Plant Growth-promoting Rhizobacteria Strains Research Articles

Morphological, Biochemical and Molecular Characterization of Plant Growth Promoting Rhizobacteria with Synergistic Effect against Fusarium oxysporum

Currently, plant growth-promoting rhizosphere bacteria (PGPR) are heavily exploited as microbial inoculants in agricultural production among which Pseudomonas sp. and Bacillus sp. are widely used in plant growth promotion and disease control as excellent inoculum strains. Plant growth-promoting rhizobacteria has the potential to be used in agriculture to promote plant growth and health through various mechanisms. Therefore, identifying novel strains tailored to specific agricultural requirements continues to be a key area of research. In the present research work, the strains of PGPR isolated from soybean rhizosphere at Department of Plant Pathology and characterized at morphological, biochemical and molecular level. These strains are now used in ‘Biomix’ formulation a microbial consortium product developed at the Department of Plant Pathology VNMKV, Parbhani to benefit the farmers. Studying the molecular, biochemical, and morphological traits of these microbial strains which are now involved in VNMKV produced ‘Biomix formulation is crucial to maintain its consistency, purity and optimize its formulation. Molecular characterization and phylogenetic analysis will ensure accurate identification and classification of the isolates, enhancing the biofertilizer's effectiveness in agriculture. All three isolates have been identified as per Bergey’s Manual for the determination of bacteriology. All isolates tested positive for starch hydrolysis, citrate utilization, catalase activity, indole production, gelatine hydrolysis, and tetrazolium reduction. Additionally, the 16S rDNA gene sequences of the various potential isolates were validated as P. fluorescens VNMKV1, P. putida VNMKV1, and B. subtilis VNMKV1. The potential of these novel strains of plant growth-promoting rhizobacteria as biocontrol agents was explored by screening against soil-borne pathogenic fungi highlighting their potential to be used as bioinoculant agents.

Flg22-facilitated PGPR colonization in root tips and control of root rot.

Plant root border cells (RBCs) prevent the colonization of plant growth-promoting rhizobacteria (PGPR) at the root tip, rendering the PGPR unable to effectively control pathogens infecting the root tip. In this study, we engineered four strains of Pseudomonas sp. UW4, a typical PGPR strain, each carrying an enhanced green fluorescent protein (EGFP)-expressing plasmid. The UW4E strain harboured only the plasmid, whereas the UW4E-flg22 strain expressed a secreted EGFP-Flg22 fusion protein, the UW4E-Flg(flg22) strain expressed a non-secreted Flg22, and the UW4E-flg22-D strain expressed a secreted Flg22-DNase fusion protein. UW4E-flg22 and UW4E-flg22-D, which secreted Flg22, induced an immune response in wheat RBCs and colonized wheat root tips, whereas the other strains, which did not secrete Flg22, failed to elicit this response and did not colonize wheat root tips. The immune response revealed that wheat RBCs synthesized mucilage, extracellular DNA, and reactive oxygen species. Furthermore, the Flg22-secreting strains showed a 33.8%-93.8% higher colonization of wheat root tips and reduced the root rot incidence caused by Rhizoctonia solani and Fusarium pseudograminearum by 24.6%-35.7% compared to the non-Flg22-secreting strains in pot trials. There was a negative correlation between the incidence of wheat root rot and colonization of wheat root tips by these strains. In contrast, wheat root length and dry weight were positively correlated with the colonization of wheat root tips by these strains. These results demonstrate that engineered secretion of Flg22 by PGPR is an effective strategy for controlling root rot and improving plant growth.

Isolation and characterization of plant growth-promoting rhizobacteria (PGPR) from eelgrass Zostera marina rhizosphere: implications for bioremediation

Plant growth-promoting rhizobacteria (PGPR) play a crucial role in enhancing plant growth. However, investigations into the presence and function of PGPR in seagrass rhizospheres remain relatively limited. In this study, we isolated 45 strains of PGPR from Zostera marina rhizospheres and assessed their functional attributes. Additionally, we evaluated the performance of these candidate strains under varying environmental conditions, such as temperature, salinity, and pH. Out of the 45 analyzed strains, 6 were found to possess the nifH gene; some strains also exhibited the ability to solubilize inorganic phosphorus, with dissolved phosphorus content ranging widely from 14.6 to 393.9 mg l-1. Eleven strains demonstrated indole-3-acetic acid production, with yields spanning from 16.3 to 42.8 mg l-1. Siderophores and ammoniated proteins were produced by 32 and 20 strains, respectively. Notably, 5 PGPR strains (F65, G84, G85, G86, and I109) displayed multiple growth-promoting properties along with strong adaptability to a wide range of physicochemical conditions. This study highlights the potential reservoir of PGPR in the eelgrass rhizosphere and provides significant implications for utilizing these bacteria to enhance the success rate of restoring degraded seagrass meadows.

