Characterization of soybean Bradyrhizobium strains adapted to the Brazilian savannas

Abstract

Brazilian soils are originally free from soybean bradyrhizobia and the first inoculants were brought to the country in this century, but a search for adapted strains started immediately and still continues. A strain selection program was established at Embrapa based on the reisolation of strains after a long period of adaptation to the soils followed by a search for variant genotypes with higher N 2 fixation capacity and competitiveness. A second approach of this program consists of searching for variant colonies of a single strain with higher N 2 fixation rates and competitiveness, following a short period of adaptation to the soil. In this study, using both approaches, strains belonging to three serogroups, CB 1809, 532C and SEMIA 5020, were obtained. In general, the variant strains showed differences in colony morphology (mucoidy) but produced similar protein and lipopolysaccharide profiles. Within serogroup CB 1809, containing variants obtained via the second approach, a low level of DNA polymorphism was detected relative to the parental genotype by ERIC and REP-PCR. However, within the two other serogroups, containing variant strains obtained via the first approach, a high level of polymorphism in ERIC and REP-PCR fingerprints was observed relative to the putative serologically related parental genotypes. These results show that a great variability can be detected following adaptation of Bradyrhizobium strains to the soil, although other potential explanations for the DNA polymorphisms observed are discussed. Some of the variant strains obtained by both methodologies were found to have higher rates of N 2 fixation and almost all were more competitive than the parental genotypes, suggesting that it is possible to select variant strains which can contribute to an improved plant N nutrition status. ß

Differentiating Indigenous Soybean Bradyrhizobium and Rhizobium spp. of Indian Soils

“…The effects of inoculation during early years of soybean introduction in India, with the imported bradyrhizobia were very convincing. But the responses declined gradually with time [4] due to the naturalization of the introduced slow growing strains (evolution of native or indigenous strains) similar to that observed in other soybean growing countries like Brazil [2].…”

“…To understand loss of inoculant effectiveness, it is necessary to distinguish how introduced bradyrhizobia (slow growing) and indigenous adapted rhizobia (slow as well as fast growing) differ and whether simple biochemical methods could be used to distinguish them. Santos et al [2] reported how soybean rhizobia that were native, genetically very distant from parental strains, became good nodulators and N 2 fixers, possibly via transfer of nodulation, nitrogen fixation and other competition-related genes from the inoculum strain. In our study there was close similarity of some of the slow and all fast growing rhizobia to R. radiobacter (Agrobacterium radiobacter type strain ATCC 19358).…”

“…The fast growers belong to Sinorhizobium fredii as well as saprophytic bacteria resembling Agrobacterium spp. reported in effectively nodulated soybean fields in South America [2,3]. In India, large scale cultivation of soybean began in late 1960s with the import of yellow seeded modern soybean varieties from USA along with their slow growing symbiont-B.…”

“…The rhizobial population structure in soils is a resultant of the interactions among the bacteria, host plant and environmental factors [5]. Large diversities of soybean rhizobia (in countries for which soybean was an exotic crop) could have arisen due to transfer of symbiotic genes from the inoculant strains to indigenous non-symbiotic rhizobia [2]. In earlier studies in India, effective nodulation of soybean was reported to be mostly by slow growing bradyrhizobia; nodulation by fast growing rhizobia was to a much lesser extent but ineffective [6].…”

Microevolution of symbiotic Bradyrhizobium populations associated with soybeans in east North America

“…Interestingly, two isolates from D. canadense and 10 isolates from A. bracteata had the same core and symbiotic genotypes as, respectively, inoculant strains 532C (B. japonicum 1; lineage V) and 61A101 (lineage I; in a clade with B. elkani). Strain 61A101 was originally isolated from soybeans grown in the United States (Table 1), whereas 532C was isolated from soybeans grown in Brazil that were inoculated with strains from the United States (Santos et al 1999), suggesting that both of these strains might have originated from legumes native to North America. Further support for this proposition was provided by our finding that a minority of isolates from field site B had the same ST and symbiotic genotype as, respectively, 532C and 61A101, but exhibited poorly effective nitrogen-fixing phenotypes atypical of inoculant strains that are invariably selected for high symbiotic effectiveness with soybeans.…”