Molecular diversity of nitrogen-fixing bacteria associated with Chrysopogon zizanioides (L.) Roberty (vetiver), an essential oil producer plant

The essential oil produced by vetiver can vary in amount and composition depending on the bacterial community associated with its roots. Some of these bacteria could also promote plant growth by fixing nitrogen. This study aimed to analyze the diversity of diazotrophic bacteria tightly associated with roots of different vetiver genotypes. nifH-based PCR-denaturing gradient gel electrophoresis (DGGE) and clone libraries were used. DGGE profiles obtained from bulk and rhizosphere soils and root DNA amplified with nifH primers showed that samples from rhizosphere soil and root were separated at 68% similarity. Twelve bands were excised from the DGGE and sequenced. High similarity with nifH sequences of Bradyrhizobium sp., Pseudacidovorax sp. and Xanthobacter sp. was observed. Moreover, three nifH clone libraries were generated using polF/polR-primers from root DNA samples obtained from vetiver genotypes UFS-VET001, UFS-VET003 and UFS-VET004. In UFS-VET001, 24.2% of 95 clones were affiliated with sequences of Mesorhizobium loti while in UFS-VET003 41.5% of 89 clones were affiliated with Sphingomonas azotifigens, and in UFS-VET004 36.4% of 85 clones were affiliated with Klebsiella pneumoniae. The data obtained can be used to guide the isolation of diazotrophic bacteria, which may contribute to plant growth promotion and improvement of the production of essential oil in vetiver.     

Genotypic diversity among rhizospheric bacteria of three legumes assessed by cultivation-dependent and cultivation-independent techniques

The genotypic diversity of rhizospheric bacteria of 3 legumes including Vigna radiata, Arachis hypogaea and Acacia mangium was compared by using cultivation-dependent and cultivation-independent methods. For cultivation-dependent method, Random amplified polymorphic DNA (RAPD) profiles revealed that the bacterial genetic diversity of V. radiata and A. mangium rhizospheres was higher than that of A. hypogaea rhizosphere. For cultivation-independent method, Denaturing gradient gel electrophoresis (DGGE) profiles of PCR-amplified 16S rRNA genes revealed the difference in bacterial community and diversity of rhizospheres collected from 3 legumes. The ribotype richness which indicates species diversity, was highest in V. radiata rhizosphere, followed by A. hypogaea and A. mangium rhizospheres, respectively. Three kinds of media were used to cultivate different target groups of bacteria. The result indicates that the communities of cultivable bacteria in 3 rhizospheres recovered from nutrient agar (NA) medium were mostly different from each other, while Bradyrhizobium selective medium (BJSM) and nitrogen-free medium shaped the communities of cultivable bacteria. Nine isolates grown on BJSM were identified by 16S rRNA gene sequence analysis. These isolates were very closely related (with 96% to 99% identities) to either one of the three groups including Cupriavidus-Ralstonia group, Bacillus group and Bradyrhizobium-Bosea-Afipia group. The rhizospheres were also examined for their enzymatic patterns. Of 19 enzymes tested, 3 rhizospheres were distinguishable by the presence or the absence of leucine acrylamidase and acid phosphatase. The selected cultivable bacteria recovered from NA varied in their abilities to produce indole-acetic acid and ammnonia. The resistance to 10 antibiotics was indistinguishable among bacteria isolated from different rhizospheres.     

Molecular and phenotypic characterization of potential plant growth-promoting <Emphasis Type="Italic">Pseudomonas</Emphasis> from rice and maize rhizospheres

In this study, Pseudomonas species were isolated from the rhizospheres of two plant hosts: rice (Oryza sativa cultivar Pathum Thani 1) and maize (Zea mays cultivar DK888). The genotypic diversity of isolates was determined on basis of amplified rDNA restriction analysis (ARDRA). This analysis showed that both plant varieties selected for two distinct populations of Pseudomonas. The actual biocontrol and plant promotion abilities of these strains was confirmed by bioassays on fungal (Verticillum sp., Rhizoctonia solani and Fusarium sp.) and bacterial (Ralstonia solanacearum and Bacillus subtilis) plant pathogens, as well as indole-3-acetic acid (IAA) production and carbon source utilization. There was a significant difference between isolates from rice and maize rhizosphere in terms of biological control against R.  solanacearum and B.  subtilis. Interestingly, none of the pseudomonads isolated from maize rhizosphere showed antagonistic activity against R.  solanacearum. This study indicated that the percentage of pseudomonad isolates obtained from rice rhizosphere which showed the ability to produce fluorescent pigments was almost threefold higher than pseudomonad isolates obtained from maize rhizosphere. Furthermore, the biocontrol assay results indicated that pseudomonad isolated from rice showed a higher ability to control bacterial and fungal root pathogens than pseudomonad isolates obtained from maize. This work clearly identified a number of isolates with potential for use as plant growth-promoting and biocontrol agents on rice and maize.     

