Plant Growth-promoting Rhizobacteria Strains Research Articles

Screening of heavy metal-resistant rhizobial and non-rhizobial microflora isolated from Trifolium sp. growing in mining areas.

Plant growth-promoting rhizobacteria (PGPR) can promote plant growth and development with several beneficial effects, especially in challenging environmental conditions, such as the presence of toxic contaminants. In this study, 49 isolates obtained from Trifolium sp. nodules growing on a Pb/Zn mine site were characterized for PGP traits including siderophores production, phosphate solubilization, extracellular enzymes production, and antifungal activity. The isolates were also screened for their ability to grow at increasing concentrations of NaCl and heavy metals, including lead, zinc, cobalt, copper, nickel, cadmium, and chromium. The findings of our study indicated that isolates Cupriavidus paucula RSCup01-RSCup08, Providencia rettgeri RSPro01, Pseudomonas putida RSPs01, Pseudomonas thivervalensis RSPs03-RSPs09, and Acinetobacter beijerinckii RSAci01 showed several key traits crucial for promoting plant growth, thus demonstrating the greatest potential. Most isolates displayed resistance to salt and heavy metals. Notably, Staphylococcus xylosus RSSta01, Pseudomonas sp. RSPs02, Micrococcus yunnanensis RSMicc01, and Kocuria dechangensis RSKoc01 demonstrated a significant capacity to grow at salt concentrations ranging from 10 to 20%, and isolates including Cupravidus paucula RSCup01-RSCup08 exhibited resistance to high levels of heavy metals, up to 1300 mg/L Pb++, 1200 mg/L Zn++, 1000 mg/L Ni++, 1000 mg/L Cd++, 500 mg/L Cu++, 400 mg/L Co++, and 50 mg/L CrVI+. Additionally, the analysis revealed that metal-resistant genes pbrA, czcD, and nccA were exclusively detected in the Cupriavidus paucula RSCup01 strain. The results of this study provide insights into the potential of plant growth-promoting rhizobacteria strains that might be used as inoculants to improve phytoremediation in heavy metal-contaminated soils.

A novel PGPR strain, Streptomyces lasalocidi JCM 3373T, alleviates salt stress and shapes root architecture in soybean by secreting indole-3-carboxaldehyde.

While soybean (Glycine max L.) provides the most important source of vegetable oil and protein, it is sensitive to salinity, which seriously endangers the yield and quality during soybean production. The application of Plant Growth-Promoting Rhizobacteria (PGPR) to improve salt tolerance for plant is currently gaining increasing attention. Streptomycetes are a major group of PGPR. However, to date, few streptomycetes has been successfully developed and applied to promote salt tolerance in soybean. Here, we discovered a novel PGPR strain, Streptomyces lasalocidi JCM 3373T, from 36 strains of streptomycetes via assays of their capacity to alleviate salt stress in soybean. Microscopic observation showed that S. lasalocidi JCM 3373T does not colonise soybean roots. Chemical analysis confirmed that S. lasalocidi JCM 3373T secretes indole-3-carboxaldehyde (ICA1d). Importantly, IAC1d inoculation alleviates salt stress in soybean and modulates its root architecture by regulating the expression of stress-responsive genes GmVSP, GmPHD2 and GmWRKY54 and root growth-related genes GmPIN1a, GmPIN2a, GmYUCCA5 and GmYUCCA6. Taken together, the novel PGPR strain, S. lasalocidi JCM 3373T, alleviates salt stress and improves root architecture in soybean by secreting ICA1d. Our findings provide novel clues for the development of new microbial inoculant and the improvement of crop productivity under salt stress.

