Submitted to: North American Conference on Symbiotic Nitrogen Fixation
Publication Type: Abstract Only
Publication Acceptance Date: June 27, 2004
Publication Date: June 27, 2004
Citation: Vance, C.P. 2004. Root nodule N and C metabolism: nutrient cycling required for development and function [abstract]. 19th North American Conference on Symbiotic Nitrogen Fixation, June 27-July 1, 2004, Bozeman, MT. p. 39. Technical Abstract: Nitrogen (N) and carbon (C) metabolism is highly integrated in N2-fixing root nodules to provide N and C sources for both partners. Nodule N assimilation requires some 6 grams of C for each 1 gram of N fixed. Although sucrose is the primary C compound translocated to nodules, it is not used by N2-fixing bacteroids to support the energy requirements of nitrogenase. Instead, sucrose is metabolized to malate and succinate via sucrose synthase and glycolysis in coordination with the TCA cycle, phosphoenolpyruvate carboxylase, and malate dehydrogenase (MDH). Nodules have at least five forms of MDH found in a variety of subcellular locations. A nodule-enhanced NAD-dependent MDH may be the critical step regulating plant malate accumulation in root nodules. Transgenic modification of nodule-enhanced MDH may lead to improved N2 fixation. Because N metabolism is uniquely distributed between the plant and microbe, with the bacteria releasing fixed N2 in the form of NH+4 to the plant and the plant providing a source of N to the bacteria, it is imperative to identify the plant N product supplied to the bacteria. Current hypotheses suggest that the plant provides the bacteria glutamate in exchange for aspartate. While glutamate is highly abundant in root nodules, asparagine, alanine, gamma-aminobutyric acid (GABA) and glutamine are also found in high concentrations. GABA may comprise up to 10% of the nodule N. Glutamate can be easily converted to gamma-amino butyric acid and further to succinate via the GABA shunt. Moreover, GABA is important in plant defense, O2 protection, and signaling. Metabolic studies are urgently needed to define the pathways involved in root nodule N and C exchange between symbiotic partners.