Transport of C(4)-Dicarboxylates and Amino Acids by Rhizobium meliloti Bacteroids. D. G. McRae. Department of Biology and Institute of Biochemistry, Carleton University, Ottawa, Ontario, Canada, K1S 5B6.. R. W. Miller(2), W. B. Berndt(2), and K. Joy(1). (1)Department of Biology and Institute of Biochemistry, Carleton University, Ottawa, Ontario, Canada, K1S 5B6; and (2)Plant Research Centre, Agriculture Canada, Ottawa, Canada, K1A OC6.. MPMI 2:273-278. Accepted 10 May 1989. Copyright 1989 The American Phytopathological Society.

The rates of transport of C₄-dicarboxylic acids, aspartate, glutamate, alanine, and other compounds into mature bacteroids isolated from alfalfa nodules were determined. Of the compounds tested, only malate, succinate, pyruvate, and aspartate were accumulated at significant rates. Apparent K_m values for these compounds indicated that the concentrations of the dicarboxylates which half-saturated the transport carrier (60-70 μ/M) were at least 200-fold less than that required for aspartate. Malate, succinate, and aspartate were found to be present in the cytoplasm of nodule cells in the range of 150-810 nmol per gram fresh weight (0.1-0.6 mM). At these concentrations malate and succinate completely inhibited aspartate uptake by mature bacteroids. It is unlikely that any active uptake of aspartate occurs in vivo in view of the relative K_m values and physiologic concentrations of these metabolites. Major differences in the uptake of amino acids were noted in direct comparisons of bacteroids isolated from soybean and alfalfa nodules. Only dicarboxylates supported nitrogenase activity of the R. meliloti bacteroids, while succinate, aspartate, and glutamate all supported activity with B. japonicum bacteroids. The significance of these experimental findings is discussed in terms of energy supply, the fate of carbon sources, and nutrient exchange between host and microsymbiont.