A functional myo-inositol dehydrogenase gene is required for efficient nitrogen fixation and competitiveness of Sinorhizobium fredii USDA191 to nodulate soybean (Glycine max [L.] Merr.)

Inositol derivative compounds provide a nutrient source for soil bacteria t hat possess the ability to degrade such compounds. Rhizobium strains that a re capable of utilizing certain inositol derivatives are better colonizers of their host plants. We have cloned and determined the nucleotide sequence of the myo-inositol dehydrogenase gene (idhA) of Sinorhizobium fredii USDA 191, the first enzyme responsible for inositol catabolism. The deduced IdhA protein has a molecular mass of 34,648 Da and shows significant sequence s imilarity with protein sequences of Sinorhizobium meliloti IdhA and MocA; B acillus subtilis IolG, YrbE, and YucG; and Streptomyces griseus StrI. S. fr edii USDA191 idhA mutants revealed no detectable myo-inositol dehydrogenase activity and failed to grow on myo-inositol as a sole carbon source. North ern blot analysis and idhA-lacZ fusion expression studies indicate that idh A is inducible by myo-inositol. S, fredii USDA191 idhA mutant was drastical ly affected in its ability to reduce nitrogen and revealed deteriorating ba cteroids inside the nodules. The number of bacteria recovered from such nod ules was about threefold lower than the number of bacteria isolated from no dules initiated by S. fredii USDA191. In addition, the idhA mutant was also severely affected in its ability to compete with the wild-type strain in n odulating soybean. Under competitive conditions, nodules induced on soybean roots were predominantly occupied by the parent strain, even when the idhA mutant was applied at a 10-fold numerical advantage. Thus, we conclude tha t a functional idhA gene is required for efficient nitrogen fixation and fo r competitive nodulation of soybeans by S. fredii USDA191.