Plant Growth-promoting Rhizobacteria Pseudomonas Research Articles

Nitric oxide synthase expression in Pseudomonas koreensis MME3 improves plant growth promotion traits

The development of novel biotechnologies that promote a better use of N to optimize crop yield is a central goal for sustainable agriculture. Phytostimulation, biofertilization, and bioprotection through the use of bio-inputs are promising technologies for this purpose. In this study, the plant growth–promoting rhizobacteria Pseudomonas koreensis MME3 was genetically modified to express a nitric oxide synthase of Synechococcus SyNOS, an atypical enzyme with a globin domain that converts nitric oxide to nitrate. A cassette for constitutive expression of synos was introduced as a single insertion into the genome of P. koreensis MME3 using a miniTn7 system. The resulting recombinant strain MME3:SyNOS showed improved growth, motility, and biofilm formation. The impact of MME3:SyNOS inoculation on Brachypodium distachyon growth and N uptake and use efficiencies under different N availability situations was analyzed, in comparison to the control strain MME3:c. After 35 days of inoculation, plants treated with MME3:SyNOS had a higher root dry weight, both under semi-hydroponic and greenhouse conditions. At harvest, both MME3:SyNOS and MME3:c increased N uptake and use efficiency of plants grown under low N soil. Our results indicate that synos expression is a valid strategy to boost the phytostimulatory capacity of plant-associated bacteria and improve the adaptability of plants to N deficiency.Key points• synos expression improves P. koreensis MME3 traits important for rhizospheric colonization• B. distachyon inoculated with MME3:SyNOS shows improved root growth• MME3 inoculation improves plant N uptake and use efficiencies in N-deficient soil

Genomic content of wheat has a higher influence than plant domestication status on the ability to interact with Pseudomonas plant growth-promoting rhizobacteria.

Plant evolutionary history has had profound effects on belowground traits, which is likely to have impacted the ability to interact with microorganisms, but consequences on root colonization and gene expression by plant growth-promoting rhizobacteria (PGPR) remain poorly understood. Here, we tested the hypothesis that wheat genomic content and domestication are key factors determining the capacity for PGPR interaction. Thus, 331 wheat representatives from eight Triticum or Aegilops species were inoculated under standardized conditions with the generalist PGPR Pseudomonas ogarae F113, using an autofluorescent reporter system for monitoring F113 colonization and expression of phl genes coding for the auxinic inducing signal 2,4-diacetylphloroglucinol. The interaction with P. ogarae F113 was influenced by ploidy level, presence of genomes AA, BB, DD, and domestication. While root colonization was higher for hexaploid and tetraploid species, and phl expression level higher for hexaploid wheat, the diploid Ae. tauschii displayed higher phl induction rate (i.e., expression:colonisation ratio) on roots. However, a better potential of interaction with F113 (i.e., under non-stress gnotobiotic conditions) did not translate, after seed inoculation, into better performance of wheat landraces in non-sterile soil under drought. Overall, results showed that domestication and especially plant genomic content modulate the PGPR interaction potential of wheats.

Isolation, Identification, and Biocontrol Potential of Plant Growth Promoting Rhizobacteria Pseudomonas fluorescens

Isolation, Identification, and Biocontrol Potential of Plant Growth Promoting Rhizobacteria Pseudomonas fluorescens

Impact assessment of cold-adapted Pseudomonas jesenii MP1 and Pseudomonas palleroniana N26 on Phaseolus vulgaris yield and soil health

