Distribution And Symbiotic Specificity Of Astragalus Rhizobia Influenced By Soil Environment And Host

Astragalus mongholicus and A.membranaceus,two species of the important medicinal legumes,are collectively called Astragalus or "Huangqi" in Chinese.The officinal components of Astraglus are Astragalus polysaccharides,having the function of tonifying,anti-aging and immunity enhancement.Since the widely applied in medicine and the increase in demands,the wild resources of Astragalus becomes exhaustion and cultivation of medicinal legumes artificially developed.The Astragalus could establish symbiotic nitrogen fixation with rhizobia.There is no system research about the diversity of rhizobia that could form nodules with Astragalus.The current job focused firstly and systematically on the nodulation and nitrogen fixation of the astragalus cultivated in different ecological regions and symbiotic nitrogen fixation between them.Firstly,survey on the nodulation of these two plants was carried out and the root nodules were collected.Rhizobal strains were isolated,purified and were subjected to the analyses of genetic and symbiotic gene diversity.Then the distribution of rhizobia influenced by soil envirnments and the host plants was analyzed.Results showed that,root nodules of astragalus decreased with the increasing of application of fertilizers by farmers.The more the fertilizers used,the less the nodule numbers and the predominant bacteria in nodules were non-symbiotic endophytes in these conditions.While the less the fertilizers used,the more the nodule numbers and most of the bacteria belonged to symbiotic Mesorhizobium,with M.septentrionale,M.temperatum,M.tianshanense and M.muleiense as the predomiant microsymbionts in the nodules.The symbiotic genes(nodC and nifH)were possessed by the rhizobia associating with Subtribe of Astragalinae specificially.Some endophytes isolated from root nodules of Astragalus mongholicus in Bozhou city,Anhui Province,where the fertilizer was used heavily.These endophytes,together with some isolates from soybean in Hebei Province,were identified as a novel species in the genus of Ensifer(formally Sinorhizobium)by using polyphasic taxonomy and whole genomic comparison.The novel species could not nodulae its original host plant but could form nodules in wild soybean and cultivated soybean.It was named as Ensifer glycinis with the type strain of CCBAU 23380T.To identify the common genome features of rhizobia that selected Astragalus as host and the differences of other Mesorhizobium species that could not form nodules with Astragalus,whole genome sequences of were sequenced and compared.A largest gene cluster comprising 11 genes related to hydrogenase function and one nodO gene were identified in the esorhizobium species associting with Astragalus.nodO gene,encoding for a secretory protein,was only studied in Rhizobium leguminosarum bv.viciae and was identified as determing the host range.Rhizobium yanglingense CCBAU 01603,a microsymbionts with Astragalus had nodO gene highly homologous to these Asgragalus mesorhizobia.Mutant of the nodO deletion in strain CCBAU 01603 formed malformal nodules and nitrogen fixation was lower.However,the mutant could form morphologically normal nodules with Caragana and Sophora flavescens.Further studies showed that though belonging to just one species,Mesorhizobium silamurunense,the two strains,CCBAU 45272 and CCBAU 01550T had different symbiotic nitrogen fixation efficiency and the different development of the bacteroids in nodules greatly.They shared 87.1%homologous genes but they had 12.9%differential genes themselves.Bacteroids of CCBAU 01550 were small and the efficiency of nitrogen fixation was lower while strain CCBAU 45272 had swollen bacteroids and the efficiency of nitrogen fixation was higher.Transcriptome of these symbiotic bacteroids showed they had different genes expressing up-or down-regulated.Both the swollen and non-swollen bacteroids formed by different rhizobia on Astragalus were found firstly.One highly effective rhizobium was selected and it can promote the growth of astragalus and improve the concentration of astraglus polysaccharides.