NADH-Glutamate Synthase in Alfalfa Root Nodules. Genetic Regulation and Cellular Expression¹

Gian B. Trepp, Martijn van de Mortel, Hirofumi Yoshioka, Susan S. Miller, Deborah A. Samac, J. Stephen Gantt, and Carroll P. Vance^*

Institut für Pflanzenwissenschaften Eidgenössische Technische Hochschule-Zürich, 8092 Zürich, Switzerland (G.B.T.); Department of Agronomy and Plant Genetics (G.B.T., M.v.d.M., H.Y., S.S.M., C.P.V.), Department of Plant Pathology (D.A.S.), and Department of Plant Biology (J.S.G.), University of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, Minnesota 55108; University of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, Minnesota 55108Plant Pathology Laboratory, Nagoya University, Chikusa, Nagoya 464-01, Japan (H.Y.); and United States Department of Agriculture, Agricultural Research Service, Plant Science Research Unit, 411 Borlaug Hall, 1991 Upper Buford Circle, University of Minnesota, St. Paul, Minnesota 55108 (D.A.S., C.P.V)

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 nodules. Here we report that in alfalfa, a single gene, probably with multiple alleles, encodes for NADH-GOGAT. In situ hybridizations were performed to assess the location of NADH-GOGAT transcript in alfalfa root nodules. In wild-type cv Saranac nodules the NADH-GOGAT gene is predominantly expressed in infected cells. Nodules devoid of bacteroids (empty) induced by Sinorhizobium meliloti 7154 had no NADH-GOGAT transcript detectable by in situ hybridization, suggesting that the presence of the bacteroid may be important for NADH-GOGAT expression. The pattern of expression of NADH-GOGAT shifted during root nodule development. Until d 9 after planting, all infected cells appeared to express NADH-GOGAT. By d 19, a gradient of expression from high in 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 early symbiotic zone. This pattern of expression was also observed for the nifH transcript but not for leghemoglobin. The promoter of NADH-GOGAT was evaluated in transgenic alfalfa plants carrying chimeric -glucuronidase promoter 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.

Plant Physiol. (1999) 119: 817-828
Copyright Clearance Center:   0032-0889/99/119//12
© 1999 American Society of Plant Physiologists

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