盐条件下产胞外多糖植物促生细菌研究

Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can play an important role in alleviating soil salinity stress during plant growth and bacterial exopolysaccharide (EPS) can also help to mitigate salinity stress by reducing the content of Na + available for plant uptake.In this study,native bacterial strains of wheat rhizosphere in soils of Varanasi,India,were screened to identify the EPS-producing salt-tolerant rhizobacteria with plant growth-promoting traits.The various rhizobacteria strains were isolated and identified using 16S rDNA sequencing.The plant growth-promoting effect of inoculation of seedlings with these bacterial strains was evaluated under soil salinity conditions in a pot experiment.Eleven bacterial strains which initially showed tolerance up to 80 g L -1 NaCl also exhibited an EPS-producing potential.The results suggested that the isolated bacterial strains demonstrated some of the plant growth-promoting traits such as phosphate solubilizing ability and production of auxin,proline,reducing sugars,and total soluble sugars.Furthermore,the inoculated wheat plants had an increased biomass compared to the un-inoculated plants.     

Plant growth-promoting rhizobacteria as transplant amendments and their effects on indigenous rhizosphere microorganisms

Field trials were conducted in Florida on bell pepper (Capsicum annuum) to monitor the population dynamics of two plant growth-promoting rhizobacteria (PGPR) strains (Bacillus subtilis strain GBO3 and Bacillus amyloliquefaciens strain IN937a) applied in the potting media at seeding and at various times after transplanting to the field during the growing season. In-field drenches of an aqueous bacterial formulation were used for the mid-season applications. The effects of the applied PGPR and application methods on bacterial survival, rhizosphere colonization, plant growth and yield, and selected indigenous rhizosphere microorganisms were assessed. The Gram-positive PGPR applied to the potting media established stable populations in the rhizosphere that persisted throughout the growing season. Additional aqueous applications of PGPR during the growing season did not increase the population size of applied strains compared to treatments only receiving bacteria in the potting media; however, they did increase plant growth compared to the untreated control to varying degrees in both trials. Most treatments also reduced disease incidence in a detached leaf assay, indicating that systemic resistance was induced by the PGPR treatments. However, treatments did not result in increased yield, which was highly variable. Application of the PGPR strains did not adversely affect populations of beneficial indigenous rhizosphere bacteria including fluorescent pseudomonads and siderophore-producing bacterial strains. Treatment with PGPR increased populations of fungi in the rhizosphere but did not result in increased root disease incidence. This fungal response to the PGPR product was likely due to an increase in nonpathogenic chitinolytic fungal strains resulting from the application of chitosan, which is a component of the PGPR formulation applied to the potting media.     

Yield and N assimilation of winter wheat (Triticum aestivum L., var. Norstar) inoculated with rhizobacteria

The effects of bacterial inoculants on the growth of winter wheat were studied in a growth chamber. Azospirillum brasilense, Azotobacter chroococcum, Bacillus polymyxa, Enterobacter cloacae, or a mixture of the four rhizobacteria were the inoculants tested. Inoculation effects on yield, yield components, and N-derived from fertilizer (Ndff) were assessed. The response of plants inoculated with individual bacteria was inconsistent and varied with treatment. At the first harvest (58 days after planting-DAP) plants inoculated with the mixture exhibited increases in plant dry weight, total-N and Ndff. At the second harvest (105 DAP), plants inoculated with A. brasilense and the mixture exhibited increases in shoot biomass, whereas at maturity (170 DAP), the inoculated plants showed no differences in total-N or shoot dry matter yield, as compared to the uninoculated controls. Inoculation with A. brasilense, however, increased the Ndff in the shoots, and B. polymyxa tended to enhance grain yield. Practical use of these rhizobacteria as inoculants for winter wheat may have limited value until such time as we better understand factors which influence rhizosphere competence of bacterial inoculants.     