The poor agriculture practices, fragmented land holdings, fluctuating climatic conditions, and minimal external inputs lead to nutrient deficiency in the Himalayan agroecosystems. Because of the risks associated with chemical fertilizers, their implication is a big question mark. Therefore, two previously characterized plant growth-promoting rhizobacteria Pseudomonas jesenii MP1 and Pseudomonas palleroniana N26 were employed to enhance kidney bean productivity and soil health at farmer’s fields of Harsil and Chakrata regions of Uttarakhand Himalayas. The study revealed that MP1 and N26 treatment resulted in 25.62% and 37.23% higher grain yield than respective uninoculated controls at the trial fields of Harsil and Chakrata regions, respectively. Further, the bacterial treatments have significantly increased nitrogen, phosphorus, and potassium levels in the soils. The soil diversity analysis revealed the dominance of Proteobacteria and Actinobacteria at Harsil and Chakrata, respectively. Further, the MP1 treatment had increased Firmicutes percentage over uninoculated control at both locations. Conclusively, the application of cold adaptive Pseudomonas jesenii MP1 and Pseudomonas palleroniana N26 improved the grain yield and soil health status of the Himalayan agroecosystems. Therefore, they can be explored as an eco-friendly alternative for the commercial production of kidney beans.

Growth enhancement and extenuation of drought stress in maize inoculated with multifaceted ACC deaminase producing rhizobacteria

IntroductionMaize is a major staple cereal crop grown and consumed globally. However, due to climate change, extreme heat and drought stresses are greatly affecting its production especially in sub-Saharan Africa. The use of a bio-based approach to mitigate drought stress is therefore suggested using plant growth-promoting rhizobacteria (PGPR).MethodsThis study investigated the abilities of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing PGPR Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13 and Bacillus sp. MRBP10 isolated from maize rhizosphere soil, to ameliorate the effect of drought stress in maize genotypes MR44 and S0/8/W/I137TNW//CML550 under two water regimes; mild drought stress (50% FC) and well-watered conditions (100% FC). The rhizobacterial strains were identified by 16S rRNA sequencing and biochemical tests, and evaluated for plant growth-promoting and abiotic stress tolerance traits.Results and discussionThe synergistic effect of the bacterial strains had a highly significant (p < 0.001) effect on the total soluble sugar, soil moisture content and relative water content, which were enhanced under water-stress in the inoculated plants. Relative water content was significantly highest (p < 0.001) in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 (60.55%). Total chlorophyll content was significantly enhanced in maize seedlings sole inoculated with Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13, and co-inoculated with Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 by 15.91%, 14.99% and 15.75% respectively, over the un-inoculated control. Soil moisture content increased by 28.67% and 30.71% compared to the un-inoculated control when plants were inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 and Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 respectively. The interactive effect of genotype × bacteria significantly enhanced biomass production. Leaf area was highest in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Pseudomonas sp. MRBP13 (212.45 ± 0.87 cm2) under drought stress. Treatment of maize seeds with Pseudomonas sp. MRBP 4 + Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 significantly increased the root length (10.32 ± 0.48 cm) which enhanced survival of the maize seedlings. Bioinoculation of maize seeds with these strains could boost maize production cultivated in arid regions.

Rhizobacteria Mitigate the Negative Effect of Aluminum on Pea Growth by Immobilizing the Toxicant and Modulating Root Exudation.

High soil acidity is one of the main unfavorable soil factors that inhibit the growth and mineral nutrition of plants. This is largely due to the toxicity of aluminum (Al), the mobility of which increases significantly in acidic soils. Symbiotic microorganisms have a wide range of beneficial properties for plants, protecting them against abiotic stress factors. This report describes the mechanisms of positive effects of plant growth-promoting rhizobacteria Pseudomonas fluorescens SPB2137 on four pea (Pisum sativum L.) genotypes grown in hydroponics and treated with 80 µM AlCl3. In batch culture, the bacteria produced auxins, possessed 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, alkalized the medium and immobilized Al, forming biofilm-like structures and insoluble phosphates. Inoculation with Ps. fluorescens SPB2137 increased root and/or shoot biomass of Al-treated plants. The bacteria alkalized the nutrient solution and transferred Al from the solution to the residue, which contained phosphorus that was exuded by roots. As a result, the Al concentration in roots decreased, while the amount of precipitated Al correlated negatively with its concentration in the solution, positively with the solution pH and negatively with Al concentration in roots and shoots. Treatment with Al induced root exudation of organic acids, amino acids and sugars. The bacteria modulated root exudation via utilization and/or stimulation processes. The effects of Al and bacteria on plants varied depending on pea genotype, but all the effects had a positive direction and the variability was mostly quantitative. Thus, Ps. fluorescens SPB2137 improved the Al tolerance of pea due to immobilization and exclusion of toxicants from the root zone.

