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Background: The plant growth-promoting bacteria (PGPB) Azospirillum brasilense is widely used as an inoculant for important grass crops, providing numerous benefits to the plants. However, one limitation to develop viable commercial inoculants is the control of PGPB survival, requiring strategies that guarantee their survival during handling and field application. The application of sublethal stress appears to be a promising strategy to increase bacterial cells tolerance to adverse environmental conditions since previous stress induces the activation of physiological protection in bacterial cell. In this work, we evaluated the effects of thermal and salt stresses on the survival of inoculant containing A. brasilense Ab-V5 and Ab-V6 strains and we monitored A. brasilense viability in inoculated maize roots after stress treatment of inoculant., Results: Thermal stress application (> 35 °C) in isolated cultures for both strains, as well as salt stress [sodium chloride (NaCl) concentrations > 0.3 mol L -1 ], resulted in growth rate decline. The A. brasilense enumeration in maize roots obtained by propidium monoazide quantitative polymerase chain reaction (PMA-qPCR), for inoculated maize seedlings grown in vitro for 7 days, showed that there is an increased number of viable cells after the salt stress treatment, indicating that A. brasilense Ab-V5 and Ab-V6 strains are able to adapt to salt stress (0.3 mol L -1 NaCl) growth conditions., Conclusion: Azospirillum brasilense Ab-V5 and Ab-V6 strains had potential for osmoadaptation and salt stress, resulting in increased cell survival after inoculation in maize plants. © 2024 Society of Chemical Industry., (© 2024 Society of Chemical Industry.)

Motile plant-associated bacteria use chemotaxis and dedicated chemoreceptors to navigate gradients in their surroundings and to colonize host plant surfaces. Here, we characterize a chemoreceptor that we named Tlp2 in the soil alphaproteobacterium Azospirillum brasilense . We show that the Tlp2 ligand-binding domain is related to the 4-helix bundle family and is conserved in chemoreceptors found in the genomes of many soil- and sediment-dwelling alphaproteobacteria. The promoter of tlp2 is regulated in an NtrC- and RpoN-dependent manner and is most upregulated under conditions of nitrogen fixation or in the presence of nitrate. Using fluorescently tagged Tlp2 (Tlp2-YFP), we show that this chemoreceptor is present in low abundance in chemotaxis-signaling clusters and is prone to degradation. We also obtained evidence that the presence of ammonium rapidly disrupts Tlp2-YFP localization. Behavioral experiments using a strain lacking Tlp2 and variants of Tlp2 lacking conserved arginine residues suggest that Tlp2 mediates chemotaxis in gradients of nitrate and nitrite, with the R159 residue being essential for Tlp2 function. We also provide evidence that Tlp2 is essential for root surface colonization of some plants (teff, red clover, and cowpea) but not others (wheat, sorghum, alfalfa, and pea). These results highlight the selective role of nitrate sensing and chemotaxis in plant root surface colonization and illustrate the relative contribution of chemoreceptors to chemotaxis and root surface colonization.IMPORTANCEBacterial chemotaxis mediates host-microbe associations, including the association of beneficial bacteria with the roots of host plants. Dedicated chemoreceptors specify sensory preferences during chemotaxis. Here, we show that a chemoreceptor mediating chemotaxis to nitrate is important in the beneficial soil bacterium colonization of some but not all plant hosts tested. Nitrate is the preferred nitrogen source for plant nutrition, and plants sense and tightly control nitrate transport, resulting in varying nitrate uptake rates depending on the plant and its physiological state. Nitrate is thus a limiting nutrient in the rhizosphere. Chemotaxis and dedicated chemoreceptors for nitrate likely provide motile bacteria with a competitive advantage to access this nutrient in the rhizosphere., Competing Interests: The authors declare no conflict of interest.

