The Induction of Nitrate Reductase in Mung Bean Seedlings

Abstract

Nitrate reductase is induced by its substrate in the root and hypocotyl of mung bean seedlings (Phaseolus aureus Roxb.). This induction is considerably enhanced by NH₄⁺ and to a lesser extent by an amino acid mixture, and is also promoted by the presence of sucrose in the incubation medium. There is a lag period between the addition of nitrate and the establishment of a steady rate of increase in enzyme activity. The lag period decreases with increasing temperature. The enzyme activity continues to increase only in the continued presence of nitrate in the incubation medium. However, a constant level of activity is maintained for at least 6 h after removal of substrate. Re-addition of substrate causes an immediate resumption of the normal rate of increase without manifestation of a lag period.

A number of inhibitors of RNA and protein synthesis inhibit the induction. However, chloramphenicol and 5-fluorouracil at appropriate concentrations promote induction. The inhibitor of DNA synthesis, fluorodeoxyuridine, is without effect.

Gibberellic acid, naphthaleneacetic acid, benzyladenine and abscisic acid have no effect in the induction process in the absence of substrate. However, abscisic acid and high concentrations of naphthaleneacetic acid (10^-6M) and benzyladenine (10^-5M) inhibit the response to nitrate.

Cyclic 3',5'-AMP slightly enhances the induction of the enzyme. Related nucleotides cause varying degrees of inhibition: cyclic 2',3'-AMP is strongly inhibitory.

The distribution of enzyme activity and nitrate along the roots after growth in the induction medium has been examined. Maximum enzyme activity is associated with minimum internal nitrate concen- tration. The highest nitrate reductase activity occurs at the root tip.