Identification of nitrogen-fixing <Emphasis Type="Italic">Paenibacillus</Emphasis> from different plant rhizospheres and a novel <Emphasis Type="Italic">nifH</Emphasis> gene detected in the <Emphasis Type="Italic">P. stellifer</Emphasis>

A total of 534 isolates were selectively obtained from different plant rhizospheres based on their growth on nitrogen-free medium and their resistance to 80°C for 15 min. Of the 534 isolates, 23 isolates had nifH gene and exhibited nitrogenase activities. Based on 16S rDNA sequence, G + C content assay and DNA-DNA hybridization, the 23 isolates which divided into four monophyletic clusters were all belonged to the Paenibacillus genus. nifH gene deduced amino acid alignment aLnalysis revealed that cluster I, including 15 isolates, showed the highest NifH identity with Paenibacillus genus; while cluster II identified as P. stellifer by DNA-DNA hybridization was consistent with four uncultured bacterial clones. This study suggested that the nitrogen-fixing Paenibacillus were distributed in various ecosystems and prevalent in different plant rhizospheres. It was the first demonstration that nitrogen fixation existed in P. jamilae and P. stellifer. In eight isolates identified as P. stellifer species, a novel nifH gene was detected in Paenibacillus.     

Dynamics of Fungal Communities in Bulk and Maize Rhizosphere Soil in the Tropics

The fungal population dynamics in soil and in the rhizospheres of two maize cultivars grown in tropical soils were studied by a cultivation-independent analysis of directly extracted DNA to provide baseline data. Soil and rhizosphere samples were taken from six plots 20, 40, and 90 days after planting in two consecutive years. A 1.65-kb fragment of the 18S ribosomal DNA (rDNA) amplified from the total community DNA was analyzed by denaturing gradient gel electrophoresis (DGGE) and by cloning and sequencing. A rhizosphere effect was observed for fungal populations at all stages of plant development. In addition, pronounced changes in the composition of fungal communities during plant growth development were found by DGGE. Similar types of fingerprints were observed in two consecutive growth periods. No major differences were detected in the fungal patterns of the two cultivars. Direct cloning of 18S rDNA fragments amplified from soil or rhizosphere DNA resulted in 75 clones matching 12 dominant DGGE bands. The clones were characterized by their HinfI restriction patterns, and 39 different clones representing each group of restriction patterns were sequenced. The cloning and sequencing approach provided information on the phylogeny of dominant amplifiable fungal populations and allowed us to determine a number of fungal phylotypes that contribute to each of the dominant DGGE bands. Based on the sequence similarity of the 18S rDNA fragment with existing fungal isolates in the database, it was shown that the rhizospheres of young maize plants seemed to select the Ascomycetes order Pleosporales, while different members of the Ascomycetes and basidiomycetic yeast were detected in the rhizospheres of senescent maize plants.     

Diversity and functional traits of culturable microbiome members,including cyanobacteria in the rice phyllosphere

The diversity and abundance of culturable microbiome members of the rice phyllosphere was investigated using cv. Pusa Punjab Basmati 1509. Both diversity and species richness of bacteria were significantly higher in plants in pots in a semi‐controlled environment than those in fields. Application of fertilisers reduced both diversity and species richness in field‐grown plants under a conventional flooded system of rice intensification (SRI) and in dry‐seeded rice (DSR) modes. Sequence analyses of 16S rDNA of culturable bacteria, those selected after amplified ribosomal DNA restriction analysis (ARDRA), showed the dominance of α‐proteobacteria (35%) and actinobacteria (38%); Pantoea, Exiguobacterium and Bacillus were common among the culturable phyllospheric bacteria. About 34% of 83 culturable bacterial isolates had higher potential (>2 μg·ml^?1) for indole acetic acid production in the absence of tryptophan. Interestingly, the phyllosphere bacterial isolates from the pot experiment had significantly higher potential for nitrogen fixation than isolates from the field experiment. Enrichment for cyanobacteria showed both unicellular forms and non‐heterocystous filaments under aerobic as well as anaerobic conditions. PCR‐DGGE analysis of these showed that aerobic and anaerobic conditions as well as the three modes of cultivation of rice in the field strongly influenced the number and abundance of phylotypes. The adaptability and functional traits of these culturable microbiome members suggest enormous diversity in the phyllosphere, including potential for plant growth promotion, which was also significantly influenced by the different methods of growing rice.     