Growth Promoting Characteristics of Bacillus amyloliquefaciens GZA69 and Its Biocontrol Potential Against Soft Rot of Amorphophallus konjac

Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria colonizing the plant rhizosphere and can promote plant growth. PGPR have important application potential in the field of microbial fertilizers. This study isolated and characterized a PGPR strain GZA69 from konjac rhizosphere soil collected from Damogu Village, Luliang County, Qujing City, Yunnan Province, China. The strain was identified as Bacillus amyloliquefaciens. GZA69 showed diverse plant growth-promoting abilities, including nitrogen fixation, phosphorus solubilization, siderophore production, and indole-3-acetic acid (IAA) production. Tomato was used as an indicator crop to evaluate its growth-promoting effect, two GZA69 suspension concentrations (A1, OD600 = 0.3 and A2, 0.6) were used to treat tomato seeds and seedlings. Plate confrontation assay and konjac corm tissue inoculation were conducted to identify the antagonistic effect of GZA69 strain on the pathogen of konjac soft rot disease. The results showed that, both GZA69 concentrations significantly promoted tomato seed germination (A) and seedling growth, with growth increased of 8.2% and 9.66% in height, 20.87% and 22.77% in root length, and 90% and 130% in fresh weight, respectively. Additionally, GZA69 demonstrated a significant inhibitory effect on the konjac soft rot pathogen, with an inhibition zone of 1.47±0.07 cm. Furthermore, GZA69 effectively reduced disease incidence in inoculated corm tissues, with disease index decreased by 8.00%, 16.23%, 24.80% in co-inoculated with different concentrations of GZA69 suspensions (solution of soft rot pathogen and GZA69 bacterial at ratios of 1:1, 1:2, and 1:3, respectively) comparted to the control (sterile water). In summary, the B. amyloliquefaciens GZA69 screened from konjac rhizosphere soil has various plant growth promoting characteristics and has the potential to prevent and control konjac soft rot disease, which has important application value for developing konjac microbial fertilizers.

Metagenomic Analysis to Assess the Impact of Plant Growth-Promoting Rhizobacteria on Peanut (Arachis hypogaea L.) Crop Production and Soil Enzymes and Microbial Diversity.

Peanut production could be increased through plant growth-promoting rhizobacteria (PGPR). In this regard, the present field research aimed at elucidating the impact of PGPR on peanut yield, soil enzyme activity, microbial diversity, and structure. Three PGPR strains (Bacillus velezensis, RI3; Bacillus velezensis, SC6; Pseudomonas psychrophila, P10) were evaluated, along with Bradyrhizobium japonicum (BJ), taken as a control. PGPR increased seed yield by 8%, improving the radiation use efficiency (4-14%). PGPR modified soil enzymes (fluorescein diacetate activity by 17% and dehydrogenase activity by 28%) and microbial abundance (12%). However, PGPR did not significantly alter microbial diversity; nonetheless, it modified the relative abundance of key phyla (Actinobacteria > Proteobacteria > Firmicutes) and genera (Bacillus > Arthrobacter > Pseudomonas). PGPRs modified the relative abundance of genes associated with N-fixation and nitrification while increasing genes related to N-assimilation and N-availability. PGPR improved agronomic traits without altering rhizosphere diversity.

Identification of Stutzerimonas stutzeri volatile organic compounds that enhance the colonization and promote tomato seedling growth.

Volatile organic compounds (VOCs) have an important function in plant growth-promoting rhizobacteria (PGPR) development and plant growth. This study aimed to identify VOCs of the PGPR strain, Stutzerimonas stutzeri NRCB010, and investigate their effects on NRCB010 biofilm formation, swarming motility, colonization, and tomato seedling growth. Solid-phase microextraction and gas chromatography-mass spectrometry were performed to identify the VOCs produced during NRCB010 fermentation. A total of 28 VOCs were identified. Among them, seven (e.g. γ-valerolactone, 3-octanone, mandelic acid, 2-heptanone, methyl palmitate, S-methyl thioacetate, and 2,3-heptanedione), which smell well, are beneficial for plant, or as food additives, and without serious toxicities were selected to evaluate their effects on NRCB010 and tomato seedling growth. It was found that most of these VOCs positively influenced NRCB010 swarming motility, biofilm formation, and colonization, and the tomato seedling growth. Notably, γ-valerolactone and S-methyl thioacetate exhibited the most positive performances. The seven NRCB010 VOCs, essential for PGPR and crop growth, are potential bioactive ingredients within microbial fertilizer formulations. Nevertheless, the long-term sustainability and replicability of the positive effects of these compounds across different soil and crop types, particularly under field conditions, require further investigation.