The Importance of Considering Levels of P and N Fertilization to Promote Beneficial Interaction between Rapeseed and Phosphate-Solubilizing Bacteria

Biointrants constitute a promising opportunity to lower mineral input on rapeseed, characterized by high nutrient requirements. As bio-inoculants, phosphate-solubilizing bacteria (PSB) could increase the amount of available P in a soil solution. However, the deployment of these bio-inoculants in fields is not always successful. Disentangling the factors conditioning their reliability is necessary. Because the activities of microorganisms are particularly subject to nutrient availability, the N fertilization level could represent a key factor for the success of PSB inoculation in the early stages of plant growth. In this study, Pfaba (Pseudomonas sp.), a promising plant growth-promoting rhizobacteria (PGPR) strain isolated from soil, was inoculated on rapeseed grown in rhizotrons under two N fertilization levels (N160 or N80) in P labile or P complexed conditions. Pfaba confirmed its PSB potential to solubilize recalcitrant P complexed forms for the benefit of plant growth, but only when the N supply is adequate (N80). In a P complexed environment, Pfaba tended to increase root and shoot biomass (respectively, from 2.17 ± 0.47 g for control modality to 2.88 ± 0.85 g, and from 6.06 ± 1.67 g for control modality to 8.33 ± 1.70 g), increase the P and N contents in roots (respectively, from 0.15 ± 0.09 mg for control modality to 0.70 ± 0.51 mg, and from 37.90 ± 11.09 mg for control modality to 41.34 ± 14.16 mg), and restore root length at a comparable level than plants supplemented with labile P. Conversely, these positive effects were inhibited with lower levels of N fertilization. Our results highlight the importance of nutrient availability to promote beneficial interaction between plants and microorganisms. These findings could also contribute to ensuring the successful deployment of microbial biointrants.

Characterization of Rhizosphere Microbial Diversity and Selection of Plant-Growth-Promoting Bacteria at the Flowering and Fruiting Stages of Rapeseed.

Plant rhizosphere microorganisms play an important role in modulating plant growth and productivity. This study aimed to elucidate the diversity of rhizosphere microorganisms at the flowering and fruiting stages of rapeseed (Brassica napus). Microbial communities in rhizosphere soils were analyzed via high-throughput sequencing of 16S rRNA for bacteria and internal transcribed spacer (ITS) DNA regions for fungi. A total of 401 species of bacteria and 49 species of fungi in the rhizosphere soil samples were found in three different samples. The composition and diversity of rhizosphere microbial communities were significantly different at different stages of rapeseed growth. Plant-growth-promoting rhizobacteria (PGPRs) have been widely applied to improve plant growth, health, and production. Thirty-four and thirty-one PGPR strains were isolated from the rhizosphere soil samples collected at the flowering and fruiting stages of rapeseed, respectively. Different inorganic phosphorus- and silicate-solubilizing and auxin-producing capabilities were found in different strains, in addition to different heavy-metal resistances. This study deepens the understanding of the microbial diversity in the rapeseed rhizosphere and provides a microbial perspective of sustainable rapeseed cultivation.

Utilization of Stenotrophomonas koreensis and Bacillus amyloliquefaciens for Improving Growth, Reducing Nitrogen Fertilization and Controlling Bipolaris sorokiniana in Wheat

Wheat (<em>Triticum aestivum</em> L.), a vital cereal, faces significant challenges from common root rot and spot blotch diseases caused by <em>Bipolaris sorokiniana</em>. This study aimed to explore the potential of plant growth promoting rhizobacteria (PGPR) to enhance wheat growth, reduce fertilizer input, and combat <em>Bipolaris </em>diseases. Two PGPR isolates, selected for their superior antagonistic properties, were identified as <em>Stenotrophomonas koreensis</em> RB11 and <em>Bacillus amyloliquefaciens </em>RB12. These PGPR strains displayed multiple plant growth promoting and biocontrol attributes, including phosphate solubilization, indole-3-acetic acid production, nitrogen fixation and antagonism against <em>B. sorokiniana</em> and other fungi. Wheat seed priming with the PGPR significantly improved germination, plant growth, nutrient content and biomass carbon accumulation in the rhizosphere soil. Importantly, the application of RB11 and RB12 allowed for a 25% and 50% reduction in nitrogen fertilizer usage, respectively, without compromising the yield. RB11 and RB12 also demonstrated potent inhibitory effects on <em>B. sorokiniana</em> conidial germination and significantly controlled common root rot and spot blotch in wheat, similar to those observed with the fungicide Protaf 250EC. Overall, this study underscores the multifaceted roles of <em>S. koreensis</em> RB11 and <em>B. amyloliquefaciens</em> RB12 in promoting wheat growth, reducing fertilizer inputs and effectively suppressing wheat pathogens. These findings contribute to the development of PGPR-based strategies for sustainable crop production and disease control.

