Interaction of Rhizobium fredii USDA257 and nodulation mutants derived from it with the agronomically improved soybean cultivar McCall

Rhizobium fredii USDA257 does not nodulate McCall soybean (Glycine max (L.) Merr.), but two transposon-mutants derived from it, 257DH4 and 257DH5, do. All three organisms cause curling of McCall root hairs and induce the formation of underlying cortical cell divisions. The mutants produce infection threads, and many of the meristematic foci develop into nodules. In contrast, root hairs that deform in response to USDA257 lack infection threads, and meristematic activity ceases prior to the appearance of nodule meristems. Root systems nodulated by mutant 257DH4 reduce acetylene at rates similar to those of roots nodulated by reference R. fredii strain USDA191. The presence of living cells of USDA257 in inocula leads to strong inhibition of nodulation by 257DH4 but not by 257DH5. This blocking effect depends on the ratio of USDA257 cells to 257DH4 cells in the inoculum; nodules that form contain cells of 257DH4, but not those of parental strain USDA257.