Biocontrol Potential of Soybean Bacterial Endophytes Against Charcoal Rot Fungus, <Emphasis Type="Italic">Rhizoctonia bataticola</Emphasis>

A total of 137 bacterial isolates from surface sterilized root, stem, and nodule tissues of soybean were screened for their antifungal activity against major phytopathogens like Rhizoctonia bataticola, Macrophomina phaseolina, Fusarium udam, and Sclerotium rolfsii. Nine bacterial endophytes suppressed the pathogens under in vitro plate assay. These were characterized biochemically and identified at the genus level based on their partial sequence analysis of 16S rDNA. Eight of the isolates belonged to Bacillus and one to Paenibacillus. The phylogenetic relationship among the selected isolates was studied and phylogenetic trees were generated. The selected isolates were screened for biocontrol traits like production of hydrogen cyanide (HCN), siderophore, hydrolytic enzymes, antibiotics, and plant growth promoting traits like indole 3-acetic acid production, phosphate solubilization, and nitrogen fixation. A modified assessment scheme was used to select the most efficient biocontrol isolates Paenibacillus sp. HKA-15 (HKA-15) and Bacillus sp. HKA-121 (HKA-121) as potential candidates for charcoal rot biocontrol as well as soybean plant growth promotion.     

Pseudomonads: major antagonistic endophytic bacteria to suppress bacterial wilt pathogen, <Emphasis Type="Italic">Ralstonia solanacearum</Emphasis> in the eggplant (<Emphasis Type="Italic">Solanum melongena</Emphasis> L.)

Endophytic bacteria of eggplant, cucumber and groundnut were isolated from different locations of Goa, India. Based on in vitro screening, 28 bacterial isolates which effectively inhibited Ralstonia solanacearum, a bacterial wilt pathogen of the eggplant were characterized and identified. More than 50% of these isolates were Pseudomonas fluorescens in which a vast degree of variability was found to exist when biochemical characteristics were compared. In greenhouse experiments, the plants treated with Pseudomonas isolates (EB9, EB67), Enterobacter isolates (EB44, EB89) and Bacillus isolates (EC4, EC13) reduced the wilt incidence by more than 70%. All the selected isolates reduced damping off by more than 50% and improved the growth of seedlings in the nursery stage. Most of the selected antagonists produced an antibiotic, DAPG, which inhibited R. solanacearum under in vitro conditions and might have been responsible for reduced wilt incidence under in vivo conditions. Also production of siderophores and IAA in the culture medium by the antagonists was recorded, which could be involved in biocontrol and growth promotion in crop plants. From our study we conclude that Pseudomonas is the major antagonistic endophytic bacteria from eggplants which have the potential to be used as a biocontrol agent as well as plant growth-promoting rhizobacteria. Large scale field evaluation and detailed knowledge on antagonistic mechanism could provide an effective biocontrol solution for bacterial wilt of solanaceous crops.     

Assessment of root-associated paenibacillus polymyxa groups on growth promotion and induced systemic resistance in pepper

Twenty-nine P. polymyxa strains isolated from rhizospheres of various crops were clustered into five genotypic groups on the basis of BOX-PCR analysis. The characteristics of several plant growth-promoting factors among the isolates revealed the distinct attributes in each allocated group. Under gnotobiotic conditions, inoculation of pepper roots with P. polymyxa isolates significantly increased the biomass in 17 of total 29 treated plants with untreated plants. Experiments on induced systemic resistance (ISR) against bacterial spot pathogen Xanthomonas axonopodis pv. vesicatoria in pepper by P. polymyxa strains were conducted and only one isolate (KNUC265) was selected. Further studies into ISR mediation by the KNUC265 strain against the soft-rot pathogen Erwinia carotovora subsp. carotovora in tobacco demonstrated that the tobacco seedlings exposed to either bacterial volatiles or diffusible metabolites exhibited a reduction in disease severity. In conclusion, ISR and plant growth promotion triggered by P. polymyxa isolates were systemically investigated on pepper for the first time. The P. polymyxa KNUC265 strain, which elicited both ISR and plant growth promotion, could be potentially used in improving the yield of pepper and possibly of other crops.     

