Diversity and structural differences of bacterial microbial communities in rhizocompartments of desert leguminous plants.

By assessing diversity variations of bacterial communities under different rhizocompartment types (i.e., roots, rhizosphere soil, root zone soil, and inter-shrub bulk soil), we explore the structural difference of bacterial communities in different root microenvironments under desert leguminous plant shrubs. Results will enable the influence of niche differentiation of plant roots and root soil on the structural stability of bacterial communities under three desert leguminous plant shrubs to be examined. High-throughput 16S rRNA genome sequencing was used to characterize diversity and structural differences of bacterial microbes in the rhizocompartments of three xeric leguminous plants. Results from this study confirm previous findings relating to niche differentiation in rhizocompartments under related shrubs, and they demonstrate that diversity and structural composition of bacterial communities have significant hierarchical differences across four rhizocompartment types under leguminous plant shrubs. Desert leguminous plants showed significant hierarchical filtration and enrichment of the specific bacterial microbiome across different rhizocompartments (P < 0.05). The dominant bacterial microbiome responsible for the differences in microbial community structure and composition across different niches of desert leguminous plants mainly consisted of Proteobacteria, Actinobacteria, and Bacteroidetes. All soil factors of rhizosphere and root zone soils, except for NO3-N and TP under C. microphylla and the two Hedysarum spp., recorded significant differences (P < 0.05). Moreover, soil physicochemical factors have a significant impact on driving the differentiation of bacterial communities under desert leguminous plant shrubs. By investigating the influence of niches on the structural difference of soil bacterial communities with the differentiation of rhizocompartments under desert leguminous plant shrubs, we provide data support for the identification of dominant bacteria and future preparation of inocula, and provide a foundation for further study of the host plants-microbial interactions.

Physiological and symbiotic variation of a long-term evolved Rhizobium strain under alkaline condition.

Physiological variation and adaptation of the long-term evolved rhizobia to alkaline environments where no host plant existence and the stability of their symbiotic properties when they are reinoculated to legume host remain unclear. A highly effective N2-fixing Rhizobium yanglingense strain CCBAU 01603 was used as the ancestral strain and was cultured continuously with/without addition of extra alkaline reagent (KOH) in laboratory conditions for approximately 500 generations. Total 60 evolved clones obtained were checked for their adaptation to higher alkaline pH level and inoculated to their host plant Caragana microphylla to evaluate their symbiotic efficiencies. Most of the evolved clones showed increased adaptation to higher alkaline pH but all of them decreased symbiotic efficiencies, resulting in the formation of irregular root nodules with lower nitrogenase activity, production of abnormal bacteroids, and accumulation more starch grains in uninfected nodule cells. Further demonstration of lower symbiotic efficiencies came from the down-regulated expression of genes related to nitrogen fixation in the bacteroids by transcriptome comparison. In addition, genes related to transporters and other diverse functions were up- or down-regulated in the evolved clones in free-living conditions (like yjiS gene) or in symbiotic situations, demonstrating the significant variations in cellular physiology and symbiosis. Our study revealed that the enhancement of alkaline adaptation but loss of symbiotic efficiencies of the evolved clones had happened during the long-term evolution in alkaline environments where no selective pressures from host plant, offering new insight into the molecular mechanism and direction of rhizobial evolution in nature.

Effect of aridity and dune type on rhizosphere soil bacterial communities of Caragana microphylla in desert regions of northern China.

Understanding the response of soil properties and bacterial communities in rhizosphere soil to aridity and dune types is fundamental to desertification control. This study investigated soil properties and bacterial communities of both rhizosphere and bulk soils of Caragana microphylla from four sites with different aridity indices, and one site with three different types of dunes. All sites were located in the desert regions of northern China. The results indicated that compared with the bulk soil, the soil nutrient content of rhizosphere, especially the content of total phosphorus, was generally significantly improved in different desertification environments. The bacterial richness and diversity were also higher than those of bulk soil, especially in arid regions and fixed dunes. Firmicutes, Actinobacteria, Proteobacteria, and Acidobacteria were the most dominant phyla in all samples. The regression analyses showed that at different sites, soil total organic C, total N, Na+, and total P played key roles in determining the bacterial community structure while total organic carbon, electronic conductivity, pH and total phosphorus were the dominant factors at the different dunes. The results further revealed that the dominant phyla strongly affected by environmental factors at different sites were Acidobacteria, Gemmatimonadetes, and Actinobacteria among which, Acidobacteria and Gemmatimonadetes were negatively correlated with Na+ content. At different types of dunes, Actinobacteria, Planctomycetes, and Gemmatimonadetes were particularly affected by environmental factors. The increased abundance of Actinobacteria in the rhizosphere soil was mainly caused by the decreased soil pH.