In this work, we tested if inoculation with the plant growth-promoting rhizobacteria Azospirillum brasilense strain Az39 alleviates Cd 2+ stress in wheat seedlings grown under controlled conditions. Growth, total N, N-related metabolites/enzymes, and oxidative stress parameters were measured. Additionally, the usefulness of a real-time PCR protocol to screen the preferred colonization site of the introduced microorganism was evaluated. Inoculated plants demonstrated mitigation of cadmium-induced adverse effects on plant growth and less reactive oxygen species accumulation in their roots by the end of the experiment, 28 days after sowing. Cd addition resulted in lower NO 3 - content in the leaves and higher NO 3 - content in the roots, and a significant rise in NH 4 + concentration in both organs in uninoculated plants; in inoculated plants, NH 4 + content in the roots did not vary. A. brasilense Az39 enhanced NO levels in wheat root tips, and more adventitious roots and root hairs were observed in inoculated plants. Despite having a more developed root system, inoculated plants showed lower Cd levels in their roots compared to non-inoculated plants. Inoculation with this PGPR favored ion homeostasis in the roots of metal-exposed plants, decreasing Cd/Fe ratio. We corroborated A. brasilense Az39 preference for wheat exorhizosphere using a real-time PCR-based method targeting the nifA gene., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Plant roots shape the rhizosphere community by secreting compounds that recruit diverse bacteria. Colonization of various plant roots by the motile alphaproteobacterium Azospirillum brasilens e causes increased plant growth, root volume, and crop yield. Bacterial chemotaxis in this and other motile soil bacteria is critical for competitive colonization of the root surfaces. The role of chemotaxis in root surface colonization has previously been established by endpoint analyses of bacterial colonization levels detected a few hours to days after inoculation. More recently, microfluidic devices have been used to study plant-microbe interactions, but these devices are size limited. Here, we use a novel slide-in chamber that allows real-time monitoring of plant-microbe interactions using agriculturally relevant seedlings to characterize how bacterial chemotaxis mediates plant root surface colonization during the association of A. brasilens e with Triticum aestivum (wheat) and Medicago sativa (alfalfa) seedlings. We track A. brasilense accumulation in the rhizosphere and on the root surfaces of wheat and alfalfa. A. brasilense motile cells display distinct chemotaxis behaviors in different regions of the roots, including attractant and repellent responses that ultimately drive surface colonization patterns. We also combine these observations with real-time analyses of behaviors of wild-type and mutant strains to link chemotaxis responses to distinct chemicals identified in root exudates to specific chemoreceptors that together explain the chemotactic response of motile cells in different regions of the roots. Furthermore, the bacterial second messenger c-di-GMP modulates these chemotaxis responses. Together, these findings illustrate dynamic bacterial chemotaxis responses to rhizosphere gradients that guide root surface colonization. IMPORTANCE Plant root exudates play critical roles in shaping rhizosphere microbial communities, and the ability of motile bacteria to respond to these gradients mediates competitive colonization of root surfaces. Root exudates are complex chemical mixtures that are spatially and temporally dynamic. Identifying the exact chemical(s) that mediates the recruitment of soil bacteria to specific regions of the roots is thus challenging. Here, we connect patterns of bacterial chemotaxis responses and sensing by chemoreceptors to chemicals found in root exudate gradients and identify key chemical signals that shape root surface colonization in different plants and regions of the roots., (Copyright © 2020 American Society for Microbiology.)

This study reports the introduction of egfp or mCherry markers to the Sp245, Sp7, and M40 wild-type strains of Azospirillum brasilense and the hhkB (encoding for a putative hybrid histidine kinase) minus mutant an isogenic strain of A. brasilense Sp245 to monitor colonization of wheat (Triticum aestivum). Two plasmids were constructed: (1) the pJMS-2 suicide plasmid derived from pSUP202 and harboring the mCherry gene expressed under the constitutive kanamycin resistance promoter to create a cis tag and (2) the broad-range plasmid pMP2449-5 that carries the mCherry gene under the lac promoter, which is derived from the plasmid pMP2444; to create the in trans tag. The stability of the plasmids encoding egfp and mCherry were confirmed in vitro for seven days of bacterial growth, and then, the A. brasilense strains harboring the plasmids were studied under nonselective conditions for adherence to seeds and, at seven or 14 days post-inoculation, for wheat root colonization. The utility of the labeled strains was proven by observation, using fluorescence microscopy and confocal laser scanning microscopy (CLSM) in wheat plants inoculated with the labeled strains and compared with the CFU g -1 for seed and wheat root. The method was suitable for observation of the in situ formation of mini-colonies, enabled visualization of bacterial colonization sites on large root fragments, and showed adherence to germinated seeds and root colonization of all strains by cell counts and direct microscopic examination. Thus, we are able to quantify the structures of the biofilms formed by each strain., (Copyright © 2018 Elsevier GmbH. All rights reserved.)

A plant growth-promoting bacteria, Azospirillum sp. B510, isolated from rice, can enhance growth and yield and induce disease resistance against various types of diseases in rice. Because little is known about the interaction between other plant species and this strain, we have investigated the effect of its colonization on disease resistance in tomato plants. Treatment with this strain by soil-drenching method established endophytic colonization in root tissues in tomato plant. The endophytic colonization with this strain-induced disease resistance in tomato plant against bacterial leaf spot caused by Pseudomonas syringae pv. tomato and gray mold caused by Botrytis cinerea. In Azospirillum-treated plants, neither the accumulation of SA nor the expression of defense-related genes was observed. These indicate that endophytic colonization with Azospirillum sp. B510 is able to activate the innate immune system also in tomato, which does not seem to be systemic acquired resistance.

