|Samac, Deborah - Debby|
Submitted to: Nitrogen Fixation International Congress
Publication Type: Proceedings
Publication Acceptance Date: 8/25/1997
Publication Date: N/A
Technical Abstract: The energy burden carbon (C) cost imposed on plants for symbiotic nitrogen (N2) fixation is approximately 6 mg C per mg N reduced. Photosynthate, in the form of sucrose, is the ultimate source of carbon required for both N2 fixation and assimilation. Although sucrose is the initial nodule product with the greatest amount of label derived from shoot CO2 fixation, it is rapidly metabolized to the organic acids malate and succinate accompanied by subsequent CO2 evolution. The enhanced metabolism of sucrose to C4-dicarboxylic acids coupled to their uptake and use by bacteroids reflect exquisite symbiotic adaptations in carbon metabolism for energy production in the low O2 environment of root nodules. These plant adaptations involve coordinated expression and control of three critical enzymes, sucrose synthase (SS; EC 22.214.171.124), phosphoenolpyruvate carboxylase (PEPC; 126.96.36.199), and malate dehydrogenase (MDH; EC 188.8.131.52). In this report we document the isolation and characterization of cDNAs from alfalfa encoding the nodule enhanced forms of SS, PEPC, and MDH. Both SS and PEPC contain conserved serine residues that can be phosphorylated. Evidence suggests that phosphorylation is a major mechanism for posttranslational regulation of these enzymes. In situ hybridization was used to localize and map the expression of transcripts in alfalfa nodules.