Plant Growth Promoting Rhizobacteria Species Research Articles

Real-Time PCR (qtPCR) to Discover the Fate of Plant Growth-Promoting Rhizobacteria (PGPR) in Agricultural Soils.

To optimize the application of plant growth-promoting rhizobacteria (PGPR) in field trials, tracking methods are needed to assess their shelf life and to determine the elements affecting their effectiveness and their interactions with plants and native soil microbiota. This work developed a real-time PCR (qtPCR) method which traces and quantifies bacteria when added as microbial consortia, including five PGPR species: Burkholderia ambifaria, Bacillus amyloliquefaciens, Azotobacter chroococcum, Pseudomonas fluorescens, and Rahnella aquatilis. Through a literature search and in silico sequence analyses, a set of primer pairs which selectively tag three bacterial species (B. ambifaria, B. amyloliquefaciens and R. aquatilis) was retrieved. The primers were used to trace these microbial species in a field trial in which the consortium was tested as a biostimulant on two wheat varieties, in combination with biochar and the mycorrhizal fungus Rhizophagus intraradices. The qtPCR assay demonstrated that the targeted bacteria had colonized and grown into the soil, reaching a maximum of growth between 15 and 20 days after inoculum. The results also showed biochar had a positive effect on PGPR growth. In conclusion, qtPCR was once more an effective method to trace the fate of supplied bacterial species in the consortium when used as a cargo system for their delivery.

Thymol-based Dual Chemosensor for Optical Detection of Cu2+ and Pb2+ ions and Evaluation of its Toxicity Against Plant Growth Promoting Rhizobacteria

Abstract: Thymol is a naturally occurring monoterpenoid phenol, and its derivatives may emerge as eco-friendly materials for the development of chemosensing probes. To prepare a non-toxic chemosensing probe based on thymol moiety for the detection of metal ions. : A chemosensor (TPC) based on thymol was afforded by the reaction of thymol and piperidine in methanol. The structure elucidation of TPC was carried out with UV-Vis, 1H-NMR, 13C-NMR, and FT-IR analysis. The chemosensing properties of the synthesized probe were determined with UVvisible spectroscopy, and further, it was exploited for the determination of the concentration of Cu2+ and Pb2+ ions in spiked tap water samples. The toxicity studies of the chemosensing probe were conducted against the plant growth-promoting rhizobacteria (PGPR), i.e., Rhizobium sp., Pseudomonas sp., and Azospirillum sp. The absorption spectrum of TPC showed a band at 277 nm. The presence of Cu2+ and Pb2+ ions induced an increase in the absorption intensity at 277 nm by 2-fold and 1.5-fold, respectively. The binding constant for both TPC-Cu2+ and TPC-Pb2+ was calculated as 1.55 × 105 M-1 and 1.47 × 105 M-1, respectively. The respective limit of detection (LOD) and limit of quantification (LOQ) for Cu2+ were 623.64 × 10-6 mol L-1 and 1889.85 × 10-6 mol L-1. For Pb2+ ions, LOD and LOQ were calculated as 676.70 × 10-6 mol L-1 and 2050.60 × 10-6 mol L-1, respectively. In spiked tap water samples, percent recovery was observed in the range of 80.1 to 81.0 % and 80.1 to 81.9 % for Cu2+ and Pb2+ ions, respectively. Toxicity studies of the synthesized probe inferred that TPC was non-toxic against the tested PGPR at all the tested concentrations. In this work, we have synthesized a thymolbased chemosensor, which has been evaluated as a non-toxic sensor for Pb2+ and Cu2+ ions.

Toxicity of copper oxide nanoparticles on agriculturally important soil rhizobacteria Bacillus megaterium

Copper oxide nanoparticles are an integral part of modern agrochemical industries, such as pesticides and nano fertilizers. It has been extensively reviewed and reported that the interaction of nanoparticles with plant growth-promoting rhizobacteria (PGPRs) may be toxic due to their accumulation in the soil and may remain in contact for a long time which causes a negative impact on the soil microbial community. The study presented here, investigates the impact of the copper oxide nanoparticles on the isolated PGPR species Bacillius megaterium AGNTL MM7 in invitro conditions. We observed that the formation of Reactive Oxygen Species (ROS) at ¼ of Minimum Inhibitory Concentrations (MIC) of CuO nanoparticles resulted in a decrease in the Indole Acetic Acid (IAA) production (83.02 μg/ml) as the marker of the nanoparticles mediated stress on the Bacillus megaterium AGNTL MM7. The minimum inhibitory concentration (MIC) assay was observed at 4 mg/ml. The solubility of CuO nanoparticles was 0.014% and the Nanotracking analyzer estimated the mean average size 188 nm in a liquid medium such as water. Transmission Electron Microscopic (TEM) studies observed that the concentration of 1 mg/ml nanoparticles showed a strong affinity with cell envelope and caused a significant negative impact on the microorganism, resulting in 58% inhibition in IAA production was observed in comparison to the control i.e., 197.8 μg/ml. The flow cytometric analysis also significantly validated the toxicity at the same concentration.