Formulation of Soil Beneficial Microbes Solid Inoculant for Controlling Nematode in Coffee

Plant Growth Promoting Rhizobacteria Pseudomonas and Bacillus are well known biofertilizer due to their ability for solubilizing inorganic phosphate and producing phytohormones in soil. However, their potential to decreased diseases incidence caused by Pratylenchus coffeae has been documented. The objective of this laboratory experiment was to formulate solid biofertilizer contained Pseudomonas diminuta and Bacillus subtilis. The laboratory experiments consisted of three-step experiment: 1) selection of the molasses-based liquid media for P. diminuta and B. subtilis growth, 2) organic liquid inoculant formulation and 3) organic carrier-based inoculant formulation. The population of both bacteria in all steps was count by serial dilution plate method with specific medium. The result verifeid that cell density of P. diminuta and B. subtilis in liquid culture with 2% molasses were higher than cell count in the culture contained 4% molasses. Bagasse-based carrier inoculated with liquid culture of mixed bacteria has higher bacterial count compared to manure-based carrier. After 90-day incubation, bagasse-based solid inoculant maintained P. diminuta and B. subtilis density up to 108 CFU/mL and the acidity of solid inculaant was about neutral. The bacterial count and acidity of solid inoculant agreed with Indonesian standard for solid biofertilizer

Co-inoculation of rhizobacteria promotes growth, yield, and nutrient contents in soybean and improves soil enzymes and nutrients under drought conditions

Drought stress is the major abiotic factor limiting crop production. Co-inoculating crops with nitrogen fixing bacteria and plant growth-promoting rhizobacteria (PGPR) improves plant growth and increases drought tolerance in arid or semiarid areas. Soybean is a major source of high-quality protein and oil for humans. It is susceptible to drought stress conditions. The co-inoculation of drought-stressed soybean with nodulating rhizobia and root-colonizing, PGPR improves the root and the shoot growth, formation of nodules, and nitrogen fixation capacity in soybean. The present study was aimed to observe if the co-inoculation of soybean (Glycine max L. (Merr.) nodulating with Bradyrhizobium japonicum USDA110 and PGPR Pseudomonas putida NUU8 can enhance drought tolerance, nodulation, plant growth, and nutrient uptake under drought conditions. The results of the study showed that co-inoculation with B. japonicum USDA110 and P. putida NUU8 gave more benefits in nodulation and growth of soybean compared to plants inoculated with B. japonicum USDA110 alone and uninoculated control. Under drought conditions, co-inoculation of B. japonicum USDA 110 and P. putida NUU8 significantly enhanced the root length by 56%, shoot length by 33%, root dry weight by 47%, shoot dry weight by 48%, and nodule number 17% compared to the control under drought-stressed. Co-inoculation with B. japonicum, USDA 110 and P. putida NUU8 significantly enhanced plant and soil nutrients and soil enzymes compared to control under normal and drought stress conditions. The synergistic use of B. japonicum USDA110 and P. putida NUU8 improves plant growth and nodulation of soybean under drought stress conditions. The results suggested that these strains could be used to formulate a consortium of biofertilizers for sustainable production of soybean under drought-stressed field conditions.

Heavy Metals Immobilization in Soil with Plant-growthpromoting Precipitation in Support of Radish Growth

The application of plant-growth-promoting rhizobacteria (PGPR) supports the growth of plants in contaminated soil while ureolytic bacteria can immobilise heavy metals by carbonate precipitation. Thus, dual treatment with such bacteria may be beneficial for plant growth and bioremediation in contaminated soil. This study aimed to determine whether the PGPR Pseudomonas fluorescens could work in synergy with ureolytic bacteria to assist with the remediation of cadmium (Cd)- and lead (Pb)-contaminated soils. Pot experiments were conducted to grow radish plants in Cd- and Pb-contaminated soils treated with PGPR P. fluorescens and the results were compared with dual inoculation of P. fluorescens combined with ureolytic Staphylococcus epidermidis HJ2. The removal rate of the metals from the soil was more than 83% for Cd and Pb by the combined treatment compared to 17% by PGPR alone. Further, the dual treatment reduced the metal accumulation in the roots by more than 80%. The translocation factors for Cd and Pb in plant tissues in both treatments remained the same, suggesting that PGPR combined with the carbonate precipitation process does not hamper the transfer of essential metal ions into plant tissues from the soil.

