NADH-glutamate synthase in alfalfa root nodules. Immunocytochemical localization

In root nodules of alfalfa (Medicago sativa L.), N-2 is reduced to NH4+ in the bacteroid by the nitrogenase enzyme and then released into the plant cy tosol. The NH4+ is then assimilated by the combined action of glutamine syn thetase (EC 6.3.1.2) and NADH-dependent Glu synthase (NADH-GOGAT; EC 1.4.1. 14) into glutamine and Glu. The alfalfa nodule NADH-GOGAT protein has a 101 -amino acid presequence, but the subcellular location of the protein is unk nown. Using immunocytochemical localization, we determined first that the N ADH-GOGAT protein is found throughout the infected cell region of both 19- and 33-d-old nodules. Second, in alfalfa root nodules NADH-GOGAT is localiz ed predominantly to the amyloplast of infected cells. This finding, togethe r with earlier localization and fractionation studies, indicates that in al falfa the infected cells are the main location for the initial assimilation of fixed N-2.