Rhizospheric Strains Research Articles

Stimulation, purification, and chemical characterization of siderophores produced by the rhizospheric bacterial strain Pseudomonas putida

Most of the microorganisms generate siderophores in iron deficient condition. Siderophores are low-molecular-weight chelating agents produced by microorganisms and higher plants to encourage take-up of iron. It is reported that heavy metals in the milieu influence siderophores biosynthesis by microorganisms. In this study, we evaluated the effect of zinc ions, in the case of iron deficiency, on the siderophores generation by an array of rhizobacteria. Sixteen strains were specifically isolated from the rhizosphere of ginseng plant (Panax schinseng). The siderophores-producing rhizobacteria were investigated using chrome azurol S (CAS)-liquid media provided with various concentrations of ZnSO4 of (50–250µgmL−1). The siderophore-producing strain Pseudomonas KNUK9 indicated high identity (99% using 16S-rRNA sequencing) to P. putida and showed potent antagonistic effect in vitro against the phytopathogenic fungal strain Aspergillus niger. The Sephadex LH-20 purified siderophore of Pseudomonas KNUK9 strain was chemically investigated using UV, FTIR, and MALDI-TOF-MS spectra. Results demonstrated that the pyoverdine is the iron-binding compound produced by Pseudomonas KNUK9 strain, and it's greatly influenced by the presence of Zn2+ ions in the medium. 100µgmL−1 Zn2+ ions concentration improved the siderophores production in all siderophores-producing bacterial strains.

Synergetic effect of rhamnolipid from Pseudomonas aeruginosa C1501 and phytotoxic metabolite from Lasiodiplodia pseudotheobromae C1136 on Amaranthus hybridus L. and Echinochloa crus-galli weeds

Rhamnolipid (Rh) is a biosurfactant produced by the bacterial Pseudomonas aeruginosa. This present study investigates rhizospheric strain C1501 of P. aeruginosa with an accession number KF976394 with the best production of rhamnolipid: a biosurfactant. The partially purified rhamnolipid from strain C1501 and Tween 80 was tested on mycelial growth of wild strain C1136. The enzyme activities involved in biodegradation, as well as necrosis induction on the tested weeds, were performed using scanning electron microscopy. It was observed that the different concentrations of rhamnolipid tested enhanced the dry mycelia weight yield of Lasiodiplodia pseudotheobromae which has been established to be producing a phytotoxic metabolite for killing weeds. It was observed that strain C1136 had a high level of cellulase and xylanase enzyme activities during this study. The scanning electron microscopy showed that the mutant strain of C1136 combined with 0.003% v/v of rhamnolipid enhances biodegradability and a high level of necrosis on the tested weeds compared with that on the untreated weeds. The highest CMCase activities and xylanase activities were obtained on the fourth day from the phytotoxic metabolite produced from the mutant strain of L. pseudotheobromae when combined with 0.003% v/v of rhamnolipid. This study has shown that rhamnolipid can serve as an adjuvant in order to enhance the penetrability of bioherbicide active ingredient for controlling weeds.

Evaluation of indigenous rhizobacterial strains with reduced dose of chemical fertilizer towards growth and yield of mustard ( Brassica campestris ) under old alluvial soil zone of West Bengal, India

Abstract A field experiment had been carried out in the Crop Research and Seed Multiplication Farm of The University of Burdwan, West Bengal, India during the two consecutive winter seasons of 2011-2012 and 2012-2013 to study the effect of indigenous rhizospheric bacterial strains on growth, physiology and yield of mustard variety. Pseudomonas putida, Burkholderia cepacia, Burkholderia sp. and their mixture were used as seed inoculants for mustard cultivation. The experiment was laid down in a randomized complete block design (RCBD) with three replications. Results revealed that indigenous inoculation (with reduced dose of chemical fertilizer) significantly increased (p

Application of Plant Growth Promoting Rhizobacteria in Promising Agriculture: An Appraisal