A Multiphasic Approach for the Identification of Endophytic Bacterial in Strawberry Fruit and their Potential for Plant Growth Promotion

This study used a multiphasic approach, characterized by the simultaneous use of culture-dependent and culture-independent methods, to investigate endophytic bacterial communities in strawberry (Fragaria ananassa) fruit. A total of 92 bacterial endophytes were isolated and initially grouped by their repetitive extragenic palindromic (rep)-PCR banding pattern and biochemical features. Phylogenetic analysis of the 16S rRNA gene sequences of 45 representatives showed that the isolates belonged to the species Bacillus subtilis (eight isolates), Bacillus sp. (seven isolates), Enterobacter sp. (seven isolates), Enterobacter ludwigii (six isolates), Lactobacillus plantarum (six isolates), Pseudomonas sp. (five isolates), Pantoea punctata (three isolates), and Curtobacterium citreum (three isolates). Nucleic acids were extracted from the strawberry fruit and subjected to 16S rRNA gene directed polymerase chain reaction denaturing gradient gel electrophoresis (16S rRNA PCR-DGGE). The species B. subtilis, Enterobacter sp., and Pseudomonas sp. were detected both by isolation and DGGE. The DGGE fingerprints of total bacterial DNA did not exhibit bands corresponding to several of the representative species isolated in the extinction dilution (L. plantarum, C. citreum, and P. punctata). In contrast, bands in the DGGE profile that were identified as relatives of Arthrobacter sp. and one uncultivable Erythrobacter sp. were not recovered by cultivation techniques. After isolation, the nitrogen fixation ability and the in vitro production of indole-3-acetic acid (IAA) equivalents and siderophores were evaluated. A high percentage of isolates were found to possess the ability to produce siderophores and IAA equivalents; however, only a few isolates belonging to the genera Pseudomonas and Enterobacter showed the ability to fix nitrogen. Plant growth promotion was evaluated under greenhouse conditions and revealed the ability of the Bacillus strains to enhance the number of leaves, shoot length, root dry weight, and shoot dry weight. The activity of the bacterial isolate identified as B. subtilis NA-108 exerted the greatest influence on strawberry growth and showed a 42.8% increase in number of leaves, 15.26% for high shoot, 43.5% increase in root dry weight, and a 77% increase in shoot dry weight when compared with untreated controls.     

Seasonal Variability of Diazotroph Assemblages Associated with the Rhizosphere of the Salt Marsh Cordgrass, Spartina alterniflora

Nitrogen fixation is the primary N source in the highly productive but N-limited North Inlet, SC, USA salt marsh system. The diverse assemblages of nitrogen-fixing (diazotrophic) bacteria associated with the rhizospheres of the short and tall growth forms of Spartina alterniflora were analyzed at two sites, Crab Haul Creek and Goat Island, which are in different tidal creek drainage systems in this marsh. The sites differed in proximity to the main channel for tidal intrusion and in several edaphic parameters. We hypothesized that either the differing abiotic environmental regimes of the two sites or the variation due to seasonal effects result in differences in the diazotroph assemblage. Rhizosphere samples were collected seasonally during 1999 and 2000. DNA was purified and nifH amplified for denaturing gradient gel electrophoresis (DGGE) analysis of diazotroph assemblage composition. Principal components analysis was used to analyze the binary DGGE band position data. Season strongly influenced assemblage composition and biplots were used to identify bands that significantly affected the seasonal and site-specific clustering. The types of organisms that were most responsive to seasonal or site variability were identified on the basis of DGGE band sequences. Seasonally responsive members of the anaerobic diazotrophs were detected during the winter and postsenescence conditions and may have been responsible for elevated pore water sulfide concentrations. Sequences from a diverse assemblage of Gammaproteobacteria were predominant during growth periods of S. alterniflora. Abiotic environmental parameters strongly influenced both the S. alterniflora and the diazotrophic bacterial assemblages associated with this keystone salt marsh plant species.     