Bosea caraganae sp. nov. a new species of slow-growing bacteria isolated from root nodules of the relict species Caragana jubata (Pall.) Poir. originating from Mongolia.

Two Gram-stain-negative strains, RCAM04680T and RCAM04685, were isolated from root nodules of the relict legume Caragana jubata (Pall.) Poir. originating from the south-western shore of Lake Khuvsgul (Mongolia). The 16S rRNA gene (rrs) sequencing data showed that these novel isolates belong to the genus Bosea and are phylogenetically closest to the type strains Bosea lathyri LMG 26379T, Bosea vaviloviae LMG 28367T, Bosea massiliensis LMG 26221T and Bosea lupini LMG 26383T (the rrs-similarity levels were 98.7-98.8 %). The recA gene of strain RCAM04680T showed the highest sequence similarity to the type strain B. lupini LMG 26383T (95.4 %), while its atpD gene was closest to that of B. lathyri LMG 26379T (94.4 %). The ITS, dnaK and gyrB sequences of this isolate were most similar to the B. vaviloviae LMG 28367T (86.8 % for ITS, 90.4 % for the other genes). The most abundant fatty acid was C18 : 1ω7c (40.8 %). The whole genomes of strains RCAM04680T and RCAM04685 were identical (100 % average nucleotide identity). The highest average nucleotide identity value (82.8 %) was found between the genome of strain RCAM04680T and B. vaviloviae LMG 28367T. The common nodABC genes required for legume nodulation were absent in both strains; however, some other symbiotic nol, nod, nif and fix genes were detected. Based on the genetic study, as well as analyses of the whole-cell fatty acid compositions and phenotypic properties, a new species, Boseacaraganae sp. nov. (type strain RCAM04680T (=LMG 31125T), is proposed.

Genetic divergence and gene flow among Mesorhizobium strains nodulating the shrub legume Caragana.

Although the biogeography of rhizobia has been investigated extensively, little is known about the adaptive molecular evolution of rhizobia influenced by soil environments and selected by legumes. In this study, microevolution of Mesorhizobium strains nodulating Caragana in a semi-fixing desert belt in northern China was investigated. Five core genes-atpD, glnII, gyrB, recA, and rpoB, six heat-shock factor genes-clpA, clpB, dnaK, dnaJ, grpE, and hlsU, and five nodulation genes-nodA, nodC, nodD, nodG, and nodP, of 72 representative mesorhizobia were studied in order to determine their genetic variations. A total of 21 genospecies were defined based on the average nucleotide identity (ANI) of concatenated core genes using a threshold of 96% similarity, and by the phylogenetic analyses of the core/heat-shock factor genes. Significant genetic divergence was observed among the genospecies in the semi-fixing desert belt (areas A-E) and Yunnan province (area F), which was closely related to the environmental conditions and geographic distance. Gene flow occurred more frequently among the genospecies in areas A-E, and three sites in area B, than between area F and the other five areas. Recombination occurred among strains more frequently for heat-shock factor genes than the other genes. The results conclusively showed that the Caragana-associated mesorhizobia had divergently evolved according to their geographic distribution, and have been selected not only by the environmental conditions but also by the host plants.

Diversity of endophytic bacteria in Caragana microphylla grown in the desert grassland of the Ningxia Hui autonomous region of China.

The diversity of endophytic bacteria in the sand-fixation plant Caragana microphylla was investigated by amplified rDNA restriction analysis and by sequence and phylogenetic comparisons of the 16S rRNA genes. A total of 24, 19, and 17 operational taxonomic units were identified from 16S rDNA libraries of the plant roots, stems and leaves, respectively. Homology analysis revealed a 92-100% identity of bacterial 16S rDNA sequences compared with those in the GenBank database. The bacteria identified by sequence homology fell into the following groups: α-Proteobacteria, ß-Proteobacteria, γ-Proteobacteria, bacilli, and uncultured bacterium. Sequence analysis demonstrated that the roots were colonized predominantly by Bradyrhizobiaceae, while bacteria from Burkholderiaceae and Sphingomonadaceae were predominant in the stems and leaves, respectively. Additionally, the endophytic bacterial community in leaves was more diverse than those in the roots and stems. Overall, the most abundant bacteria in all three tissues analyzed were from the Sphingomonadaceae and Burkholderiaceae families, although many bacterial populations were found in only a single tissue. These results suggest that the bacterial population of C. microphylla is diverse.

