HOST-CONTROLLED RESTRICTION OF NODULATION BY BRADYRHIZOBIUM-JAPONICUMSTRAINS IN SEROGROUP-110

We previously reported the identification of a soybean plant introduct ion (PI) genotype, PI 417566, which restricts nodulation by Bradyrhizo bium japonicum MN1-1c (USDA 430), strains in serogroup 129, and USDA 1 10 (P. B. Cregan, H. H. Keyser, and M. J. Sadowsky, Appl. Environ. Mic robiol. 55:2532-2536, 1989, and Crop Sci. 29:307-312, 1989). In this s tudy, we further characterized nodulation restriction by PI 417566. Tw enty-four serogroup 110 isolates were tested for restricted nodulation on PI 417566. Of the 24 strains examined, 62.5% were restricted in no dulation by the PI genotype. The remainder of the serogroup 110 strain s tested (37.5%), however, formed significant numbers of nodules on PI 417566, suggesting that host-controlled restriction of nodulation by members of serogroup 110 is strain dependent, Analysis of allelic vari ation at seven enzyme encoding loci by multilocus enzyme electrophores is indicated that the serogroup 110 isolates can be divided into two m ajor groups. The majority of serogroup 110 isolates which nodulated PI 417566 belonged to the same multilocus enzyme electrophoresis group. B. japonicum USDA 110 and USDA 123 were used as coinoculants in compet ition-for-nodulation studies using PI 417566. Over 98% of the nodules formed on PI 417566 contained USDA 123, whereas less than 2% contained USDA 110. We also report the isolation of a Tn5 mutant of USDA 110 wh ich has overcome nodulation restriction conditioned by Pr 417566. This mutant, D4.2-5, contained a single Tn5 insertion acid nodulated PI 41 7566 to an extent equal to that seen with the unrestricted strain USDA 123, The host range of D4.2-5 on soybean plants and other legumes was unchanged relative to that of USDA 110, except that the mutant nodula ted Glycine max cv. Hill more efficiently. While strain USDA 110 has t he ability to block nodulation by D4.2-5 on PI 417566, the nodulation- blocking phenomenon was not seen unless strain USDA 110 was inoculated at a 100-fold greater concentration than the mutant strain.