Azospirillum is a plant growth-promoting rhizobacteria (PGPR) able to enhance the growth of wheat. The aim of this study was to test the effect of Azospirillum brasilense cell wall components on superoxide (O2·(-)) production in wheat roots and the effect of oxidative stress on A. brasilense viability. We found that inoculation with A. brasilense reduced O2·(-) levels by approx. 30 % in wheat roots. Inoculation of wheat with papain-treated A. brasilense, a Cys protease, notably increased O2·(-) production in all root tissues, as was observed by the nitro blue tetrazolium (NBT) reduction. However, a 24-h treatment with rhizobacteria lipopolysaccharides (50 and 100 μg/mL) alone did not affect the pattern of O2·(-) production. Analysis of the effect of plant cell wall components on A. brasilense oxidative enzyme activity showed no changes in catalase activity but a decrease in superoxide dismutase activity in response to polygalacturonic acid treatment. Furthermore, A. brasilense growth was only affected by high concentrations of H2O2 or paraquat, but not by sodium nitroprusside. Our results suggest that rhizobacterial cell wall components play an important role in controlling plant cell responses and developing tolerance of A. brasilense to oxidative stress produced by the plant.

Azospirillum-plant cooperation has been mainly studied from an agronomic point of view leading to a wide description of mechanisms implicated in plant growth-promoting effects. However, little is known about genetic determinants implicated in bacterial adaptation to the host plant during the transition from free-living to root-associated lifestyles. This study aims at characterizing global gene expression of Azospirillum lipoferum 4B following a 7-day-old interaction with two cultivars of Oryza sativa L. japonica (cv. Cigalon from which it was originally isolated, and cv. Nipponbare). The analysis was done on a whole genome expression array with RNA samples obtained from planktonic cells, sessile cells, and root-adhering cells. Root-associated Azospirillum cells grow in an active sessile-like state and gene expression is tightly adjusted to the host plant. Adaptation to rice seems to involve genes related to reactive oxygen species (ROS) detoxification and multidrug efflux, as well as complex regulatory networks. As revealed by the induction of genes encoding transposases, interaction with root may drive bacterial genome rearrangements. Several genes related to ABC transporters and ROS detoxification display cultivar-specific expression profiles, suggesting host specific adaptation and raising the question of A. lipoferum 4B/rice cv. Cigalon co-adaptation., (© 2013 Federation of European Microbiological Societies. Published by John Wiley & Sons Ltd. All rights reserved.)

Root research often uses rhizotron systems where their main characteristic is to possess a transparent wall allowing the in situ observation of plants' root system. Rhizotrons can vary in size and be filled with different substrates based on soil, sand and other components. Here, a simple and low cost rhizotron system without soil substrate was developed, allowing the observation of wheat (Triticum aestivum L.) root system treated with bioproducts. To show the effectiveness of the rhizotrons, hydrated wheat seeds were treated with plant growthpromoting bioproducts such as Azospirillum argentinense REC3 (106 CFU.ml-1), flagellin AzFlap (200 nM), or strawberry hydroalcoholic extract (0.01 mg.mL-1). This rhizotron system allowed to non-destructively evaluate the roots (length, density and root hair proliferation), where the values of the bioproducts surpassed the control. [ABSTRACT FROM AUTHOR]

The Azospirillum genus includes plant growth-promoting bacteria found in different soil regions worldwide. When associated with plant roots these bacteria help augment productivity by increasing both the aerial and root portions. These benefits derive from excretion of growth phytonutrients, especially auxins. Use of Azospirillum spp. can help to bridge the gap between productivity and sustainability since inoculants based on this microorganism can reduce use of nitrogen fertilizers without affecting productivity, and generate savings and greater profitability. Inoculation of Azospirillum spp. strains with forage grasses can result in greater forage mass gains and less need for nitrogen fertilizer, improving pasture production system sustainability. Co-inoculation with other strains such as Bradyrhizobium sp. apparently potentiates growth promotion. Proper application methods must be followed for these growth-promoting bacteria to be effective. Growth promotion in response to Azospirillum has been described in grasses such as sugarcane, maize and forages, but further research is needed under different conditions to support adoption by producers. Application of bacterial inoculates can increase competitiveness vis-'-vis conventional agriculture methods. Inoculation of grasses in livestock systems can raise forage mass production, mitigate degradation risks and improve productive indices. [ABSTRACT FROM AUTHOR]