Characterization and genus identification of rhizobial symbionts from Caragana arborescens in western Canada.

Naturally occurring nitrogen-fixing symbionts from root nodules of caragana (Caragana arborescens) growing in central Saskatchewan were isolated following surface sterilization of caragana root nodules and squashing and spreading of the contents on yeast extract - mannitol medium. The symbiotic nature of the strains was confirmed following inoculation onto surface-sterilized C. arborescens seed in a gnotobiotic Leonard jar system. The Rhizobium isolates from C. arborescens root nodules were intermediate in generation time (g) (mean g of 5 isolates was 6.41 h) compared with the fast growers, Rhizobium leguminosarum NRG457 (g: 4.44 h), Rhizobium tropici 899 (g: 3.19 h), and Sinorhizobium meliloti BALSAC (g: 3.45 h), but they were faster than the slow-growing Bradyrhizobium japonicum USDA 110 (g: 13.86 h) and similar to Mesorhizobium amorphae (g: 7.76 h). Nitrogen derived from fixation by measuring changes in δ(15)N natural abundance in plant tissue confirmed the effectiveness of the strains; approximately 80% N2 from fixation. Strain identification was carried out by determining the sequences of 3 genes: 16S rRNA-encoding genes, cpn60, and recA. This analysis determined that the symbiotic partner of Canadian C. arborescens belongs to the genus Mesorhizobium and seems more related to M. loti than to previously described caragana symbionts like M. caraganae. This is the first report of Mesorhizobium sp. nodulating C. arborescens in western Canada.

Genetic diversity and phylogeny of rhizobia isolated from Caragana microphylla growing in desert soil in Ningxia, China.

Rhizobia are soil bacteria with the capacity to induce nitrogen-fixing nodules on the roots or stems of legume plants. A total of 40 bacterial isolates from the root nodules of Caragana microphylla growing in desert soil in Ningxia, China, were analyzed for genetic diversity and phylogenetic position. These isolates were classified into 7 types of 16S ribosomal DNA (rDNA) using polymerase chain reaction-restriction fragment length polymorphism analysis. They were grouped into 4 clades, Rhizobium-Agrobacterium, Sinorhizobium, Phyllobacterium, and Bradyrhizobium, when the phylogenies of 16S rDNA, recA, and atpD genes were applied. Phylogenetic analysis showed that the tree generated from the 16S rDNA sequencing agreed with that produced from the recA and atpD genes. By analyzing phylogenetic relationship using the 3 loci, the isolates in the branches of Phyllobacterium and Sinorhizobium could be identified as P. brassicacearum and S. meliloti. The isolates in the branch of Rhizobium-Agrobacterium were the most abundant microsymbiont of C. microphylla and were designated R. leguminosarum, R. galegae, R. alamii, and A. tumefaciens. Two isolates with low sequence similarity to the known species of Bradyrhizobium might be novel species in this genus.

Paenibacillus taohuashanense sp. nov., a nitrogen-fixing species isolated from rhizosphere soil of the root of Caragana kansuensis Pojark.

A nitrogen-fixing bacterium, designated strain gs65(T), was isolated from a rhizosphere soil sample of Caragana kansuensis Pojark. Phylogenetic analysis based on a fragment of the nifH gene and the full-length 16S rRNA gene sequence revealed that strain gs65(T) is a member of the genus Paenibacillus. High levels of 16S rRNA gene similarity were found between strain gs65(T) and Paenibacillus borealis DSM 13188(T) (97.5 %), Paenibacillus odorifer ATCC BAA-93(T) (97.3 %), Paenibacillus durus DSM 1735(T) (97.0 %) and Paenibacillus sophorae DSM23020(T) (96.9 %). Levels of 16S rRNA gene sequence similarity between strain gs65(T) and the type strains of other recognized members of the genus Paenibacillus were below 97.0 %. Levels of DNA-DNA relatedness between strain gs65(T) and P. borealis DSM 13188(T), P. odorifer ATCC BAA-93(T) (97.3 %), P. durus DSM 1735(T) and P. sophorae DSM23020(T) were 35.9, 38.0, 34.2 and 35.5 % respectively. The DNA G+C content of strain gs65(T) was determined to be 51.6 mol%. The major fatty acids were found to be iso-C(14:0), anteiso-C(15:0) and iso-C(16:0). On the basis of its phenotypic characteristics and levels of DNA-DNA hybridization, strain gs65(T) is considered to represent a novel species of the genus Paenibacillus, for which the name Paenibacillus taohuashanense sp. nov. is proposed. The type strain is gs65(T) (=CGMCC 1.12175(T) = DSM 25809(T)).

