Nodule Carbon Metabolism: Organic Acids for N2 Fixation

摘要: The energy burden (carbon (C) cost) imposed on plants for symbiotic nitrogen (N2) fixation is approximately 6 mg C · N reduced (Day, Copeland, 1991). Photosynthate, in the form of sucrose, ultimate source carbon required both N2 and assimilation. Labeling studies show that sucrose derived from shoot transported to nodule within 15 min, reaching steady state concentrations 3.6 gfw (Streeter, Although initial product with greatest amount label CO2 fixation, it rapidly metabolized organic acids malate succinate accompanied by subsequent 14CO2 evolution. Several lines evidence make apparent, however, C4-dicarboxylic rather than provide nitrogenase activity skeletons Rhlzobium/Bradyrhizobium mutants incapable utilizing glucose fructose continue effective nodules, while unable take up ineffective nodules (Ronson et al., 1981; Finan 1983; Vance, Heichel, Bacteroids have a high affinity uptake system but not sugars (Udvardi, Day, 1997). Bacterial lacking malic enzyme are Fix (Driscoll, Finan, 1993). Symbiosome membranes acid transporters capable mediating flux lack comparable systems amino Furthermore 14C-malate - synthesized through directly incorporated into bacteroids respired concomitant metabolism does occur (Rosendahl 1990). Lastly, strikingly or other (Anthon, Emerich 1990; Rosendahl Romanov 1995). enhanced coupled their use reflect exquisite adaptations production low O2 environment. These plant involve coordinated expression control three critical enzymes, synthase (SS; EC 2.4.1.13), phosphoenolpyruvate carboxylase (PEPC; 4.1.1.31), dehydrogenase (MDH; 1.1.1.82).