Inducing Rhizosphere Acidification in White Willow with Bacillus sp. ZV6 Enhances Ni Phytoextraction from Soil and Soil Quality

Chelating agents may decrease the extent of Ni phytoextraction by reducing plant growth and soil health due to Ni toxicity during enhanced phytoextraction. Contrarily, inducing acidity in the rhizosphere of Ni-accumulating plants with plant growth-promoting rhizobacteria (PGPR) having rhizosphere acidification ability can enhance Ni phytoextraction by increasing Ni bioavailability in the soil, plant growth, and plant stress tolerance. We investigated the efficacy of a PGPR species with rhizosphere acidification potential, named Bacillus sp. ZV6 (ARB), in enhancing Ni phytoextraction by white willow (Salix alba) from a Ni-affected soil. The plants were grown for 120 days in soil with zero, threshold, and moderate Ni pollution levels (0, 50, and 100 mg Ni kg−1 soil, respectively) with and without ARB inoculation. After harvest, the effects of the treatments on rhizosphere acidification and associated Ni bioavailability in this zone, Ni distribution in plants, and Ni removal from the soil were investigated. Moreover, enzyme activity, count of bacteria, biomass of microbes, and organic C in the soil, together with indices of plant growth and antioxidant defense, were evaluated. The ARB inoculation significantly improved the plant parameters and soil health and reduced plant oxidative stress at each Ni level compared to the treatments lacking ARB. Besides lowering the soil pH and increasing Ni bioavailability in the rhizosphere with respect to the bulk zone, ARB inoculation exerted additional effects. Surprisingly, the Ni 100 + ARB treatment induced the highest decrease in soil pH (0.32 unit) and an increase in DPTA-extractable Ni (0.45 mg kg−1 soil) between that measured in the bulk zones and that obtained in the rhizosphere zone. Ni distribution in plant parts and Ni removal (% of total Ni) from the soil were also significantly improved with ARB inoculation, compared to the Ni treatments without ARB. The extent of Ni removal was similar for the Ni 50 + ARB (0.27%) and Ni 100 + ARB (0.25%) treatments. Concluding, ARB-inoculated Salix alba can remove similar amounts of Ni from the soil, irrespective of the Ni pollution level.

Plant Growth-Promoting Rhizobacteria (PGPR) Reduce Adverse Effects of Salinity and Drought Stresses by Regulating Nutritional Profile of Barley

With the growing emphasis on sustainable agriculture, food security, and environmental protection, the use of beneficial soil microbes is imperative, as the use of chemicals such as fertilizers, pesticides, and herbicides has resulted in food contamination, disease, weed resistance, and negative environmental consequences, which ultimately impacted human health. Climate change is a major factor and is of great concern for crop production. Abiotic stresses, including salt and drought stress, restrain the crop yield. The aim of this particular study is to understand what role do plant growth-promoting rhizobacteria (PGPR) play in combating the salinity and drought stresses through modification of nutritional profile. In the current study, inoculated barley (Hordeum vulgare L.) plants were subjected to various stresses such as 200 mM and 1000 mM salinity stress as well as drought stress, and then their various parameters such as seed germination as well as shoot and root biomasses and photosynthetic activity were compared with non-treated stressed barley plants. Our data depicted an improvement or significant enhancement of these parameters in PGPR (Pseudomonas fluorescens SBW25 and Pseudomonas putida KT2440) applied barley plants. Furthermore, the particle-induced X-ray emission (PIXE) technique was used for the elemental analysis of PGPR-inoculated and non-inoculated plants under stress vs. no stress conditions. Our PIXE analysis of various macro- and micronutrients revealed an enhancement of Ca, Mg, K, P, S, Al, and Si uptake in PGPR-treated plants. PGPR applications depicted reduced Cl− contents in 200 mM salt-stressed barley roots (KT2440 = 7.7 mg/kg and SBW25 = 6.3 mg/kg) and stems (KT2440 = 406.4 mg/kg and SBW25 = 365.5 mg/kg) as compared to controls (roots = 8.9 and stems = 469.5), while they displayed a significant increase in the barley leaves (KT2440 = 405 mg/kg and SBW25 = 416.4 mg/kg) when compared to control (110.6 mg/kg) under the same stress condition. In 1000 mM salt stress, a significant reduction in the Cl− content was observed in PGPR-applied barley roots (KT2440 = 7.6 mg/kg), stems (KT2440 = 1205.8 mg/kg and SBW25 = 1008.3 mg/kg), and leaves (KT2440 = 967.8 mg/kg and SBW25 = 530.8 mg/kg) when compared to controls (roots = 15.2 mg/kg, stems = 1605.2 mg/kg, and leaves = 1165.2 mg/kg). On the other hand, a significant increase in the Cl− content was noticed in PGPR-applied barley roots (KT2440 = 29.5 mg/kg and SBW25 = 25.8 mg/kg), stems (KT2440 = 1023.8 mg/kg and SBW25 = 894.9 mg/kg), and leaves (KT2440 = 369.2 mg/kg and SBW25 = 409.8 mg/kg) when compared to controls (roots = 13.5 mg/kg, stems = 505.3 mg/kg, and leaves = 219.9 mg/kg) under drought stress condition. PGPR application was also found to be effective for enhancing the uptake of micronutrients (Mn, Fe, Co, Ni, Cu, and Zn) in barley plant parts under control and also under stressed conditions. Overall, our findings revealed an improvement in the uptake of macro- and micronutrients for the enhancement of salinity and drought stress tolerance. Conclusively, these PGPR species are an effective source of plant stress tolerance and elevated growth of barley and related plants under stress conditions.