[Stress resistance and genetic diversity of endophytic bacteria isolated from Caragana spp. root nodules].

By adopting PCR-RFLP and 16S rDNA sequencing, this paper analyzed the genetic diversity and phylogeny of 40 endophytic bacterial strains isolated from Caragana spp. root nodules, and determined the salt resistance, acid- and alkali resistance, and growth temperature range of the strains. A total of 9 genotypes were obtained from the 40 strains by RFLP. The 16S rDNA sequencing, morphological observation, and biochemical test of representative strains showed that the strains belonged to Bacillus, Inquilinus, Shinella and Acinetobacter, respectively, and had rich genetic diversity. 57.5% of the strains could tolerate 4% NaCl stress, 75% of the strains could grow in YMA medium with an initial pH 11.0, and 85% of the strains could survive after heat shock treatment at 60 degrees C, suggesting that the endophytic bacteria of Caragana spp. had strong resistance capacity. Among the strains, LWEN 07 and LWEN 15 were most resistant.

Genetic diversity, community structure and distribution of rhizobia in the root nodules of Caragana spp. from arid and semi-arid alkaline deserts, in the north of China.

The genetic diversity of 88 Caragana nodule rhizobial isolates, collected from arid and semi-arid alkaline sandy soils in the north of China, was assessed by PCR-RFLP of the 16S rRNA gene and the 16S-23S IGS, as well as the phylogenies of housekeeping genes (atpD, glnII and recA) and symbiotic genes (nodC and nifH). Of the 88 strains, 69 were placed in the genus Mesorhizobium, 16 in Rhizobium and 3 in Bradyrhizobium. Mesorhizobium amorphae, Mesorhizobium septentrionale, Mesorhizobium temperatum and Rhizobium yanglingense were the four predominant microsymbionts associated with Caragana spp. in the surveyed regions, and M. septentrionale was widely distributed among the sampling sites. Phylogenies of nodC and nifH genes showed that two kinds of symbiotic genes existed, corresponding to Mesorhizobium and Rhizobium, respectively. Available phosphorous (P) and potassium (K) contents were the main soil factors correlated with the distribution of these rhizobia in the sampling regions. Positive correlations between the available higher P content/lower K content and the dominance of Mesorhizobium species (M. temperatum, M. amorphae and M. septentrionale), and between the lower P content/higher K content and the dominance of R. yanglingense were found.

[Characterization of soil microbial community function and structure in rhizosphere of typical tree species and the meaning for environmental indication in the Loess Plateau].

Determination of the soil microbial community structure in rhizosphere of typical tree species in the Loess Plateau can be of great theoretical significance for correctly assessing the characteristics of soil ecological rehabilitation of the Loess Plateau. In this study,spore density analysis, microbial cultivation and BIOLOG were employed to evaluate the AMF spore density and soil microbial community diversity under four tree species with vesicular-arbuscular mycorrhizae in ecological rehabilitation area of the Loess Plateau, north Shaanxi Province. The results show that the different tree species differed significantly in both soil microbial number and microbial functional diversity, AMF spore density of Hippophae rhamnoides soil is 2.24 times than that of the Robinia pseudoacacia soil,and the rank as following order: Hippophae rhamnoides > Sophora viciifolia > Caragana microphylla > Robinia pseudoacacia . The statistical significant are detected in the bacteria and actinomyces numbers, however, there is no statistical significance in fungi number among the treatments. The principle component analyses indicates that scatter of Caragana microphylla and Hippophae rhamnoides are smaller than that of Sophora viciifolia and Robinia pseudoacacia, these results suggest that the soil community structure strongly varied among the different tree species. Numbers of carbon sources related to the first two components are 14 and 8. Correlation analysis shows that the AMF spore density appeared extremely significantly and positively correlated with the number of bacteria,and the metabolic of amino acids,amines and aromatic compounds, respectively. Moreover,AMF spore density positively correlated with the average well color development (AWCD), nevertheless, no correlations are found among AMF spore density, carboxylic acids,carbohydrates and polymers. These results suggest that AMF spore density is shown to be an important environmental biology parameter used in correctly assessing the soil bacteria metabolic community and diversity under the tree species in ecological rehabilitation of the Loess Plateau, north Shaanxi Province.

