Soil amendment and rhizobacterial inoculation improved Cu phytostabilization, plant growth and microbial activity in a bench-scale experiment

Mine driven trace elements' pollution entails environmental risks and causes soil infertility. In the last decades, in situ techniques such as phytostabilization have become increasingly important as ways to tackle these negative impacts. The aim of this study was to test the individual and combined effects of different aided phytostabilization techniques using substrate from barren tailings of a Cu mine, characterized by extreme infertility (high acidity and deficiency of organic matter and nutrients). The experiment analyzed the growth of Populus nigra L. planted alone (P) or in co-cropping with Trifolium repens L. (PT), in pots containing mine soil amended with compost (1, 10, compost, soil, w/w) non inoculated (NI) or inoculated with plant growth promoting rhizobacteria (PGP), mycorrhizae (MYC) or a combination of bacterial and fungal inocula (PGPMYC). Non-amended, non-planted and non-inoculated reference ports were also prepared. Plants were harvested after 110 days of plant development and several biometric and phytopathological parameters (stem height, aerial biomass, root biomass, wilting, chlorosis, pest and death) and macro and micronutrient composition were determined. The growth substrate was analyzed for several physicochemical (pH, CEC_e, and exchangeable cations, total C and N, P Olsen and availability of trace elements) and microbiological (community level physiological profiles: activity, richness and diversity) parameters. The use of the amendment, P. nigra plantation, and inoculation with rhizobacteria were the best techniques to reduce toxicity and improve soil fertility, as well as to increase the plant survival and growth. Soil bacterial functional diversity was markedly influenced by the presence of plants and the inoculation with bacteria, which suggests that the presence of plant regulated the configuration of a microbial community in which the inoculated bacteria thrive comparatively better. The results of this study support the use of organic amendments, tolerant plants, and plant growth promoting rhizobacteria to reduce environmental risk and improve fertility of soils impacted by mining.

Plant -microbe assisted emerging contaminants (ECs) removal and carbon cycling

Continuous increase in the level of atmospheric CO₂ and environmental contaminates has aggravated various threats resulting from environmental pollution and climate change. Research into plant -microbe interaction has been a central concern of ecology for over the year. However, despite the clear contribution of plant -microbe to the global carbon cycle, the role of plant -microbe interaction in carbon pools, fluxes and emerging contaminants (ECs) removal are still a poorly understood. The use of plant and microbes in ECs removal and carbon cycling is an attractive strategy because microbes operate as biocatalysts to remove contaminants and plant roots offer a rich niche for their growth and carbon cycling. However, bio-mitigation of CO₂ and removal of ECs is still under research phase because of the CO₂ capture and fixation efficiency is too low for industrial purposes and cutting-edge removal methods have not been created for such emerging contaminants.

Physiological and Genetic Modifications Induced by Plant-Growth-Promoting Rhizobacteria (PGPR) in Tomato Plants under Moderate Water Stress

Physiological, metabolic, and genetic changes produced by two plant growth promoting rhizobacteria (PGPR) Pseudomonas sp. (internal code of the laboratory: N 5.12 and N 21.24) inoculated in tomato plants subjected to moderate water stress (10% polyethylene glycol-6000; PEG) were studied. Photosynthesis efficiency, photosynthetic pigments, compatible osmolytes, reactive oxygen species (ROS) scavenging enzymes activities, oxidative stress level and expression of genes related to abscisic acid synthesis (ABA; 9-cis-epoxycarotenoid dioxygenase NCDE1 gene), proline synthesis (Pyrroline-5-carboxylate synthase P5CS gene), and plasma membrane ATPase (PM ATPase gene) were measured. Photosynthetic efficiency was compromised by PEG, but bacterial-inoculated plants reversed the effects: while N5.12 increased carbon fixation (37.5%) maintaining transpiration, N21.24 increased both (14.2% and 31%), negatively affecting stomatal closure, despite the enhanced expression of NCDE1 and plasma membrane ATPase genes, evidencing the activation of different adaptive mechanisms. Among all parameters evaluated, photosynthetic pigments and antioxidant enzymes guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) responded differently to both strains. N 5.12 increased photosynthetic pigments (70% chlorophyll a, 69% chlorophyll b, and 65% carotenoids), proline (33%), glycine betaine (4.3%), and phenolic compounds (21.5%) to a greater extent, thereby decreasing oxidative stress (12.5% in Malondialdehyde, MDA). Both bacteria have highly beneficial effects on tomato plants subjected to moderate water stress, improving their physiological state. The use of these bacteria in agricultural production systems could reduce the amount of water for agricultural irrigation without having a negative impact on food production.

Draft Genome Sequence of Enterobacter cloacae S23 a Plant Growth-promoting Passenger Endophytic Bacterium Isolated from Groundnut Nodule Possesses Stress Tolerance Traits

Aim

This study aims to reveal the passenger endophytic bacterium Enterobacter cloacae S23 isolated from groundnut nodules and to underpin the molecular mechanism and genes responsible for abiotic stress tolerance.