Azospirillum brasilense, which has the potential to stimulate plant growth, belongs to the group of plant-growth-promoting rhizobacteria. The surface lectins of A. brasilense strains Sp7 and Sp245 can bind specific carbohydrates and ensure adhesion of the bacteria to the root surface. We investigated possible effects of the Sp7 and Sp245 lectins on the activities of proteases and their inhibitors in wheat seedling roots. Spectrophotometry was used to determine the protease and trypsin inhibitor activities in the cells of wheat-root seedlings. The Sp7 lectin decreased the activities of acidic (pH 3.5), neutral (pH 6.8), and alkaline (pH 7.8) proteases, whereas the Sp245 lectin activated all enzymes but acidic proteases, whose activity remained unchanged. Both lectins changed the ratio between the activities of the three types of proteases in germinating seeds. The trypsin-inhibiting activity in wheat seedling roots increased in the presence of either lectin. The results suggest that the range of effects of Azospirillum lectins on host plant metabolism is wider then previously thought. Together with the already existing evidence, they allow correction of the current views about the mechanisms that govern the interaction between the plant and the bacterium involved in an associative relationship. [ABSTRACT FROM AUTHOR]

Azospirillum-plant cooperation has been mainly studied from an agronomic point of view leading to a wide description of mechanisms implicated in plant growth-promoting effects. However, little is known about genetic determinants implicated in bacterial adaptation to the host plant during the transition from free-living to root-associated lifestyles. This study aims at characterizing global gene expression of Azospirillum lipoferum 4 B following a 7-day-old interaction with two cultivars of Oryza sativa L. japonica (cv. Cigalon from which it was originally isolated, and cv. Nipponbare). The analysis was done on a whole genome expression array with RNA samples obtained from planktonic cells, sessile cells, and root-adhering cells. Root-associated Azospirillum cells grow in an active sessile-like state and gene expression is tightly adjusted to the host plant. Adaptation to rice seems to involve genes related to reactive oxygen species ( ROS) detoxification and multidrug efflux, as well as complex regulatory networks. As revealed by the induction of genes encoding transposases, interaction with root may drive bacterial genome rearrangements. Several genes related to ABC transporters and ROS detoxification display cultivar-specific expression profiles, suggesting host specific adaptation and raising the question of A. lipoferum 4 B/rice cv. Cigalon co-adaptation. [ABSTRACT FROM AUTHOR]

Bacteria of the genus Azospirillum are well known as plant growth promoting rhizobacteria. The ability to synthesize phytohormones is considered one of the most important mechanisms to promote plant growth and is widely distributed among plantassociated rhizobacteria. The most important phytohormone produced by Azospirillum is the auxin indole-3-acetic acid, with the L-tryptophan as the precursor. In the present study, we evaluate the capacity of eight Azospirillum strains isolated from rice and wheat, to produce in vitro auxins using plant exudates. Our results show that isolates produced auxins in tryptophan free media, but, generally, the amount produced increased when the tryptophan concentration increased. Some plants root exudates had a similar effect to tryptophan for the auxin production. In this sense, bean, rice and canola root extracts produced, respectively, 93.3%, 96.2%, and 88.31% more auxin than L-tryptophan. Azospirillum sp. isolate A3 had the maximal capacity to produce auxin. Therefore, the effect of cell free supernatant was studied on rice root development. Statistical analysis did not show any significant difference between root number and dry weight of the treated and control seedlings. However, significant differences were observed in root length and wet weight at α=0.01 and α=0.05, respectively. [ABSTRACT FROM AUTHOR]

Background and aims: Many plant-beneficial microorganisms can influence secondary plant metabolism, but whether these effects add up when plants are co-inoculated is unclear. This issue was assessed, under field conditions, by comparing the early impacts of seed inoculation on secondary metabolite profiles of maize at current or reduced mineral fertilization levels. Methods: Maize seeds were inoculated singly with selected strains from bacterial genera Pseudomonas and Azospirillum or mycorrhizal genus Glomus, or with these strains combined two by two or all three together. At 16 days, maize root methanolic extracts were analyzed by RP-HPLC and secondary metabolites (phenolics, flavonoids, xanthones, benzoxazionoids, etc.) identified by LC/MS. Results: Inoculation did not impact on plant biomass but resulted in enhanced total root surface, total root volume and/or root number in certain inoculated treatments, at reduced fertilization. Inoculation led to qualitative and quantitative modifications of root secondary metabolites, particularly benzoxazinoids and diethylphthalate. These modifications depended on fertilization level and microorganism(s) inoculated. The three selected strains gave distinct results when used alone, but unexpectedly all microbial consortia gave somewhat similar results. Conclusions: The early effects on maize secondary metabolism were not additive, as combining strains gave effects similar to those of Glomus alone. This is the first study demonstrating and analyzing inoculation effects on crop secondary metabolites in the field. [ABSTRACT FROM AUTHOR]

