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The genetic diversity of ten Bradyrhizobium strains was evaluated for tolerance to high temperatures, to different salinity levels and for the efficiency of symbiosis with cowpea plants (Vigna unguiculata (L.) Walp.). Eight of these strains were isolated from nodules that appeared on cowpea after inoculation with suspensions of soil sampled from around the root system of Sesbania virgata (wand riverhemp) in ecosystems of South Minas Gerais. The other two strains used in our analyses as references, were from the Amazon and are currently recommended as cowpea inoculants. Genetic diversity was analyzed by amplifying repetitive DNA elements with the BOX primer, revealing high genetic diversity with each strain presenting a unique band profile. Leonard jar assays showed that the strains UFLA 03-30 and UFLA 03-38 had the highest N2-fixing potentials in symbiosis with cowpea. These strains had more shoot and nodule dry matter, more shoot N accumulation, and a higher relative efficiency than the strains recommended as inoculants. All strains grew in media of pH levels ranging from 4.0 to 9.0. The strains with the highest N2-fixing efficiencies in symbiosis with cowpea were also tolerant to the greatest number of antibiotics. However, these strains also had the lowest tolerance to high salt concentrations. All strains, with the exceptions of UFLA 03-84 and UFLA 03-37, tolerated temperatures of up to 40 ºC. The genetic and phenotypic characteristics of the eight strains isolated from soils of the same region were highly variable, as well as their symbiotic efficiencies, despite their common origin. This variability highlights the importance of including these tests in the selection of cowpea inoculant strains. Dez estirpes de rizóbios, sendo oito isoladas de amostras de solos coletadas próximo ao sistema radicular de Sesbania virgata, no Sul de Minas Gerais, e duas recomendadas como inoculante para o caupi (Vigna unguiculata (L.) Walp.) usadas como referências, foram avaliadas quanto a: diversidade genética, eficiência simbiótica em caupi, tolerância a altas temperaturas e concentrações salinas, valores extremos de pH e 15 tipos de antibióticos. A análise de diversidade genética utilizando a técnica do Rep-PCR, com o primer BOX, revelou alta diversidade, pois cada estirpe apresentou perfil único de DNA. O teste de eficiência simbiótica conduzido em vasos de Leonard indicou que as estirpes UFLA 03-30 e UFLA 03-38 demonstraram alto potencial em fixar N2 em simbiose com o caupi, proporcionando resultados de matéria seca da parte aérea e dos nódulos, eficiência relativa e acúmulo de N na parte aérea superiores aos das estirpes recomendadas como inoculantes. Todas as estirpes cresceram em meios com valores de pH variando de 4,0 a 9,0. Em relação à tolerância aos antibióticos, foi observado que as estirpes eficientes em fixar N2 em simbiose com o caupi foram as que apresentaram tolerância a um maior número desses compostos. No entanto, estas estirpes mostraram comportamentos semelhantes em relação a tolerância a salinidade, constituindo o grupo de maior sensibilidade. Com exceção das estirpes UFLA 03-84 e UFLA 03-37, as demais toleraram até 40 ºC. Embora as estirpes estudadas tenham sido isoladas de solos da mesma região, com exceção das recomendadas para inoculante oriundas da Amazônia, foi observado que elas apresentaram comportamentos distintos quando submetidas aos diferentes testes de diversidade genética, fenotípica e simbiótica, justificando a importância de incluir estes testes no processo de seleção de estirpes simbióticas em caupi

revista brasileira de ciência do solo

Diversity and efficiency of bradyrhizobium strains isolated from soil samples collected from around sesbania virgata roots using cowpea as trap species