SA, ABA and Pseudomonas fluorescens elicit defense responses in tomato against Alternaria blight

Alternaria blight causes significant losses in tomato. High disease severity during fruiting stage restricts the applicability of chemicals. Chemical pesticides can be substituted by resistance inducing agents. In present investigation, two resistance inducing agents namely Salicylic acid and Abscisic acid and a plant growth promoting rhizobacteria Pseudomonas fluorescens strain PBAT-2 (Psf) were used to assess their effect on Alternaria blight disease severity through the activity of defense-related compounds i.e. Peroxidase (POD), Polyphenol oxidase (PPO), Phenyl ammonia lyase (PAL), total phenolic content along with pathogenesis-related gene PR-1 and β-1,3-glucanase (GLU) gene expression in tomato. SA, ABA and Psf reduced disease severity significantly as compared to control. A significant increase in the activity of POD, PPO, PAL and total phenol content was recorded in all the three treatments over control. POD, PPO, PAL levels were significantly high at 24 h and 48 h post Alternaria solani inoculation, whereas total phenolic content increased up to 72 h. The expression of PR-1 and GLU gene was upregulated at 24 h post A. solani inoculation in SA and ABA treated leaves. However, these genes were unexpressed in Psf treated plants which confirms the role of SA and ABA in systemic acquired resistance. These findings suggest that SA, ABA and Psf can be used in early blight management in tomato without affecting the quality of the fruit.

Characterization of a versatile plant growth-promoting rhizobacterium Pseudomonas mediterranea strain S58

Characterization of a versatile plant growth-promoting rhizobacterium Pseudomonas mediterranea strain S58

Inheritance of IncP-9 Catabolic Plasmids in Pseudomonas Bacteria

This work investigated the capability of the conjugative transfer of epsilon-caprolactam biodegradation (CAP) plasmids, belonging to incompatibility (Inc) groups P-2, P-7, P-9, and IncP-9 naphthalene biodegradation (NAH) plasmids to various species of Pseudomonas bacteria, including strains of plant growth-promoting rhizobacteria (PGPR), and the character of plasmid inheritance in these strains. The frequency of the conjugative transfer of all CAP and NAH plasmids (irrespective of their Inc groups) into Pseudomonas PGPR was shown to be lower than that into P. putida soil strains by more than an order of magnitude. The low frequency of conjugative transfer was found not to be caused by secretion of antibiotically active metabolites by Pseudomonas PGPR strains. Stability of inheritance of catabolic plasmids under nonselective conditions was established to depend on their Inc group and bacterial host strain. Thus, all investigated plasmids were stably maintained in P. putida KT2442 and BS394 strains for at least 100 generations. However, only CAP plasmids of IncP-2 and IncP-7 groups were stably maintained in Pseudomonas PGPR cells. CAP and NAH plasmids of different IncP-9 subgroups were eliminated from cells of rhizosphere pseudomonads within several generations. Thus, the work first revealed inheritance features of IncP-9 catabolic plasmids in Pseudomonas PGPR cells.

Rhizosphere-Associated Pseudomonas Suppress Local Root Immune Responses by Gluconic Acid-Mediated Lowering of Environmental pH

