NADH-glutamate synthase in alfalfa root nodules. Genetic regulation and cellular expression

NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14) is a key enzyme in primary nitrogen assimilation in alfalfa (Medicago sativa L.) root nodu les. Here we report that in alfalfa, a single gene, probably with multiple alleles, encodes for NADH-GOGAT. In situ hybridizations were performed to a ssess the location of NADH-GOGAT transcript in alfalfa root nodules. In wil dtype cv Saranac nodules the NADH-GOGAT gene is predominantly expressed in infected cells. Nodules devoid of bacteroids (empty) induced by Sinorhizobi um meliloti 7154 had no NADH-GOGAT transcript detectable by in situ hybridi zation, suggesting that the presence of the bacteroid may be important for NADH-COGAT expression. The pattern of expression of NADH-GOGAT shifted duri ng root nodule development. Until d 9 after planting, all infected cells ap peared to express NADH-GOGAT. By d 19, a gradient of expression from high i n the early symbiotic zone to low in the late symbiotic zone was observed. In 33-d-old nodules expression was seen in only a few cell layers in the ea rly symbiotic zone. This pattern of expression was also observed for the ni fH transcript but not for leghemoglobin. The promoter of NADH-GOGAT was eva luated in transgenic alfalfa plants carrying chimeric beta-glucuronidase pr omoter fusions. The results suggest that there are at least four regulatory elements. The region responsible for expression in the infected cell zone contains an 88-bp direct repeat.