Abstract: Azospirillum brasilense SR55, isolated from the rhizosphere of Triticum durum, was classified as serogroup II on the basis of serological tests. Such serogroup affiliation is uncharacteristic of wheat-associated Azospirillum species. The lipid A of A. brasilense SR55 lipopolysaccharide contained 3-hydroxytetradecanoic, 3-hydroxyhexadecanoic, hexadecanoic and octadecenoic fatty acids. The structure of the lipopolysaccharide''s O polysaccharide was established, with the branched octasaccharide repeating unit being represented by l-rhamnose, l-3-O-Me-rhamnose, d-galactose and d-glucuronic acid. The SR55 lipopolysaccharide induced deformations of wheat root hairs. The lipopolysaccharide was not involved in bacterial cell aggregation, but its use to pretreat wheat roots was conducive to cell adsorption. This study shows that Azospirillum bacteria can utilise their own lipopolysaccharide as a carbon source, which may give them an advantage in competitive natural environments. [Copyright &y& Elsevier]

Free-living soil bacteria beneficial to plant growth are usually referred to as plant growth promoting rhizobacteria (PGPR), capable of promoting plant growth by colonizing the plant root. Application of PGPR to increase the yield is limited by variability among the results obtained in the laboratory, in greenhouse and field. Rhizobacteria that promote plant growth (PGPR) participate in interactions with plants (rice, wheat, maize, sugarcane, sugar beet, cotton) and significantly increase their vegetative growth and yield. Apart from Azotobacter and Azospirillum, PGPR also include Acetobacter, Azoarcus and several species of Enterobacteriaceae (Klebsiella, Enterobacter, Citrobacter, and Pseudomonas). PGPR represent an alternative to plant growth enhancement chemicals. [ABSTRACT FROM AUTHOR]

Aims: To assess the applicability of sequence characterized amplified region (SCAR) markers obtained from BOX, ERIC and RAPD fragments to design primers for real-time PCR quantification of the phytostimulatory maize inoculants Azospirillum brasilense UAP-154 and CFN-535 in the rhizosphere. Methods and Results: Primers were designed based on strain-specific SCAR markers and were screened for successful amplification of target strain and absence of cross-reaction with other Azospirillum strains. The specificity of primers thus selected was verified under real-time PCR conditions using genomic DNA from strain collection and DNA from rhizosphere samples. The detection limit was 60 fg DNA with pure cultures and 4 × 103 (for UAP-154) and 4 × 104 CFU g−1 (for CFN-535) in the maize rhizosphere. Inoculant quantification was effective from 104 to 108 CFU g−1 soil. Conclusion: BOX-based SCAR markers were useful to find primers for strain-specific real-time PCR quantification of each A. brasilense inoculant in the maize rhizosphere. Significance and Impact of the Study: Effective root colonization is a prerequisite for successful Azospirillum phytostimulation, but cultivation-independent monitoring methods were lacking. The real-time PCR methods developed here will help understand the effect of environmental conditions on root colonization and phytostimulation by A. brasilense UAP-154 and CFN-535. [ABSTRACT FROM AUTHOR]

Lectins were extracted from the surface of nitrogen-fixing soil bacteria Azospirillum brasilense Sp7 and from its mutant A. brasilense Sp7.2.3 defective in lectin activity. The ability of lectins to stimulate the rapid formation of hydrogen peroxide related to increase of oxalate oxidase and peroxidase activity in the roots of wheat seedlings has been demonstrated. The most rapid induced pathway of hydrogen peroxide formation in the roots of wheat seedlings was the oxalic acid oxidation by oxalate oxidase which is the effect of lectin in under 10 min in a concentration of 10 μg/ml. The obtained results show that lectins from Azospirillum are capable of inducing the adaptation processes in the roots of wheat seedlings. [ABSTRACT FROM AUTHOR]

Abstract   Azospirillum brasilense (strains REC3, RLC1, PEC5) were root inoculated in strawberry plants of the cultivars ‘Milsei’, ‘Selva’ and ‘Camarosa’ to assess plant growth-promoting effects. The bacteria were able to promote plant growth (expressed as root length, root area, and dry weight of root and shoot), depending on the genotypes of plants and bacteria used, whereas the stolon production (3–4) depended only on the strawberry cultivar. To explain whether root exudates plays any role on the growth-promotion observed herein, total protein and sugar were determined, and chemotaxis properties were evaluated. The strains showed positive chemotaxis toward the root exudates, being influenced by the total sugars content, suggesting that the latter plays an important role in the chemotaxis effect and may contribute to enhance the root capacity to recruit azospirilla from rhizosphere, thus improving the growth-promoting effect exerted by these bacteria. [ABSTRACT FROM AUTHOR]