Enhancing Soybean Growth with Phytohormone-enriched PGPR Bioinoculants and Biofertilizers in Plant Stress Physiology

Background: The present investigation aimed to evaluate the effects of plant growth-promoting rhizobacteria (PGPR) bioinoculants and their carrier-based biofertilizers on the physiology of soybean. Methods: Seeds of two soybean varieties (cv. NARC-1 and cv. William-82) were either soaked in broth cultures of two PGPR strains, Planomicrobium chinense (MF616408) and Pseudomonas putida (KX574857), containing 106 cells/mL, or coated with biofertilizers. Three biofertilizers, namely biostimulant, biozote and biofert, were used. Result: Significant increases in root and shoot weight were observed with both liquid bioinoculants and carrier-based biofertilizers compared to the control. The consortium of P. chinense and P. putida resulted in the highest proline content and superoxide dismutase (SOD) activity. Biostimulant-treated plants exhibited higher catalase (CAT) activity and phenolics content. Indole-3-acetic acid (IAA) and gibberellic acid (GA) levels were enhanced in plants treated with PGPR and biofertilizers. Specifically, P. chinense and the biostimulant significantly promoted IAA content in cv. William-82, while biozote and P. chinense treatments increased GA content in both varieties. The consortium of P. chinense and P. putida showed the lowest ABA levels. The highest IAA/ABA and GA/ABA ratios were observed in plants treated with P. chinense. Liquid bioinoculants had a more pronounced effect on plant growth than carrier-based biofertilizers, though biofertilizers had stronger residual effects on rhizosphere soil organic matter. The combined effects of phytohormones, sugar, protein and proline production and antioxidant enzyme activity influenced plant growth and development.

Alleviation of salt stress on Zea mays L. plant by PGPR isolates as an effective sustainable strategy

Among the major constraints for the production of economic crops, such as maize, is salinity. Plenty of research has suggested the use of various strains of plant growth-promoting rhizobacteria (PGPR) as a sustainable approach to improve plant tolerance to salinity. Nonetheless, the exploration of the rhizosphere in Egyptian agricultural lands with salt affected soil remains incomplete. In this research, two newly isolated local bacterial species were finally selected and identified as Alcaligenes sp. strains. E1 (BS-45) and E2 (BS-57). IAA production by both isolates was confirmed by TLC. Several bioactive metabolites were identified in Alcaligenes sp. E1 and E2 by gas chromatography-mass spectroscopy (GC-MS). These include 9,12,15-octadecatrienoic acid, 2,3-bis [(trimethylsilyl)oxy] propyl ester, (z,z,z), 2 (1H)-Naphthalenone, octahydro-1-methyl-1-(2-propenyl)-, (1à,4aá,8aà), dodecane and isopulegol which have a defensive role against biotic and abiotic stress of the plant. Interestingly, the selected bacteria Alcaligenes sp. E1 and E2 significantly improve the overall maize growth attributes, antioxidant scavenging activities and photosynthetic pigments in maize plants grown under salt stress (150 mM NaCl) compared to the control plants as analyzed by greenhouse experiment. Therefore, this study recommends the Alcaligenes sp. strains (E1 and E2) to alleviate the negative impact of salinity on the growth and development of maize seedlings.

Pseudochrobactrum asaccharolyticum mitigates arsenic induced oxidative stress of maize plant by enhancing water status and antioxidant defense system