Fractionation and Speciation of Manganese in Rhizosphere Soils of Pseudomonas sp. Rhizobacteria Inoculated Pistachio (Pistacia vera L.) Seedlings Under Salinity Stress

Enhancement of manganese (Mn) availability in saline and Mn-deficient soils is very important for plant growth. An experiment was carried out to evaluate the effect of Pseudomonas sp. rhizobacteria (P₀ (control), P₁, P₂ and P₃) and Mn (0 and 10 mg Mn kg^?1 soil) on the distribution of Mn in the rhizosphere of pistachio seedlings under salinity stress (0, 1000 and 2000 mg NaCl kg^?1 soil). The results showed that salinity decreased the dry weight, Mn uptake and chlorophyll content of the pistachio seedlings. However, inoculation with rhizobacteria increased these parameters in saline conditions. Application of rhizobacteria increased the availability of Mn in the rhizosphere soil. The use of rhizobacteria decreased the residual-Mn form in the rhizosphere. Inoculation with rhizobacteria increased the percent of Mn²⁺ and MnCl⁺ species in the soil solution. However, pistachio seedlings inoculation with rhizobacteria increased the contents of Mn available forms in the rhizosphere soil.     

Hydrocarbon degradation potential and activity of endophytic bacteria associated with prairie plants

Phytoremediation systems for organic compounds such as petroleum hydrocarbons rely on a synergistic relationship between plants and their root-associated microbial communities. To determine the probable role of endophytic bacterial communities in these systems, this study examined both rhizosphere and endophytic communities of five different plant species at a long-term phytoremediation field site. Hydrocarbon degradation potential and activity were assessed using MPN assays, PCR analysis of catabolic genes associated with hydrocarbon degradation, and mineralization assays with C-14 labeled hydrocarbons. Microbial community structure in each niche was assessed by DGGE analysis of 16S rRNA gene fragments and subsequent band sequencing. Both endophytic degrader populations and endophytic degrader activity showed substantial inter-species variation, largely independent of that shown by the respective rhizosphere populations. Endophytic hydrocarbon degradation was linked to dominant bacterial endophytes. Pseudomonas spp. dominated endophytic communities exhibited increased alkane hydrocarbon degradation potential and activity, while Brevundimonas and Pseudomonas rhodesiae dominated endophytic communities were associated with increased PAH degradation potential and activity. In one plant species, Lolium perenne, increased endophytic alkane hydrocarbon degradation was associated with increased rhizosphere alkane degradation and decreased rhizosphere PAH degradation. Our results show that diverse plant species growing in weathered-hydrocarbon contaminated soil maintain distinct, heterogeneously distributed endophytic microbial populations, which may impact upon the ability of plants to promote the degradation of specific types of hydrocarbons.     

Überleben inokulierter Rhizosphärenbakterien und deren Einfluß auf native Bakterienpopulationen im Wurzelraum von Luzerne

Survival of inoculated rhizosphere bacteria and their influence on native bacterial populations in the rhizosphere of alfalfa The survival of inoculated bacteria and their influence on native bacterial populations in the rhizosphere of alfalfa were investigated in a greenhouse experiment. The plant growth promoting strains Rhizobium meliloti me18 and Pseudomonas fluorescens PsIA12 were reisolated from the rhizosphere about 7 weeks after single and mixed strain inoculation. They did not induce lasting changes in the diversity of the native bacterial communities of the rhizosphere. Only within the first week after inoculation was an increase in total bacterial abundance observed. In general, the diversity of bacterial communities increased with plant age and with proximity to the root tip.     

Influence of enhanced malate dehydrogenase expression by alfalfa on diversity of rhizobacteria and soil nutrient availability

Transgenic alfalfa over-expressing a nodule-enhanced malate dehydrogenase (neMDH) cDNA and untransformed alfalfa plants were grown at the same field site and rhizosphere soils collected after 53 weeks of plant growth. These alfalfa lines differ in the amount and composition of root organic acids produced and exuded into the rhizosphere. Nucleotide sequencing of PCR-based 16S ribosomal DNA (rDNA) clone libraries and Biolog™ GN microtiter plates were employed to assess the activity of naturally occurring rhizobacteria in the two alfalfa rhizospheres. Selected macro- and micro-elements in the two alfalfa rhizosphere soils were also measured. Analysis of 240 16S rDNA clone sequences indicated the existence of about 11 bacterial phyla and their major subdivisions in the two alfalfa rhizosphere samples. There were qualitative changes in the abundance of bacterial phylogenetic groups between rhizosphere soils of transgenic and untransformed alfalfa. Carbon substrate utilization profiles suggested that rhizosphere samples from transgenic alfalfa had significantly greater microbial functional diversity compared with rhizosphere samples from untransformed alfalfa. The concentrations of nitric acid extractable P, K, Mn, Zn and Cu increased significantly in the transgenic alfalfa rhizosphere compared with the untransformed alfalfa rhizosphere. These observations indicate that organic acids produced by plant roots significantly influence rhizosphere microbial diversity and availability of macro- and micro-nutrients and demonstrate the utility of such trangenic plants as tools for studying the potential impact of plant root exudates on soil microbial ecosystems.     