The Potential Use of Plant Growth Promoting Rhizobacteria (PGPR) for Medicinal Plant Cultivation in Meghalaya: A Review

Beneficial PGPR have been identified in a range of plant species, including those with medicinal properties. These PGPR have been shown to enhance the growth and development of medicinal plants through the improvement of nutrient availability in rhizospheric root systems. Some medicinal plants have been found to harbour specific strains of PGPR that are associated with increased medicinal properties and enhanced bioactive compound production. So, the use of PGPR can minimise the application of chemical fertilizer for the production of rare species of medicinal plants. This review highlights the future research works which are needed in many areas of Meghalaya by the use of various beneficial strains of PGPR to reduce the use of pesticide for commercial application in the cultivation of medicinal plants. The PGPR has the ability to increase the soil fertility be decreasing the concentration of inorganic fertilizer and thereby they are regarded as the potential strains to be used as biofertilizer which in turn will also improve the cultural practices of Meghalaya to conserve a large number of rare and endangered medicinal plants.

Plant growth-promoting rhizobacteria: Peribacillus frigoritolerans 2RO30 and Pseudomonas sivasensis 2RO45 for their effect on canola growth under controlled as well as natural conditions.

Even though canola is one of the most important industrial crops worldwide, it has high nutrient requirements and is susceptible to pests and diseases. Therefore, natural methods are sought to support the development of these plants. One of those methods could be a plant growth-promoting rhizobacteria (PGPR) that have a beneficial effect on plant development. The aim of this study was a genomic comparison of two PGPR strains chosen based on their effect on canola growth: Peribacillus frigoritolerans 2RO30, which stimulated canola growth only in sterile conditions, and Pseudomonas sivasensis 2RO45, which promoted canola growth in both sterile and non-sterile conditions. First of all, six bacterial strains: RO33 (Pseudomonas sp.), RO37 (Pseudomonas poae), RO45 (Pseudomonas kairouanensis), 2RO30 (Peribacillus frigoritolerans), 2RO45 (Pseudomonas sivasensis), and 3RO30 (Pseudomonas migulae), demonstrating best PGP traits in vitro, were studied for their stimulating effect on canola growth under sterile conditions. P. frigoritolerans 2RO30 and P. sivasensis 2RO45 showed the best promoting effect, significantly improving chlorophyll content index (CCI) and roots length compared to the non-inoculated control and to other inoculated seedlings. Under non-sterile conditions, only P. sivasensis 2RO45 promoted the canola growth, significantly increasing CCI compared to the untreated control and to other inoculants. Genome comparison revealed that the genome of P. sivasensis 2RO45 was enriched with additional genes responsible for ACC deaminase (acdA), IAA (trpF, trpG), and siderophores production (fbpA, mbtH, and acrB) compared to 2RO30. Moreover, P. sivasensis 2RO45 showed antifungal effect against all the tested phytopathogens and harbored six more biosynthetic gene clusters (BGC), namely, syringomycin, pyoverdin, viscosin, arylpolyene, lankacidin C, and enterobactin, than P. frigoritolerans 2RO30. These BGCs are well known as antifungal agents; therefore, it can be assumed that these BGCs were responsible for the antifungal activity of P. sivasensis 2RO45 against all plant pathogens. This study is the first report describing P. sivasensis 2RO45 as a canola growth promoter, both under controlled and natural conditions, thus suggesting its application in improving canola yield, by improving nutrient availability, enhancing stress tolerance, and reducing environmental impact of farming practices.

Polysaccharides Produced by Plant Growth-Promoting Rhizobacteria Strain Burkholderia sp. BK01 Enhance Salt Stress Tolerance to Arabidopsis thaliana.