BackgroundOxidative stress mediated by reactive oxygen species (ROS) is a common denominator in arsenic toxicity. Arsenic stress in soil affects the water absorption, decrease stomatal conductance, reduction in osmotic, and leaf water potential, which restrict water uptake and osmotic stress in plants. Arsenic-induced osmotic stress triggers the overproduction of ROS, which causes a number of germination, physiological, biochemical, and antioxidant alterations. Antioxidants with potential to reduce ROS levels ameliorate the arsenic-induced lesions. Plant growth promoting rhizobacteria (PGPR) increase the total soluble sugars and proline, which scavenging OH radicals thereby prevent the oxidative damages cause by ROS. The main objective of this study was to evaluate the potential role of Arsenic resistant PGPR in growth of maize by mitigating arsenic stress.MethodologyArsenic tolerant PGPR strain MD3 (Pseudochrobactrum asaccharolyticum) was used to dismiss the ‘As’ induced oxidative stress in maize grown at concentrations of 50 and 100 mg/kg. Previously isolated arsenic tolerant bacterial strain MD3 “Pseudochrobactrum asaccharolyticum was used for this experiment. Further, growth promoting potential of MD3 was done by germination and physio-biochemical analysis of maize seeds. Experimental units were arranged in Completely Randomized Design (CRD). A total of 6 sets of treatments viz., control, arsenic treated (50 & 100 mg/kg), bacterial inoculated (MD3), and arsenic stress plus bacterial inoculated with three replicates were used for Petri plates and pot experiments. After treating with this MD3 strain, seeds of corn were grown in pots filled with or without 50 mg/kg and 100 mg/kg sodium arsenate.ResultsThe plants under arsenic stress (100 mg/kg) decreased the osmotic potential (0.8 MPa) as compared to control indicated the osmotic stress, which caused the reduction in growth, physiological parameters, proline accumulation, alteration in antioxidant enzymes (Superoxide dismutase-SOD, catalase-CAT, peroxidase-POD), increased MDA content, and H2O2 in maize plants. As-tolerant Pseudochrobactrum asaccharolyticum improved the plant growth by reducing the oxidation stress and antioxidant enzymes by proline accumulation. PCA analysis revealed that all six treatments scattered differently across the PC1 and PC2, having 85.51% and 9.72% data variance, respectively. This indicating the efficiency of As-tolerant strains. The heatmap supported the As-tolerant strains were positively correlated with growth parameters and physiological activities of the maize plants.ConclusionThis study concluded that Pseudochrobactrum asaccharolyticum reduced the ‘As’ toxicity in maize plant through the augmentation of the antioxidant defense system. Thus, MD3 (Pseudochrobactrum asaccharolyticum) strain can be considered as bio-fertilizer.

Mining rhizobacteria from indigenous halophytes to enhance alfalfa (Medicago sativa L.) growth and soil reclamation in saline soils of Northwest China

Enhancing the growth of alfalfa (Medicago sativa L.) through inoculation with rhizobacteria represents a sustainable strategy for reclaiming saline soils. However, the lack of suitable strains and practical application guidelines poses significant challenges to the utilization of Plant Growth-Promoting Rhizobacteria (PGPR) in salt-affected soils of Northwest China. In this study, we selected four PGPR strains derived from indigenous halophytes based on their growth-promoting characteristics. These strains underwent further selection via a petri dish assay. Subsequently, the effects of the selected PGPR strains on alfalfa growth and soil fertility were rigorously examined through pot trials. The results demonstrated that Bacillus filamentosus HL3, B. filamentosus HL6, Bacillus subtilis subsp. stercoris HG12, and Paenibacillus peoriae HG24 significantly produced indole-3-acetic acid (IAA), solubilized phosphorus, and fixed nitrogen (except for B. filamentosus HL6, which did not significantly fix nitrogen). Compared to non-inoculated plants, B. filamentosus HL6 and B. subtilis subsp. stercoris HG12 significantly enhanced seed germination, root elongation, and seedling biomass in a 150 mM NaCl saline solution. In saline-alkaline soils, PGPR inoculation under brackish water irrigation did not restore alfalfa growth to the levels observed under freshwater irrigation. Principal Component Analysis (PCA) condensed ten indicators into two indices, explaining 86.85% of the variance. Using these two indices as weights, an evaluation model for the PGPR-alfalfa symbiosis indicated that B. subtilis subsp. stercoris HG12 had the most substantial effect under freshwater irrigation, while co-inoculation with B. subtilis subsp. stercoris HG12 and B. filamentosus HL6 had the most significant impact on alfalfa growth and soil improvement under brackish water irrigation. Available phosphorus was identified as the primary factor influencing alfalfa growth, contributing 82.3% to the growth variation. These findings provide suitable microbial strains for the utilization of saline-alkali land and underscore the potential of applying indigenous PGPR-alfalfa symbiotic techniques to improve soil fertility and crop yield in the arid regions of Northwest China.

Macrogenome-based study on the mechanism of Bacillus velezensis SX13 in regulating rhizophere environment and cucumber growth under different cultivation environments