Modes of Action of Biocontrol Agents and Elicitors for sustainable Protection against Bacterial Canker of Tomato.

Tomato is one of the world's most commonly grown and consumed vegetables. However, it can be attacked by the Gram-positive bacterium Clavibacter michiganensis subsp. michiganensis (Cmm), which causes bacterial canker on tomato plants, resulting in significant financial losses in field production and greenhouses worldwide. The current management strategies rely principally on the application of various chemical pesticides and antibiotics, which represent a real danger to the environment and human safety. Plant growth-promoting rhizobacteria (PGPR) have emerged as an attractive alternative to agrochemical crop protection methods. PGPR act through several mechanisms to support plant growth and performance, while also preventing pathogen infection. This review highlights the importance of bacterial canker disease and the pathogenicity of Cmm. We emphasize the application of PGPR as an ecological and cost-effective approach to the biocontrol of Cmm, specifying the complex modes of biocontrol agents (BCAs), and presenting their direct/indirect mechanisms of action that enable them to effectively protect tomato crops. Pseudomonas and Bacillus are considered to be the most interesting PGPR species for the biological control of Cmm worldwide. Improving plants' innate defense mechanisms is one of the main biocontrol mechanisms of PGPR to manage bacterial canker and to limit its occurrence and gravity. Herein, we further discuss elicitors as a new management strategy to control Cmm, which are found to be highly effective in stimulating the plant immune system, decreasing disease severity, and minimizing pesticide use.

Endophytic PGPR from Tomato Roots: Isolation, In Vitro Characterization and In Vivo Evaluation of Treated Tomatoes (Solanum lycopersicum L.).

Plant-growth-promoting rhizobacteria (PGPR) are soil bacteria colonizing the rhizosphere and the rhizoplane which have an effect on plant growth through multiple chemical compounds. Rhizobacteria with beneficial effects for plants could therefore be used to reduce the dependence on synthetic chemical fertilizers in conventional agriculture. Within this study, 67 endophytic fungi and 49 bacteria were isolated from root samples from 3 different commercial productions: an off-ground tomato production in a greenhouse, an organic production and a conventional production, both in a soil tunnel. Following morphological selection, 12 fungal and 33 bacterial isolates were genetically identified. Thirteen bacterial isolates belonging to nine potential PGPR species were then applied to tomato seedlings established in sterile substrate. The ability of these bacteria to produce indole acetic acid (IAA) and solubilize phosphate was also evaluated. They all were IAA producers and solubilized phosphate. The most interesting strains for growth promotion were found to be the isolates Pseudomonas palleroniana B10, Bacillus subtilis B25, Bacillus aryabhattai B29 and Pseudomonas fluorescens B17. The isolates P. fluorescens B17, B. aryabhattai B29, B. subtilis B18 and Pseudomonas moraviensis B6 also increased root growth. This study proposed a quick protocol for isolating and testing potential endophytic PGPR that should be characterized further for the direct and indirect mechanisms of growth promotion.

Newly Isolated Paenibacillus monticola sp. nov., a Novel Plant Growth-Promoting Rhizobacteria Strain From High-Altitude Spruce Forests in the Qilian Mountains, China.

