Genetic diversity of indigenous tropical fast-growing rhizobia isolated from soybean nodules

Abstract

This study characterized genetically 30 fast-growing rhizobial strains isolated from nodules of Asian and modern soybean genotypes that had been inoculated with soils from disparate regions of Brazil. Analyses by rep-PCR (ERIC and REP) and RAPD indicated a high level of genetic diversity among the strains. The RFLP-PCR and sequencing analysis of the 16S rRNA genes indicated that none of the strains was related to Sinorhizobium (Ensifer) fredii, whereas most were related to Rhizobium tropici (although they were unable to nodulate Phaseolus vulgaris) and to Rhizobium genomic species Q. One strain was related to Rhizobium sp. OR 191, while two others were closely related to Agrobacterium (Rhizobium) spp.; furthermore, symbiotic effectiveness with soybean was maintained in those strains. Five strains were related to Bradyrhizobium japonicum and B. elkanii, with four of them being similar to strains carried in Brazilian inoculants, therefore modifications in physiological properties, as a shorter doubling time might have resulted from adaptation to local conditions. Phospholipid fatty acid analysis (PFLA) was less precise in delineating taxonomic relationships. The strains fit into eight Nod-factor profiles that were related to rhizobial species, but not to N 2 -fixation capacity or competitiveness. The data obtained highlight the diversity and promiscuity of rhizobia in the tropics, being capable of nodulating exotic legumes and might reflect ecological strategies to survive in N-poor soils; in addition, the diversity could also represent an important source of efficient and competitive rhizobial strains for the tropics. Putative new rhizobial species were detected only in undisturbed soils. Three species (R. tropici, B. japonicum and B. elkanii) were found under the more sustainable management system known as no-till, while the only species isolated from soils under conventional till was R. tropici. Those results emphasize that from the moment that agriculture was introduced into undisturbed soils rhizobial diversity has changed, being drastically reduced when a less sustainable soil management system was adopted.

Variações qualitativas e quantitativas na microbiota do solo e na fixação biológica do nitrogênio sob diferentes manejos com soja

“…Em relação às alterações qualitativas, no caso de rizóbios microssimbiontes da soja (Glycine max) e do feijoeiro (Phaseolus vulgaris), foi relatada maior diversidade de estirpes em solos sob PD, em comparação com o PC Hungria et al, 2006a;Kaschuk et al, 2006a,b), havendo relação positiva, inclusive, com o rendimento de grãos (Kaschuk et al, 2006b). Contudo, nem sempre foi verificada maior diversidade de espécies de rizóbios (Kaschuk et al, 2006a), indicando que vários parâmetros precisam ser analisados em conjunto, para evitar conclusões precipitadas.…”

Genome Sequence of the Soybean Symbiont Sinorhizobium fredii HH103

“…oybean-nodulating bacteria are alphaproteobacteria belonging to the genera Bradyrhizobium, Mesorhizobium, Rhizobium, and Sinorhizobium (5,(9)(10)(11). Sinorhizobium fredii HH103 is a fast-growing rhizobial strain with a generation time of about 3 to 4 h, which was isolated from Chinese soil from Hubei Province (3).…”

Ensifer sojae sp. nov., isolated from root nodules of Glycine max grown in saline-alkaline soils

“…Soybean forms root nodules with various symbiotic bacteria (Chen et al, 2005) including the slow-growing species Bradyrhizobium japonicum, Bradyrhizobium elkanii, Bradyrhizobium liaoningense and Bradyrhizobium yuanmingense (Vinuesa et al, 2008) and the fast-growing species Ensifer (Sinorhizobium) fredii, Rhizobium tropici, Rhizobium sp. (Hungria et al, 2006), Rhizobium oryzae (Peng et al, 2008) and Mesorhizobium tianshanense (Chen et al, 1995). Most of these bacteria, except Rhizobium tropici and Rhizobium sp.…”