Microbial activities are the dynamic core of nutrient cycling in organic substrates, and the exploitation of plant growth-promoting rhizobacteria strains contributes to sustainable agricultural development. This study aimed to investigate the effect and mechanism of Bacillus velezensis SX13 in nutrient cycling and plant promotion under different substrate supply conditions. The effects of reduced substrate amount (sCK) and inoculation of SX13 strain under both substrate supply conditions (Bv and sBv) on rhizosphere microenvironment and plant growth were investigated using conventional substrate amount (CK) as a control. Results showed no significant difference in the α-diversity indexes (Chao1 and Shannon) of the rhizospheric microbial community among the four treatments. However, nonmetric multidimensional scaling analysis and principal coordinate analysis revealed that compared with CK treatment, the inoculation of SX13 strain and reduced substrate supply reshaped the β-diversity structure of microbial communities. Furthermore, inoculation with B. velezensis SX13 under both substrate supply conditions increased the abundance of Proteobacteria (1.64%–2.46 %), Acidobacteria (14.09%–43.07 %), and Firmicutes (179.29%–861.29 %). The results of the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the metabolic pathway with the highest abundance of enriched genes was also the pathway with the most enriched differential genes caused by reducing substrate supply or inoculation of B. velezensis SX13. The rhizosphere inoculation of B. velezensis SX13 significantly up-regulated the top genes related to carbohydrate esterases, carbohydrate binding modules, glycoside hydrolases, glycoside transferases, and polysaccharide lyases. As a result, the activities of carbon and nitrogen cycle-related enzymes such as cellobiohydrolase, β-glucosidase, urease, l-leucine amino peptidase, and β-1 4-N-acetylglucosaminidase were increased, which in turn accelerated nutrient cycling. B. velezensis SX13 and its mediated improvement of the rhizospheric microenvironment resulted in the up-regulation of root CsNRT family genes (such as CsNRT1.1, CsNRT1.4a, CsNRT1.4b, CsNRT1.5a, CsNRT1.5b, CsNRT1.5c, and CsNRT1.8), which accelerated nitrogen uptake, accumulation, and utilization efficiency and ultimately improved the yield and quality of cucumber. The effect of SX13 strain was more stable and efficient under conventional substrate supply conditions than under reduced substrate supply conditions.

Enhancing saffron (Crocus sativus L.) growth in the Kashmir valley with resilient and widely effective Plant Growth-Promoting Rhizobacteria (PGPR) under field conditions

This study provides a thorough examination of stress-tolerant plant growth-promoting rhizobacteria (PGPR) and their significance in enhancing plant growth and biocontrol activities, and influence on saffron cultivation in the Kashmir Valley. Quantitative analysis of 35 efficient PGPR revealed significant variability in plant growth promotion (PGP) attributes ranging from 60 to 653 µg/ml tri-calcium phosphate (TCP) solubilization, 1548–4311 nM α-ketobutyrate h/mg/protein ACC-deaminase activity, 1.4–74.8 µg/ml IAA-like auxins production, and 94–214 % siderophore production. Molecular fingerprinting disclosed 52 ERIC types that belonged to 25 genera, mainly Bacillus. Bacillus species including B. aryabhattai, B. halotolerans, B. methylotrophicus, B. safensis, B. siamensis and B. simplex dominated the PGPR population, along with Pseudomonas spp., such as P. azotoformans and P. chlororaphis, indicating a selective buildup of PGPR in the saffron rhizosphere. In-vivo and In-vitro dual inoculation test of these widespread PGPR and stress-tolerant also inhibited the mycelial growth of Fusarium species including F. oxysporum, F. oxysporum f. sp. ciceris, and F. proliferatum. The field application of seven selected PGPR strains resulted in significant improvements in saffron productivity, including increased sprouted corms, plant height, stigma length, and stigma fresh and dry weight, which ranged from 0.6-1.8 %, 0.4–0.7 %, 0.3–0.5 %, and 6–19 %, respectively. These strains have the potential to stimulate cormlet growth and ultimately increase saffron production. Therefore, the PGP traits of these stress-tolerant rhizobacteria contributing to nutrients solubilization, auxin biosynthesis, and fungicidal attributes can ameliorate the saffron agricultural practices.

Plant-microbe interactions in the rhizosphere for smarter and more sustainable crop fertilization: the case of PGPR-based biofertilizers.

Biofertilizers based on plant growth promoting rhizobacteria (PGPR) are nowadays gaining increasingly attention as a modern tool for a more sustainable agriculture due to their ability in ameliorating root nutrient acquisition. For many years, most research was focused on the screening and characterization of PGPR functioning as nitrogen (N) or phosphorus (P) biofertilizers. However, with the increasing demand for food using far fewer chemical inputs, new investigations have been carried out to explore the potential use of such bacteria also as potassium (K), sulfur (S), zinc (Zn), or iron (Fe) biofertilizers. In this review, we update the use of PGPR as biofertilizers for a smarter and more sustainable crop production and deliberate the prospects of using microbiome engineering-based methods as potential tools to shed new light on the improvement of plant mineral nutrition. The current era of omics revolution has enabled the design of synthetic microbial communities (named SynComs), which are emerging as a promising tool that can allow the formulation of biofertilizers based on PGPR strains displaying multifarious and synergistic traits, thus leading to an increasingly efficient root acquisition of more than a single essential nutrient at the same time. Additionally, host-mediated microbiome engineering (HMME) leverages advanced omics techniques to reintroduce alleles coding for beneficial compounds, reinforcing positive plant-microbiome interactions and creating plants capable of producing their own biofertilizers. We also discusses the current use of PGPR-based biofertilizers and point out possible avenues of research for the future development of more efficient biofertilizers for a smarter and more precise crop fertilization. Furthermore, concerns have been raised about the effectiveness of PGPR-based biofertilizers in real field conditions, as their success in controlled experiments often contrasts with inconsistent field results. This discrepancy highlights the need for standardized protocols to ensure consistent application and reliable outcomes.