Biocontrol bacteria selected by a direct plant protection strategy against avocado white root rot show antagonism as a prevalent trait

Aim: This study was undertaken to study bacterial strains obtained directly for their efficient direct control of the avocado white root rot, thus avoiding prescreening by any other possible mechanism of biocontrol which could bias the selection. Methods and Results: A collection of 330 bacterial isolates was obtained from the roots and soil of healthy avocado trees. One hundred and forty‐three representative bacterial isolates were tested in an avocado/Rosellinia test system, resulting in 22 presumptive protective strains, all of them identified mainly as Pseudomonas and Bacillus species. These 22 candidate strains were screened in a more accurate biocontrol trial, confirming protection of some strains (4 out of the 22). Analyses of the potential bacterial traits involved in the biocontrol activity suggest that different traits could act jointly in the final biocontrol response, but any of these traits were neither sufficient nor generalized for all the active bacteria. All the protective strains selected were antagonistic against some fungal root pathogens. Conclusions: Diverse bacteria with biocontrol activity could be obtained by a direct plant protection strategy of selection. All the biocontrol strains finally selected in this work were antagonistic, showing that antagonism is a prevalent trait in the biocontrol bacteria selected by a direct plant protection strategy. Significance and Impact of the Study: This is the first report on the isolation of biocontrol bacterial strains using direct plant protection strategy in the system avocado/Rosellinia. Characterization of selected biocontrol bacterial strains obtained by a direct plant protection strategy showed that antagonism is a prevalent trait in the selected strains in this experimental system. This suggests that antagonism could be used as useful strategy to select biocontrol strains.     

Plant growth-promoting rhizobacteria modulate root-system architecture in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> through volatile organic compound emission

Extensive communication occurs between plants and microorganisms during different stages of plant development in which signaling molecules from the two partners play an important role. Volatile organic compounds (VOCs) emission by certain plant-growth promoting rhizobacteria (PGPR) has been found to be involved in plant growth. However, little is known about the role of bacterial VOCs in plant developmental processes. In this work, we investigated the effects of inoculation with twelve bacterial strains isolated from the rhizosphere of lemon plants (Citrus aurantifolia) on growth and development of Arabidopsis thaliana seedlings. Several bacterial strains showed a plant growth promoting effect stimulating biomass production, which was related to differential modulation of root-system architecture. The isolates L263, L266, and L272a stimulated primary root growth and lateral root development, while L254, L265a and L265b did not significantly alter primary root growth but strongly promoted lateral root formation. VOC emission analysis by SPME-GC-MS identified aldehydes, ketones and alcohols as the most abundant compounds common to most rhizobacteria. Other VOCs, including 1-octen-3-ol and butyrolactone were strain specific. Characterization of L254, L266 and L272a bacterial isolates by 16S rDNA analysis revealed the identity of these strains as Bacillus cereus, Bacillus simplex and Bacillus sp, respectively. Taken together, our data suggest that rhizospheric bacterial strains can modulate both plant growth promotion and root-system architecture by differential VOC emission.     

Rhizosphere <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. strains reduce occurrence of pre- and post-emergence damping-off in chile and tomato in Central Himalayan region

Based on in vitro screening for PGP and anti-mycelial activity against three zoosporic pathogenic oomycetes, Pythium aphanidermatum 123, P. aphanidermatum 4746, and Phytophthora nicotianae 4747, seven bacterial isolates were selected for field trials on tomato and chile to test for plant growth promotion under natural and artificial disease-infested field sites in both winter and wet seasons. The effectiveness of isolates in the field trials correlated with the in vitro antagonism screening data. Pseudomonas sp. FQP PB-3, FQA PB-3 and GRP₃ showed substantial beneficial effects on plant growth promotion and lowered considerably the incidence of pre- and post-emergence damping-off in both the crops under various disease scenarios. For example, seed bacterization with these bacterial strains reduced pre-emergence-damping off by ca. 60–70% in the two natural sites, with and without histories of fungicide use in the winter season, and to a lesser extent, ca. 20–40%, in the warmer wet (high humidity; 85–92%) season. The suppression efficacy for post-emergence damping-off was less compared to pre-emergence damping-off although still significant (P > 0.05). Our data unambiguously show that screening of a large number of bacterial pool identifies promising isolates that show beneficial effects on all stages of plant growth in natural oomycete-infested regimes.     

