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.)

摘要

Inositol derivative compounds provide a nutrient source for soil bacteria that possess the ability to degrade such compounds.Rhizobium strains that are capable of utilizing certain inositol derivatives are better colonizers of their host plants. We have cloned and determined the nucleotide sequence of themyo-inositol dehydrogenase gene (idhA) ofSinorhizobium fredii USDA191, the first enzyme responsible for inositol catabolism. The deduced IdhA protein has a molecular mass of 34,648 Da and shows significant sequence similarity with protein sequences of Sinorhizobium meliloti IdhA and MocA; Bacillus subtilis IolG, YrbE, and YucG; andStreptomyces griseus StrI. S. frediiUSDA191 idhA mutants revealed no detectablemyo-inositol dehydrogenase activity and failed to grow on myo-inositol as a sole carbon source. Northern blot analysis and idhA-lacZ fusion expression studies indicate that idhA is inducible bymyo-inositol. S. fredii USDA191idhA mutant was drastically affected in its ability to reduce nitrogen and revealed deteriorating bacteroids inside the nodules. The number of bacteria recovered from such nodules was about threefold lower than the number of bacteria isolated from nodules initiated by S. fredii USDA191. In addition, theidhA mutant was also severely affected in its ability to compete with the wild-type strain in nodulating soybean. Under competitive conditions, nodules induced on soybean roots were predominantly occupied by the parent strain, even when theidhA mutant was applied at a 10-fold numerical advantage. Thus, we conclude that a functional idhA gene is required for efficient nitrogen fixation and for competitive nodulation of soybeans by S. fredii USDA191.