Submitted to: Plant Cell and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/7/1999
Publication Date: N/A
Citation: Interpretive Summary: Nitrogen (N) is a major nutrient required by all plants. Legumes, like pea, obtain a substantial amount of N through a process known as symbiotic N2 fixation. In this process soil bacteria infect the roots and form small wart-like structures called nodules. Within the nodules the bacteria provide the plant with symbiotically fixed N fertilizer, while the plant gives the bacteria carbon in the form of sugars for the energy needed to fuel N2 fixation. Symbiotic N2 fixation requires large quantities of carbon from the plant to be transferred and then metabolized in nodules. Although plant physiologists know a great deal about carbon metabolism in leaves, much less is known about that process in root nodules. In this study the genes encoding five enzymes (protein catalysts) involved in pea root nodule carbon metabolism were identified and characterized. The expression and cellular location of genes encoding aldolase, phosphoenolpyruvate carboxylase, sucrose synthase, malic dehydrogenase, and alcohol dehydrogenase were evaluated. Each gene was shown to have a unique expression pattern and location within the nodule. Expression of all five genes occurred prior to the detection of active N2 fixation, yet the maximum expression of malic dehydrogenase, sucrose synthase, and phosphoenolpyruvate carboxylase required actively N2 fixing nodules. This research is the first to report the isolation of complete genes from pea encoding malic dehydrogenase, aldolase, and alcohol dehydrogenase. The findings are the first to show that the nodulation process in pea induces expression of genes involved in carbon metabolism. Isolation of the carbon assimilation genes from pea can be used by biotechnologists to modify either N2 fixation or carbon metabolism in pea nodules.
Technical Abstract: Full length cDNAs encoding alcohol dehydrogenase (ADH), fructose-1, 6- biphosphate aldolase (ALD), nodule-enhanced malate dehydrogenase (neMDH), phosphoenolpyruvate carboxylase (PEPC), and nodule-enhanced sucrose synthase (neSS) were isolated from a pea (Pisum sativum L.) root nodule cDNA library and characterized. Transcript abundance and cellular expression patterns for each gene were examined at different stages of nodule development. All the genes were expressed prior to the induction of nitrogenase suggesting a developmental signal as the initial trigger for expression. RNA tissue blots demonstrated that all the genes except ALD exhibit enhanced expression in effective nodules. In situ hybridization studies showed contrasting patterns of gene expression within various nodule zones. The highest expression of ADH was observed in interzone. ALD was expressed predominantly in nodule meristem, invasion zone, and interzone. neSS transcripts were found rather uniformly throughout the nodule. Expression of neMDH and PEPC was also detected throughout the nodule, but the highest levels were associated with interzone and N2-fixing zone.