Background

A variety of microorganisms that contribute to nodulation and encourage plant development activity in addition to the nodulating Rhizobium. Passenger endophytes (PE) are endophytes that accidentally penetrate the plant without any selective pressure keeping them in the interior tissue of the plant. PE possesses characteristics that encourage plant development and boost output while reducing pathogen infection and improving biotic and abiotic stress tolerance. However, there is a lack of molecular evidence on the passenger endophyte-mediated alleviation of abiotic stresses.

Objective

This study was formulated to reveal the draft genome sequence of Enterobacter cloacae S23, as well as genes and characteristics involved in plant growth promotion and stress tolerance.

Method

The data were submitted to PATRIC and the TORMES-1.0 Unicyclker tools were used to conduct a complete genome study of Enterobacter cloacae S23. The TORMES-1.0 platform was used to process the reads. RAST tool kit (RASTtk) was used to annotate the S23 sequence. The plant growth-promoting traits such as indole acetic acid production, siderophore secretion, production of extracellular polysaccharides, biofilm formation, phosphate solubilization, and accumulation of osmolytes were examined under normal, 7% NaCl and 30% polyethylene glycol amended conditions to determine their ability to withstand salt and moisture stressed conditions, respectively.

Result

We report the size of Enterobacter cloacae S23 is 4.82Mb which contains 4511 protein-coding sequences, 71 transfer RNA genes, and 3 ribosomal RNA with a G+C content of DNA is 55.10%. Functional analysis revealed that most of the genes are involved in the metabolism of amino acids, cofactors, vitamins, stress response, nutrient solubilization (kdp, pho, pst), biofilm formation (pga) IAA production (trp), siderophore production (luc, fhu, fep, ent, ybd), defense, and virulence. The result revealed that E. cloacae S23 exhibited multiple plant growth-promoting traits under abiotic stress conditions.

Conclusion

Our research suggested that the discovery of anticipated genes and metabolic pathways might characterise this bacterium as an environmentally friendly bioresource to support groundnut growth through several mechanisms of action under multi-stresses.

Symbiotic Variations among Wheat Genotypes and Detection of Quantitative Trait Loci for Molecular Interaction with Auxin-Producing Azospirillum PGPR

Crop varieties differ in their ability to interact with Plant Growth-Promoting Rhizobacteria (PGPR), but the genetic basis for these differences is unknown. This issue was addressed with the PGPR Azospirillum baldaniorum Sp245, using 187 wheat accessions. We screened the accessions based on the seedling colonization by the PGPR and the expression of the phenylpyruvate decarboxylase gene ppdC (for synthesis of the auxin indole-3-acetic acid), using gusA fusions. Then, the effects of the PGPR on the selected accessions stimulating Sp245 (or not) were compared in soil under stress. Finally, a genome-wide association approach was implemented to identify the quantitative trait loci (QTL) associated with PGPR interaction. Overall, the ancient genotypes were more effective than the modern genotypes for Azospirillum root colonization and ppdC expression. In non-sterile soil, A. baldaniorum Sp245 improved wheat performance for three of the four PGPR-stimulating genotypes and none of the four non-PGPR-stimulating genotypes. The genome-wide association did not identify any region for root colonization but revealed 22 regions spread on 11 wheat chromosomes for ppdC expression and/or ppdC induction rate. This is the first QTL study focusing on molecular interaction with PGPR bacteria. The molecular markers identified provide the possibility to improve the capacity of modern wheat genotypes to interact with Sp245, as well as, potentially, other Azospirillum strains.

Schizotrophic Sclerotinia sclerotiorum-Mediated Root and Rhizosphere Microbiome Alterations Activate Growth and Disease Resistance in Wheat

Sclerotinia sclerotiorum, a widespread pathogen of dicotyledons, can grow endophytically in wheat, providing protection against Fusarium head blight and stripe rust and enhancing wheat yield. In this study, we found that wheat seed treatment with strain DT-8, infected with S. sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1) and used as a "plant vaccine" for brassica protection, could significantly increase the diversity of the fungal and bacterial community in rhizosphere soil, while the diversity of the fungal community was obviously decreased in the wheat root. Interestingly, the relative abundance of potential plant growth-promoting rhizobacteria (PGPR) and biocontrol agents increased significantly in the DT-8-treated wheat rhizosphere soil. These data might be responsible for wheat growth promotion and disease resistance. These results may provide novel insights for understanding the interaction between the schizotrophic microorganism and the microbiota of plant roots and rhizosphere, screening and utilizing beneficial microorganisms, and further reducing chemical pesticide utilization and increasing crop productivity. IMPORTANCE Fungal pathogens are seriously threatening food security and natural ecosystems; efficient and environmentally friendly control methods are essential to increase world crop production. S. sclerotiorum, a widespread pathogen of dicotyledons, can grow endophytically in wheat, providing protection against Fusarium head blight and stripe rust and enhancing wheat yield. In this study, we discovered that S. sclerotiorum treatment increased the diversity of the soil fungal and bacterial community in rhizosphere soil, while the diversity of the fungal community was obviously decreased in the wheat root. More importantly, the relative abundance of potential PGPR and bio-control agents increased significantly in the S. sclerotiorum-treated wheat rhizosphere soil. The importance of this work is that schizotrophic S. sclerotiorum promotes wheat growth and enhances resistance against fungal diseases via changes in the structure of the root and rhizosphere microbiome.