Application of Plant Growth Promoting Rhizobacteria in Promising Agriculture: An Appraisal Gradually agriculture is facing struggle to meet the various threaten of producing more food for an increasing world population food security. Large quantities of synthetic fertilizers and pesticides are necessitated for high productivity which can smash up bionetwork structures and functions, including the soil microbial community which plays imperative function in agriculture sustainability. Soil is an excellent niche of growth of many bacteria in rhizosphere. Many rhizospheric bacterial strains possess plant growth-promoting mechanisms. These bacteria can be applied as biofertilizers in agriculture and forestry, enhancing crop yields. Plant growthpromoting rhizobacteria can improve plant growth through several different mechanisms: (a) the synthesis of plant nutrients or phytohormones (Indole acetic acid and cytokinin), which can be uptake by plants, (b) the mobilization of soil compounds like phosphorous and metals, making them available for the plant to be used as nutrients, (c) the protection of plants under stressful conditions, thereby counteracting the negative impacts of stress, or (d) defense against phytopathogens, reducing plant diseases or death by producing antibiotics and HCN. Several plant growth-promoting rhizobacteria have been used worldwide as biofertilizers, contributing to increasing crop yields and soil fertility and hence having the potential to contribute to more sustainable agriculture and forestry. Scientific researchers involve various moves towards for identification of plant growthpromoting rhizobacteria adaptation, upshot on physiological and growth aspects on plants and induced systemic resistance, biocontrol and biofertilization in agriculture.

Compatible salt-tolerant rhizosphere microbe-mediated induction of phenylpropanoid cascade and induced systemic responses against Bipolaris sorokiniana (Sacc.) Shoemaker causing spot blotch disease in wheat (Triticum aestivum L.)

Cell wall is one of the first lines of defence used by plants to restrict invading fungal pathogens. Lignin is a complex polymer of hydroxylated and methoxylated phenylpropane units (monolignols). Cell wall lignification can establish mechanical barriers to pathogen invasion and renders the cell wall more resistant to pathogen attack. Compatible salt-tolerant rhizosphere microbe-mediated induction of phenylpropanoid cascade and induced systemic responses against Bipolaris sorokiniana (Sacc.) Shoemaker causing spot blotch disease in wheat (Triticum aestivum L.) is demonstrated and the details are being shared through this paper. Twelve rhizospheric microbial strains were tested against Bipolaris sorokiniana under in vitro condition on dual plate. Bacillus amyloliquefaciens B-16 and Trichoderma harzianum UBSTH-501 showed maximum inhibition of mycelial growth of B. sorokiniana and was used in further in planta assay. These selected antagonists were tested alone and in combination for induction of phenylpropanoid cascade in wheat infected with B. sorokiniana. Results showed that plants co-inoculated with B. amyloliquefaciens B-16 and T. harzianum UBSTH-501 up-regulated the phenylpropanoid cascade and manifold increase was recorded in phenylalanine ammonia lyase (PAL), peroxidase, chitinase and other enzymes related to induced systemic resistance. Results also showed that significantly higher amounts of phenolic acids viz. gallic acid, ferulic acid were accumulated in the plant leaves co-inoculated with B. amyloliquefaciens B-16 and T. harzianum UBSTH-501 as compared to individually inoculated and uninoculated control plants. Histopathological studies showed significantly higher cell wall lignification in plant leaves co-inoculated with B. amyloliquefaciens B-16 and T. harzianum UBSTH-501 than the plants under control. These results illustrate that microbe-mediated up-regulation of phenylpropanoid biosynthesis pathway is of critical importance for host defence against spot blotch pathogen invasion in wheat.

Cellulosimicrobium funkei-like enhances the growth of Phaseolus vulgaris by modulating oxidative damage under Chromium(VI) toxicity