In‐situ remediation of contaminated farmland by horizontal transfer of degradative plasmids among rhizosphere bacteria

Horizontal transfer of catabolic genes for pollutant degradation among rhizobacteria plays an important role in environmental bioremediation, but lacks support from field trial data. To address this problem, Pseudomonas fluorescens strain TP13 was inoculated into the soils of a phenol‐contaminated farmland on which tomato seedlings were growing in April 2009, 2010, 2011 and 2012. Results consistently showed that introduction of TP13 strain significantly reduced phenol content and increased plant biomass after 20 days, compared with controls. Strain TP13 was able to colonize the plant rhizosphere and the number of rhizosphere bacteria which were grown on phenol and contained the plasmids containing the gene encoding for catechol 2, 3‐dioxygenase (C23O) increased gradually in the later stages of the experiment. The increase in magnitude of the plasmid‐containing rhizosphere bacteria correlated well with plant biomass, while the number of plasmid‐containing rhizosphere bacteria and phenol content was strongly negatively correlated. Furthermore, six strains (T1‐T6) of rhizosphere bacteria were isolated and found to possess large plasmids containing identical C23O genes and similar HindIII restriction patterns. Sequence alignment showed that the C23O genes from strains T1‐T6 contained almost identical sequences and the sequence of the C23O of strain T1 was the same as that of strain TP13. These data indicated that the plasmids were transferred from strain TP13 to these rhizosphere bacteria and that horizontal gene transfer stimulated phenol degradation and plant growth in the contaminated farmland. This is an important finding for in situ remediation of contaminated farmland.     

Grazing of protozoa on rhizosphere bacteria alters growth and reproduction of Arabidopsis thaliana

Plant roots are densely colonized by bacteria which form the basis of the rhizosphere bacterial food web with protozoa as most effective predators. We established a well defined laboratory system with Arabidopsis thaliana as model plant allowing to investigate in detail the effect of rhizosphere interactions on plant performance. We used this system to analyse separate and combined effects of natural rhizobacteria and the protozoa Acanthamoeba castellanii on plants.Protozoa and bacteria increased plant growth with the effect of protozoa markedly exceeding that of bacteria only. Arabidopsis immediately responded to the presence of protozoa by increasing carbon but not nitrogen uptake. Later protozoa enhanced plant uptake of nitrogen from organic material and prolonged vegetative growth of Arabidopsis resulting in strongly increased seed production. It is concluded that the immediate plant response was based on changes in rhizosphere signalling inducing increased plant carbon fixation rather than on protozoa-mediated increase in nitrogen availability. The subsequently increased plant nitrogen uptake presumably originated from nitrogen fixed in bacterial biomass made available by protozoan grazing, i.e. the microbial loop in soil. The results suggest that Arabidopsis prepared for the upcoming mobilization of nitrogen by increasing carbon fixation and root carbon allocation which paid-off later by increased nutrient capture and strongly increased plant reproduction.     

盐沼湿地中植物根围土壤细菌群落结构和多样性的16S rDNA分析

The structure and diversity of the bacterial communities in rhizosphere soils of native Phragmites australis and Scirpus rnariqueter and alien Spartina alterniflora in the Yangtze River Estuary were investigated by constructing 16S ribosomal DNA （rDNA） clone libraries. The bacterial diversity was quantified by placing the clones into operational taxonomic unit （OTU） groups at the level of sequence similarity of 〉 97%. Phylogenetic analysis of the resulting 398 clone sequences indicated a high diversity of bacteria in the rhizosphere soils of these plants. The members of Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria of the phylum Proteobacteria were the most abundant in rhizobacteria. Chao 1 nonpaxametric diversity estimator coupled with the reciprocal of Simpson＇s index （l/D） was applied to sequence data obtained from each library to evaluate total sequence diversity and quantitatively compare the level of dominance. The results showed that Phragmites, Scirpus, and Spartina rhizosphere soils contained 200, 668, and 382 OTUs, respectively. The bacterial communities in the Spartina and Phragraites rhizosphere soils displayed species dominance revealed by 1/D, whereas the bacterial community in Scirpus rhizosphere soil had uniform distributions of species abundance. Overall, analysis of 16S rDNA clone libraries from the rhizosphere soils indicates that the changes in bacterial composition may occur concomitantly with the shift of species composition in plant communities.     