Mechanistic insight into the free radical scavenging and xanthine oxidase (XO) inhibitor potent of monoacetylphloroglucinols (MAPGs)

Three novel antioxidant candidates based on phenolic polyketide, monoacetylphloroglucinol (MAPG), a natural antibiotic compound produced by plant growth-promoting rhizobacteria (PGPR), Pseudomonas fluorescens F113 have been proposed. Initially, a green and highly efficient route to the synthesis of MAPG and its two analogues from phloroglucinol (PG) has been developed. Afterward, their rational mechanism of antioxidant activity has been investigated based on thermodynamic descriptors involved in the double ( 2H+/2e-) radical trapping processes. These calculations have been performed using the systematic density functional theory (DFT) method at the B3LYP/Def2-SVP level of theory in the gas phase and aqueous solution. Our findings reveal that the double formal hydrogen atom transfer (df-HAT) mechanism is preferred in the gas phase, while the double sequential proton loss electron transfer (dSPLET) mechanism is preferred in aqueous solution for all MAPGs. The 6-OH group represents the most favourable site for trapping radical species for all MAPGs, which is supported by the pK_a values obtained from DFT calculations. The role of acyl substituents on the PG ring has been comprehensively discussed. The presence of acyl substituents has a strong influence on the thermodynamic parameters of the phenolic O-H bond in PG. These results are supported by frontier molecular orbitals (FMOs) analysis, where the addition of acyl substituents increases the chemical reactivity of MAPGs significantly. Based on molecular docking and molecular dynamics simulations (MDs), MAPGs are also predicted to be promising candidates for xanthine oxidase (XO) inhibition.HighlightsThe antioxidant activity of the three synthesised MAPGs has been investigated using the DFT method.Acyl substituents increase the chemical reactivity and antioxidant activity of MAPGs.df-HAT is the preferred mechanism in the gas phase.dSPLET seems to be more favoured in aqueous solution.MAPGs are expected to be promising xanthine oxidase (XO) inhibitors.

Isolation and Identification Antagonistic Bacterium Paenibacillus tianmuensis YM002 against Acidovorax citrulli

In this study, we aimed to screen antagonistic microorganisms against Acidovorax citrulli, the causal agent of bacterial fruit blotch, which is known to induce sever diseases in cucurbit crops. From 240 bacterial strains isolated, only one unknown bacterial isolate, named YM002, showed significant antagonistic activity against A. citrulli KACC17909. Further experiments revealed that YM002 shows antagonistic activity against all tested A. citrulli strains, including KACC17000, KACC17001 and KACC17005, to different degrees. The phylogenetic analysis of 16S rRNA sequences identified YM002 as Paenibacillus tianmuensis. Importantly, pretreatment of cucumber (Cucumis sativus) leaves with YM002 enhanced disease resistance as observed by significantly reduced necrotic symptom development and bacterial growth. YM002-induced resistance accompanied by enhanced expression of defense-related genes, such as PAL1, PR1-1a and CTR1. Importantly, culture filtrate of YM002 significantly suppressed biofilm formation and swimming motility of A. citrulli, which is indispensable for its full virulence. In addition to its antagonistic activity, YM002 showed a various plant growth promotion (PGP)-related traits, such as production of ammonia production, amylase production, ACC deaminase production, inodole-3-acetic acid production, extracellular protease production, siderophore production, and zinc solubilization activities. Indeed, treatment of cucumber roots with YM002 significantly enhanced plant growth parameters, such as fresh and dry weight of leaves or roots. This study suggests the potential of YM002 as an effective PGPR with biological control activity against Acidovorax citrulli in cucumber plants.

Isolation and Evaluation of the Antagonistic Activity of Cnidium officinale Rhizosphere Bacteria against Phytopathogenic fungi (Fusarium solani)

Cnidium officinale Makino, a perennial crop in the Umbeliperae family, is one of Korea's representative forest medicinal plants. However, the growing area of C. officinale has been reduced by plant disease and soil sickness caused by fusarium wilt. This study isolated rhizosphere bacteria from C. officinale, and their antagonistic activity was evaluated against Fusarium solani. Particularly, four isolated strains, namely, PT1, ST7, ST8, and SP4, showed a significant antagonistic activity against F. solani. An in planta test showed that the mortality rates of shoots were significantly low in the PT1-inoculated group. The fresh and dry weights of the inoculated plants were also higher than that of the other groups. The 16S rRNA gene sequencing identified the strain PT1 as Leclercia adecarboxylata, and downstream studies confirmed the production of antagonism-related enzymes such as siderophore and N-acetyl-β-glucosaminidase. The phosphorous solubilizing ability and secretion of related enzymes were also analyzed. The results showed that PT1 strain could be utilized as promising plant growth-promoting rhizobacteria (PGPR) and biocontrol agents (BCAs).

