished from the wild type by loss of thermal stability in vitro. Presumably the reduced thermal stability may also indicate to some degree an increased in vivo turnover rate, hence a regulatory effect on enzymatic activity. Although regulation of enzymatic activity by allelic differences in regulator loci, structural loci, add a locus controlling in vivo turnover rate has been demonstrated, no observation of multiple loci controlling enzymatic activity has been reported for higher plants. In view of the number of sites at which control of activity has been demonstrated in various organisms, the existence of multiple loci controlling enzymatic activity is to be expected. This communication describes the inheritance of nitrate reductase activity in populations derived from two inbred lines of maize. These two inbreds, when