Enhanced root uptake of acibenzolar-S-methyl (ASM) by tomato plants inoculated with selected Bacillus plant growth-promoting rhizobacteria (PGPR)

The combination of plant growth-promoting rhizobacteria (PGPR) and plant resistance inducers is an alternative crop protection approach in modern agricultural systems. Despite the numerous reports regarding the improved suppression of plant pathogens by their combined application, little is known about the interactions among these components. In the present study, the persistence behavior of the plant activator acibenzolar-S-methyl (ASM) in the rhizosphere of tomato plants and its root uptake as well as systemic translocation ability in aboveground parts after combined use with certain Bacillus PGPR strains (B. amyloliquefaciens IN937a, B. pumilus SE34, B. subtilis FZB24 and GB03) were investigated. Additionally, the population dynamics of the PGPR strain B. subtilis GB03 at the tomato root system and rhizosphere soil applied with or without the pesticide were studied. The results showed that the addition of PGPR inocula did not affect the dissipation rate of ASM in rhizosphere soil. Also, the formation of its major metabolite CGA 210007 in soil was rapid, since it was detected one hour after root drench and it was maintained at high levels during the sampling period without considerable variations among the bacterial treatments compared to the control. The uptake and systemic translocation of ASM and its metabolite CGA 210007 from root to shoot was rapid and maximum concentrations were observed at 48–96 h after its application. It was revealed that in plants treated with the PGPR strains B. subtilis GB03 and B. pumilus SE34 the uptake and systemic translocation of ASM and CGA 210007 in the aerial parts of the tomato plants was significantly higher compared to the control receiving no bacterial treatment. Also, the populations of the strain B. subtilis GB03 showed high colonizing ability in the root system and the rhizosphere soil. PGPR strains that lead to enhanced pesticide uptake by plants should be further evaluated as components in integrated management systems.     

Plant growth-promoting bacteria in the rhizo- and endosphere of plants: Their role, colonization, mechanisms involved and prospects for utilization

In both managed and natural ecosystems, beneficial plant-associated bacteria play a key role in supporting and/or increasing plant health and growth. Plant growth-promoting bacteria (PGPB) can be applied in agricultural production or for the phytoremediation of pollutants. However, because of their capacity to confer plant beneficial effects, efficient colonization of the plant environment is of utmost importance. The majority of plant-associated bacteria derives from the soil environment. They may migrate to the rhizosphere and subsequently the rhizoplane of their hosts before they are able to show beneficial effects. Some rhizoplane colonizing bacteria can also penetrate plant roots, and some strains may move to aerial plant parts, with a decreasing bacterial density in comparison to rhizosphere or root colonizing populations. A better understanding on colonization processes has been obtained mostly by microscopic visualisation as well as by analysing the characteristics of mutants carrying disfunctional genes potentially involved in colonization. In this review we describe the individual steps of plant colonization and survey the known mechanisms responsible for rhizosphere and endophytic competence. The understanding of colonization processes is important to better predict how bacteria interact with plants and whether they are likely to establish themselves in the plant environment after field application as biofertilisers or biocontrol agents.     

Interactions between a plant growth‐promoting rhizobacterium and smoke‐derived compounds and their effect on okra growth

Plant growth‐promoting rhizobacteria (PGPR) are used in agriculture to improve crop yield. Crude smoke–water (made by bubbling plant‐derived smoke through water) stimulates germination and improves seedling growth. Some active compounds have been isolated from smoke with karrikinolide (KAR₁) stimulating plant growth and trimethylbutenolide (TMB) being inhibitory. These smoke compounds have great potential in agriculture but their interaction with PGPR is unknown. In the present study, a two‐factorial pot trial with three replicates per treatment was designed to investigate the interactions between Bacillus licheniformis and two concentrations each of smoke–water, KAR₁, and TMB on okra (Abelmoschus esculentus). Growth and physiological parameters (chlorophyll, carotenoid, protein, sugar and α‐amylase) of okra as well as bacterial abundance in the rhizosphere were measured after 5 weeks. Application of B. licheniformis and 10^?7 M KAR₁ significantly improved the shoot biomass and 10^?7 M KAR₁ also significantly improved leaf area of okra. However, when 10^?7 M KAR₁ was applied in combination with B. licheniformis, there was an antagonistic effect on plant growth. While TMB had a negative effect on plant growth, a combination treatment of TMB and B. licheniformis overcame the inhibitory effect of TMB resulting in plant growth similar to the control plants. All treatments had no effect on chlorophyll, carotenoid, protein and sugar concentrations, while α‐amylase activity was significantly elevated in okra root treated with 1:500 (v/v) smoke–water. Determining the rhizobacteria populations at harvest showed that all treatments had no significant effect on the rhizosphere microbial abundance. The modes of interaction between PGPR and smoke‐derived compounds need to be further elucidated.     