Effect of Heavy-Metal-Resistant PGPR Inoculants on Growth, Rhizosphere Microbiome and Remediation Potential of Miscanthus × giganteus in Zinc-Contaminated Soil

Microbial-assisted phytoremediation is considered a more effective approach to soil rehabilitation than the sole use of plants. Mycolicibacterium sp. Pb113 and Chitinophaga sp. Zn19, heavy-metal-resistant PGPR strains originally isolated from the rhizosphere of Miscanthus × giganteus, were used as inoculants of the host plant grown in control and zinc-contaminated (1650 mg/kg) soil in a 4-month pot experiment. The diversity and taxonomic structure of the rhizosphere microbiomes, assessed with metagenomic analysis of rhizosphere samples for the 16S rRNA gene, were studied. Principal coordinate analysis showed differences in the formation of the microbiomes, which was affected by zinc rather than by the inoculants. Bacterial taxa affected by zinc and the inoculants, and the taxa potentially involved in the promotion of plant growth as well as in assisted phytoremediation, were identified. Both inoculants promoted miscanthus growth, but only Chitinophaga sp. Zn19 contributed to significant Zn accumulation in the aboveground part of the plant. In this study, the positive effect of miscanthus inoculation with Mycolicibacterium spp. and Chitinophaga spp. was demonstrated for the first time. On the basis of our data, the bacterial strains studied may be recommended to improve the efficiency of M. × giganteus phytoremediation of zinc-contaminated soil.

Synergistic effects of biofilm-producing PGPR strains on wheat plant colonization, growth and soil resilience under drought stress

Drought stress substantially impedes crop productivity throughout the world. Microbial based approaches have been considered a potential possibility and are under study. Based on our prior screening examination, two distinct and novel biofilm-forming PGPR strains namely Bacillus subtilis-FAB1 and Pseudomonas azotoformans-FAP3 are encompassed in this research. Bacterial biofilm development on glass surface, microtiter plate and seedling roots were assessed and characterized quantitatively and qualitatively by light and scanning electron microscopy. Above two isolates were further evaluated for their consistent performance by inoculating on wheat plants in a pot-soil system under water stresses. Bacterial moderate tolerance to ten-day drought was recorded on the application of individual strains with wheat plants; however, the FAB1 + FAP3 consortium expressively improved wheat survival during drought. The strains FAB1 and FAP3 displayed distinct and multifunctional plant growth stimulating attributes as well as effective roots and rhizosphere colonization in combination which could provide sustained wheat growth during drought. FAB1 and FAP3-induced alterations cooperatively conferred improved plant drought tolerance by controlling physiological traits (gs, Ci, E, iWUE and P_N), stress indicators (SOD, CAT, GR, proline and MDA content) and also maintained physico-chemical attributes and hydrolytic enzymes including DHA, urease, ALP, protease, ACP and β glucosidase in the soil. Our findings could support future efforts to enhance plant drought tolerance by engineering the rhizobacterial biofilms and associated attributes which requires in-depth exploration and exploiting potential native strains for local agricultural application.

Counteracting action of Bacillus stratosphericus and Staphylococcus succinus strains against deleterious salt effects on Zea mays L

The salinization of soil is the process of progressive accumulation of salts such as sulfates, sodium, or chlorides into the soil. The increased level of salt has significant effects on glycophyte plants, such as rice, maize, and wheat, which are staple foods for the world's population. Consequently, it is important to develop biotechnologies that improve crops and clean up the soil. Among other remediation methods, there is an environmentally friendly approach to ameliorate the cultivation of glycophyte plants in saline soil, namely, the use of microorganisms tolerant to salt with growth-promoting features. Plant growth-promoting rhizobacteria (PGPR) can improve plant growth by colonizing their roots and playing a vital role in helping plants to establish and grow in nutrient-deficient conditions. Our research aimed to test in vivo halotolerant PGPR, isolated and characterized in vitro in a previous study conducted in our laboratory, inoculating them on maize seedlings to improve their growth in the presence of sodium chloride. The bacterial inoculation was performed using the seed-coating method, and the produced effects were evaluated by morphometric analysis, quantization of ion contents (sodium, potassium), produced biomass, both for epigeal (shoot) and hypogeal (root) organs, and by measuring salt-induced oxidative damage. The results showed an increase in biomass and sodium tolerance and even a reduction of oxidative stress in seedlings pretreated with a PGPR bacterial consortium (Staphylococcus succinus + Bacillus stratosphericus) over the control. Moreover, we observed that salt reduces growth and alters root system traits of maize seedlings, while bacterial treatment improves plant growth and partially restores the root architecture system in saline stress conditions. Therefore, the PGPR seed-coating or seedling treatment could be an effective strategy to enhance sustainable agriculture in saline soils due to the protection of the plants from their inhibitory effect.