Contamination of agriculture land by heavy metals is a worldwide risk that has sped up noticeably since the beginning of the industrial revolution. Hence, there arise the demands of heavy metal tolerant plant growth promoting bacterial strains for specific metal contaminated agricultural sites restoration. In this study, 36 bacterial isolates were screened out from the rhizospheric soil of Phaseolus vulgaris. Among these, two bacterial strains AR6 and AR8 were selected based on their higher Cr(VI) tolerance (1200 and 1100μg/mL, respectively) and the maximum production of plant growth promoting substances. In the molecular characterization study, both the bacterial strains showed 99% homology with Cellulosimicrobium funkei KM032184. In greenhouse experiments, the exposure of Cr(VI) to P.vulgaris inhibited the growth and photosynthetic pigments and increased the enzymatic and non-enzymatic antioxidant expressions. However, rhizosphere bacterial inoculations alleviated the negative effect of Cr(VI) and enhanced the seed germination rate (89.54%), shoot (74.50%),root length (60%), total biomass (52.53%), chlorophyll a (15.91%), chlorophyll b (17.97%), total chlorophyll (16.58%) and carotenoid content (3.59%). Moreover, bacterial inoculations stabilized and modulated the antioxidant system of P. vulgaris by reducing the accumulation of Cr in plant tissues. The present finding shows the Cr(VI) tolerance and plant growth promoting properties of the rhizosphere bacterial strains which might make them eligible as biofertilizer of metal-contaminated soils.

Enhanced Cd extraction of oilseed rape (Brassica napus) by plant growth-promoting bacteria isolated from Cd hyperaccumulator Sedum alfredii Hance

ABSTRACTFour plant growth-promoting bacteria (PGPB) were used as study materials, among them two heavy metal-tolerant rhizosphere strains SrN1 (Arthrobacter sp.) and SrN9 (Bacillus altitudinis) were isolated from rhizosphere soil, while two endophytic strains SaN1 (Bacillus megaterium) and SaMR12 (Sphingomonas) were identified from roots of the cadmium (Cd)/zinc (Zn) hyperaccumulator Sedum alfredii Hance. A pot experiment was carried out to investigate the effects of these PGPB on plant growth and Cd accumulation of oilseed rape (Brassica napus) plants grown on aged Cd-spiked soil. The results showed that the four PGPB significantly boosted oilseed rape shoot biomass production, improved soil and plant analyzer development (SPAD) value, enhanced Cd uptake of plant and Cd translocation to the leaves. By fluorescent in situ hybridization (FISH) and green fluorescent protein (GFP), we demonstrated the studied S. alfredii endophytic bacterium SaMR12 were able to colonize successfully in the B. napus roots. However, all four PGPB could increase seed Cd accumulation. Due to its potential to enhance Cd uptake by the plant and to restrict Cd accumulation in the seeds, SaMR12 was selected as the most promising microbial partner of B. napus when setting up a plant–microbe fortified remediation system.

Efficacy of some rhizospheric and endophytic bacteria in vitro and as seed coating for the control of Fusarium culmorum infecting durum wheat in Tunisia

Sixty two rhizospheric and endophytic bacterial strains were evaluated for their biocontrol effect on two aggressive Fusarium culmorum isolates (Fc2 and Fc3). We observed that 35 % and 23 % of the tested strains inhibited the in vitro growth of Fc2 and Fc3 respectively. The observed antagonism was due to inhibition by contact (13–19 % of the strains) or at distance (10–16 % of the strains) for both fungal isolates. Some of the antagonistic bacteria showed the ability to produce diffuse and/or volatile compounds that inhibit the growth, the sporulation and macroconidia germination of F. culmorum. None of the tested antagonistic bacteria showed chitinase activity on synthetic medium. The sequencing of the 16S rDNA genes of some antagonistic bacteria showed that they belong to the genera Bacillus, Pseudomonas and Microbacterium. The double inoculation of durum wheat seeds by the antagonistic bacterial strains (B13, B18, BSE1, BSE3 and B16E) and the two F. culmorum isolates showed that germination and seedling vigor were generally improved in vitro. The percentage of infected seeds was also reduced. In greenhouse trials, the biocontrol effectiveness of F. culmorum was dependant from the virulence of the fungal strain and the specificity of the antagonistic interaction between bacterial and fungal strains. The bacterial strains B18 and B16E reduced F. culmorum infection on durum wheat plants probably due to their antagonistic and plant growth promoting activities and they may be used in a mixture as seed biopriming inoculum for plant growth bio-promoting and Fusarium wheat diseases biocontrol.