Population variation of invasive <Emphasis Type="Italic">Spartina</Emphasis> <Emphasis Type="Italic">alterniflora</Emphasis> can differentiate bacterial diversity in its rhizosphere

While several studies have documented that invasive plants can change the microbial communities, little is known about how soil microbial communities respond to population variation of invasive plants. Here, nine populations of Spartina alterniflora were selected from the east coast of China along latitudinal gradient to compare bacterial diversity of rhizospheres among these populations. The bacterial diversity in S. alterniflora rhizospheres was valued by denaturing gradient gel electrophoresis (DGGE) analysis. Shannon–Weaver diversity index (H′) and number of DGGE bands showed that rhizosphere bacterial diversity of S. alterniflora populations increased along a latitudinal gradient when all the populations were grown in a common garden. These findings suggest that population variation of S. alterniflora can differentiate the rhizosphere bacterial diversity, and the latitudinal gradient can shape the specific plant–bacterial diversity relationship. Our results adding to the recent literature suggest that invasive plant–soil biota interactions would have clinal variation with environmental gradients and improve our understanding of the mechanisms and processes of plant invasions.     

Effect of bacterial inoculation,plant genotype and developmental stage on root-associated and endophytic bacterial communities in potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>)

Beneficial bacteria interact with plants by colonizing the rhizosphere and roots followed by further spread through the inner tissues, resulting in endophytic colonization. The major factors contributing to these interactions are not always well understood for most bacterial and plant species. It is believed that specific bacterial functions are required for plant colonization, but also from the plant side specific features are needed, such as plant genotype (cultivar) and developmental stage. Via multivariate analysis we present a quantification of the roles of these components on the composition of root-associated and endophytic bacterial communities in potato plants, by weighing the effects of bacterial inoculation, plant genotype and developmental stage. Spontaneous rifampicin resistant mutants of two bacterial endophytes, Paenibacillus sp. strain E119 and Methylobacterium mesophilicum strain SR1.6/6, were introduced into potato plants of three different cultivars (Eersteling, Robijn and Karnico). Densities of both strains in, or attached to potato plants were measured by selective plating, while the effects of bacterial inoculation, plant genotype and developmental stage on the composition of bacterial, Alphaproteobacterial and Paenibacillus species were determined by PCR-denaturing gradient gel-electrophoresis (DGGE). Multivariate analyses revealed that the composition of bacterial communities was mainly driven by cultivar type and plant developmental stage, while Alphaproteobacterial and Paenibacillus communities were mainly influenced by bacterial inoculation. These results are important for better understanding the effects of bacterial inoculations to plants and their possible effects on the indigenous bacterial communities in relation with other plant factors such as genotype and growth stage.     

Comparison of Soil Bacterial Communities in Rhizospheres of Three Plant Species and the Interspaces in an Arid Grassland

Soil bacteria are important contributors to primary productivity and nutrient cycling in arid land ecosystems, and their populations may be greatly affected by changes in environmental conditions. In parallel studies, the composition of the total bacterial community and of members of the Acidobacterium division were assessed in arid grassland soils using terminal restriction fragment length polymorphism (TRF, also known as T-RFLP) analysis of 16S rRNA genes amplified from soil DNA. Bacterial communities associated with the rhizospheres of the native bunchgrasses Stipa hymenoides and Hilaria jamesii, the invading annual grass Bromus tectorum, and the interspaces colonized by cyanobacterial soil crusts were compared at three depths. When used in a replicated field-scale study, TRF analysis was useful for identifying broad-scale, consistent differences in the bacterial communities in different soil locations, over the natural microscale heterogeneity of the soil. The compositions of the total bacterial community and Acidobacterium division in the soil crust interspaces were significantly different from those of the plant rhizospheres. Major differences were also observed in the rhizospheres of the three plant species and were most apparent with analysis of the Acidobacterium division. The total bacterial community and the Acidobacterium division bacteria were affected by soil depth in both the interspaces and plant rhizospheres. This study provides a baseline for monitoring bacterial community structure and dynamics with changes in plant cover and environmental conditions in the arid grasslands.     