The root microbiome consists of commensal, pathogenic, and plant-beneficial microbes [1]. Most members of the root microbiome possess microbe-associated molecular patterns (MAMPs) similar to those of plant pathogens [2]. Their recognition can lead to the activation of host immunity and suppression of plant growth due to growth-defensetradeoffs [3, 4]. We found that 42% of the tested root microbiota, including the plant growth-promoting rhizobacteria Pseudomonas capeferrum WCS358 [5, 6] and Pseudomonas simiae WCS417 [6, 7], are able to quench local Arabidopsis thaliana root immune responses that aretriggered by flg22 [8], an immunogenic epitope of the MAMP flagellin [9], suggesting that this is animportant function of the root microbiome. Ina screen for WCS358 mutants that lost their capacity to suppress flg22-induced CYP71A12pro:GUS MAMP-reporter gene expression, we identified the bacterial genes pqqF and cyoB in WCS358, which are required for the production of gluconic acid and its derivative 2-keto gluconic acid. Both WCS358 mutants are impaired in the production of these organic acids and consequently lowered their extracellular pH to a lesser extent thanwild-type WCS358. Acidification of the plant growth medium similarly suppressed flg22-induced CYP71A12pro:GUS and MYB51pro:GUS expression, and the flg22-mediated oxidative burst, suggesting a role for rhizobacterial gluconic acid-mediated modulation of the extracellular pH in the suppression of root immunity. Rhizosphere population densities of the mutants were significantly reduced compared to wild-type. Collectively, these findings show thatsuppression of immune responses is an important function of the root microbiome, as it facilitates colonization by beneficial root microbiota.

Plant-Beneficial <i>Pseudomonas</i> Spp. Suppress Local Root Immune Responses by Gluconic Acid-Mediated Lowering of Environmental pH

The plant root microbiome consists of commensal, pathogenic and plant-beneficial microbes. Most members of the root microbiome possess microbe-associated molecular patterns (MAMPs) similar to those of plant pathogens. Their recognition by the cell surface-localized pattern recognition receptors can lead to the initiation of host immune signaling and suppression of plant growth due to growth-defense tradeoffs. We found that 42% of the tested root microbiota, including the plant growth-promoting rhizobacteria Pseudomonas capeferrum WCS358 (WCS358) and Pseudomonas simiae WCS417, are able to quench local Arabidopsis thaliana root immune responses that were triggered by the synthetic MAMP flg22, suggesting that this is an important function of the root microbiome. In a screen for WCS358 mutants that lost their capacity to suppress flg22-induced CYP71A12pro:GUS MAMP-reporter gene expression, we identified the bacterial genes pqqF and cyoB in WCS358. CyoB and pqqF are required for the production of gluconic acid and its derivative 2-keto gluconic acid. Both WCS358 mutants are impaired in the production of these organic acids and consequently lowered their extracellular pH to a lesser extent than wild-type WCS358. Acidification of the plant growth medium similarly suppressed flg22-induced CYP71A12pro:GUS and MYB51pro:GUS expression as well as the flg22-mediated oxidative burst, suggesting a role for rhizobacterial gluconic acid-mediated modulation of the extracellular pH in the suppression of local root immunity. Rhizosphere population densities of the mutants were significantly reduced compared to wild-type. Collectively, these findings show that suppression of local root immune responses is an important function of the root microbiome, as it facilitates colonization by beneficial root microbiota.

Draft Genome Sequence of the Plant Growth-Promoting Rhizobacterium Pseudomonas protegens Strain BNJ-SS-45, Isolated from Rhizosphere Soil of Wheat (Triticum aestivum).

Here, we present the draft genome sequence of Pseudomonas protegens strain BNJ-SS-45, which was isolated from wheat rhizosphere. The genome is assembled with 7,116,445 bp with a GC content of 63.34% consisting of 32 scaffolds. The genome is useful in prediction of secondary metabolites, particularly rhizoxin, pyoverdine, and bacteriocin.

Salt stress alleviation in citrus plants by plant growth-promoting rhizobacteria Pseudomonas putida and Novosphingobium sp.

