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

A comparison of induction and inactivation of nitrate reductase and two of its component activities, namely FMNH(2)-nitrate reductase and NO(3) (-)-induced NADH-cytochrome c reductase, was made in roots and leaves of corn (Zea mays L. var. W64A x 182E). The three activities were induced in parallel in both tissues when NO(3) (-) was supplied. WO(4) (=) suppressed the induction of NADH- and FMNH(2)-nitrate reductase activities in root tips and leaves. The NO(3) (-)-induced NADH-cytochrome c reductase activity showed a normal increase in roots treated with WO(4) (=). In leaves, on the other hand, there was a marked superinduction of the NO(3) (-)-induced NADH-cytochrome c reductase in the presence of WO(4) (=).The half-life values of NADH-nitrate reductase and FMNH(2)-nitrate reductase measured by removing NO(3) (-) and adding WO(4) (=) to the medium, were 4 hours in root tips and 6 hours in excised leaves. Addition of NO(3) (-) in the induction medium together with WO(4) (=) gave partial protection of NADH-nitrate reductase and FMNH(2)-nitrate reductase activities in both root tips and leaves with a t(0.5) of 6 and 8 hours, respectively. NO(3) (-) also reduced the loss of nitrate reductase activity from mature root sections. In the presence of cycloheximide, both NADH-nitrate reductase and NO(3) (-)-induced NADH-cytochrome c reductase activities were lost at similar rates in root tips. NO(3) (-) protected the loss of NO(3) (-)-induced NADH-cytochrome c reductase to the same extent as that of NADH-nitrate reductase.