[Seasonal variation and related affecting factors of arbuscular mycorrhizal fungi in Caragana korshinskii roots].

With the combination of root staining and PCR-DGGE, the seasonal variation of arbuscular mycorrhizal fungi (AMF) in Caragana korshinskii roots was investigated; and by the methods of principal component analysis and canonical correspondence analysis, the relationships between this variation and soil factors were analyzed. It was found that the total infection rate of AMF and the infection rates of arbuscules and vesicles in C. korshinskii roots as well as the spore density of AMF in soil varied significantly among seasons. From spring to autumn, the total infection rate of AMF had a decreasing trend, whereas the other three indicators were in adverse. A total of nine AMF phylotypes were detected in spring, summer, and autumn, but the community composition and diversity indices of AMF had definite differences among the seasons. The infection rate of vesicles decreased with increasing soil total N, P, and organic C, while the spore density of AMF increased with decreasing soil moisture and available P. The seasonal variation of AMF community composition in C. korshinskii roots was mainly affected by soil total P, available P, and organic C.

Mesorhizobium shangrilense sp. nov., isolated from root nodules of Caragana species.

Five strains of bacteria isolated from nodules of Caragana bicolor and Caragana erinacea in Yunnan Province of China were classified within the genus Mesorhizobium in the class Alphaproteobacteria. The highest degree of 16S rRNA gene sequence similarity was determined to be to Mesorhizobium loti LMG 6125(T) (99.7 %) and Mesorhizobium ciceri UPM-Ca7(T) (99.7 %). Polyphasic taxonomic methods including SDS-PAGE of whole-cell soluble proteins, comparative housekeeping sequence analysis of atpD, glnII and recA, fatty acid profiles and a series of phenotypic and physiological tests allowed us to cluster the five strains into a coherent group while differentiating them from all previously established Mesorhizobium species. The DNA-DNA relatedness between the representative strain CCBAU 65327(T) and the type strains of M. loti and M. ciceri was 26.5 and 23.4 %, respectively, clearly indicating that strain CCBAU 65327(T) represents a novel species for which we propose the name Mesorhizobium shangrilense sp. nov. Strain CCBAU 65327(T) (=LMG 24762(T) =HAMBI 3050(T)) is designated as the type strain, and could nodulate Caragana microphylla, Caragana intermedia, Glycyrrhiza uralensis, Astragalus adsurgens, Vigna unguiculata, Vigna radiata and Phaseolus vulgaris in cross-nodulation tests.

Rhizobium alkalisoli sp. nov., isolated from Caragana intermedia growing in saline-alkaline soils in the north of China.

Three rhizobial strains (CCBAU 01393(T), CCBAU 01389 and CCBAU 03239) isolated from nodules of Caragana intermedia grown in saline-alkaline soils in the north of China had identical 16S rRNA genes that showed 99.7 and 99.5 % sequence similarities with those of Rhizobium huautlense SO2(T) and Rhizobium galegae USDA 4128(T), respectively. Phylogenies of the housekeeping genes atpD, recA and glnII confirmed their distinct position, differing from recognized Rhizobium species. SDS-PAGE of whole-cell soluble proteins and a series of phenotypic and physiological tests allowed us to differentiate the novel group from all closely related recognized Rhizobium species. The levels of DNA-DNA relatedness between strain CCBAU 01393(T) and R. huautlense SO2(T) and R. galegae USDA 4128(T) were 34.9 and 20.5 %, respectively. Therefore, we propose that strains CCBAU 01393(T), CCBAU 01389 and CCBAU 03239 represent a novel species, Rhizobium alkalisoli sp. nov., with strain CCBAU 01393(T) (=LMG 24763(T)=HAMBI 3051(T)) as the type strain. This strain could form effective nodules on Caragana microphylla, Phaseolus vulgaris and Vigna radiata.

Genetic diversity and biogeography of rhizobia associated with Caragana species in three ecological regions of China.

Twenty-two genospecies belonging mainly to Mesorhizobium, and occasionally to Rhizobium and Bradyrhizobium, were defined among the 174 rhizobia strains isolated from Caragana species. Highly similar nodC genes were found in the sole Bradyrhizobium strain and among all the detected Mesorhizobium strains. A clear correlation between rhizobial genospecies and the eco-regions where they were isolated was found using homogeneity analysis. All these results demonstrated that Caragana species had stringently selected the rhizobia symbiotic genotype, but not the genomic background; lateral transfer of symbiotic genes from Mesorhizobium to Bradyrhizobium and among the Mesorhizobium species has happened in the Caragana rhizobia; and biogeography of Caragana rhizobia exists. Furthermore, a combined cluster analysis, based upon the patterns obtained from amplified 16S rRNA gene and 16S-23S intergenic spacer restriction analyses, BOX PCR and SDS-PAGE of proteins, was reported to be an efficient method to define the genospecies.

