Horizontal gene transfer and recombination shape mesorhizobial populations in the gene center of  the host plantsAstragalus luteolusandAstragalus ernestiiin Sichuan, China

Abstract

Thirty-three rhizobial strains isolated from the root nodules of Astragalus luteolus and Astragalus ernestii growing on the west plateau at two different altitudes in Sichuan province, China, were characterized by amplified rDNA restriction analysis (ARDRA), amplified fragment length polymorphism (AFLP), and by sequencing of rrs, glnA, glnII and nifH. The ARDRA analysis revealed considerable genomic diversity. In AFLP analysis, 20 of 33 Astragalus rhizobia formed three distinct clades, with others dispersed into different groups with the reference strains. Phylogenetic analysis of the rrs gene of six representative strains showed that the isolates were members of the genus Mesorhizobium. Three of the isolates formed a sister clade to Mesorhizobium loti and Mesorhizobium ciceri, whereas the other three formed a sister clade to a clade harboring the species Mesorhizobium huakuii, Mesorhizobium plurifarum, Mesorhizobium septentrionale and Mesorhizobium amorphae, indicating the existence of two new species. Phylogenetic analysis of glnA and glnII confirmed the rrs phylogenies for four strains, but the trees were incongruent. The nifH sequences of the strains formed a monophyletic clade and were typical of those of mesorhizobia forming symbioses with inverted repeat lacking clade legume species. The incongruent phylogenies of the genes studied suggest that horizontal gene transfer and recombination shape mesorhizobial populations in the gene center of the host plants.

Mesorhizobium sangaii sp. nov., isolated from the root nodules of Astragalus luteolus and Astragalus ernestii

“…A total of 28 strains were classified as members of the genus Mesorhizobium by using amplified 16S rRNA gene restriction analysis, amplified fragment length polymorphism, BOX element-PCR fingerprinting, 16S rRNA gene sequence analysis and other molecular characterization methods (Li et al, 2009b). The results revealed that the two strains SCAU7 T and SCAU27 exhibited distinct characteristics and differed from the known species of the genus Mesorhizobium.…”

“…Astragalus luteolus and Astragalus ernestii are wild plants that grow at altitudes above 3000 m. Astragalus ernestii grows only in western Sichuan, north-west of Yunnan and in eastern Tibet. Astragalus luteolus is an indigenous endemic plant species grown in western Sichuan province (Li et al, 2009b).…”

“…Seeds were incubated to allow germination in sterile Petri dishes at 25 u C (Vidal et al, 2009). Plants were transferred into pots filled with vermiculite moistened with an N-free plant nutrient solution (Vincent, 1970).The pots were maintained in a growth chamber at 28 u C with a photoperiod of 15 hours light: 9 hours dark (Li et al, 2009b). After 7 weeks, plants were harvested and analysed for nodule occurrence.…”

Trade-Offs, Spatial Heterogeneity, and the Maintenance of Microbial Diversity

“…To understand the maintenance of heritable phenotypic diversity within host‐associated assemblages, this study considers rhizobia symbiotic with either a native or an invasive legume species across serpentine soil boundaries. Although the rhizobia associated with a given host species may be composed of multiple species, such assemblages are an evolutionarily cohesive unit of microbial biodiversity because they often exhibit high frequencies of horizontal gene transfer, particularly for loci conferring physiological tolerance (Sullivan et al 1995; van Berkum et al 2003; Bailly et al 2007; Lakzian et al 2007; Prosser et al 2007; Li et al 2009; Philippot et al 2010; Sachs et al 2010; Tian et al 2010). The legume Acmispon wrangelianus is native to California and associates with rhizobia in the genus Mesorhizobium (Porter, unpubl.…”

Diverse nodule bacteria were associated with Astragalus species in arid region of northwestern China

“…Currently, diverse rhizobia belonging to the genera Rhizobium, Mesorhizobium, Ensifer, and Bradyrhizobium had been isolated from Astragalus spp. grown in different geographic regions in North America, Europe, or Asia [3][4][5][6][7].…”