Species in the genus Paenibacillus from special habitats have attracted great attention due to their plant growth-promoting traits. A novel plant growth-promoting rhizobacteria (PGPR) species in the genus Paenibacillus was isolated from spruce forest at the height of 3,150 m in the Qilian Mountains, Gansu province, China. The phylogenetic analysis based on 16S rRNA, rpoB, and nifH gene sequences demonstrated that strain LC-T2T was affiliated in the genus Paenibacillus and exhibited the highest sequence similarity with Paenibacillus donghaensis KCTC 13049T (97.4%). Average nucleotide identity (ANIb and ANIm) and digital DNA–DNA hybridization (dDDH) between strain LC-T2T and P. donghaensis KCTC 13049T were 72.6, 83.3, and 21.2%, respectively, indicating their genetic differences at the species level. These differences were further verified by polar lipids profiles, major fatty acid contents, and several distinct physiological characteristics. Meanwhile, the draft genome analysis provided insight into the genetic features to support its plant-associated lifestyle and habitat adaptation. Subsequently, the effects of volatile organic compound (VOC) emitted from strain LC-T2T on the growth of Arabidopsis were evaluated. Application of strain LC-T2T significantly improved root surface area, root projection area, and root fork numbers by 158.3, 158.3, and 241.2%, respectively, compared to control. Also, the effects of LC-T2T on the growth of white clover (Trifolium repens L.) were further assessed by pot experiment. Application of LC-T2T also significantly improved the growth of white clover with root fresh weight increased over three-folds compared to control. Furthermore, the viable bacterial genera of rhizosphere soil were detected in each treatment. The number of genera from LC-T2T-inoculated rhizosphere soil was 1.7-fold higher than that of control, and some isolates were similar to strain LC-T2T, indicating that LC-T2T inoculation was effective in the rhizosphere soil of white clover. Overall, strain LC-T2T should be attributed to a novel PGPR species within the genus Paenibacillus based on phylogenetic relatedness, genotypic features, and phenotypic and inoculation experiment, for which the name Paenibacillus monticola sp. nov. is proposed.

Effect of plant growth promoting rhizobacteria (PGPR) on the growth of habanero pepper (Capsicum chinense Jacq.)

Objective: To evaluate the effect of plant growth promoting rhizobacteria (PGPR) on the growth and production of habanero pepper (Capsicum chinense Jacq.). Design/Methodology/Approach: Twelve strains of PGPR were evaluated in habanero pepper seeds of the orange variety. The species of PGPR were Rhizobium leguminosarum: (CP Méx 46), Pseudomonas spp: (P fluorescens, C2, A7, A9, A9m, Avm); Azospirillum, (Sp7, Sp 59, UAP 40, UAP154), plus a control treatment, giving a total of 13 treatments. The study variables were seedling emergence (SE), plant height (PH), white fruit incidence (WFI), virotic plants (VP), days to flowering (DF) and fresh fruit yield (FFY). The experimental design was random blocks with four repetitions. Results: An effect on the growth of habanero pepper from PGPRs was found in all the variables studied. Seedling emergence and their height was favored by strains Sp9 (84.16%) and A7 (73.44). The number of white flies decreased with the inoculation of CP Méx 46, while the incidence of virosis decreased in plants inoculated with SP9 (32.00%). The highest yield of fresh fruit was found in plants with the strain AVM with 16636 kg ha-1. Findings/Conclusions: The effect of inoculation with PGPR is in function of the strain used and the study variable, growth stage and development stage of the habanero pepper plant.

Effects of Phosphonate Herbicides on the Secretions of Plant-Beneficial Compounds by Two Plant Growth-Promoting Soil Bacteria: A Metabolomics Investigation.

Plant growth-promoting rhizobacteria (PGPR) that colonize plant roots produce a variety of plant-beneficial compounds, including plant-growth regulators, metal-scavenging compounds, and antibiotics against plant pathogens. Adverse effects of phosphonate herbicides, the most extensively used herbicides, on the growth and metabolism of PGPR species have been widely reported. However, the potential consequence of these effects on the biosynthesis and secretion of PGPR-derived beneficial compounds still remains to be investigated. Here, using high-resolution mass spectrometry and a metabolomics approach, we investigated both the intracellular metabolome and the extracellular secretions of biomass-normalized metabolite levels in two PGPR species (Pseudomonas protegens Pf-5, a Gram-negative bacterium; Priestia megaterium QM B1551, a Gram-positive bacterium) exposed to three common phosphonate herbicides (glyphosate, glufosinate, and fosamine; 0.1-1 mM) in either iron (Fe)-replete or Fe-deficient nutrient media. We quantified secreted auxin-type plant hormone compounds (phenylacetic acid and indole-3-acetic acid), iron-scavenging compounds or siderophores (pyoverdine and schizokinen), and antibiotics (2,4-diacetylphloroglucinol and pyoluteorin) produced by these PGPR species. The Fe-replete cells exposed to the phosphonate herbicides yielded up to a 25-fold increase in the production of both auxin and antibiotic compounds, indicating that herbicide exposure under Fe-replete conditions triggered metabolite secretions. However, the herbicide-exposed Fe-deficient cells exhibited a near 2-fold depletion in the secretion of these auxin and antibiotic compounds as well as a 77% decrease in siderophore production. Intracellular metabolomics analysis of the Fe-deficient cells further revealed metabolic perturbations in biosynthetic pathways consistent with the impaired production of the plant-beneficial compounds. Our findings implied that compromised cellular metabolism during nutrient deficiency may exacerbate the adverse effects of phosphonate herbicides on PGPR species.

