HOST GENETIC-CONTROL OF SYMBIOSIS IN SOYBEAN (GLYCINE-MAX L)

Genes controlling nitrogen-fixing symbioses of legumes with specialize d bacteria known as rhizobia are presumably the products of many milli ons of years of evolution. Different adaptative solutions evolved in r esponse to the challenge of survival in highly divergent complexes of symbionts. Whereas efficiency of nitrogen fixation appears to be contr olled by quantitative inheritance, genes controlling nodulation are qu alitatively inherited. Genes controlling nodulation include those for non-nodulation, those that restrict certain microsymbionts, and those conditioning hypernodulation, or supernodulation. Some genes are natur ally occurring polymorphisms, while others were induced or were the re sult of spontaneous mutations. The geographic patterns of particular a lleles indicate the role of coevolution in determining symbiont specif icities and compatibilities. For example, the Rj4 allele occurs with h igher frequency (over 50%) among the soybean (G. max) from Southeast A sia. DNA homology studies of strains of Bradyrhizobium that nodulate s oybean indicated two groups so distinct as to warrant classification a s two species. Strains producing rhizobitoxine-induced chlorosis occur only in Group II, now classified as B. elkanii. Unlike B, japonicum, B, elkanii strains are characterized by (1) the ability to nodulate th e rjl genotype, (2) the formation of nodule-like structures on peanut, (3) a relatively high degree of ex planta nitrogenase activity, (4) d istinct extracellular polysaccharide composition, (5) distinct fatty a cid composition, (6) distinct antibiotic resistance profiles, and (7) low DNA homology with B. japonicum. Analysis with soybean lines near i sogenic for the Rj4 versus rj4 alleles indicated that the Rj4 allele e xcludes a high proportion of B. elkanii strains and certain strains of B. japonicum such as strain USDA62 and three serogroup 123 strains. T hese groups, relatively inefficient in nitrogen fixation with soybean, tend to predominate in soybean nodules from many US soils. The Rj4 al lele, the most common allelic form in the wild species, has a positive value for the host plants in protecting them from nodulation by rhizo bia poorly adapted for symbiosis.