Osmotolerant Cytokinin Producing Microbes Enhance Tomato Growth in Deficit Irrigation Conditions

The authors have reported the evaluation of two osmotolerant rhizospheric microbial strains for their abilities to promote tomato growth and alleviate deficit irrigation stress effects at 100, 50 and 25 % water holding capacity (WHC) of a red sandy loam soil in pot culture conditions. It was observed that osmotolerant microbial inoculation significantly enhanced the stomatal conductivity, rates of transpiration, photosynthesis and the relative water contents of tomato plants across stress levels. A similar trend was also observed with respect to the biomass production and marketable fruit yields at harvest. Plants inoculated with Actinobacterium, Citrococcus zhacaiensis recorded 24 and 9 % higher yields, while Bacillus amyloliquefaciens inoculated plants recorded 42.0 and 12.7 % higher yield at 50 and 25 % WHC, respectively, as compared to uninoculated controls.

Isolation of plant-growth-promoting and metal-resistant cultivable bacteria from Arthrocnemum macrostachyum in the Odiel marshes with potential use in phytoremediation

Arthrocnemum macrostachyum is a halophyte naturally growing in southwest coasts of Spain that can tolerate and accumulate heavy metals. A total of 48 bacteria (30 endophytes and 18 from the rhizosphere) were isolated from A. macrostachyum growing in the Odiel River marshes, an ecosystem with high levels of contamination. All the isolates exhibited plant-growth-promoting (PGP) properties and most of them were multiresistant to heavy metals. Although the presence of heavy metals reduced the capability of the isolates to exhibit PGP properties, several strains were able to maintain their properties or even enhance them in the presence of concrete metals. Two bacterial consortia with the best-performing endophytic or rhizospheric strains were selected for further experiments. Bacterial inoculation accelerated germination of A. macrostachyum seeds in both the absence and presence of heavy metals. These results suggest that inoculation of A. macrostachyum with the selected bacteria could ameliorate plant establishment and growth in contaminated marshes.

Thailandins A and B, New Polyene Macrolactone Compounds Isolated from Actinokineospora bangkokensis Strain 44EHW(T), Possessing Antifungal Activity against Anthracnose Fungi and Pathogenic Yeasts.

Two new polyene macrolactone antibiotics, thailandins A, 1, and B, 2, were isolated from the fermentation broth of rhizosphere soil-associated Actinokineospora bangkokensis strain 44EHW(T). The new compounds from this strain were purified using semipreparative HPLC and Sephadex LH-20 gel filtration while following an antifungal activity guided fractionation. Their structures were elucidated through spectroscopic techniques including UV, HR-ESI-MS, and NMR. These compounds demonstrated broad spectrum antifungal activity against fungi causing anthracnose disease (Colletotrichum gloeosporioides DoA d0762, Colletotrichum gloeosporiodes DoA c1060, and Colletotrichum capsici DoA c1511) as well as pathogenic yeasts (Candida albicans MT 2013/1, Candida parasilopsis DKMU 434, and Cryptococcus neoformans MT 2013/2) with minimum inhibitory concentrations ranging between 16 and 32 μg/mL. This is the first report of polyene antibiotics produced by Actinokineospora species as bioactive compounds against anthracnose fungi and pathogenic yeast strains.

Synergistic influence of Vetiveria zizanioides and selected rhizospheric microbial strains on remediation of endosulfan contaminated soil.

Application of endosulfan tolerant rhizospheric bacterial strain isolated from pesticide contaminated area, Ghaziabad in combination with V. zizanioides for the remediation of endosulfan is described herein. The dissipation of endosulfan from soil was considerably enhanced in the presence of bacterial strain and Vetiveria zizanioides together when compared to the dissipation in presence of either of them alone. Four strains- EAG-EC-12 (M1), EAG-EC-13(M2), EAG-EC-14(M3) and EAG-EC-15(M4) are used for this purpose. V. zizanioides was grown in garden soil spiked with 1500µgg(-1) of endosulfan and inoculated with 100ml of microbial culture of above motioned strains. Effect of microbial inoculation on plant growth, endosulfan uptake and endosulfan removal efficiency were analyzed. The microbial inoculation significantly enhances the growth of test plant and endosulfan dissipation from soil (p<0.05). The addition of bacterial strain M1, M2, M3 and M4 in treated pots showed enhanced root length by 13, 33 35, 20.2 and 4.3%, above ground plant length by 16.38, 35.56, 24.92 and 9.8% and biomass by 33.69, 49.63, 39.24 and 17.09% respectively when compared with endosulfan treated plants. After 135days of exposure, a decline in endosulfan concentration by 59.12, 64.56, 62.69 and 56.39% was obtained in the spiked soil inoculated with bacterial strains M1, M2, M3 and M4 respectively whereas, decrease in endosulfan concentration by 72.78, 85.25, 76.91 and 65.44% in the vegetative spiked soil inoculated with same strains was observed during same exposure period. After 135days of growth period, enhanced removal of endosulfan from experimental soil by 13.66, 20.69, 14.22 and 9.05% was found in vegetative experiment inoculated with same strains when compared with non vegetative experiment. Result of the study showed that use of toletant plant and tolerant bacterial strains could be the better strategy for the remediation of endosulfan contaminated soil.