↵​Impact of A Selected Mycorrhizal Complex and A Rhizobacterial Species on Tomato Plants’ Growth under Water Stress Conditions

Background: Plant strategies for adapting to drought could be improved by associations between plant roots and soil microorganisms, including arbuscular mycorrhizal fungi (AMF) and plant growth promoting rhizobacteria (PGPR). In this study, the impact of a selected AMF complex and a selected PGPR species on the growth of tomato (Lycopersicum esculentum Mill.) under induced water stress was evaluated. Methods: Three different inoculation treatments were applied to tomato seedlings (a complex of AMF composed mainly of Glomus genus a Bacillus sp. PGPR treatment and a combination of both) and three different water levels (75%, 50% and 25% of field capacity). Result: A significant damaging impact of drought on tomato growth parameters and root mycorrhizal colonization, although the presence of microbes stimulated tomato plants growth and decreased the impact ofdrought stress. Indeed inoculated plants presented greater heights, fresh and dry weights, leaves number and area; greater water status; and greater proteins, sugars and chlorophylls contents either with the AMF complex or the Bacillus sp. in normal and drought stress conditions compared to the non-inoculated plants. However dual inoculation recorded the highest values under all water levels treatments.

Quorum Quenching Activity of the PGPR Bacillus subtilis UD1022 Alters Nodulation Efficiency of Sinorhizobium meliloti on Medicago truncatula.

Plant growth-promoting rhizobacteria (PGPR) have enormous potential for solving some of the myriad challenges facing our global agricultural system. Intense research efforts are rapidly moving the field forward and illuminating the wide diversity of bacteria and their plant beneficial activities. In the development of better crop solutions using these PGPR, producers are including multiple different species of PGPR in their formulations in a “consortia” approach. While the intention is to emulate more natural rhizomicrobiome systems, the aspect of bacterial interactions has not been properly regarded. By using a tri-trophic model of Medicago truncatula A17 Jemalong, its nitrogen (N)-fixing symbiont Sinorhizobium meliloti Rm8530, and the PGPR Bacillus subtilis UD1022, we demonstrate indirect influences between the bacteria affecting their plant growth-promoting activities. Co-cultures of UD1022 with Rm8530 significantly reduced Rm8530 biofilm formation and downregulated quorum sensing (QS) genes responsible for symbiotically active biofilm production. This work also identifies the presence and activity of a quorum quenching lactonase in UD1022 and proposes this as the mechanism for non-synergistic activity of this model “consortium.” These interspecies interactions may be common in the rhizosphere and are critical to understand as we seek to develop new sustainable solutions in agriculture.

The Role of Microbial Inoculants on Plant Protection, Growth Stimulation, and Crop Productivity of the Olive Tree (Olea europea L.).

The olive tree (Olea europaea L.) is an emblematic, long-living fruit tree species of profound economic and environmental importance. This study is a literature review of articles published during the last 10 years about the role of beneficial microbes [Arbuscular Mycorrhizal Fungi (AMF), Plant Growth Promoting Rhizobacteria (PGPR), Plant Growth Promoting Fungi (PGPF), and Endophytes] on olive tree plant growth and productivity, pathogen control, and alleviation from abiotic stress. The majority of the studies examined the AMF effect using mostly Rhizophagus irregularis and Glomus mosseae species. These AMF species stimulate the root growth improving the resistance of olive plants to environmental and transplantation stresses. Among the PGPR, the nitrogen-fixing bacteria Azospirillum sp. and potassium- and phosphorous-solubilizing Bacillus sp. species were studied extensively. These PGPR species were combined with proper cultural practices and improved considerably olive plant’s growth. The endophytic bacterial species Pseudomonas fluorescens and Bacillus sp., as well as the fungal species Trichoderma sp. were identified as the most effective biocontrol agents against olive tree diseases (e.g., Verticillium wilt, root rot, and anthracnose).

