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