• The decline of take-all disease ( Gaeumannomyces graminis var. tritici), which may take place during wheat monocropping, involves plant-protecting, root-colonizing microorganisms. So far, however, most work has focused on antagonistic fluorescent pseudomonads. Our objective was to assess the changes in rhizobacterial community composition during take-all decline of field-grown wheat. • The study was based on the development and utilization of a taxonomic 16S rRNA-based microarray of 575 probes, coupled with cloning–sequencing and quantitative PCR. Plots from one experimental field grown with wheat for 1 yr (low level of disease), 5 yr (high level of disease) or 10 yr (low level of disease, suppressiveness reached) were used. • Microarray data discriminated between the three stages. The outbreak stage (5 yr) was mainly characterized by the prevalence of Proteobacteria, notably Pseudomonas ( Gammaproteobacteria), Nitrosospira ( Betaproteobacteria), Rhizobacteriaceae, Sphingomonadaceae, Phyllobacteriaceae ( Alphaproteobacteria), as well as Bacteroidetes and Verrucomicrobia. By contrast, suppressiveness (10 yr) correlated with the prevalence of a broader range of taxa, which belonged mainly to Acidobacteria, Planctomycetes, Nitrospira, Chloroflexi, Alphaproteobacteria (notably Azospirillum) and Firmicutes (notably Thermoanaerobacter). • In conclusion, take-all decline correlated with multiple changes in rhizobacterial community composition, far beyond the sole case of pseudomonads. [ABSTRACT FROM AUTHOR]

Bacteria of the genus Azospirillum are free-living nitrogen-fixing, rhizobacteria that are found in close association with plant roots, where they exert beneficial effects on plant growth and yield in many crops of agronomic importance. Unlike other bacteria, little is known about the genetics and biochemistry of exopolysaccharides in Azospirillum brasilense. In an attempt to characterize genes associated with exopolysaccharides production, we generated an A. brasilense Cd Tn 5 mutant that showed exopolysaccharides overproduction, decreased tolerance to saline conditions, altered cell morphology, and increased sensitivity to detergents. Genetic characterization showed that the Tn 5 was inserted within a ddlB gene encoding for ad-alanine–d-alanine ligase, and located upstream of the ftsQAZ gene cluster responsible for cell division in different bacteria. Heterologous complementation of the ddlB Tn 5 mutant restored the exopolysaccharides production to wild-type levels and the ability to grow in the presence of detergents, but not the morphology and growth characteristics of the wild-type bacteria, suggesting a polar effect of Tn 5 on the fts genes. This result and the construction of a nonpolar ddlB mutant provide solid evidence of the presence of transcriptional coupling between a gene associated with peptidoglycan biosynthesis and the fts genes required to control cell division. [ABSTRACT FROM AUTHOR]

Abstract: Inoculation with Azospirillum brasilense Sp245 exerts beneficial effects on micropropagated plants of Prunus cerasifera L. clone Mr.S 2/5, as seen in the results of a comparative analysis of inoculated and non-inoculated explants, during both the rooting and acclimatation phases. The presence of Azospirillum brasilense Sp245 increased root system, root hair biomass production and apical activity. Although the presence of the bacteria had a positive effect on rooting, the addition of indolebutyric acid (IBA) to Murashige and Skoog (MS) medium was seen as indispensable in order to promote the rooting of explants. Aside from the promotion of plant growth, A. brasilense Sp245 protects plants against pathogen attacks, such as Rhizoctonia spp., with a plant survival rate of nearly 100% vs. 0% as seen in the negative control. The biocontrol effect of A. brasilense Sp245 on the fungal rhizospheric community has been confirmed by denaturing gradient gel electrophoresis (DGGE) profiles of the rhizospheric microbial community. This study indicates that A. brasilense Sp245 could be employed as a tool in plant biotechnology. [Copyright &y& Elsevier]

Plant root development can be largely affected through the association of roots with plant growth-promoting rhizobacteria (PGPR). However, little is known about the identity of plant genes enabling such PGPR-plant root associations. Differences in the responsiveness to PGPR among cultivars suggest genetic variation for this trait within germplasm. In this study, two genotypes of common bean ( Phaseolus vulgaris L.), BAT477 and DOR364, were identified showing contrasting responsiveness in root development to inoculation with the PGPR Azospirillum brasilense Sp245. Inoculation with an A. brasilense Sp245 mutant strain strongly reduced in auxin biosynthesis or addition of increasing concentrations of exogenous auxin to the plant growth medium, indicated that the differential response to A. brasilense Sp245 among the bean genotypes is related to a differential response to the bacterial produced auxin. To further assess the role of the plant host in root responsiveness, a population of Recombinant Inbred Lines (RILs) of the DOR364×BAT477 cross was used to evaluate the efficacy of exogenous auxin on root development. We detected significant phenotypic variation among the RILs for basal root formation during germination upon addition of auxin to the growth medium. Genetic analysis revealed two quantitative trait loci (QTLs) associated with basal root responsiveness to auxin of which one explained 36% of the phenotypic variation among the RILs. This latter QTL mapped to the same location as a QTL for root tip formation at low P, suggesting that the host effect on root responsiveness to IAA interacts with specific root development. Also, significant correlations between basal root responsiveness to auxin and growth, root tips and root dry weight at low P were identified. To our knowledge, this is the first report on QTL detection for root responsiveness to auxin. [ABSTRACT FROM AUTHOR]

