| Bradyrhizobium strains are widely used as inoculants for soybeans around the world in acidic, alkaline or neutral soil. But in alkalin soil, Sinorhizobium strains are the dominant microsymbionts of soybeans and Sinorhizobium strains are characterized by their faster growth rate than Bradyrhizobium and thus cost less in the fermentation.Sinorhizobium inoculants have a broad prospect in alkaline soil, but the symbiosis specificity of Sinorhizobium limited its application. In order to investigate the genetic differentiation and microevolutionary mechanisms of60Sinorhizobium strains from Xinjiang and Huanghuaihai, we selected three housekeeping genes (SMc00019, truA, and thrA) and16symbiosis-related genes (chromosome:bacA, purL, nodM, gcvT, cysD, cobO, nodE; pSymB:exoA, exoY, phnC; pSymA: nodC, nodS, y4wE, nodDl, nodD2, rhcJ). Five distinct species were identified and different levels of genetic differentiation were observed among these species or different replicons:(1) S.fredii has higher intraspecific diversity and wider distribution, but S. sojae was the most divergent from the other test species and was characterized by its low intraspecies diversity and limited geographic distribution.(2) Intraspecies recombination happened frequently in housekeeping genes and symbiosis-related genes on the chromosome and pSymB, whereas pSymA genes showed a clear pattern of lateral-transfer events between different species. Moreover, pSymA genes were characterized by a lower level of polymorphism and recombination than those on the chromosome and pSymB. Taken together, genes from different replicons of rhizobia might be involved in the establishment of symbiosis with legumes, but these symbiosis-related genes might have evolved differently according to their corresponding replicons, this study laid a foundation to reveal the mechanism of symbiotic differences between Sinorhizobium strains and soybean cultivars.Can Sinorhizobium inoculants replace Bradyrhizobium inoculants in alkaline soil? The454pyrosequencing of rpoB amplicon was used to compare the microbial community structure and nodulation competition ability after inoculating S. fredii and B. liaoningense in alkaline soil. The results showed that:(1) a and β-diversity of microbial communities have changed in four treatments in flowering period (H1, no inoculation and no fertilizer control; H2, inoculating S.fredii; H3, inoculating B. liaoningense; H4, no inoculation and fertilizer control) as compared to those of corresponding soil samples collected before inoculation (Q1-Q4).(2) The abundance of OTUs has increased in genera of Thiorhodovibrio, Thioalkalivibrio, Thauera, Sphingomonas, Sphingobium, Bradyrhizobium and decreased in genera of Nitrobacter, Mesorhizobium and Agrobacterium.(3) As to the abundance of OTUs belonging to Sinorhizobium, H2did not cause significant difference compared to Q2whereas H3’s value is two times of Q3; H3aslo showed higher abundance of Bradyrhizobium than Q3(1.6times, p-value=0.075).(4) Although the abundance of Bradyrhizobium is higher than Sinorhizobium in all test soil samples, we did not find any defined Bradyrhizobium species which can nodulate with soybean. But the populations of S.fredii have increased by inoculating with B. liaoningense (5.6times, P=0.058). Nodule occupation ratio of S.fredii is94.1%in the treatment of inoculating B. liaoningense, and strains isolated from the other three treatments all belonged to S. fredii. Apparently, the nodulation competition ability of S. fredii with soybean is stronger than B. liaoningense in alkaline soil.In short, S. fredii is suggested to be used as the inoculants for soybeans in alkaline soils, given the compatibility of different S. fredii strains and soybean cultivars is well considered. |
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