Certain Tomato Root Exudates Induced by Pseudomonas stutzeri NRCB010 Enhance Its Rhizosphere Colonization Capability

Plant growth-promoting rhizobacteria (PGPR) can colonize plant root surfaces or form biofilms to promote plant growth and enhance plant resistance to harsh external environments. However, plant-PGPR interactions, especially chemical signaling molecules, are poorly understood. This study aimed to gain an in-depth understanding of the rhizosphere interaction mechanisms between PGPR and tomato plants. This study found that inoculation with a certain concentration of Pseudomonas stutzeri significantly promoted tomato growth and induced significant changes in tomato root exudates. Furthermore, the root exudates significantly induced NRCB010 growth, swarming motility, and biofilm formation. In addition, the composition of the root exudates was analyzed, and four metabolites (methyl hexadecanoate, methyl stearate, 2,4-di-tert-butylphenol, and n-hexadecanoic acid) significantly related to the chemotaxis and biofilm formation of NRCB010 were screened. Further assessment showed that these metabolites positively affected the growth, swarming motility, chemotaxis, or biofilm formation of strain NRCB010. Among these, n-hexadecanoic acid induced the most remarkable growth, chemotactic response, biofilm formation, and rhizosphere colonization. This study will help develop effective PGPR-based bioformulations to improve PGPR colonization and crop yields.

Lysinibacillus spp.: an IAA-producing endospore forming-bacteria that promotes plant growth

Lysinibacillus is a bacterial genus that has generated recent interest for its biotechnological potential in agriculture. Strains belonging to this group are recognized for their mosquitocidal and bioremediation activity. However, in recent years some reports indicate its importance as plant growth promoting rhizobacteria (PGPR). This research sought to provide evidence of the PGP activity of Lysinibacillus spp. and the role of the indole-3-acetic acid (IAA) production associated with this activity. Twelve Lysinibacillus spp. strains were evaluated under greenhouse conditions, six of which increased the biomass and root architecture of corn plants. In most cases, growth stimulation was evident at 10⁸ CFU/mL inoculum concentration. All strains produced IAA with high variation between them (20-70 µg/mL). The bioinformatic identification of predicted genes associated with IAA production allowed the detection of the indole pyruvic acid pathway to synthesize IAA in all strains; additionally, genes for a tryptamine pathway were detected in two strains. Extracellular filtrates from all strain's cultures increased the corn coleoptile length in an IAA-similar concentration pattern, which demonstrates the filtrates had an auxin-like effect on plant tissue. Five of the six strains that previously showed PGPR activity in corn also promoted the growth of Arabidopsis thaliana (col 0). These strains induced changes in root architecture of Arabidopsis mutant plants (aux1-7/axr4-2), the partial reversion of mutant phenotype indicated the role of IAA on plant growth. This work provided solid evidence of the association of Lysinibacillus spp. IAA production with their PGP activity, which constitutes a new approach for this genus. These elements contribute to the biotechnological exploration of this bacterial genus for agricultural biotechnology.

Plant growth-promoting rhizobacteria are important contributors to rice yield in karst soils

The difficulty of releasing nutrients from soils in karst areas limits the yield of local crops and leads to poverty. In this study, two strains of plant growth-promoting rhizobacteria (PGPR) were isolated from the rhizosphere soil of typical plants in karst areas, which were both identified as Bacillus sp. and named GS1 and N1. And two isolates were used to construct a composite PGPR named MC1. These three strains of PGPR were used for soil inoculation in the pot experiment and field trial and their capacity to promote rice development was assessed. The results showed that MC1 inoculation exhibited notable rice growth-promoting ability in pot experiments, and, respectively, had an increment of 16.96, 18.74, and 11.50% in shoot biomass, total biomass, and rice height compared with control. This is largely attributed to PGPR's capacity to secrete phytohormones and soil enzymes, particularly urease (UE) in GS1, whose secreted UE content was significantly higher by 12.18% compared to the control. When applied to the field, MC1 inoculation not only increased rice yield by 8.52% and the available nutrient content in rice rhizosphere soil, such as available phosphorus (AP) and exchangeable magnesium (EMg); but also improved the abundance of beneficial rhizobacteria and the diversity of microbial communities in rice rhizosphere soil. Results in this study revealed that inoculated PGPR played a major role in promoting rice growth and development, and a new strategy for facilitating the growth of rice crops in agriculture was elucidated.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03593-0.

Isolation and characterization of a plant growth‑promoting rhizobacterium strain MD36 that promotes barley seedlings and growth under heavy metals stress

Plant growth, promoting, bacteria, (PGPB) can improve plant germination and growth in heavy metal-contaminated land and enhance heavy metal removal efficiency. In this study, we isolated PGPR bacterial strains which can withstand heavy metal pollution and tested their ability to improve barley germination under heavy metal stress. Out of 16 multi-resistant heavy metal isolates, strain MD36 was identified by 16S rRNA sequencing and shared close relation to different species of the genus Glutamicibacter. The new isolated strain showed other important PGPR activities, mainly IAA production and salt tolerance. The effect of adding the strain MD36 to barley grains under heavy metal stress enhanced their germination up to 100%, while the percentage of germination ranged between 0 and 20% for non-inoculated grains. In addition, in these conditions, MD36 can significantly enhance barley growth by reducing the heavy metal effect. This study strongly recommends the use of MD36 as seed coatings trials in the field to enhance growth and yield in soils contaminated with heavy metals, as well as in bioremediation of HM-contaminated salt-containing soils and water.

