Plant Genetic Study of Restricted Nodulation in Soybean

Abstract

The displacement of indigenous Bradyrhizobium japonicum in soybean [Glycine max (L.) Merr.] nodules in favor of a more effective inoculant strain has been suggested as an approach to enhance soybean productivity. The objectives of this study were to (i) screen additional soybean genotypes for ones which restrict nodulation by certain B. japonicum strains, and (ii) investigate at which stage nodule development is blocked. Preliminary screenings of 23 soybean accessions, including Glycine soja Sieb. & Zucc., indicated that ‘TN 4—86‘, BARC‐2(Rj4), and PI 468.397 showed restricted nodulation with B. japonicum strain 61A101C. Cultivar Essex gave effective nodulation. Segregation in the F2 of Essex × PI 468.397 showed no significant difference (P ≤ 0.05) from a 3 nodulation :1 restricted nodulation ratio. The results indicate that the nodulation response of PI 468.397 with strain 61A101C is controlled by a single recessive gene. In contrast, the restricted nodulation response in TN 4–86 is controlled by a dominant gene which is allelic or closely linked to gene Rj4. The kinetic study of nodule development following inoculation with strain 61A101C indicated that the blockage of nodule development on BARC‐2(Rj4), TN 4–86, and PI 468.397 did not occur during root hair curling, but during the stage of cortical cell division. Information of this nature might be useful in distinguishing among different types of nodulation restriction genes based on the stage that blockage occurs. This study suggests that accessions of G. soja should be viewed as potential sources of additional genes that restrict nodulation of soybean with specific strains of Bradyrhizobium. Introgression of such genes could result in soybean cultivars that exclude some of the indigenous strains and become nodulated with commercial strains that are more efficient at fixing nitrogen.