IMPACT OF PLANT GROWTH PROMOTING RHIZOBACTERIA AS BIO-CONTROL AGENTS ON CITRUS NEMATODE, TYLENCHULUS SEMIPENETRANS INFECTING BALADY ORANGE (CITRUS SINENSIS L.) AND IMPROVING ITS PRODUCTIVITY

Three bacterial species of plant growth promoting rhizobacteria (PGPR) namely Paenibacillus polymyxa, Methylobacterium mesophilicum and Methylobacterium radiotolerans were tested alone or combined with humic acid as bio-control agents against the citrus nematode Tylenchulus semipenetrans under laboratory and field conditions. Results cleared that; all tested PGPR species produced IAA, HCN, ammonia, chitinase and protease enzymes and also solubilized phosphate in laboratory. P. polymyxa emphasized the superiority among other species in all PGPR properties except for phosphate solubilization, whereas M. radiotolerans showed highest amount of phosphorus solubilized in culture media. On the other hand, the results of the nematode survey conducted on orange, Citrus sinensis L. cv Balady grown in different localities of Ismailia and Sharkia Governorates during season 2019, revealed the presence of seven genera and species of plant-parasitic nematodes. Among which, T. semipenetrans occurred in all examined samples (100% frequency of occurrence) with a relatively high population density of 2330 and 2640 infected juveniles (J2) /250 g soil in Ismailia and Sharkia Governorates, respectively. Field experiments were conducted in two different locations, at Ismailia and Sharkia Governorates to assess the effectiveness of PGPR strains alone or combined to reduce the numbers of T. semipenetrans during season 2019. It was found that, all treatments caused significant (P≤0.05) reduction in T. semipenetrans population, compared to control treatment. The nematicide, Nemathorin® 10% G followed by P. polymyxa (20 L.fed-1 ) + foliarspraying of M. mesophilicm (5 L.fed-1 ) gave the highest efficacy in controlling the citrus nematode. Percentage reduction in numbers of J2/250g soil and adult females/1g roots for these treatments in Ismailia Governorate were 91% (85.7%) and 91.4% (89.5%), respectively. While the parallel values in Sharkia Governorate were 90% (87%) and 94% (90%), respectively. The combination of P. polymyxa and humic acid (20 L.fed-1 ) with foliar spraying of M. mesophilicm (5 L.fed-1 ) gained the third position. All treatments increased the fruit yield compared to control treatment. The highest percentages of increase were determined with Nemathorin (160% and 206%) followed by P. polymyxa + Humic acid (20 L.fed-1 ) + foliar spraying of M. mesophilicum (155.7% and 193%) and M. radiotolerans + P. polymyxa + foliar spraying of M. mesophilicum (153% and182%) in Ismailia, and Sharkia Governorates respectively.

Plant growth promoting rhizobacteria improve the antioxidant status in Mentha piperita grown under drought stress leading to an enhancement of plant growth and total phenolic content

An experiment was conducted to investigate the effects of plant growth promoting rhizobacteria (PGPR) on Mentha piperita grown under drought stress. We performed root inoculation with strains of two PGPR species (Pseudomonas fluorescens WCS417 r and Bacillus amyloliquefaciens GB03) on peppermint plants and subjected them to moderate (MS) or severe drought stress (SS). To determine the growth-promoting potential and ability of PGPR to increase the drought tolerance in peppermint, different plant growth parameters were measured, along with the activities of antioxidant enzymes such as peroxidase (PX) and superoxide dismutase (SOD), as well as the non-enzymatic antioxidant proline and total phenolic content (TPC). In addition, to determine whether drought stress induces oxidative damage in peppermint, membrane lipid peroxidation was analyzed. An increment in the level of drought stress produced a reduction in plant growth, fresh weight, leaf number and leaf area. However, these negative effects of drought were mitigated in plants exposed to PGPR inoculation, resulting in significantly less reduction in the above growth traits related to plants not treated with PGPR, regardless of the severity of the drought treatment. In addition, drought-stressed plants treated with PGPR had a significantly higher total phenolic content than water-stressed plants without PGPR. Higher enzymatic activities were also observed in drought-stressed plants inoculated with PGPR. The proline content did not change in stressed plants, but inoculation reduced the amount of proline in the different stressed conditions tested. Membrane lipid peroxidation was also decreased in inoculated plants grown under drought conditions.These results are important as they illustrate the potential of PGPR to mitigate the adverse consequences of drought stress, and offer a way of increasing the tolerance of peppermint plants grown and TPC under water deficit conditions.

