Root Nodule Enzymes of Ammonia Metabolism from Medicago sativa L. as Influenced by Nitrate Levels

Abstract

Plants of alfalfa (Medicago sativa L.) were subjected to six levels of nitrate ranging from 0 to 200 mM. Then, nodule content of soluble protein and activities of the nodule enzymes, nitrogenase (C(2)H(2) reduction activity), glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.1.14), and NADH/NAD(+)-glutamate dehydrogenases (EC 1.4.1.2) were determined. Decline of glutamate synthase activity closely paralleled that of nitrogenase. In contrast, glutamine synthetase activity reached a minimum at 20mM nitrate but began to recover at higher nitrate concentrations. Patterns of NADH-(aminating) and NAD(+)-(deaminating) glutamate dehydrogenase activities were opposed with a great increase (2- to 5-fold) in the aminating direction at highest levels of nitrate. Results indicate a main role of glutamine synthetase/glutamate synthase pathway in the assimilation of fixed N. In addition, increase of the activities of glutamine synthetase and NADH-glutamate dehydrogenase concomitant with an almost fully suppressed nitrogenase and decreased NAD(+)-glutamate dehydrogenase activities at above 20mM nitrate reflect: (a) rapid induced senescence of nodules and (b) the production of ammonia by alternative means as nitrate reduction or oxidative deamination of amino acids.