MICROSCOPIC CHARACTERIZATION OF EARLY NODULATION EVENTS IN A NONNODULATING SOYBEAN MUTANT (NN5) AND LACK OF RESPONSE TO HIGH INOCULUM DOSE

The microscopic events leading to nodulation in normally nodulating so ybean [Glycine max (L.) Merr.] genotypes, and the effects of Bradyrhiz obium strain and inoculum dose on nodulation, were examined in the NN5 non-nodulating mutant derived from cv. Williams. The NN5 mutant posse sses the recessive genes rj(5) and rj(6). Bradyrhizobium japonicum str ain USDA 110 cells attached normally to the root surface of NN5, many in a polar manner as in its wild-type parent, but failed to induce roo t hair curling and sub-epidermal cell division in the root. Co-culturi ng NN5 and Williams did not modify nodulation of either genotype. Hydr oponically-grown NN5 seedlings did not nodulate at a high inoculum dos e (1 x 10(10) cells seedling(-1)) of any B. japonicum strain tested (U SDA 110, USDA 26, USDA 136, and the tryptophan metabolic variants B-14 075 and ta 11 Nod(+)). A higher inoculum dose of 3 x 10(11) USDA 136 c ells seedling(-1) also failed to induce nodulation in NN5 and nod139 ( a nonnodulating mutant of cv. Bragg). The lack of nodulation of NN5 at any inoculum dose is contrary to previous observations of sparse nodu lation of other non-nodulating mutants at high inoculum dose. Genetic control of non-nodulation in NN5 is probably similar to nod139.