Salt stress is one of the most serious abiotic stresses leading to reduced agricultural productivity. Polysaccharides from seaweed have been used as biostimulants to promote crop growth and improve plant resistance to abiotic stress. In this study, PGPR strain Burkholderia sp. BK01 was isolated from the rhizosphere of wheat, and it was characterized for phosphorus (Pi) dissolution, indole-3-acetic acid (IAA) production, ammonia (NH3) and exopolysaccharides (EPS). In particular, strain BK01 can efficiently produce extracellular polysaccharide with a yield of 12.86 g/L, using sorbitol as carbon source. BK01 EPS was identified as an heteropolysaccharide with Mw 3.559 × 106 Da, composed of (D)-galactose (75.3%), (D)-glucose (5.5%), (L)-rhamnose (5.5%), (D)-galactouronic acid (4.9%) and (D)-glucuronic acid (8.8%). The present work aims to highlight the effect of the BK01 EPS on growth and biochemical changes in Arabidopsis thaliana under salt stress (100 mM). The purified BK01 EPS at a concentration of 100 mg/L efficiently promoted the growth of plants in pot assays, improved the chlorophyll content, enhanced the activities of SOD, POD and CAT, and decreased the content of MDA. This results suggested that the polysaccharides produced by PGPR strain Burkholderia sp. BK01 can be used as biostimulants to promote plant growth and improve plant resistance to salt stress.

Long-Term Field Application of a Plant Growth-Promoting Rhizobacterial Consortium Suppressed Root-Knot Disease by Shaping the Rhizosphere Microbiota.

Root-knot nematodes (Meloidogyne spp.) are one of the most economically important plant parasitic nematodes, infecting almost all cultivated plants and resulting in severe yield losses every year. Plant growth-promoting rhizobacteria (PGPR) have been extensively used to prevent and control root-knot diseases and increase yield. In this study, the effect of a consortium of three PGPR strains (Bacillus cereus AR156, B. subtilis SM21, and Serratia sp. XY21; hereafter "BBS") on root-knot disease of cucumber was evaluated. The application of BBS significantly reduced the severity of root-knot disease by 56 to 72%, increased yield by 36 to 55%, and improved fruit quality by 14 to 90% and soil properties by 1 to 90% relative to the control in the cucumber fields of the Nanjing suburb, Jiangsu Province, from 2015 to 2018. BBS altered the rhizosphere bacterial community. Compared with the control group, it significantly (false discovery rate, P < 0.05) increased the abundance of 14 bacterial genera that were negatively correlated with disease severity. Additionally, the redundancy analysis suggested that BBS-treated rhizosphere soil samples were dominated by disease-suppressive bacteria, including the genera Iamia, Kutzneria, Salinibacterium, Mycobacterium, Kribbella, Pseudonocardia, Sporichthya, Sphaerisporangium, Actinomadura, Flavisolibacter, Phenylobacterium, Bosea, Hyphomicrobium, Agrobacterium, Sphingomonas, and Nannocystis, which were positively related to total organic carbon, total nitrogen, total organic matter, dissolved organic carbon, [Formula: see text]-N, and available phosphorus contents. This suggests that BBS suppresses root-knot nematodes and improves the soil chemical properties of cucumber by altering the rhizosphere microbial community.

Mechanism on the promotion of host growth and enhancement of salt tolerance by Bacillaceae isolated from the rhizosphere of Reaumuria soongorica.

Salt stress is a major abiotic stress that affects the growth of Reaumuria soongorica and many psammophytes in the desert areas of Northwest China. However, various Plant Growth-Promoting Rhizobacteria (PGPR) have been known to play an important role in promoting plant growth and alleviating the damaging effects of salt stress. In this study, three PGPR strains belonging to Bacillaceae were isolated from the rhizosphere of Reaumuria soongorica by morphological and molecular identification. All isolated strains exhibited capabilities of producing IAA, solubilizing phosphate, and fixing nitrogen, and were able to tolerate high levels of NaCl stress, up to 8-12%. The results of the pot-based experiment showed that salt (400 mM NaCl) stress inhibited Reaumuria soongorica seedlings' growth performance as well as biomass production, but after inoculation with strains P2, S37, and S40, the plant's height significantly increased by 26.87, 17.59, and 13.36%, respectively (p < 0.05), and both aboveground and root fresh weight significantly increased by more than 2 times compared to NaCl treatment. Additionally, inoculation with P2, S37, and S40 strains increased the content of photosynthetic pigments, proline, and soluble protein in Reaumuria soongorica seedlings under NaCl stress, while reducing the content of malondialdehyde and soluble sugars. Metabolomic analysis showed that strain S40 induces Reaumuria soongorica seedling leaves metabolome reprogramming to regulate cell metabolism, including plant hormone signal transduction and phenylalanine, tyrosine, and tryptophan biosynthesis pathways. Under NaCl stress, inoculation with strain S40 upregulated differential metabolites in plant hormone signal transduction pathways including plant hormones such as auxins (IAA), cytokinins, and jasmonic acid. The results indicate that inoculation with Bacillaceae can promote the growth of Reaumuria soongorica seedlings under NaCl stress and enhance salt tolerance by increasing the content of photosynthetic pigments, accumulating osmoregulatory substances, regulating plant hormone levels This study contributes to the enrichment of PGPR strains capable of promoting the growth of desert plants and has significant implications for the psammophytes growth and development in desert regions, as well as the effective utilization and transformation of saline-alkali lands.