Phosphate Solubilization and Plant Growth Promotion by Pantoea brenneri Soil Isolates

Phosphate solubilizing microorganisms (PSMs) in soil have been shown to reduce mineral phosphate fertilizer supplementation and promote plant growth. Nevertheless, only several P-solubilizing microorganisms capable of solubilizing both organic and mineral sources of soil phosphorus have been identified up to now. The aim of this study was to evaluate the inorganic soil phosphate solubilizing activity of phytate-hydrolyzing Pantoea brenneri soil isolates. We showed that the strains efficiently solubilize a variety of inorganic phosphates. We optimized the media composition and culturing conditions to improve the solubilization efficiency of the strains and investigated the mechanisms of their phosphate solubilization. Through HPLC analysis, it was determined that P. brenneri produce oxalic, malic, formic, malonic, lactic, maleic, acetic, and citric acids as well as acid and alkaline phosphatases while growing on insoluble phosphate sources. Finally, we analyzed the influence of P. brenneri strains with multiple PGP-treats on plant growth in greenhouse experiments and showed their ability to promote growth of potato.

Rhizobacteria control damping-off and promote growth of lima bean with and without co-inoculation with Rhizobium tropici CIAT899

Rhizoctonia solani compromises the production of lima bean, an alternative and low-input food source in many tropical regions. Inoculation of bacterial strains has been used, but research on their biocontrol and growth promotion potential on lima bean is scarce. The objective of this study was to evaluate the effects of inoculation with rhizobacterial strains of the genera Bacillus, Brevibacillus, Paenibacillus, Burkholderia, Pseudomonas, and Rhizobium in combination or not with N₂-fixing Rhizobium tropici on the control of damping-off disease and growth promotion in lima bean plants. Greenhouse experiments were conducted to evaluate the inoculation with bacterial strains with biocontrol potential in combination or not with R. tropici in substrate infected with R. solani CML 1846. Growth promotion of these strains was also assessed. Strains of Brevibacillus (UFLA 02-286), Pseudomonas (UFLA 02-281 and UFLA 04-885), Rhizobium (UFLA 04-195), and Burkholderia (UFLA 04-227) co-inoculated with the strain CIAT 899 (Rhizobium tropici) were the most effective in controlling R. solani, reducing the disease incidence in 47-60% on lima bean. The promising strains used in the biocontrol assays were also responsive in promoting growth of lima bean under disease and sterile conditions. A positive synergistic effect of co-inoculation of different genera contributed to plant growth, and these outcomes are important first steps to improve lima bean production.

Yield and Nutraceutical Value of Lettuce and Basil Improved by a Microbial Inoculum in Greenhouse Experiments

Members of Bacillus spp. have been widely used to enrich the soil/root interface to provide plant growth promoting activities. A new isolate, namely to Bacillus sp. VWC18, has been tested under greenhouse conditions in lettuce (Lactuca sativa L.) pots at different concentrations (10³, 10⁵, 10⁷, and 10⁹ CFU·mL^-1) and application time (single inoculum at transplant and multiple inoculum every ten days) to evaluate the best application dose and frequency. Analysis of foliar yield, main nutrients, and minerals evidenced a significant response for all applications. The lowest (10³ CFU·mL^-1) and the highest doses (10⁹ CFU·mL^-1), applied every ten days until harvest, had the greatest efficacy; the nutrient yield (N, K, P, Na, Ca, Fe, Mg, Mn, Cu, and B) increased more than twice. A new randomized block design with three replicates was then performed in lettuce and basil (Ocinum basilicum L.), with the two best performing concentrations applied every ten days. In addition to previous analysis, root weight, chlorophyll, and carotenoids were also examined. Both experiments confirmed the previous results: inoculation of the substrate with Bacillus sp. VWC18 promoted plant growth, chlorophyll, and mineral uptake in both crop species. Root weight duplicated or triplicated compared to control plants, and chlorophyll concentration reached even higher values. Both parameters had a dose-dependent increase.

