Regulation of the inducible soybean nitrate reductase isoform in mutants lacking constitutive isoform(s).

In soybean, three nitrate reductase isoforms have been identified based on metabolic regulation, substrate specificity, and kinetic parameters. Two isoforms have been termed constitutive, as their activities are present without the addition exogenous nitrate to soybean seedlings. The third activity is termed inducible, as its activity is present only when soybean plants have been supplied with nitrate. The purpose of this study was to examine the regulation of the inducible nitrate reductase isoform in soybean mutants lacking one or both of the constitutive isoforms. Based on evidence obtained through measurements of enzyme activity, Western blotting, and RNA determinations, the absence of one or both of the constitutive nitrate reductase isoforms has no effect on the metabolite regulation of the inducible nitrate reductase isoform.