Photosynthetic activity and growth of poblano pepper biofertilized with plant growth promoting rhizobacteria and arbuscular mycorrhizal fungi

The rhizosphere of plants are natural hosts for beneficial microorganisms such as plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF). The objective of this work was to determine the effect of a consortium of AMF and three strains of PGPR on growth, gas exchange and phosphorus content in poblano pepper plants. An experiment was established in a completely randomized design with factorial arrangement, with two factors: AMF [Funneliformis geosporum and Claroideoglomus sp. (AM) and without AM (WM)] and PGPR [Rhizobium nepotum (B1), Serratia plymuthica (B2), Pseudomonas tolaasii (B3) and without PGPR (WB)]; generating eight treatments: T1) AM+B1, T2) AM+B2, T3) AM+B3, T4) AM+WB, T5) WM+B1, T6) WM+B2, T7) WM+B3 and T8) WM+WB. Plant height, number of leaves, leaf area, number of flowers, dry biomass, phosphorus content and AMF colonization were measured; internal CO2 concentration (Ci), transpiration rate (E), stomatal conductance (gS) and photosynthesis rate (Pn) were determined in leaves. Co-inoculation with AM+B3 promoted greater height (35%), number of leaves (66%), leaf area (62%), dry biomass (140%), phosphorus content (195%) and mycorrhizal colonization (26%); AM+B2 improved Ci (5%), E (8%), gS (5%) and Pn (9%) in poblano pepper leaves, compared to the control treatment (WM+WB). Biofertilization with AMF and PGPR improved gas exchange and growth of poblano pepper.

Genotypic and Phenotypic Characterization of Pseudomonas atacamensis EMP42 a PGPR Strain Obtained from the Rhizosphere of Echinocactus platyacanthus (Sweet Barrel).

Plant growth-promoting rhizobacteria (PGPR) are a group of bacteria that associate with the rhizosphere of plants; one of the most abundant bacterial genera in this ecological niche is Pseudomonas, which is constantly expanding due to the emergence of new species such as Pseudomonas atacamensis, whose discovery in 2019 has led to the characterization of several strains from different environments but taxonomically related. The objective of this work was to phenotypically and molecularly characterize P. atacamensis strain EMP42, isolated from the rhizosphere of Echinocactus platyacanthus. The strain EMP42 is able to use different substrates and reduce oxidative stress in plants. It is capable of improving growth parameters such as the number of inflorescences and the height of the aerial body of Arabidopsis thaliana, as well as the germination and seedling survival of the cacti Echinocactus platyacanthus and Astrophytum capricorne. The genetic structure of P. atacamensis EMP42 consists of a closed chromosome of 6.14 Mbp, and 61.1% GC content. It has 5572 genes, including those associated with PGPR activities, such as the trpABCDE, SAP, phoABPRU and acsABC genes, among others, and three ncRNA loci, nine regulatory regions, five complete rRNA operons and three CRISPR-Cas loci, showing phylogenomic similarities with the reference strain P. atacamensis B21-026. Therefore, this study contributes to the understanding of genomic diversity within P. atacamensis and, particularly, highlights the potential application of strain EMP42 as a PGPR.

Halotolerant Bacillus sp. strain RA coordinates myo-inositol metabolism to confer salt tolerance to tomato.

Soil salinity is a worldwide problem threatening crop yields. Some plant growth-promoting rhizobacteria (PGPR) could survive in high salt environment and assist plant adaptation to stress. Nevertheless, the genomic and metabolic features, as well as the regulatory mechanisms promoting salt tolerance in plants by these bacteria remain largely unknown. In the current work, a novel halotolerant PGPR strain, namely, Bacillus sp. strain RA can enhance tomato tolerance to salt stress. Comparative genomic analysis of strain RA with its closely related species indicated a high level of evolutionary plasticity exhibited by strain-specific genes and evolutionary constraints driven by purifying selection, which facilitated its genomic adaptation to salt-affected soils. The transcriptome further showed that strain RA could tolerate salt stress by balancing energy metabolism via the reprogramming of biosynthetic pathways. Plants exude a plethora of metabolites that can strongly influence plant fitness. The accumulation of myo-inositol in leaves under salt stress was observed, leading to the promotion of plant growth triggered by Bacillus sp. strain RA. Importantly, myo-inositol serves as a selective force in the assembly of the phyllosphere microbiome and the recruitment of plant-beneficial species. It promotes destabilizing properties in phyllosphere bacterial co-occurrence networks, but not in fungal networks. Furthermore, interdomain interactions between bacteria and fungi were strengthened by myo-inositol in response to salt stress. This work highlights the genetic adaptation of RA to salt-affected soils and its ability to impact phyllosphere microorganisms through the adjustment of myo-inositol metabolites, thereby imparting enduring resistance against salt stress in tomato.

