Characterization of rhizobia isolates obtained from nodules of wild genotypes of common bean

Abstract This study aimed to evaluate the tolerance to salinity and temperature, the genetic diversity and the symbiotic efficiency of rhizobia isolates obtained from wild genotypes of common bean cultivated in soil samples from the States of Goiás, Minas Gerais and Paraná. The isolates were subjected to different NaCl concentrations (0%, 1%, 2%, 4% and 6%) at different temperatures (28 °C, 33 °C, 38 °C, 43 °C and 48 °C). Genotypic characterization was performed based on BOX-PCR, REP-PCR markers and 16S rRNA sequencing. An evaluation of symbiotic efficiency was carried out under greenhouse conditions in autoclaved Leonard jars. Among 98 isolates about 45% of them and Rhizobium freirei PRF81 showed a high tolerance to temperature, while 24 isolates and Rhizobium tropici CIAT899 were able to use all of the carbon sources studied. Clustering analysis based on the ability to use carbon sources and on the tolerance to salinity and temperature grouped 49 isolates, R. tropici CIAT899 and R. tropici H12 with a similarity level of 76%. Based on genotypic characterization, 65% of the isolates showed an approximately 66% similarity with R. tropici CIAT899 and R. tropici H12. About 20% of the isolates showed symbiotic efficiency similar to or better than the best Rhizobium reference strain (R. tropici CIAT899). Phylogenetic analysis of the 16S rRNA revealed that two efficient isolates (ALSG5A1 and JPrG6A8) belong to the group of strains used as commercial inoculant for common bean in Brazil and must be assayed in field experiments.

Phenotypic and molecular characterization of indigenous rhizobia nodulating chickpea in India.

In a combined approach of phenotypic and genotypic characterization, 28 indigenous rhizobial isolates obtained from different chickpea growing regions in peninsular and northern India were analyzed for diversity. The field isolates were compared to two reference strains TAL620 and UPM-Ca142 representing M. ciceri and M. mediterraneum respectively. Phenotypic markers such as resistance to antibiotics, tolerance to salinity, temperature, pH, phosphate solubilization ability, growth rate and also symbiotic efficiency showed considerable diversity among rhizobial isolates. Their phenotypic patterns showed adaptations of rhizobial isolates to abiotic stresses such as heat and salinity. Two salt tolerant strains (1.5% NaCl by T1 and T4) with relatively high symbiotic efficiency and two P-solubilising strains (66.7 and 71 mg/ml by T2 and T5) were identified as potential bioinoculants. Molecular profiling by 16S ribosomal DNA Restriction Fragment Length Polymorphism (RFLP) revealed three clusters at 67% similarity level. Further, the isolates were differentiated at intraspecific level by 16S rRNA gene phylogeny. Results assigned all the chickpea rhizobial field isolates to belong to three different species of Mesorhizobium genus. 46% of the isolates grouped with Mesorhizobium loti and the rest were identified as M. ciceri and M. mediterraneum, the two species which have been formerly described as specific chickpea symbionts. This is the first report on characterization of chickpea nodulating rhizobia covering soils of both northern and peninsular India. The collection of isolates, diverse in terms of species and symbiotic effectiveness holds a vast pool of genetic material which can be effectively used to yield superior inoculant strains.