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.

Soybean [Glycine max (L.) Merrill] rhizobial diversity in Brazilian oxisols under various soil, cropping, and inoculation managements

“…After some years of soybean cropping, a high percentage of strains belonging to unknown serogroups or showing highly dissimilar DNA profiles have been detected in Brazil (Santos et al 1999;Ferreira et al 2000;GalliTerasawa et al 2003;Hungria et al 2006;Batista et al 2006). For example, in the South Region, Ferreira et al (2000) reported that 38% of the strains belonged to unknown serogroups after 17 years of soybean cropping, and in the Cerrados that percentage was about 50% after some years of cropping (Galli-Terasawa et al 2003;Mendes et al 2004).…”

“…However, dispersal of inoculant strains did occur (Ferreira and Hungria 2002;Mendes et al 2004) and Andrade and Hungria (2002) demonstrated that viable cells on the soybean seed coat can be transported to new cropping areas. After introduction through inoculation and some years of adaptation to the soil conditions, a high level of diversity in morphological, physiological, genetic, and symbiotic properties has been reported especially under the stressful conditions of the Cerrados (Hungria et al , 1998Nishi et al 1996;Boddey and Hungria 1997;Santos et al 1999;Hungria and Vargas 2000;Galli-Terasawa et al 2003;Batista et al 2006). In the USA, Streeter (1994) has also noted substantial "genetic adjustment" during the period of adaptation, leading to wide genetic diversity in field populations.…”

“…SEMIA 566 is the putative parental strain of SEMIA 5079 (Hungria and Vargas 2000;Hungria et al 2006), yet the strains show differences in several morphological, physiological, symbiotic and genetic properties, including DNA profiles with specific or random primers (Boddey and Hungria 1997;Hungria et al 1996Hungria et al , 1998Hungria et al , 2006Nishi et al 1996;Santos et al 1999;Ferreira et al 2000;Ferreira and Hungria 2002). Together, those two very competitive strains, which belong to the same serogroup as USDA 123 (Mendes et al 2004;Hungria et al 2006), represented 18% of all the nodules sampled (Table 3).…”

“…Two B. elkanii strains were included: SEMIA 587 (used in commercial inoculants from 1968-1975 and since 1979) and SEMIA 5019 (used since 1979). The strains have been described in detail elsewhere Santos et al 1999;Hungria and Vargas 2000;Hungria et al 2006), and they represent the majority of the strains applied to the soybean-cropped area of the Cerrados ).…”

Genetic diversity of rhizobia nodulating Trifolium ambiguum in North America

“…Identification and selection of highly efficient, locally adapted strains, which have established in soil after introduction of the inoculated host plant, is therefore possible. This approach has been used successfully with other species (e.g., Santos et al 1999), but the chances for success may be limited where the host is highly specific in its Rhizobium requirements.…”