Isolation, identification, and mechanism analysis of plant growth-promoting rhizobacteria in tobacco.

Plant growth, crop yield, and pest and disease control are enhanced by PGPR (Plant growth promoting rhizobacteria), which are beneficial microorganisms found in a close symbiosis with plant roots. Phytohormones are secreted, nutrient uptake is improved, and soil properties along with the microbiological environment are regulated by these microorganisms, making them a significant focus in agricultural research. In this study, the efficient PGPR strain T1 was isolated and screened from tobacco inter-root soil, and identified and confirmed by ITS sequencing technology. Tobacco growth indicators and soil property changes were observed and recorded through potting experiments. The activities of key enzymes (e.g., sucrase, catalase, urease) in soil were further determined. High-throughput sequencing technology was utilized to sequence the soil microbial community, and combined with macro-genomics analysis, the effects of T1 strain on soil microbial diversity and metabolic pathways were explored. Following the application of T1, significant improvements were observed in the height, leaf length, and width of tobacco plants. Furthermore, the physical and chemical properties of the soil were notably enhanced, including a 26.26% increase in phosphorus availability. Additionally, the activities of key soil enzymes such as sucrase, catalase, and urease were significantly increased, indicating improved soil health and fertility. Comprehensive joint microbiomics and macrogenomics analyses revealed a substantial rise in the populations of beneficial soil microorganisms and an enhancement in metabolic pathways, including amino acid metabolism, synthesis, and production of secondary metabolites. These increase in beneficial microorganisms and the enhancement of their metabolic functions are crucial for plant growth and soil fertility. This study provides valuable references for the development of innovative microbial fertilizers and offers programs for the sustainable development of modern agriculture.

The effect of plant-associated rhizobacteria on the resistance of white mustard to soil drought

The paper discusses the results of a vegetation experiment with Plant Growth-Promoting Rhizobacteria (PGPR), namely such products as Agrofil, Mizorin, Flavobakterin, and Ekstrasol, used as organic fertilizers for white mustard of the Rapsodiya/Rhapsody variety (k-4278) grown in normal soil moisture and drought. The study aims to determine the role of plant-associated PGPR strains in physiological processes and productivity of white mustard grown in normal moisture and soil drought during the critical stages of plant development. The growth stimulators showed to increase plant productivity under normal moisture facilitating growth and an increase in the number of leaves and seeds. Insufficient soil moisture disrupts water balance in plants. The seeds treated with PGPR were more physiologically stable and retained more water under short-term drought facilitating sufficient seed yield. Mizorin and Flavobakterin proved to be most effective under normal moisture, while the seeds inoculated with Agrofil performed best—both biologically and economically—in drought.