English

To identify the phenazine antibiotics from the selected UPMP3 strain of Pseudomonas aeruginosa and assessment of its antagonistic activity of Ganoderma boninense, three antibiotics phenazine (PHZ), phenazine-l-carboxylic acid (PCA) and pyocyanin (PYO) were extracted from the bacterial fermented broth using benzene and chloroform and detected through High-performance liquid chromatography (HPLC). The effects of three antibiotics on suppressing mycelial growth of G. boninense were assayed. The maximum amount of PHZ, PCA and PYO were recorded as 1.36, 9.62 and 15.48 μg/mL, respectively and showed in vitro antagonistic activity against G. boninense. Phenazine was found to be more effective than PCA and PYO to inhibit the mycelial growth of G. boninense. It was 100% for PHZ at 1.0 mg/mL, while 78.61% for PCA and 91.87% for PYO after 7 days of incubation. Phenazine, PCA and PYO were identified as first time from oil palm rhizospheric UPMP3 strain of P. aeruginosa. The results indicated that the identified PHZ, PCA and PYO compound had good antifungal activity against G. boninense, however the PHZ can be developed as an ideal biofungicide for the control basal stem rot disease of oil palm and helps to overcome the concerns about the residual effects of synthetic fungicides.

Influence of volatile organic compounds emitted by Pseudomonas and Serratia strains on Agrobacterium tumefaciens biofilms

The ability to form biofilms plays an important role in bacteria-host interactions, including plant pathogenicity. In this work, we investigated the action of volatile organic compounds (VOCs) produced by rhizospheric strains of Pseudomonas chlororaphis 449, Pseudomonas fluorescens B-4117, Serratia plymuthica IC1270, as well as Serratia proteamaculans strain 94, isolated from spoiled meat, on biofilms formation by three strains of Agrobacterium tumefaciens which are causative agents of crown-gall disease in a wide range of plants. In dual culture assays, the pool of volatiles emitted by the tested Pseudomonas and Serratia strains suppressed the formation of biofilms of A. tumefaciens strains grown on polycarbonate membrane filters and killed Agrobacterium cells in mature biofilms. The individual VOCs produced by the tested Pseudomonas strains, that is, ketones (2-nonanone, 2-heptanone, 2-undecanone), and dimethyl disulfide (DMDS) produced by Serratia strains, were shown to kill A. tumefaciens cells in mature biofilms and suppress their formation. The data obtained in this study suggest an additional potential of some ketones and DMDS as protectors of plants against A. tumefaciens strains, whose virulence is associated with the formation of biofilms on the infected plants.

Burkholderia novacaledonica sp. nov. and B. ultramafica sp. nov. isolated from roots of Costularia spp. pioneer plants of ultramafic soils in New Caledonia