Aims: To evaluate the effect of plant variety and Azospirillum brasilense inoculation on the microbial communities colonizing roots and leaves of tomato ( Lycopersicon esculentum Mill.) plants. Methods and Results: Seeds of cherry and fresh-market tomato were inoculated with A. brasilense BNM65. Sixty days after planting, plants were harvested and the microbial communities of the rhizoplane and phyllosphere were analysed by community-level physiological profiles (CLPP) using BIOLOG® EcoPlates and denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Differences on the rhizoplane and phyllosphere bacterial communities between the two tomato types were detected by principal component analysis of the CLPP; DGGE fingerprints also showed differences at the phyllosphere level. Fresh-market tomato had a more complex phyllosphere bacterial community than cherry tomato, as determined by DGGE profiles. Physiological and genetic changes on phyllosphere and rhizoplane bacterial communities by Azospirillum seed inoculation were evident only on cherry tomato. Conclusions: Tomato genotype affects the response of native bacterial communities associated with the roots and leaves to A. brasilense seed inoculation. Significance and Impact of the study: The successful implementation of Azospirillum inoculation requires not only the consideration of the interactions between A. brasilense strains and plant genotypes, but also the plant-associated microflora. [ABSTRACT FROM AUTHOR]

The effect of cellulase and pectinase on bacterial colonization of wheat was studied by three different experiments. In the first experiment, the root colonization of 3 wheat cultivars (Ghods, Roshan and Omid) by two A. brasilense strains (Sp7 and Dol) was compared using pre-treated roots with cellulase and pectinase, and non-treated with these enzymes (control). Although the root colonization varied greatly among strain-plant combinations in controls, the pre-treatment of roots with polysaccharide degrading enzymes significantly increased the bacterial count in roots, regardless of the strain-plant combination. This might be an indication that cell wall may act as an important factor in plant- Azospirillum interaction. In the second experiment, the root cellulase activity of the same wheat cultivars treated with and without the two Azospirillum brasilense, strains (Sp7 and Dol) was compared. The pre-treatment of wheat roots with Azospirillum enhanced the cellulase activity of wheat root extracts. Thus, the cellulase activity might participate in the initial colonization of wheat roots by Azospirillum. The comparison of the cellulase activity of root extracts within inoculated and non-inoculated seedlings showed that the inoculation had enhanced the cellulase activity in root extracts, but this effect was directly dependent on the strain-plant combination. Strain Sp7 stimulated the highest cellulase activity in cv. Roshan, but strain Dol induced the highest enzyme activity in cv. Ghods. In the third experiment, several growth parameters of those 3 wheat cultivars treated with and without those two bacterial strains (Sp7 and Dol) were compared. The highest magnitude of growth responses caused by Sp7 strain was in the cv Roshan, but Dol strain stimulated the highest growth in cv Ghods. Therefore, effective colonization may contribute to more growth responses. [ABSTRACT FROM AUTHOR]

The transformation of sodium arsenite and sodium arsenate by the rhizospheric nitrogen-fixing bacterium Azospirillum brasilense Sp245 in association with wheat ( Triticum aestivum L. ‘Saratovskaya 29’) was studied. The effect produced by the A. brasilense strain on the morphological parameters of wheat in an As-polluted environment was examined. The plants were cultivated in a hydroponic system, with glass beads serving as a support for root growth. The plant-growth medium (an artificial soil solution) was deficient in P and Fe. The total initial As concentrations used were 75, 750, and 7500 µg l−1. The As compounds used contained sodium arsenate and sodium arsenite at an As(V):As(III) ratio of 1:3.6 (in terms of As) in all experiments. Inoculation of A. brasilense Sp245 led to a decrease in the overall root length and to the formation of lateral roots; both effects are possibly related to the bacteria’s ability to synthesize auxins. Inoculation also changed the As(V): As(III) ratio of the plant-growth medium. In all experiments, the concentration of As(V) in the nutrient medium increased relative to the initial one and was approximately 1.5-fold higher than that in the medium of uninoculated plants. This value slightly decreased (1.6 > 1.5 > 1.4) with increasing concentration of As in the medium. Azospirillum-inoculated plants accumulated less As than did the surface-sterilized uninoculated plants. This study shows that A. brasilense Sp245 in association with wheat changes the speciation, bioavailability, and plant uptake of As. [ABSTRACT FROM AUTHOR]