烤烟根际烟碱降解细菌的多样性及其促生特性分析

【背景】挖掘兼具烟碱降解和植物根际促生细菌(Plant Growth-Promoting Rhizobacteria,PGPR)功能的细菌资源,有助于保护土壤质量,实现绿色种植。【目的】分析烤烟根际细菌多样性,筛选可降解高浓度烟碱的PGPR。【方法】采用纯培养法在选择性培养基上分离烟碱降解细菌。通过BOXA1R-PCR分析技术、16SrRNA基因测序及系统发育树构建,对菌株的遗传多样性和分类学地位进行分析。进一步评价了菌株的吲哚乙酸(Indole-3-Acetic Acid,IAA)活性、溶磷能力、病原菌拮抗能力等PGPR指标,以筛选出高效PGPR,最后通过盆栽试验验证其促生效果。【结果】分离得到58株烟碱降解细菌,根据BOXA1R-PCR指纹图谱选取11株菌进行16S rRNA基因序列测定,结果表明,58株菌分别属于芽孢杆菌属(Bacillus)、假单胞菌属(Pseudomonas)、拉乌尔菌属(Raoultella)和短波单胞菌属(Brevundimonas)4个属,以芽孢杆菌属(Bacillus)为优势菌属。58株细菌中48.28%的菌株可产IAA,27.59%具备溶磷能力,37.93%具备纤维素降解能力,G2-13、G2-3及HT2-8因促生与抗病特性突出而被筛选为目标功能菌。盆栽试验结果表明,G2-13菌株对幼苗生长的促进作用明显,可使株高与地上部鲜重分别增加33.05%与53.32%。【结论】烤烟根际存在较为丰富多样的烟碱降解细菌,它们在种植业上具有潜在的应用价值。     

Molecular characterization of plant growth promoting rhizobacteria that enhance peroxidase and phenylalanine ammonia-lyase activities in chile (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.) and tomato (<Emphasis Type="Italic">Lycopersicon esculentum</Emphasis> Mill.)

Pythium and Phytophthora species are associated with damping-off diseases in vegetable nurseries and reduce seedling stand and yield. In this study, bacterial isolates were selected on the basis of in vitro antagonism potential to inhibit mycelial growth of damping-off pathogens along with plant growth properties for field assessment in wet and winter seasons. We demonstrate efficacy of bacterial isolates to protect chile and tomato plants under natural vegetable nursery and artificially created pathogen-infested (Pythium and Phytophthora spp.) nursery conditions. After 21 days of sowing, chile and tomato plants were harvested and analysed for peroxidase and phenylalanine ammonia-lyase activities. Pseudomonas sp. strains FQP PB-3, FQA PB-3 and GRP₃ were most effective in increasing shoot length (P > 0.05%) in both artificial and natural field sites. For example, Pseudomonas sp. FQA PB-3 treatment increased shoot length by 40% in the artificial Pythium 4746 infested nursery site in chile plants in the wet season. The bacterial treatments significantly increased the activity of peroxidase and phenylalanine ammonia-lyase in chile and tomato plant tissues, which are well known as indicators of an active lignification process. Thus, we conclude that treatment with potential bacterial plant growth promoting agents help plants against pathogen invasion by modulating plant peroxidase and phenylalanine ammonia-lyase activities.     

Diversity of diazotrophs in arid and semi-arid regions of Haryana and evaluation of their plant growth promoting potential on Bt-cotton and pearl millet

This study revealed cultivable diazotrophs in arid and semi-arid regions of Haryana, India, harboring multiple plant growth promoting (PGP) traits, i.e. nitrogenase activity (18.84–337.26 nmol ethylene mg^?1 protein h^?1), indole-3-acetic acid production (42.4–1162.7 μg mL^?1), ammonia excretion (0.013–2.561 μg mL^?1), phosphate solubilization (55.8 %), and siderophore production (20.58 %). High diversity among diazotrophic bacterial isolates was deciphered by using amplified ribosomal DNA restriction analysis with MspI and HaeIII. All the isolates positively influenced the growth and yield of Bt-cotton and pearl millet plants under pot house conditions. On the basis of their plant growth promoting potential, 10 efficient bacterial strains were selected. Sequence analysis of these strains revealed two phyla of bacteria in the 16S rRNA library, which consisted of α and γ subclasses of the Proteobacteria and Firmicutes. The dominant group was Firmicutes (80 % of the total isolates), and the most dominant genus was Bacillus. Overall, our study reported bacterial strains with biofertilizer potential and could be considered as an excellent addition to existing beneficial microbes’ consortium for growth promotion of Bt-cotton and pearl millet plants in arid and semi-arid regions.