This work reveals the protective role of two rhizobacteria, Pseudomonas putida and Novosphingobium sp., on citrus plants subjected to salt stress conditions. Detrimental salt stress effects on crops are likely to increase due to climate change reducing the quality of irrigation water. Plant growth-promoting rhizobacteria (PGPRs) can mitigate stress-induced damage in plants cultivated under high salinity conditions. In this work, Citrus macrophylla (alemow) plants inoculated with the rhizobacteria Pseudomonas putida KT2440 or Novosphingobium sp. HR1a were subjected to salt stress for 30 days. Results showed that in absence of salt stress, Novosphingobium sp. HR1a induced a decrease of transpiration (E) and stomatal conductance (gs). Both rhizobacteria reduced salt stress-induced damage. Levels of abscisic acid (ABA) and salicylic acid (SA) were lower in inoculated plants under salt stress conditions. Similarly, under stress conditions maximum efficiency of photosystem II (Fv/Fm) in inoculated plants decreased to a lower extent than in non-inoculated ones. In stressed plants, Novosphingobium sp. HR1a also induced leaf accumulation of 3-indole acetic acid (IAA) and a delay in the decrease of quantum yield (ΦPSII). P. putida KT2440 inhibited root chloride and proline accumulation in response to salt stress. Although both bacterial species had beneficial effects on salt-stressed citrus plants, Novosphingobium sp. HR1a induced a better plant performance. Therefore, both strains could be candidates to be used as PGPRs in programs of inoculation for citrus protection against salt stress.

Indole acetic acid overproduction transformants of the rhizobacterium Pseudomonas sp. UW4.

The plant growth-promoting rhizobacterium Pseudomonas sp. UW4 was transformed to increase the biosynthesis of the auxin, indole-3-acetic acid (IAA). Four native IAA biosynthesis genes from strain UW4 were individually cloned into an expression vector and introduced back into the wild-type strain. Quantitative real-time polymerase chain reaction analysis revealed that the introduced genes ami, nit, nthAB and phe were all overexpressed in these transformants. A significant increase in the production of IAA was observed for all modified strains. Canola plants inoculated with the modified strains showed enhanced root elongation under gnotobiotic conditions. The growth rate and 1-aminocyclopropane-1-carboxylate deaminase activity of transformant strains was lower compared to the wild-type. The indoleacetic acid biosynthesis pathways and the role of this phytohormone in the mechanism of plant growth stimulation by Pseudomonas sp. UW4 is discussed.

Bacteria and smoke-water extract improve growth and induce the synthesis of volatile defense mechanisms in Vitis vinifera L.

Sustainable agricultural practices have been developed as alternative to the use of agrochemicals, and viticulture is not exempt of that. Plant growth promoting rhizobacteria (PGPR) and smoke water extracts (SW) are environmentally-friendly alternative to those agrochemicals. The aim of this study was to investigate the single or combined effects of SW and the PGPR Pseudomonas fluorescens (Pf) and Bacillus licheniformis (Bl) on the physiology and biochemistry of grapevines plants. After 38 days, single applications of SW solutions and bacterial suspensions increase rooting and root length. Combined treatments had a slight positive effect compared to the water control. At the end of 60-days pot trial, grapevine treated with 1:1000 SW and Pf applied alone showed increases in stem length, leaf area and fresh weight of the roots, shoot and leaves, although not significantly differences from the water control were found. In addition, Pf augmented chlorophyll relative content, all treatments decreased the stomatal conductance (mainly 1:500 SW, Pf and 1:1000 SW + Bl), as well as lipid peroxidation in roots (mainly in bacterial treatments), and induced the synthesis of mono and sesquiterpenes in leaves, where the effect was enhanced in combined treatments. In conclusion, PGPR and SW are effective to improve growth of V. vinifera cuttings as well as to increase the plants defense mechanisms that may help them to cope with biotic and abiotic stresses.

Plant-mediated restriction of Salmonella enterica on tomato and spinach leaves colonized with Pseudomonas plant growth-promoting rhizobacteria