Modulation of plant polyamine and ethylene biosynthesis; and brassinosteroid signaling during Bacillus endophyticus J13-mediated salinity tolerance in Arabidopsis thaliana

Salinity stress adversely impacts plant growth and development. Plant growth-promoting rhizobacteria (PGPR) are known to confer salinity stress tolerance in plants through several mechanisms. Here, we report the role of an abiotic stress-tolerant PGPR strain, Bacillus endophyticus J13, in promoting salinity stress tolerance in Arabidopsis thaliana, by elucidating its impact on physiological responses, polyamine (PA) and ethylene biosynthesis, and brassinosteroid signaling. Physiological analysis revealed that J13 can significantly improve the overall plant growth under salt stress by increasing the biomass, relative water content, and chlorophyll content, decreasing membrane damage and lipid peroxidation, and modulating proline homeostasis in plants. Evaluation of shoot polyamine levels upon J13 inoculation revealed an overall decrease in the levels of the three major PAs, putrescine (Put), spermidine (Spd), and spermine (Spm), under non-stressed conditions. Salt stress significantly increased the levels of Put and Spm, while decreasing the Spd levels in the plants. J13 inoculation under salt-stressed conditions, significantly decreased the Put levels, bringing them closer to those of the untreated control plants, whereas Spd and Spm levels did not change relative to the non-inoculated salt-stressed plants. The modulation of PA levels was accompanied by changes in the expressions of key PA biosynthetic genes under all treatments. Among the ethylene biosynthetic genes that we studied, ACS1 was induced by J13 inoculation under salt stress. J13 inoculation under salt stress resulted in the modulation of the expressions of BR-signaling genes, upregulating the expressions of the positive regulators of BR-signaling (BZR1 and BES2) and downregulating that of the negative regulator (BIN2). Our results provide a new avenue for J13-mediated salt stress amelioration in Arabidopsis, via tight control of polyamine and ethylene biosynthesis and enhanced brassinosteroid signaling.

Isolation and characterization of plant growth promoting rhizobacteria (PGPR) from rhizosphere of Helianthus annuus L.

Plant growth-promoting rhizobacteria (PGPR) support plant growth through direct and indirect mechanisms. To investigate PGPR strains that support plant growth, 21 bacterial isolates, mostly Bacillus ssp. and Pseudomonas ssp., were isolated from different rhizospheric soils of sunflowers in Kırşehir districts in 2020. All isolates were characterized morphologically, biochemically by screening under in vitro conditions for plant growth-promoting properties such as nitrogen fixation, IAA (indoleacetic acid) production, siderophore production, HCN (hydrogen cyanide) production, inorganic phosphate solubility. It was also screened for extracellular enzyme production and antifungal activity against Fusarium oxysporum. Among the 21 isolates, 3 isolates (MH-35-4, MH-49-4, MH-64-3) fixed nitrogen, 2 isolates (MH-59-6, MH-64-3), produced siderophores, 8 isolates (MH-35-4, MH-35-6, MH-54-3, MH-54-4, MH-59-1, MH-59-2, MH-59-4, MH-59-8) produced HCN, 6 isolates (MH-35-6, MH-54-4, MH-59-1, MH-59-2, MH-59-4, MH-59-8) produced IAA, and 7 isolates (MH-35-4, MH-35-6, MH-59-1, MH-59-2, MH-59-4, MH-59-8, MH-64-3) solubilized inorganic phosphate. Additionally, only 2 isolates (MH-54-3, MH-54-4) were positive amylase tests, 8 isolates (MH-35-6, MH-54-4, MH-59-1, MH-59-2, MH-59-4, MH-59-6, MH-59-7, MH-59-8) were positive citrate tests, 8 isolates (MH-35-1, MH-35-4, MH-35-7, MH-49-4, MH-54-4, MH-59-6, MH-59-7, MH-64-3) were positive protease tests, and 6 isolates (MH-35-1, MH-35-3, MH-35-7, MH-54-3, MH-54-4, MH-59-7) were positive gelatin hydrolysis tests. Among 21 isolates, 38% were determined as hydrogen cyanide producers, 10% as siderophore producers, 29% IAA producers, 33% as phosphate solubilizers and 14% as nitrogen fixers. Isolate MH-35-6 showed the highest antifungal activity against Fusarium oxysporum with an inhibition rate of 53.57%. This was followed by isolates MH-54-1 (51.19%), MH-54-3 (47.61%) and MH-59-2 (38.09%), respectively. Therefore, our study reveals that bacteria that promote plant growth in sunflowers can be used to increase crop yield and as a biocontrol agent.