不同盐分条件下植物促生菌对燕麦和黑麦草幼苗的促生效应

从盐生植物根际土中分离得到4株含1-氨基环丙烷-1-羧酸(ACC)脱氨酶的植物促生菌(PGPR),通过无菌育种袋栽培试验,考查其在不同盐分条件下对燕麦和黑麦草幼苗的促生效应。结果表明,4株菌对5 g/kg或10 g/kg NaCl盐分胁迫下的燕麦和黑麦草幼苗均表现出显著地缓解促生效应,其中假单胞菌属S1最显著,10 g/kg NaCl比无NaCl时促生作用更大。4株PGPR的ACC脱氨酶活性与植物生长参数(根长和下胚轴长)之间具有极显著的正相关性(Pearson相关系数>0.81)。     

Rhizodeposition and net release of soluble organic compounds by pine and beech seedlings inoculated with rhizobacteria and ectomycorrhizal fungi

Summary A lysimetric experiment was performed in a greenhouse to evalute root deposition and net release of soluble organic compounds after 1 and 2 years from pine and beech seedlings inoculated with an ectomycorrhizal fungus (Laccaria laccata) and/or rhizobacteria (Agrobacterium radiobacter for beech and Agrobacterium sp. for pine). Total C compounds released in the rhizosphere of both plants increased after inoculation with the bacteria or ectomycorrhizal fungus. The rhizobacteria increased root and plant growth and rhizodeposition, but the mycorrhizal fungi appeared to increase only root deposition. Soluble C compounds, collected after 2 years, represented only 0.1–0.3% of the total C compounds released into the rhizosphere, and were modified by inoculation with the microorganisms. After inoculation with the bacteria, levels of sugars and amino acids decreased in pine and beech rhizospheres, whereas organic acids increased, especially in the pine rhizosphere. In the rhizosphere of mycorrhizal beeches, sugar and amino acids increased, and organic acids differed from those released from non-mycorrhizal beeches. In the mycorrhizal pine rhizosphere, however, all compounds decreased. Following dual inoculations, mycorrhizal colonization increased, no effect on plant growth was observed, and virtually no organic acids were detected.     

黑土区大豆基因型的根际细菌群落结构时空动态变化

The dynamics of rhizosphere microbial communities is important for plant health and productivity, and can be influenced by soil type, plant species or genotype, and plant growth stage. A pot experiment was carried out to examine the dynamics of microbial communities in the rhizosphere of two soybean genotypes grown in a black soil in Northeast China with a long history of soybean cultivation. The two soybean genotypes, Beifeng 11 and Hai 9731, differing in productivity were grown in a mixture of black soil and siliceous sand. The bacterial communities were compared at three zone locations including rhizoplane, rhizosphere, and bulk soil at the third node (V3), early flowering (R1), and early pod (R3) stages using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) of 16S rDNA. The results of principal component analyses (PCA) showed that the bacterial community structure changed with growth stage. Spatially, the bacterial communities in the rhizoplane and rhizosphere were significantly different from those in the bulk soil. Nevertheless, the bacterial communities in the rhizoplane were distinct from those in the rhizosphere at the V3 stage, while no obvious differences were found at the R1 and R3 stages. For the two genotypes, the bacterial community structure was similar at the V3 stage, but differed at the R1 and R3 stages. In other words, some bacterial populations became dominant and some others recessive at the two later stages, which contributed to the variation of the bacterial community between the two genotypes. These results suggest that soybean plants can modify the rhizosphere bacterial communities in the black soil, and there existed genotype-specific bacterial populations in the rhizosphere, which may be related to soybean productivity.     