The taxonomic status of eleven rhizospheric bacterial strains belonging to the genus Burkholderia and isolated from roots of Costularia (Cyperaceae), tropical herbaceous pioneer plants growing on ultramafic soils in New Caledonia, was investigated using a polyphasic taxonomic approach. The genetic analyses (16S rRNA genes, gyrB, recA, nreB and cnr) confirmed that all strains are Burkholderia and cluster into two separated groups. The DNA hybridization results showed low relatedness values to the closest relatives Burkholderia species. The phenotypic analyses confirmed that the two groups of strains could be differentiated from each other and from other known Burkholderia species. This polyphasic study revealed that these two groups of strains represent each a novel species of Burkholderia, for which the names Burkholderia novacaledonica sp. nov. (type strain STM10272T=LMG28615T=CIP110887T) and B. ultramafica sp. nov. (type strain STM10279T=LMG28614T=CIP110886T) are proposed, respectively. These strains of Burkholderia presented specific ecological traits such as the tolerance to the extreme edaphic constraints of ultramafic soils: they grew at pH between 4 and 8 and tolerate the strong unbalanced Ca/Mg ratio (1/19) and the high concentrations of heavy metals i.e. Co, Cr, Mn and Ni. Noteworthy B. ultramafica tolerated nickel until 10mM and B. novacaledonica up to 5mM. The presence of the nickel (nreB) and cobalt/nickel (cnr) resistance determinants encoding for protein involved in metal tolerance was found in all strains of both groups. Moreover, most of the strains were able to produce plant growth promoting molecules (ACC, IAA, NH3 and siderophores). Such ecological traits suggest that these new species of Burkholderia might be environmentally adaptable plant-associated bacteria and beneficial to plants.

Production, characterization, gene cloning, and nematocidal activity of the extracellular protease from Stenotrophomonas maltophilia N4

A rhizosphere strain of the bacterium Stenotrophomonas maltophilia N4 secretes the serine protease PN4, whose molecular mass is approximately 42kDa. The optimal temperature for the enzyme activity of the 11-fold purified protein was 50°C and the optimal pH was 10.5. The activity of the enzyme was strongly inhibited by specific serine protease inhibitors, which allowed for its classification as an alkaline serine protease family. Ca(2+) ions stimulated the activity of the protease PN4, while Mg(2+) ions stabilized its activity, and Zn(2+) and Cd(2+) ions strongly inhibited its activity. The enzyme has broad substrate specificity. For example, it is able to hydrolyse casein, keratin, albumin, haemoglobin, and gelatin, as well as the insoluble modified substrates azure keratin and azocoll. The gene that encodes the 1740bp precursor form of the enzyme (accession number: LC031815) was cloned. We then deduced that its amino acid sequence includes the region of the conserved domain of the S8 family of peptidases as well as the catalytic triad Asp/His/Ser. The bacterial culture fluid as well as the purified protease PN4 demonstrated biocidal activity with regard to the nematodes Caenorhabditis elegans and Panagrellus spp.

Rhizospheric Bacterial Strain Brevibacterium casei MH8a Colonizes Plant Tissues and Enhances Cd, Zn, Cu Phytoextraction by White Mustard.

Environmental pollution by heavy metals has become a serious problem in the world. Phytoextraction, which is one of the plant-based technologies, has attracted the most attention for the bioremediation of soils polluted with these contaminants. The aim of this study was to determine whether the multiple-tolerant bacterium, Brevibacterium casei MH8a isolated from the heavy metal-contaminated rhizosphere soil of Sinapis alba L., is able to promote plant growth and enhance Cd, Zn, and Cu uptake by white mustard under laboratory conditions. Additionally, the ability of the rifampicin-resistant spontaneous mutant of MH8a to colonize plant tissues and its mechanisms of plant growth promotion were also examined. In order to assess the ecological consequences of bioaugmentation on autochthonous bacteria, the phospholipid fatty acid (PLFA) analysis was used. The MH8a strain exhibited the ability to produce ammonia, 1-amino-cyclopropane-1-carboxylic acid deaminase, indole 3-acetic acid and HCN but was not able to solubilize inorganic phosphate and produce siderophores. Introduction of MH8a into soil significantly increased S. alba biomass and the accumulation of Cd (208%), Zn (86%), and Cu (39%) in plant shoots in comparison with those grown in non-inoculated soil. Introduced into the soil, MH8a was able to enter the plant and was found in the roots and leaves of inoculated plants thus indicating its endophytic features. PLFA analysis revealed that the MH8a that was introduced into soil had a temporary influence on the structure of the autochthonous bacterial communities. The plant growth-promoting features of the MH8a strain and its ability to enhance the metal uptake by white mustard and its long-term survival in soil as well as its temporary impact on autochthonous microorganisms make the strain a suitable candidate for the promotion of plant growth and the efficiency of phytoextraction.