Reducing Salmonella enterica association with plants during crop production could reduce risks of fresh produce-borne salmonellosis. Plant growth-promoting rhizobacteria (PGPR) colonizing plant roots are capable of promoting plant growth and boosting resistance to disease, but the effects of PGPR on human pathogen-plant associations are not known. Two root-colonizing Pseudomonas strains S2 and S4 were investigated in spinach, lettuce and tomato for their plant growth-promoting properties and their influence on leaf populations of S. enterica serovar Newport. Plant roots were inoculated with Pseudomonas in the seedling stage. At four (tomato) and six (spinach and lettuce) weeks post-germination, plant growth promotion was assessed by shoot dry weight (SDW) and leaf chlorophyll content measurements. Leaf populations of S. Newport were measured after 24h of leaf inoculation with this pathogen by direct plate counts on Tryptic Soy Agar. Root inoculation of spinach cv. ‘Tyee’, with Pseudomonas strain S2 or S4 resulted in a 69% and 63% increase in SDW compared to non-inoculated controls (p<0.005 and p<0.01, respectively). Similarly, Romaine lettuce cv. ‘Parris Island Cos’ responded positively to S2 and S4 inoculation (53% and 48% SDW increase, respectively; p<0.05), and an increase in leaf chlorophyll content (p<0.001), compared to controls. Tomato cv. ‘Nyagous’ yielded significantly greater SDW (74%, p<0.01 and 54%, p<0.05 for S2 and S4, respectively), and also higher leaf chlorophyll content (19% and 29%, p<0.001, respectively) relative to controls. Leaf chlorophyll content only increased in S4-inoculated tomato cv. ‘Moneymaker’ plants (27%, p<0.001), although both S2 and S4 promoted plant growth by over 40% compared to controls (p<0.01 and p<0.05, respectively). No significant growth promotion was detected in tomato cv. ‘BHN602’, but S2-inoculated plants had elevated leaf chlorophyll content (13%, p<0.01). Root inoculation with Pseudomonas S4 restricted S. Newport populations inoculated on leaves of spinach (p<0.001) and all three tomato cultivars (p<0.05), compared to controls, 24h post Salmonella inoculation. Impairment of S. Newport leaf populations was also observed on spinach when plant roots were inoculated with S2 (p<0.01). With an initial leaf inoculum of approximately 6.0logCFU of S. Newport/plant, the significantly greater reduction of S. Newport populations on Pseudomonas-treated plants than those on non-inoculated control plants after 24h was modest with differences of one log or less. By contrast, the survival of S. Newport on the leaves of Romaine lettuce was not influenced by Pseudomonas root colonization. These findings provide evidence that root inoculation of certain specialty crops with beneficial Pseudomonas strains exhibiting PGPR properties may not only promote plant growth, but also reduce the fitness of epiphytic S. enterica in the phyllosphere. Plant-mediated effects induced by PGPR may be an effective strategy to minimize contamination of crops with S. enterica during cultivation.

Natural genetic variation in Arabidopsis for responsiveness to plant growth-promoting rhizobacteria.

The plant growth-promoting rhizobacterium (PGPR) Pseudomonas simiae WCS417r stimulates lateral root formation and increases shoot growth in Arabidopsis thaliana (Arabidopsis). These plant growth-stimulating effects are partly caused by volatile organic compounds (VOCs) produced by the bacterium. Here, we performed a genome-wide association (GWA) study on natural genetic variation in Arabidopsis for the ability to profit from rhizobacteria-mediated plant growth-promotion. To this end, 302 Arabidopsis accessions were tested for root architecture characteristics and shoot fresh weight in response to exposure to WCS417r. Although virtually all Arabidopsis accessions tested responded positively to WCS417r, there was a large variation between accessions in the increase in shoot fresh weight, the extra number of lateral roots formed, and the effect on primary root length. Correlation analyses revealed that the bacterially-mediated increase in shoot fresh weight is related to alterations in root architecture. GWA mapping for WCS417r-stimulated changes in root and shoot growth characteristics revealed 10 genetic loci highly associated with the responsiveness of Arabidopsis to the plant growth-promoting activity of WCS417r. Several of the underlying candidate genes have been implicated in important plant growth-related processes. These results demonstrate that plants possess natural genetic variation for the capacity to profit from the plant growth-promoting function of a beneficial rhizobacterium in their rhizosphere. This knowledge is a promising starting point for sustainable breeding strategies for future crops that are better able to maximize profitable functions from their root microbiome.Electronic supplementary materialThe online version of this article (doi:10.1007/s11103-016-0442-2) contains supplementary material, which is available to authorized users.