Monitoring of soil microbial inoculants and their impact on maize (Zea mays L.) rhizosphere using T-RFLP molecular fingerprint method

Plant Growth Promoting Rhizobacteria (PGPR) are commonly applied in agricultural systems as biofertilizers due to their beneficial effects on plant productivity and crop yield. Considering the growing nutritional needs of the human population, PGPR inoculants adapted to deteriorated soil conditions offer a sustainable way for crop production. The objective of this study was to detect the presence and estimate the relative abundance of the PGPR members of ‘stress-tolerant’ bacterial inocula, and to investigate their effect on the indigenous bacterial community structure of the maize rhizosphere. The acid stress tolerant bacterial inocula were compared to inocula used in mildly alkaline soil, during the 112-day growth period of maize. The Terminal Restriction Fragment Length Polymorphism (T-RFLP) molecular fingerprint method offered a possibility for monitoring the applied bacteria. Two different restriction endonucleases were selected in silico and tested in vitro for specific detection of the inoculated PGPR. The developed T-RFLP model system indicated a clear positive effect of inoculants on the relative abundance of Azospirillum species in mildly alkaline (pH 7.3) and acid (pH 5.35) plot between days 14 and 21, and also on that of Agreia pratensis in acid plot in the period between days 35 and 56. In case of Azospirillum and acid soil the positive effect proved to be statistically significant (2–5 times higher relative abundances compared to the control). Similarly, the abundances of the monitored PGPR species and the maize yield were more positively influenced in acid soil than in mildly alkaline soil treated by the inocula developed. In the case of acid soil treatments resulted in significant growth of cob number (1.29–1.44 times of the control). The bacterial community structure of the rhizosphere significantly differed depending on soil type and maize growth stage.

English

Plant growth-promoting rhizobacteria (PGPR) are well-known to influence plant growth via a variety of mechanisms such as nitrogen fixation, production of volatile organic compounds and enzymes, and bioremediation contaminants from the environment. PGPR have been previously identified by other researchers using laboratory screening methods. It was hypothesized that relying on these routine laboratory tests, some PGPR species are being overlooked. These species could promote growth through genes that encode for the synthesis of specific growth stimuli or other growth-promoting traits such as vitamins, antibiotics, and secondary metabolites. To evaluate this hypothesis, PGPR (MA-7, ON-4, SP-7, and RA-9) and previously overlooked PGPR (SE-7, LE-26, SQ-7, and SQ-9) were tested both with sterilized and non-sterilized soil in pot and greenhouse experiments. The PGPR isolates significantly increased pea plant growth, albeit to different degrees based on isolate, in both types of soil. The increases were recorded in shoot and root length and fresh matter in non-sterilized soil whereas increases in root length and root fresh weight were observed in sterilized soil. Interestingly, strains SE-7 and SQ-7 of the four overlooked PGPR isolates tested were also able to promote pea plant growth similarly to the PGPR isolates under both pot and greenhouse conditions. Morphological and biochemical characterization of the four original PGPR isolates revealed that they were rod-shaped, gram-positive, and spore-forming. Sequencing of 16S ribosomal RNA showed that these strains were mostly similar to Bacillus sp. (99% similarity). Using the EzBioCloud 16S rRNA database, it was found that one strain was likely to be Bacillus paramycoides based on 100% similarity, two strains were Bacillus wiedmannii based on 99.05 and 100% similarity, and the remaining strain was Bacillus amyloliquefaciens based on 99.64% similarity. Key words: Plant growth-promoting rhizobacteria (PGPR), pea, soil, 16S rRNA, Bacillus.

The Potential Use of Plant Growth-Promoting Rhizobacteria (PGPR) as Antagonists and Biocontrol Agents in Hevea Diseases