Chr genes from adaptive replicons are responsible for chromate resistance by Burkholderia xenovorans LB400.

The chromate ion transporter (CHR) superfamily includes proteins that confer chromate resistance by extruding toxic chromate ions from cytoplasm. Burkholderia xenovorans strain LB400 encodes six CHR homologues in its multireplicon genome and has been reported as highly chromate-resistant. The objective of this work was to analyze the involvement of chr redundant genes in chromate resistance by LB400. It was found that B. xenovorans plant rhizosphere strains lacking the megaplasmid are chromate-sensitive, suggesting that the chr gene present in this replicon is responsible for the chromate-resistance phenotype of the LB400 strain. Transformation of a chromate-sensitive B. xenovorans strain with each of the six cloned LB400 chr genes showed that genes from 'adaptive replicons' (chrA1b and chr1NCb from chromosome 2 and chrA2 from the megaplasmid) conferred higher chromate resistance levels than chr genes from 'central' chromosome 1 (chrA1a, chrA6, and chr1NCa). An LB400 insertion mutant affected in the chrA2 gene displayed a chromate-sensitive phenotype, which was fully reverted by transferring the chrA2 wild-type gene, and partially reverted by chrA1b or chr1NCb genes. These data indicate that chr genes from adaptive replicons, mainly chrA2 from the megaplasmid, are responsible for the B. xenovorans LB400 chromate-resistance phenotype.

Plant growth-promoting effects of rhizospheric and endophytic bacteria associated with different tomato cultivars and new tomato hybrids

Conventional agriculture relies on chemical pesticides and fertilizers, which can degrade ecosystems. A reduction of these harmful practices is required, replacing (or integrating) them with more eco-friendly approaches, such as microbial inoculation. Tomato is an important agricultural product, with a high content of bioactive compounds (folate, ascorbate, polyphenols, and carotenoids). The focus of this research was to investigate the plant growth-promoting (PGP) abilities of bacterial strains isolated from different tomato cultivars, with the aim to develop systems to improve plant health and crop productivity based on microbial inoculation. A pool of different tomato cultivars already available on the market and new tomato hybrids were selected based on their nutritional quality (high content of biologically active compounds). A total of 23 strains were isolated from tomato roots (11 rhizospheric strains and 12 root endophytes). The cultivable isolates were analyzed for a number of different PGP traits: organic acids (OA), indole acetic acid (IAA), ACC deaminase, and siderophore production. The effects of microbial inoculation on root growth of Arabidopsis thaliana were also evaluated using a Vertical Agar Plate assay. A high percentage of the isolated strains tested positive for the following PGP traits: 73 % were able to produce OA, 89 % IAA, 83 % ACC deaminase, and 87 % siderophores. The most striking result were remarkable increases in the formation of root hairs for most of the inoculated plants. This effect was obvious for all A. thaliana seedlings inoculated with the isolated endophytes, and for the 50 % of the seedlings inoculated with the rhizospheric strains. A better knowledge of the plant growth-promotion activity of these strains can provide an important contribution to increase environmental sustainability in agriculture.

Bacteriocins Contributing in Rhizospheric Competition among Fluorescent Pseudomonads

Aims: To examine the production of bacteriocins through the study of a group of rhizospheric Pseudomonas isolates already known to produce metabolites that are antagonistic to fungi. Methodology: Fourteen rhizospheric strains of fluorescent Pseudomonads spp., were tested as well as two referenced strains Pseudomonas protogens CHA0 and Pseudomonas aureofaciens 30-84, for their ability to produce induced bacteriocins. The induction is carried out first by UV light, and secondly by mitomycin C. Results: In addition to the reference strains, six isolates were found to produce bactericidal substances after UV light induction against Pseudomonas target bacteria but also against other genera (Escherichia and Staphylococcus). Producing strains were treated with mitomycin C, and then lysed with chloroform. Analysis of the lysates by trypsin and freezing treatments, suggests that the active compounds are of high molecular weight. Conclusion: It is therefore suggested that these bacteria could be good competitors for their introduction as biocontrol agents.