This work studied the effect of two cell-surface lectins isolated from the nitrogen-fixing soil bacterium Azospirillum brasilense Sp7 and from its mutant defective in hemagglutinating activity, A. brasilense Sp7.2.3, on the activities of α-glucosidase, β-glucosidase and β-galactosidase in the exocomponent, membrane and apoplast fractions of wheat-seedling roots. Lectin (40 μg mL−1) incubation for 1 h of the plant fractions increased the enzymes’ activities; both wild-type and mutant lectins were most stimulatory to the activities of all the exocomponent-fraction enzymes studied and to the apoplast-fraction β-glucosidase. Pretreatment of the lectins with their carbohydrate hapten, L-fucose, lowered the effect. The observed differences in the lectins’ ability to influence enzyme catalytic activity are explained by change in the antigenic properties of the mutant lectin. [ABSTRACT FROM AUTHOR]

Bacteria with the ability to grow on nitrogen-free media and with nitrogenase activity under aerobic or microaerobic conditions were isolated from sugarcane roots collected from four different agricultural locations in Granada (Spain). Isolates were Gram negative rods and were identified asAzotobacter chroococcumandAzospirillum brasilense. Our results suggest thatAzotobacterisolates do not have a particular affinity for sugarcane rhizospheres and that, on the contrary,Azospirillumisolates show specific association and perhaps endophytic colonization of sugarcane. However, obligate endophytes (Gluconacetobacter diazotrophicus) were not found in the apoplastic fluid of the stems and macerates extracts of sugarcane tissues with the procedure applied. Population of this microorganism might be in low number in the Spanish sugarcane varieties studied which is also discussed. [ABSTRACT FROM AUTHOR]

The plant growth-promoting soil bacterium Azospirillum brasilense enhances growth of economically important crops, such as wheat, corn and rice. In order to improve plant growth, a close bacterial association with the plant roots is needed. Genes encoded on a 90-MDa plasmid, denoted pRhico plasmid, present in A. brasilense Sp7, play an important role in plant root interaction. Sequencing, annotation and in silico analysis of this 90-MDa plasmid revealed the presence of a large collection of genes encoding enzymes involved in surface polysaccharide biosynthesis. Analysis of the 90-MDa plasmid genome provided evidence for its essential role in the viability of the bacterial cell. [Copyright &y& Elsevier]

The agronomic impact of genetically tagged azospirilla (Azospirillum brasilense) was assessed in open field and their fluctuation were monitored in the soil/rhizosphere. Strain performance, upon inoculation of sorghum, was evaluated over a two-years period; agronomic treatments included nitrogen application (0, 80, 160 kg ha-1), and types of inoculant (Sp245 lacZ, Sp6 gusA, Sp6 IAA++gusA). Grain yield was higher for inoculated seed plots than in non-inoculated ones, whereas nitrogen content, biomass of plant residues and nitrogen in plant residues gave values that were not statistically different. Root length density (RLD) of sorghum at the end of the stem elongation stage was affected only by the indole-3-acetic acid (IAA) overproducer Azospirillum strain (A. brasilense Sp6 IAA++gusA) with respect to the normal IAA producer (A. brasilense Sp6 gusA), being higher in the first 40 cm of depth, notwithstanding the level of nitrogen fertilization. The traceability of the released genetically modified strains enabled to monitor their ability to colonise soil and roots. Moreover, the genetic modification per se vs. the non-modified counterpart, did not affect the culturable aerobic population in soil, microfungi, streptomycetes, fluorescent pseudomonads, soil microbial biomass, or some microbial activities, all selected as important indicators. [ABSTRACT FROM AUTHOR]

Investigates the effect of compatible and incompatible Azospirillum brasilense strains on proton efflux of intact wheat roots. Root elongation; Proton extrusion; Strain-plant combination.

The surface agglutinating proteins of the soil nitrogen-fixing bacteria Bacillus polymyxa1460 and Rhizobium leguminosarum252 were found to be able to interact with the polysaccharide complexes of certain azospirilla and rhizobia. Such interactions are most likely involved in the formation of nitrogen-fixing plant-bacterial associations in the rhizosphere. [ABSTRACT FROM AUTHOR]

Azospirillum brasilense cells deprived of capsular exopolysaccharides completely lost their ability to bind wheat germ agglutinin (WGA) and much of their ability to attach to wheat seedling roots. The decapsulation of bacterial cells by washing them with a NaCl solution led to an increase in the relative hydrophobicity of the cell surface. The pretreatment of wheat seedling roots with N-acetyl-D-glucosamine (GlcNAc) or the GlcNAc-containing polysaccharide complexes stripped from Azospirillum cells reduced their attachment to the roots. Under the experimental conditions used (3-h incubation of wheat seedling roots with exponential-phase azospirilla), bacterial adsorption is mainly driven by the specific mechanisms attachment of the cells to the roots, whose operation is due to the capsular polysaccharide components and the WGA present on the wheat seedling roots. [ABSTRACT FROM AUTHOR]