NODULATION OF SOYBEAN BY A TRANSPOSON-MUTANT OF RHIZOBIUM-FREDII USDA257 IS SUBJECT TO COMPETITIVE NODULATION BLOCKING BY OTHER RHIZOBIA

Rhizobium fredii USDA257 fails to nodulate the improved soybean [Glycine max (L.)Merr.] cultivar McCall in plastic growth pouches. Mutant 257DH4, which was derived from USDA257 by transposon mutagenesis, forms nitrogen fixing nodules under these conditions. If USDA257 is present in inocula containing the mutant, most infections are arrested prior to organization of the nodule meristem, and nodule number is reduced by 95%. The improved cultivars Essex, Harosoy, Hodgson 78, and Viçoja, as well as a supernodulating mutant of Williams, respond like McCall to inoculation with such mixtures of bacteria. Modulation blocking on McCall can be elicited by rhizobia other than USDA257, provided that they meet two criteria: Blocking strains must themselves be able to induce cortical cells of McCall to divide, and such divisions must proceed to the stage of nodule meristem formation. Nodulation by the mutant remains sensitive to a challenge inoculation with USDA257 for only the first 6 to 12 hours after inoculation. Nodulation blocking involving mutant 257DH4 thus appears to be a rapid, generalized process.