Effect of Plant Growth-Promoting Rhizobacteria (PGPRs) on Tuber Storage of Two Potato Varieties

El-Allaoui N., A. Douira, A. Benbouazza, M. Ferrahi, E. Achbani and K. Habbadi. 2024. Effect of Plant GrowthPromoting Rhizobacteria (PGPRs) on Tuber Storage of Two Potato Varieties. Arab Journal of Plant Protection, 42(2): 229-233. https://doi.org/10.22268/AJPP-001242 The potato (Solanaceae family) is a cultivated plant primarily valued for its tubers, which serve as a crucial food source for numerous populations worldwide. Losses due to postharvest diseases are significantly high and need to be reduced. To preserve tuber quality without relying on chemical treatments, the application of plant growth promoting rhizobacteria (PGPR) has been employed as a means to extend the shelf life of potato tubers. The objective of this study was to assess the effect of eight PGPRs on the conservation of two potato varieties Siena and Bellini. This was carried out in two trials; in the first trial, thirty potatoes of the Siena variety were treated with PGPRs and covered with wheat straw, and in the second trial, 160 potatoes each of the two varieties Siena and Bellini were treated with PGPRs but covered with wheat straw and black plastic. Results obtained indicated that several PGPR strains performed favorably, the most promising of which was the Aureobasidium pullulans (Ach1.1) strain that kept 50% of the treated potatoes healthy for one month in the first trial, and 30% for six months in the second trial, as compared to the negative control where 100% of the potatoes showed signs of rot in both trials. It can be concluded from this study that the Ach1.1 strain of Aureobasidium pullulans could be a promising post-harvest bio-control treatment. Keywords: PGPRs, conservation, post-harvest, potato, rot

Effect of PGPR Arthrobacter sp. CTF1 and foliar iodine spraying on pea microgreens growth in hydroponic culture

The use of plant growth-promoting rhizobacteria (PGPR) along with additional biofortification of agricultural plants with scarce essential elements, such as iodine, is a promising area of agricultural biotechnology. The seeds of Pisum sativum L. (var. Madras) pre-inoculated for two hours with PGPR Arthrobacter sp. strain CTF1 (108 CFU/mL) were grown for 14 days in a hydroponic culture at foliar spraying with iodine solution (0.01% KI or KIO3) on the 7th day of the vegetation. Growth parameters such as the length of shoot, fresh and dry biomass of seedlings were studied, the germination percentage and vigor index were calculated, and the content of photosynthetic pigments in pea leaves was assessed. The results showed inoculation of pea seeds with PGPR strain CTF1 had a positive effect on the biomass of two-week-old pea microgreens and their vigor index. At the same time, a significant increase in photosynthetic pigments was also observed in the leaves of pea seedlings, especially chlorophyll a (by almost 25%) and carotenoids (by almost 40%). Additionally, application of iodine via foliar spraying, irrespective of its form (KI or KIO3), resulted in nearly a 26-fold surge in amount of microgreens. However, the significant effect of such iodine treatment had a positive effect only on the content of carotenoids.

Evaluation of antifungal activity against Corynespora cassiicola by bacteria isolated from soil in the root zone of cucumber plants

Abstract. Duy LQ. 2023. Evaluation of antifungal activity against Corynespora cassiicola by bacteria isolated from soil in the root zone of cucumber plants. Biodiversitas 24: 6584-6591. Target leaf spot caused by Corynespora cassiicola poses a major disease threat to cucumber cultivation worldwide. Reliance on chemical fungicides has raised resistance to environmental and health concerns. As cucumber production expands globally, sustainable solutions are needed. Plant Growth-Promoting Rhizobacteria (PGPR) show promise but remain underexplored against C. cassiicola in cucumber. This study aimed to assess the potency of rhizobacteria in antagonistic activity against C. cassiicola, causing cucumber target leaf spots under in vitro conditions. The present study isolated bacterial strains from cucumber soil rhizosphere and screened for antifungal activity against C. cassiicola using dual culture assays. Using quantitative assays, selected antagonistic strains were further assessed for key biocontrol traits, including ?-1,3-glucanase, chitinase, and siderophore production. Cell-free supernatants were tested for direct fungal growth inhibition using the environmental toxicity. Isolated PGPR strains showed robust C. cassiicola inhibition from 39.8% to 62.6% growth reduction. Most antagonistic isolates produced ?-1,3-glucanase, chitinase, and siderophores and suppressed pathogen growth via metabolites in their supernatants. These results demonstrate multiple biocontrol mechanisms, including enzymatic degradation of fungal cell walls, iron competition through siderophores, and antimicrobial compound secretion. Bacillus siamensis strain TV16 exhibited the strongest overall antagonism against C. cassiicola. These findings reveal the biocontrol potential of PGPR for managing cucumber-target leaf-spot disease, offering a sustainable alternative to chemical pesticides. With further optimization and field testing, PGPR-based biopesticides can be integrated into cucumber production to improve crop health while reducing fungicide reliance.