Abundance of bacterial genes encoding for proteases and chitinases in the rhizosphere of three different potato cultivars

In terrestrial ecosystems, the majority of soil N is present in organic macromolecules like proteins, nucleic acids and chitin. Because these compounds cannot be assimilated by plants, plant growth is often N-limited. Therefore, the hydrolysis of proteins and chitin is particularly important in making N available for plants. In this study, we investigated the influence of different potato cultivars and different plant developmental stages on the abundance of genes encoding for alkaline and neutral metalloproteases (apr, npr) as well as bacterial chitinases (chiA) in the rhizosphere using quantitative real-time PCR in a greenhouse trial and a field study. It could be clearly demonstrated that abundance pattern was mainly affected by the plant vegetation stage, whilst the used plant genotype had only a minor influence on the development of the two functional populations.     

Structural differences in the rhizosphere communities of legumes are not equally reflected in community-level physiological profiles

The relationship of structural diversity and differences in the functional potentials of rhizosphere communities of alfalfa, common bean and clover was investigated in microcosms. PCR-SSCP (single strand conformation polymorphism) analysis of 16S rRNA genes revealed significant differences in the composition of the leguminous rhizosphere communities at the shoot stage of plants grown in the same soil. Sequencing of dominant SSCP-bands indicated the presence of plant specific organisms. The partial rRNA gene sequences were related to members of the α- and γ-Proteobacteria, Bacteroidetes and Actinobacteria. Besides the plant species, the soil also affected the structural diversity in rhizospheres. The dominant bacterial populations of alfalfa grown in soils with different agricultural histories were assigned to different taxonomic groups. Addressing the functional potentials, community-level physiological profiles (CLPP) were generated using BIOLOG GN^®. The three leguminous rhizosphere communities could be differentiated by principle component analysis, though the overall analysis indicated that the metabolic potential of all rhizosphere samples was similar. The functional variation examined in rhizospheres of alfalfa was minor in response to the soil origin and was found not to be significant different at different growth stages. The results indicate that similar functional potentials may be provided by structurally different bacterial communities.     

Screening of rhizobacteria isolated from maize (Zea mays L.) in Rio Grande do Sul State (South Brazil) and analysis of their potential to improve plant growth

Plant growth-promoting rhizobacteria (PGPR) are considered to have a beneficial effect on host plants and may facilitate plant growth by different mechanisms. In this work, the influence of different soil types on the bacterial diversity and the stimulatory effects of selected PGPR on two cultivars of maize were investigated. A set of 292 strains was isolated from the roots and rhizosphere soil of maize cultivated in five different areas of the Rio Grande do Sul State in Brazil. 16S rDNA-PCR-RFLP and 16S rDNA partial sequencing were used for identification, and the Shannon–Weaver index was used to evaluate bacterial diversity. We evaluated the ability of each isolate to produce indole acetic acid (IAA), siderophores and solubilize phosphates. On the basis of multiple PGP traits, six isolates were selected to test their potential as plant growth-promoting rhizobacteria on maize plants. In both the roots and the rhizospheric soil of maize, the dominant bacterial genera identified were Klebsiella and Burkholderia. IAA producers were distributed widely among isolates, regardless of the sampling site. Approximately 42% of the isolates exhibited at least two attributes, and 24% showed all three PGP traits. Three strains, identified as Achromobacter, Burkholderia, and Arthrobacter, were effective as PGPR in both of the cultivars evaluated.     

Genetic variability of rhizobacteria from wild populations of four Lupinus species based on PCR-RAPDs§

The metabolic capacities of rhizosphere bacteria can depend on intraspecific genetic variability at strain level. We sampled bacteria from the rhizosphere of three populations of four different Lupinus species at two growth stages (flowering: GS1 and fruiting: GS2). Isolates were identified to the genus level by classical biochemical tests. The most abundant genera found were Bacillus, Aureobacterium, Cellulomonas, Pseudomonas, and Arthrobacter. Genetic divergence of rhizobacteria was tested by PCR-RAPDs. The genetic distances were low, with mean values of 37 % for Bacillus, 25 % for Aureobacterium, 46 % for Cellulomonas, 16 % for Pseudomonas, and 23 % for Arthrobacter. Aureobacterium, the most abundant genus, predominated in the rhizosphere of all populations and at both growth stages (GS1 and GS2) of L. angustifolius. The Aureobacterium strains consisted of 11 groups with 90 % similarity indexes. The cluster analysis of these groups shows that strains isolated from different lupin species and sampling times have extraordinary low diversity indexes, or are even identical. This fact, together with the low genetic distance detected in the rhizosphere, reveals a clear specificity in the plant-bacteria interaction. This specificity could be related with several aspects of plant physiology.