A succinate transport mutant of Bradyrhizobium japonicum forms ineffective nodules on soybeans

Abstract

Biochemical evidence has shown that dicarboxylic acids actively support symbiotic nitrogen fixation by both fast- and slow-growing Rhizobium. Mutants defective in the active uptake of succinate have been previously described only in species of the fast-growing rhizobium. This article is a report on the isolation of mutants defective in dicarboxylate transport in a slow-growing species of rhizobium, Bradyrhizobium japonicum. One of these presumptive dicarboxylate transport mutants, GTS, was characterized further. Cultured GTS was unable to accumulate [14C]succinate above background levels but possessed normal rates of malate dehydrogenase, fumarase, and hydroxybutyrate dehydrogenase activities. When inoculated onto soybeans, GTS produced a Nod+, Fix− phenotype. The bacteroids isolated from these nodules failed to accumulate labelled succinate. Electron micrographs of nodules formed by inoculation with GTS appeared normal with the exceptions of more prominent peribacteroid spaces in the infected cells and the appearance of starch granules in the noninfected cells. The phenotypical and morphological changes observed for B. japonicum are similar to those previously reported for the fast-growing species. Key words: Fix−, mutant, Rhizobium, succinate, transport.