Quorum sensing activities and genomic insights of plant growth-promoting rhizobacteria isolated from Assam tea

Secretion of quorum sensing (QS) molecules is important for the effective colonization of host plants by plant growth-promoting rhizobacteria. The current study aims at the isolation and characterization of tea rhizo bacteria, which produce the QS molecules, acyl homoserine lactone (AHLs), along with multiple plant growth-promoting (PGP) activities. Thirty-one isolates were isolated from the tea rhizosphere, and screening for PGP activities resulted in the selection of isolates RTE1 and RTE4 with multiple PGP traits, inhibiting the growth of tea fungal pathogens. Both isolates also showed production of AHL molecules when screened using two biosensor strains, Chromobacterium violaceum CV026 and Escherichia coli MT 102(jb132). The isolates identified as Burkholderia cepacia RTE1 and Pseudomonas aeruginosa RTE4 based on genome-based analysis like phylogeny, dDDH, and fastANI calculation. Detailed characterization of AHLs produced by the isolates using reverse-phase TLC, fluorometry, and LC-MS indicated that the isolate RTE1 produced a short chain, C8, and a long chain C12 AHL, while RTE4 produced short-chain AHLs C4 and C6. Confocal microscopy revealed the formation of thick biofilm by RTE1 and RTE4 (18 and 23 μm, respectively). Additionally, we found several genes involved in QS, and PGP, inducing systemic resistance (ISR) activities such as lasI/R, qscR, pqq, pvd, aldH, acdS, phz, Sod, rml, and Pch, and biosynthetic gene clusters like N-acyl homoserine lactone synthase, terpenes, pyochelin, and pyocyanin. Based on the functional traits like PGP, biofilm formation and production of AHL molecules, and genetic potential of the isolates B. cepacia RTE1 and P. aeruginosa RTE4 appear promising candidates to improve the health and growth of tea plantations.

Bacterial assisted phytoremediation of heavy metals and organic pollutants by Cannabis sativa as accumulator plants growing on distillery sludge for ecorestoration of polluted site

The aim of the study is to explore the potential rhizospheric bacterial communities associated with Cannabis sativa L. (Cannabis); growing on the complex pollutant-rich distillery sludge. Seven bacterial species were isolated, among which four potential bacterial species were characterized based on the 16s rRNA sequencing from the rhizosphere sludge of C. sativa; they are Bacillus thuringiensis (MW887525), Bacillus cereus (MW887524), Achromobacter denitrificans (MW886333), Bacillus subtilis (MW886231). The isolated bacteria showed PGPR attributes and potential for ligninolytic enzyme activity. Further, to correlate these bacteria with organic pollutants of sludge, the GC-MS analysis of fresh disposed distillery sludge and after growth of 30 and 60 days C. sativa was also analysed, which showed the conversion and disappearance of compounds by the activity of rhizospheric bacterial communities. Additionally, C. sativa showed a higher metal accumulation pattern of Fe (801.81 ± 0.123)> Cu (275.086 ± 0.069)> Zn (162.15 ± 0.085)> Mn (63.92 ± 0.093)> Pb (28.619 ± 0.192)> Ni (5.02 ± 0.078)> Cd (2.53 ± 0.085)> Cr (1.87 ± 0.079) mg kg ^-1 in their shoot, root followed by leaf. The plant also showed BCF >1 and TF > 1 for most of the metals. Thus, this showed the phytoextraction properties of C. sativa from distillery sludge polluted sites. The findings of this study will enable to understand the functional role of rhizospheric bacterial community for the detoxification and degradation of complex organometallic waste, and will thus aid in the development of adequate phytoremediation techniques for the eco-restoration of polluted industrial sites for sustainable development.

Remediation of the black-odor water body by aquatic plants with plant growth-promoting Rhizobacteria: Lab and pilot tests

To explore an effective, environmental, rapid operating method to repair black and odor water bodies, water samples and sediment samples collected from a polluted municipal lake in Daqing, China, were directly tested in transparent barrels (10 L). Seven groups of optimizing parameters obtained the optimal operating method, and the max removal rate of COD, NH₄⁺-N, NO₃^--N, and TP were achieved (89.18%, 59.65%, 69.50%, and 75.61%) by using aquatic plants with plant growth-promoting Rhizobacteria (PGPR). To further verify the method's effectiveness, lager scale tests were conducted based on a water tank (216 L), and similar removal rates were obtained within 48 h. The water quality index and microbial community structure analysis revealed the mechanisms of the interaction among plants, microorganisms, and pollutants and the main biological processes during water body remediation. Finally, the cost of water body remediation by using this method was estimated.

Performance of Plant-Growth-Promoting Rhizobacteria (PGPR) Isolated from Sandy Soil on Growth of Tomato (Solanum lycopersicum L.)