Integrated application of metal tolerant P. fluorescens and press mud for conferring heavy metal tolerance to aloe vera (Aloe barbadensis)

Soil pollution due to heavy metal (HM) contamination has emerged as a global issue worldwide owing to their adverse impacts on plant growth. Bioremediation approaches employing living organisms for HM alleviation have gained considerable attention among scientific community. Biological agents such as plant growth-promoting rhizobacteria (PGPR) offer a sustainable way to restore soil health, and their combination with different organic amendments such as press mud (PM) can serve as potential approach for immobilizing HMs in soil. We performed a pot experiment to evaluate the role of individual and combined application of press mud and PGPR strain ‘FQ6’ (identified as Pseudomonas fluorescens) in the phytoremediation of different HMs (Pb, Ni and Cd) and growth promotion of aloe vera. Combined application of FQ6 strain and PM yielded more significant outcomes in terms of all the growth and yield attributes such as leaf length (123 %), plant height (57 %), number of leaves (115 %), fresh and dry weights of gel (246 and 280 %), gel contents (96 %), root length (164 %), root diameter (220 %), no. of root tips (138 %) and root area (315 %), as compared to control. Combined application of FQ6 and PM also led to a significant improvement in different antioxidant activities i.e., CAT (129 %), SOD (48 %), APX (17 %) and POD (83 %) as compared to control. Contrastingly, mobility of these HMs was reduced under combined application of Pseudomonas strain ‘FQ6’ and PM, as there existed a considerable difference between HMs concentrations in soil and plant body. We concluded that joint application of HM-tolerant Pseudomonas FQ6 strain and PM could be an ideal option to alleviate the HM induced adverse impacts on aloe vera by immobilizing them in soil, and subsequently, improving plant growth.

Modulation of Growth and Antioxidative Defense Mechanism in Wheat (Triticum aestivum L.) Mediated by Plant-Beneficial Strain Pseudomonas veronii MR-15 under Drought Conditions

Drought stress severely influences plants in various ways and is considered an alarming threat to sustainable crop production worldwide. However, plant-growth-promoting rhizobacteria (PGPRs) have the natural ability to tolerate drought and enable plants to induce stress resistance by altering critical metabolic pathways. In this study, we isolated and characterized a drought-tolerant rhizobacterium from the ground nut (Arachis hypogaea). Sequencing of the 16S rRNA gene traced its lineage to Pseudomonas veronii, named MR-15. The strain exhibited natural capabilities to solubilize phosphate, produce indole acetic acid, and grow a drought medium containing PEG (polyethylene glycol). The seeds of two wheat varieties (Triticum aestivum) inoculated with MR-15 were grown under drought and fully hydrated conditions and showed a significant increase in plant biomass, enhanced cellular antioxidant enzyme activity, and reduced reactive oxygen species. The MR-15 strain also significantly increased pigmentation and protein contents compared to plants raised from seeds grown without inoculation. These beneficial effects were consistent under drought stress conditions, indicating that MR-15 effectively alleviated wheat plants from drought-induced cellular oxidative damage. The findings suggest that MR-15 has the potential to serve as a biofertilizer, and further experiments should be conducted to explore its role in promoting plant growth and yield under drought conditions, particularly in semi-arid and arid zones. This is the first study reporting Pseudomonas veronii as a potential PGPR strain.

