Identification and molecular mapping of a gene restricting nodulation in soybean

Successful establishment in soybean (Glycine max L. Merr.) nodules of Bradyrhizobium japonicum strains with improved nitrogen-fixing ability over indigenous strains has been suggested as an approach to enhance soybean productivity. A major obstacle to this achievement has been the superior competitive ability of indigenous strains over improved strains for developing nodules. The objectives of this study were to; (1) search for genes which restrict nodulation with certain strains of B. japonicum, (2) study the genetic pattern of restricted nodulation, (3) investigate at which stage nodulation development is blocked, and (4) use RFLP technology to map the gene restricting nodulation in soybean. Preliminary screenings indicated that 'TN 4-86', BARC-2(Rj4), and PI 468.397 (G. soja) showed restricted nodulation with B. japonicum strain 61A101C. Study of F₁, F₂, and F₃ families of Essex X PI 468.397 indicated that restricted nodulation in PI 468.397 on strain 61A101C is controlled by a recessive gene. In contrast, the restricted nodulation in TN 4-86 is controlled by a dominant gene which is allelic to gene Rj4. The kinetic study of nodule development indicated that the blockage of nodule development in BARC-2(Rj4), TN 4-86, and PI 468.397 do not occur during nodule initiation, but during further development of initiated infection. Unlike non-nodulation, restriction of nodulation in TN 4-86 and PI 468.397 is not 100%. Molecular mapping of the restricted nodulation gene in PI 468.397 was attempted. Seventy enzyme/probe combinations were tested on Essex and PI 468.397. Eighteen polymorphisms were detected. Some probes were very useful and others were relatively uninformative. Enzymes HindIII and EcoRW were more effective than DraI in producing polymorphisms between Essex and PI 468.397. Sixteen enzyme/probe combinations were tested on F₂ plants that expressed the restricted nodulation phenotype. These RFLP loci covered linkage groups A to H. No statistically significant linkage was detected between RFLP loci and the gene controlling restricted nodulation based on the data obtained. Although the segregation ratios of marker patterns in some enzyme/probe combinations are significantly different from those of independent segregation, the number of heterozygote patterns is more than that expected. This could have been due to small sample size.