The rhizosphere related bacterial species have a significant capability in plant growth promotion, referred to as Plant Growth Promoting Rhizobacteria (PGPR). Several substances produced by antagonistic rhizobacteria have been related to pathogen control and indirect promotion of growth in many plants. The objectives of this study were to identify effective PGPR bacteria in rubber soils of Sri Lanka, determine the potential use as antagonists and biocontrol agents to control economically important Hevea diseases in nurseries, and identify effective novel inoculant formulations. Soil samples were collected from the root rhizozphere of cover crops in Hevea fields in Rubber Research Institute of Sri Lanka (RRISL), Dartonfield. Bacteria were isolated by TSA plate method. Isolated PGPR bacteria were subjected to test the enzyme secretion ability. Pectinase, Lipase, and Amylase secretion ability of PGPR were tested in specifically developed media. Auxin production of PGPR were measured using freshly prepared Salkowsky reagent. Phosphorous solubilizing ability of PGPR was tested using Pikovskaya (PVK) medium. Antagonistic ability of PGPR was tested against the Rigidoporous microporus, Fusarrium sp. and Pythium sp. causing white root disease and Patch Canker disease in rubber respectively. Paper disc diffusion method was used to test the antagonism of selected bacteria and the percentage growth inhibition was calculated. Plasmid DNA extraction of antagonistic PGPR was done following the alkaline SDS method. Gene sequencing of selected bacteria was further carried out. Among 13 selected PGPR bacterial species, 3 bacteria were showing significant antagonism towards R. microporus. (Percentage growth inhibition by 51.2%, 46.8% and 44.3% respectively) Fusarium sp. was controlled by 7 PGPR species. Pythium sp. was controlled by 2 PGPR (Percentage growth inhibition by 41.6% and 38.7% respectively) 5 types of combinations of bacteria were tested. Among 5 types, 2 types were highly effective in control of R. microporus (Percentage growth inhibition by 79.8% and 72.6% respectively). All 5 combinations were moderately effective in control Fusarium sp. and 1 type was highly effective in control of Pythium sp. (Percentage growth inhibition by 67.8%). PGPR can be used as effective biocontrol agents in control of Hevea pathogens. The combinations of PGPR are more effective than the individual bacterial species. This will be leading to improve the productivity of Hevea plantations in Sri Lanka. Keywords: Plant Growth Promoting Rhizobacteria, Antagonism, Hevea pathogens

Current Research on Plant-Growth Promoting Rhizobacteria in Latin America: Meeting Report from the 2nd Latin American PGPR Workshop

Crop performance depends, to a large degree, on biological activities taking place in the root environment. By the early twentieth century, it was already recognized that plants exert key effects on the soil space just outside the root surface. This led to the concept of the rhizosphere, that is, the portion of the soil that is directly influenced by the root (Hiltner 1904). The rhizosphere contains diverse microorganisms that induce beneficial, neutral, or deleterious effects on plant growth. The beneficial rhizosphere bacteria were deemed plant-growth promoting rhizobacteria (PGPR) (Kloepper and Schroth 1978). The rhizosphere was then recognized to be an active microbial habitat with complex and multitrophic biological interactions among microbes and eukaryotic organisms, and focused studies on these interactions began. The applications of such knowledge were of direct economical relevance given that PGPRs may be utilized as agronomic inputs to increase crop yields. This gave rise to a community of scientists interested in elucidating the mechanisms displayed by PGPR species and understanding the role of the plant in shaping the microbial community of the rhizosphere. In parallel, a number of private companies started to commercialize PGPR-based products for a range of agronomical applications worldwide, with Latin American countries taking the lead in this field. With these interests in mind, the International PGPR Workshop was first held in Canada in 1987 and since then, the workshop has been held every 3 years at different locations around the world (Switzerland, Australia, Japan, Argentina, India, The Netherlands, USA and Colombia). With the selection of a Latin American venue for the last international PGPR workshop and based on a regional demand for exchange of basic and applied information about the biology of PGPR and its applications, Quirama (Medellin, Colombia) hosted the 1st official Latin American PGPR meeting as a branch of the International edition. At that time, it was decided that the Latin American PGPR would be held every two years, and that the next venue would be Argentina.

Getting the ecology into interactions between plants and the plant growth-promoting bacterium Pseudomonas fluorescens.

Plant growth-promoting rhizobacteria (PGPR) are increasingly appreciated for their contributions to primary productivity through promotion of growth and triggering of induced systemic resistance in plants. Here we focus on the beneficial effects of one particular species of PGPR (Pseudomonas fluorescens) on plants through induced plant defense. This model organism has provided much understanding of the underlying molecular mechanisms of PGPR-induced plant defense. However, this knowledge can only be appreciated at full value once we know to what extent these mechanisms also occur under more realistic, species-diverse conditions as are occurring in the plant rhizosphere. To provide the necessary ecological context, we review the literature to compare the effect of P. fluorescens on induced plant defense when it is present as a single species or in combination with other soil dwelling species. Specifically, we discuss combinations with other plant mutualists (bacterial or fungal), plant pathogens (bacterial or fungal), bacterivores (nematode or protozoa), and decomposers. Synergistic interactions between P. fluorescens and other plant mutualists are much more commonly reported than antagonistic interactions. Recent developments have enabled screenings of P. fluorescens genomes for defense traits and this could help with selection of strains with likely positive interactions on biocontrol. However, studies that examine the effects of multiple herbivores, pathogens, or herbivores and pathogens together on the effectiveness of PGPR to induce plant defenses are underrepresented and we are not aware of any study that has examined interactions between P. fluorescens and bacterivores or decomposers. As co-occurring soil organisms can enhance but also reduce the effectiveness of PGPR, a better understanding of the biotic factors modulating P. fluorescens–plant interactions will improve the effectiveness of introducing P. fluorescens to enhance plant production and defense.