The plant-growth-promoting rhizobacteria (PGPR) in the rhizosphere affect plant growth, health, and productivity, as well as soil-nutrient contents. They are considered a green and eco-friendly technology that will reduce chemical-fertilizer usage, thereby reducing production costs and protecting the environment. Out of 58 bacterial strains isolated in Qassim, Saudi Arabia, four strains were identified by the 16S rRNA as the Streptomyces cinereoruber strain P6-4, Priestia megaterium strain P12, Rossellomorea aquimaris strain P22-2, and Pseudomonas plecoglossicida strain P24. The plant-growth-promoting (PGP) features of the identified bacteria involving inorganic phosphate (P) solubilization, the production of indole acetic acid (IAA), and siderophore secretion were assessed in vitro. Regarding the P solubilization, the previous strains' efficacy reached 37.71%, 52.84%, 94.31%, and 64.20%, respectively. The strains produced considerable amounts of IAA (69.82, 251.70, 236.57, and 101.94 µg/mL) after 4 days of incubation at 30 °C. Furthermore, the rates of siderophore production reached 35.51, 26.37, 26.37, and 23.84 psu, respectively, in the same strains. The application of the selected strains in the presence of rock phosphate (RP) with tomato plants under greenhouse conditions was evaluated. The plant growth and P-uptake traits positively and significantly increased in response to all the bacterial treatments, except for some traits, such as plant height, number of leaves, and leaf DM at 21 DAT, compared to the negative control (rock phosphate, T2). Notably, the P. megaterium strain P12 (T4), followed by R. aquimaris strain P22-2 (T5), revealed the best values related to plant height (at 45 DAT), number of leaves per plant (at 45 DAT), root length, leaf area, leaf-P uptake, stem P uptake, and total plant P uptake compared to the rock phosphate. The first two components of the PCA (principal component analysis) represented 71.99% (PCA1 = 50.81% and PCA2 = 21.18%) of the variation at 45 DAT. Finally, the PGPR improved the vegetative-growth traits of the tomato plants through P solubilization, IAA, and siderophore production, and ameliorated the availability of nutrients. Thus, applying in PGPR in sustainable agriculture will potentially reduce production costs and protect the environment from contamination by chemical fertilizers and pesticides.

The peanut root exudate increases the transport and metabolism of nutrients and enhances the plant growth-promoting effects of burkholderia pyrrocinia strain P10

BACKGROUND

Burkholderia pyrrocinia strain P10 is a plant growth-promoting rhizobacterium (PGPR) that can substantially increase peanut growth. However, the mechanisms and pathways involved in the interaction between B. pyrrocinia P10 and peanut remain unclear. To clarify complex plant-PGPR interactions and the growth-promoting effects of PGPR strains, the B. pyrrocinia P10 transcriptome changes in response to the peanut root exudate (RE) were elucidated and the effects of RE components on biofilm formation and indole-3-acetic acid (IAA) secretion were analyzed.

RESULTS

During the early interaction phase, the peanut RE enhanced the transport and metabolism of nutrients, including carbohydrates, amino acids, nitrogen, and sulfur. Although the expression of flagellar assembly-related genes was down-regulated, the expression levels of other genes involved in biofilm formation, quorum sensing, and Type II, III, and VI secretion systems were up-regulated, thereby enabling strain P10 to outcompete other microbes to colonize the peanut rhizosphere. The peanut RE also improved the plant growth-promoting effects of strain P10 by activating the expression of genes associated with siderophore biosynthesis, IAA production, and phosphorus solubilization. Additionally, organic acids and amino acids were identified as the dominant components in the peanut RE. Furthermore, strain P10 biofilm formation was induced by malic acid, oxalic acid, and citric acid, whereas IAA secretion was promoted by the alanine, glycine, and proline in the peanut RE.

CONCLUSION

The peanut RE positively affects B. pyrrocinia P10 growth, while also enhancing colonization and growth-promoting effects during the early interaction period. These findings may help to elucidate the mechanisms underlying complex plant-PGPR interactions, with potential implications for improving the applicability of PGPR strains.

Evaluation of the growth-inducing efficacy of various Bacillus species on the salt-stressed tomato (Lycopersicon esculentum Mill.)

Plants are affected by salt stress in a variety of ways, including water deficiency, ion toxicity, nutrient imbalance, and oxidative stress, all of which can cause cellular damage or plant death. Halotolerant plant growth-promoting rhizobacteria (PGPR) could be a viable alternative for tomato plants growing in arid and semi-arid environments. The aim of this research was to isolate halotolerant plant growth promoting Bacillus sp. to promote tomato (Lycopersicon esculentum Mill.) growth and salt stress resistance. 107 PGPR strains were isolated from the rhizospheres of 'Kesudo' (Butea monosperma Lam.), 'Kawaria' (Cassia tora L.), and 'Arjun' (Terminalia arjuna Roxb.) plants to test their plant growth promoting abilities, including indole-3-acetic acid, phosphate solubilization, siderophore production, and ACC deaminase activity. Five bacterial strains (Bacillus pumilus (NCT4), Bacillus firmus (NCT1), Bacillus licheniformis (LCT4), Bacillus cereus (LAT3), and Bacillus safensis (LBM4)) were chosen for 16S rRNA on the basis of PGPR traits. Compared to PGPR untreated plants, tomato plants developed from PGPR-treated seeds had considerably increased germination percentage, seedling growth, plant height, dry weight, and leaf area. As comparison to PGPR non-inoculated plants, salt-stressed tomato plants treated with PGPR strains had higher levels of total soluble sugar, proline, and chlorophyll as well as higher levels of SOD, CAT, APX, and GR activity. PGPR-inoculated salt-stressed tomato plants had lower MDA, sodium, and chloride levels than non-inoculated plants. In addition, magnesium, calcium, potassium, phosphorus, and iron levels were higher in PGPR treated plants when subjected to salt stress. These results indicate that halotolerant PGPR strains can increase tomato productivity and tolerance to salt stress by removing salt stress's negative effects on plant growth.