Mesorhizobium caraganae sp. nov., a novel rhizobial species nodulated with Caragana spp. in China.

Five rhizobial strains representing the largest group in the genus Mesorhizobium associated with Caragana spp. in China were characterized taxonomically. Phylogenetic analysis based on 16S rRNA gene sequences indicated that these microsymbionts belonged to the genus Mesorhizobium, with Mesorhizobium tianshanense USDA 3592(T), Mesorhizobium temperatum SDW018(T) and Mesorhizobium mediterraneum UPM-Ca36(T) as the closest neighbours (>/=99.5 % 16S rRNA gene sequence similarity). Genotypic fingerprinting by whole-cell protein electrophoresis, DNA-DNA hybridization, comparative housekeeping sequence analysis of the atpD, glnII and recA genes, fatty acid profiles and a series of phenotypic and physiological tests allowed the novel group to be differentiated from all previously recognized species of the genus Mesorhizobium. This group therefore represents a novel species, for which the name Mesorhizobium caraganae sp. nov. is proposed with the type strain CCBAU 11299(T) (=LMG 24397(T)=HAMBI 2990(T)). Cross-inoculation tests showed that strain CCBAU 11299(T) could form effective nodules on Caragana microphylla, Caragana intermedia, Glycyrrhiza uralensis, Astragalus adsurgens and Phaseolus vulgaris.

Different Mesorhizobium species associated with Caragana carry similar symbiotic genes and have common host ranges.

Fourteen strains representing 11 Caragana-nodulating Mesorhizobium genomic species were identified as representing Mesorhizobium amorphae, Mesorhizobium huakuii, Mesorhizobium septentrionale and groups related to Mesorhizobium plurifarium, Mesorhizobium temperatum, Mesorhizobium tianshanense and Mesorhizobium mediterraneum by sequencing of the 16S rRNA gene, 16S-23S internal transcribed spacer, partial housekeeping recA gene, and previously performed sodium dodecyl sulfate polyacrylamide gel electrophoresis of proteins and BOX-PCR fingerprinting. Despite their different taxonomic affiliation, highly similar symbiotic genes (>93% similarity for nodC and >91.8% similarity for nifH) were found among the Caragana strains and the three type strains for M. tianshanense, M. temperatum and M. septentrionale. Cross nodulation tests revealed that each of these 14 Caragana mesorhizobia and the three type strains mentioned above could effectively infect each of their original host plants, Caragana microphylla, Glycyrrhiza (host for M. tianshanense type strain) and Astragalus adsurgens (host for M. temperatum and M. septentrionale type strains). These results provide evidence that different Mesorhizobium species can nodulate with Caragana, and they have similar symbiotic genes (probably acquired by a phenomenon of lateral gene transfer) and common host ranges.

[Soil microbial diversity of artificial peashrub plantation on North Loess Plateau of China].

Peashrub (Caragana korshinskii Kom) is a kind of excellent shrub used for dune-fixation in Loess Plateau of China. In order to explore relationship between peashrub and soil microorganisms, microbial communities diversity associated with rhizoplane, rhizosphere and bulk soil of peashrub in Loess Plateau of China were characterized based on a culture-independent approach. Three 16S rDNA gene libraries were constructed, respectively, and each different profile was used to define an operational taxonomic unit (OTU). The numbers of microorganisms decreased as root proximity decreased and a few OTUs became dominant. Phylogenetic analyses indicated that Proteobacteria was the predominant group in rhizoplane, which included many alpha-Proteobacteria, partially consisted of rhizobia, and gamma-Proteobacteria beneficial to plant growth. In bulk soil, the most frequent OTUs were closely related to Archaea, while Acidobacteria was the dominant group in rhizosphere of peashrub. The diversity index (H') was higher in rhizosphere than in rhizoplane and bulk soil, whereas microbial populations in rhizoplane and bulk soil had the greater dominance indices (D). It was shown that there was a significant change in microbial species composition along the root gradient, shifting from complex plant-associated bacterial community in the root habitats to a simple bacterial community in the bulk soil. These results showed that plant roots and soil conditions created a selective environment for microbial populations.