Correspondence

Poly-3-hydroxybutyrate (PHB) and glycogen are major carbon storage compounds in Sinorhizobium meliloti. The roles of PHB and glycogen in rhizobia–legume symbiosis are not fully understood. Glycogen synthase mutations were constructed by in-frame deletion (glgA1) or insertion (glgA2). These mutations were combined with a phbC mutation to make all combinations of double and triple mutants. PHB was not detectable in any of the mutants containing the phbC mutation; glycogen was not detectable in any of the mutants containing the glgA1 mutation. PHB levels were significantly lower in the glgA1 mutant, while glycogen levels were increased in the phbC mutant. Exopolysaccharide (EPS) was not detected in any of the phbC mutants, while the glgA1 and glgA2 mutants produced levels of EPS similar to the wild-type. Symbiotic properties of these strains were investigated on Medicago truncatula and Medicago sativa. The results indicated that the strains unable to synthesize PHB, or glycogen, were still able to form nodules and fix nitrogen. However, phbCmutations caused greater nodule formation delay onM. truncatula than on M. sativa. Time-course studies showed that (1) the ability to synthesize PHB is important for N2 fixation in M. truncatula nodules and younger M. sativa nodules, and (2) the blocking of glycogen synthesis resulted in lower levels of N2 fixation on M. truncatula and older nodules on M. sativa. These data have important implications for understanding how PHB and glycogen function in the interactions of S. meliloti with Medicago spp