Comparative Studies on the Induction and Inactivation of Nitrate Reductase in Corn Roots and Leaves

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Abstract
A comparison of induction and inactivation of nitrate reductase and two of its component activities, namely FMNH2-nitrate reductase and NO3−-induced NADH-cytochrome c reductase, was made in roots and leaves of corn (Zea mays L. var. W64A × 182E). The three activities were induced in parallel in both tissues when NO3− was supplied. WO4= suppressed the induction of NADH- and FMNH2-nitrate reductase activities in root tips and leaves. The NO3−-induced NADH-cytochrome c reductase activity showed a normal increase in roots treated with WO4=. In leaves, on the other hand, there was a marked superinduction of the NO3−-induced NADH-cytochrome c reductase in the presence of WO4=. The half-life values of NADH-nitrate reductase and FMNH2-nitrate reductase measured by removing NO3− and adding WO4= to the medium, were 4 hours in root tips and 6 hours in excised leaves. Addition of NO3− in the induction medium together with WO4= gave partial protection of NADH-nitrate reductase and FMNH2-nitrate reductase activities in both root tips and leaves with a t0.5 of 6 and 8 hours, respectively. NO3− also reduced the loss of nitrate reductase activity from mature root sections. In the presence of cycloheximide, both NADH-nitrate reductase and NO3−-induced NADH-cytochrome c reductase activities were lost at similar rates in root tips. NO3− protected the loss of NO3−-induced NADH-cytochrome c reductase to the same extent as that of NADH-nitrate reductase.