Carbon Metabolism and the Exchange of Metabolites Between Symbionts in Legume Nodules

Abstract

Two examples of exchange of carbon between host and bacteroids are presented. In the first example, α,α-trehalose which accumulates in nodules is synthesized in bacteroids, but substantial quantities of trehalose are released to the host cytoplasm where it is “recycled” to glucose. In comparisons across Bradyrhizobium japonicum strains, trehalose concentrations in nodules are negatively correlated with acetylene reduction activity. We still have no explanations for these unusual relationships involving trehalose. In the second example we have preliminary evidence for the operation of a malate/aspartate shuttle in B. japonicum bacteroids. Malate is taken up and, after oxidation and transamination, is returned to medium as aspartate. Glutamate which may be taken up or synthesized in bacteroids, is transaminated to α-ketoglutarate, which is exported to the medium. Evidence from the literature and from our recent labeling experiments supports this model, but it is far from being established.

We suggest that a concept of metabolite exchange between symbionts is more appropriate than a unidirectional flow of reduced carbon into bacteroids and reduced nitrogen out of bacteroids.