Analyzing Single and Combined Cultures of Plant Growth-Promoting Rhizobacteria Isolates from Afghanistan as a Potential Biofertilizer for Rice Growth and Development

Developing multi-functional, environmentally friendly biofertilizers can help reduce chemical fertilizer usage. This study examined single and combined application effects of four plant growth-promoting rhizobacteria (PGPR) strains: AF124 (Rhizobium daejeonense), AF74 (Enterobacter cloacae), AF96 (Pseudomonas putida), and AF42 (Enterobacter cloacae), isolated from alkaline salt-stressed soils from Afghanistan. Physiological features such as nitrogen (N2) fixation, indole-3-acetic acid (IAA), organic acid production, and P and K solubilization of bacteria in single or combined cultures in vitro were examined. A single application of AF124 and AF74 showed significantly higher IAA production than the combined application of dual, triple, and quadruple PGPR treatments. Regarding acetylene reduction activity, an N2-fixation activity indicator, a single application of AF124 showed the highest value, followed by a triple application of AF124 + AF96 + AF42. The P-solubilization activity and organic acid production were lower in the single application of AF124 but higher with other strains in the combined applications, which resulted in the secretion of multiple organic acids into the medium. The highest solubilized K value was obtained from the quadruple application compared to other treatments. In the rice inoculation test, all single and combined PGPR treatments increased its growth parameters, such as root length and root and shoot dry biomass. The present results indicate that these four PGPR strains can potentially contribute to developing multi-functional bio-fertilizer formulations.

Bacterial biostimulants for climate smart agriculture practices: Mode of action, effect on plant growth and roadmap for commercial products

AbstractAmidst the global food shortage and the global climate change challenge, there is an urgent need to double food production by 2050. However, the modern crop production methods, including the use of fertilizers and pesticides, have adverse environmental consequences, exacerbating the climate crisis. To address this challenge, a transition to sustainable agriculture is imperative that can harmonize the issue. Biostimulants offer an eco‐friendly solution, especially bacterial biostimulants centred on plant growth‐promoting rhizobacteria (PGPRs). These biostimulants hold the promise of offering environmentally sustainable solutions to enhance crop productivity. The adoption of PGPR‐based biostimulants in agriculture has gained significant momentum in agricultural research. PGPRs enhance plant growth through multifaceted mechanisms. This review delves into the various modes of action employed by PGPRs to improve plant growth, including their impact on nutrient availability (such as nitrogen fixation and mineral solubilization) and stress mitigation. In addition, the practical implication of PGPR strains in field research has been discussed extensively. Besides, the review outlines the roadmap for commercializing PGPR‐based biostimulants and discusses the associated challenges and limitations. A balanced perspective on the practical implementation of PGPRs in modern agriculture is presented. Exploration of future strategies and directions rounds out the review, emphasizing the necessity of a comprehensive approach to address research gaps and unlock the full potential of PGPR‐based biostimulants for sustainable agriculture. In conclusion, this review underscores the applicability of PGPR‐based biostimulants as an innovative solution to address the current food crisis in the context of climate change.

Identification of an efficient phenanthrene-degrading Pseudarthrobacter sp. L1SW and characterization of its metabolites and catabolic pathway

Phenanthrene, a typical chemical of polycyclic aromatic hydrocarbons (PAHs) pollutants, severely threatens health of wild life and human being. Microbial degradation is effective and environment-friendly for PAH removal, while the phenanthrene-degrading mechanism in Gram-positive bacteria is unclear. In this work, one Gram-positive strain of plant growth-promoting rhizobacteria (PGPR), Pseudarthrobacter sp. L1SW, was isolated and identified with high phenanthrene-degrading efficiency and great stress tolerance. It degraded 96.3% of 500 mg/L phenanthrene in 72 h and kept stable degradation performance with heavy metals (65 mg/L of Zn2+, 5.56 mg/L of Ni2+, and 5.20 mg/L of Cr3+) and surfactant (10 CMC of Tween 80). Strain L1SW degraded phenanthrene mainly through phthalic acid pathway, generating intermediate metabolites including cis-3,4-dihydrophenanthrene-3,4-diol, 1-hydroxy-2-naphthoic acid, and phthalic acid. A novel metabolite (m/z 419.0939) was successfully separated and identified as an end-product of phenanthrene, suggesting a unique metabolic pathway. With the whole genome sequence alignment and comparative genomic analysis, 19 putative genes associated with phenanthrene metabolism in strain L1SW were identified to be distributed in three gene clusters and induced by phenanthrene and its metabolites. These findings advance the phenanthrene-degrading study in Gram-positive bacteria and promote the practical use of PGPR strains in the bioremediation of PAH-contaminated environments.