Plant Growth Promoting Rhizobacteria (PGPR): Impact on peanut flowering, seed physical quality, and yield determination (Arachis hypogaea L.)

Peanut seed yield is influenced by physiological and ecophysiological crop traits, with changes in reproductive parameters such as flowering time, flower number, and fertility index significantly affecting productivity. Management practices like Plant Growth-Promoting Rhizobacteria (PGPR) could enhance crop yield. PGPR can improve flowering and yield by modulating the balance of phytohormones and enhancing reproductive processes. This study assessed PGPR effects on peanut flowering dynamics, seed yield determination, and correlation with hormone levels. Two plant-level field experiments were conducted during 2019–2020 and 2020–2021, sowing two cultivars (ASEM 400 INTA and Granoleico) on four sowing dates (Oct-05, Oct-15, Nov-29, Dec-10). Three PGPR strains (Bacillus velezensis RI3 and SC6, Pseudomonas psychrophila P10), Bradyrhizobium japonicum, and control were arranged in a split-split-split-plot design, with sowing dates as main plots, cultivars as sub-plots, and PGPRs as sub-sub-plots. PGPR improved peanut flower number (FN) and flowering rate (FR) by 20 % and 50 % compared to B. japonicum and the control. B. velezensis SC6 showed the best performance (SC6 > P10 > RI3), advancing the flowering peak by 4–12 days. PGPR also increased the fertility index (FI) to 0.31 vs. 0.24 of B. japonicum and control and pod number (PN) by 53 % compared to the control. Flowering peak advancement led to larger seeds (SF≥8 mm) (∼+8 % vs. control) due to better environmental conditions (i.e., higher temperatures). Seed number (SN) increased by 94 %–113 % under PGPR treatments. Seed yield (SY) was increased (∼75 % on average) due to higher total biomass (TB) production and improved carbon allocation to seeds (harvest index; 0.33–0.36). Hormone analysis at the R5 stage under late sowing's restrictive conditions showed hormone level increases of 15 %–90 %. This study demonstrates that PGPR significantly improves peanut flowering dynamics, seed quality, and yield by affecting hormone levels and reproductive parameters, highlighting its potential as a sustainable management practice.

Evaluation of the benefits of plant growth-promoting rhizobacteria and mycorrhizal fungi on biochemical and morphophysiological traits of Aloe barbadensis Mill under water deficit stress

Aloe barbadensis is a drought-tolerant perennial medicinal plant with both nutritional and cosmetic uses. Drought is one of the main abiotic stresses limiting plant growth and development. However, the use of drought-resistant plants combined with beneficial soil micro-organisms could improve the effectiveness of biological methods to mitigate drought damage. This research aims to evaluate the effects of Funneliformis mosseae (MF), plant growth-promoting rhizobacteria (PGPR) (including Pseudomonas putida and Pantoea agglomerans), and their co-inoculation on the macronutrient status, antioxidant enzyme activities, and other morphophysiological traits of A. barbadensis under four irrigation regimes [25%, 50%, 75% and 100% of water requirement (WR)]. Three harvests were conducted, revealing that inoculation enhanced the survival rate and shoot fresh weight (SFW) compared to the control plants. However, at 25% WR, the SFW was reduced by 43% more than the control. across all harvests, while the PGPR + MF treatment showed increases of more than 19%, 11%, and 17% compared to the control, MF, and PGPR treatments, respectively. The results also showed that A. barbadensis exhibited innate drought tolerance up to a 50% WR level by enhancing physiological defenses, such as antioxidant enzyme activity. Inoculation increased the macronutrient status of the plant at all levels of irrigation regimes especially under severe drought conditions. The highest levels of nitrogen (N) (16.24 mg g−1 DW) and phosphorus (P) (11.29 mg g−1 DW) were observed in the PGPR + MF treatment at 100% WR. The maximum relative water content under MF inoculation and 75% WR (98.24%) (98.24%) was reached. PGPR + MF treatment alleviated drought-induced osmotic stress, as indicated by reduced antioxidant enzyme activities and electrolyte leakage. However, P. putida and P. agglomerans strains alone or in combination with F. mosseae increased plant yield, macronutrient uptake and antioxidant enzyme activity. This study underscores the potential of these PGPR and MF strains as invaluable biological tools for the cultivation of A. barbadensis in regions with severe drought stress.