| This experiment was systematically performed by sampling and investigation, collecting data and organization to legume plant Sophora alopecuroides grown in part regions in different eco-regions (Gansu, Ningxia and Shaanxi) of northwestern China, which provided a base for further study of legume plant associated with rhizobia in northwestern regions. Result by comparison showed that shape of most rhizobia grown in field is claviform, spherical, minority coralliform, while most of rhizobia during nodule assay in laboratory are spherical, minority claviform and coralliform. The difference of shape characteristics may be related to environmental condition of plant growth.Based upon the combined PCR-RFLP patterns of 16S rRNA-recA-nodA-nifH, total 75 strains were defined among the rhizobia and they were classified into thirty-five genotypes, 10 rRNA types, 14 recA types, 13 nodAtypes, 12 nifH types, distributed in 5 genuses, including Mesorhizobium, Phyllobacterium, Sinorhizobium, Agrobacteium, Rhizobium, attach to 9 genomic species, including Mesorhizobium alhagi, M.gobiense, Agrobacterium tumefaciens, M. amorphae, Phyllobacterium trifolii, Rhizobium giardinii, R. indigoferae, Sinorhizobium fredii and S. meliloti. Among of them, Mesorhizobium alhagi and M. gobiense become the main groups, and the rest as the minor groups.Phyllobacteria strains ZY16-1 and ZW3-1 were first isolated from nodules of Sophora alopecuroides, and they had nodulation gene nodA and nitrogen fixation gene nifH, could form nodules with host plant during reinoculation test. Due to difference between 16S rRNA and recA phylogenetic trees, close homology with agrobacterium, it indicated that"Core"genes of Phyllobacterium and Agrobacterium can happen horizontal gene transfer (HGT). Symbiosis genes phylogeny of two Phyllobacterium strains was consistent or similar with that of M. gobiense, which mean its symbiosis genes gained by horizontal gene transfer (HGT). Agrobacterium train GZ12-1 was first isolated from nodules of Sophora alopecuroides, and they had nodulation gene nodA and nitrogen fixation gene nifH, could form nodules with host plant during reinoculation test. Compared with 16S rRNA and recA phylogenetic trees,Agrobacterium and Rhizobium exist more close relationship than Bradyrhizobium. Our study support this viewpoint. Symbiosis genes phylogeny of the strain was consistent or similar with that of M. alhagi, which mean its symbiosis genes gained by horizontal gene transfer (HGT). Another, difference between16S rRNA and recA phylogenetic trees of strain DB7 indicated they have not common origin, horizontal gene transfer (HGT) of nodA and nifH genes may separately happen.It was known by comparison nodA and16S rRNA phylogenetic trees that HGT happened between genus or species should enlarged scope of rhizobial nodule with host plant, which play a important role in ecology protect from arid, saline-alkaline, etc. evail environment, has significant value for improving survival rate of vegetation.Comparing with phylogeny of symbiosis genes nodA and nifH, We found that symbiosis genes of these species have consistent or similar with those of M. gobiense, which species including A. tumefaciens, M. amorphae, P. trifolii, R. indigoferae and R. giardinii, while they different to those of M. alhagi. The relationship suggested that HGT of symbiosis genes is considerable stable in M. alhagi, while it is very active in M. gobiense.HGT of symbiosis genes is important mechanism of enlarging diversity of related symbiotic bacteria associated with Sophora alopecuroides. In this study, four (A. tumefaciens, P. trifolii, R. indigoferae and R. giardinii) of nine species gained symbiotic genes by this way. Occurrence of HGT may be transfer result of symbiosis islands, which is an adaption of rhizobia-legumes symbiosis to environment condition, can benefit recipient rhizobia enhance adaptability, better suit nodule minienvironment.
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