Submitted to: National Congress on Plant Biochemistry and Molecular Biology
Publication Type: Abstract Only
Publication Acceptance Date: October 14, 1995
Publication Date: N/A
Technical Abstract: The enzymes phosphoenolpyruvate carboxylase (PEPC, EC 18.104.22.168) and NADH- glutamate synthase (NADH-GOGAT, EC 22.214.171.124) are pivotal in N and C metabolism. We have isolated the alfalfa genes encoding the nodule enhanced PEPC and NADH-GOGAT. During effective nodule development PEPC and NADH-GOGAT transcripts increase some 10- to 20-fold. Increased transcript accumulation is associated with two events, the first is nodule organogenesis, while the second is induction of nitrogenase activity. Ineffective nodules express PEPC but transcript levels are reduced by 60%. By comparison, NADH-GOGAT transcripts are detectable in ineffective nodules at only root background levels. PEPC is localized in the cytosol of both infected and uninfected cells of the nodule interior. The density of labeling was approximately the same in each cell type. A 5' upstream sequence between -643 and -284 in the PEPC promoter appears to be critical in regulating nodule specific expression of PEPC. This region contains a purine-rich sequence that may form H-DNA. In addition to its role in nodule N and C metabolism, PEPC may also be involved in mechanisms controlling the nodule oxygen diffusion barrier. The NADH-GOGAT enzyme has a 101-amino acid presequence but it is unclear as to the subcellular localization of the enzyme. Promoter::GUS reporter gene constructs target B-glucuronidase enzyme activity to both infected and uninfected cells. However, activity appears to be higher in the infected cells. Of all the enzymes we have evaluated, to date, only NADH-GOGAT expression appears to have an absolute requirement for some factor(s) associated with effective nodules. We postulate that NADH-GOGAT is the limiting step in nodule N metabolism.