Isolation and characterization of phosphate solubilizing bacterium Pseudomonas aeruginosa KUPSB12 with antibacterial potential from river Ganga, India

Drought-tolerant rhizobacteria with predicted functional traits enhanced wheat growth and P uptake under moderate drought and low P-availability

This study aims to investigate the effect of isolated drought-tolerant rhizobacteria, spanning various groups, such as nitrogen-fixing bacteria (NFB), phosphate solubilizing bacteria (PSB), and other plant growth promoting rhizobacteria (PGPR), on the growth of wheat (Triticum durum) plants, focusing on various morphological and physiological responses under moderate drought and low-P availability. Among 343 rhizobacterial morphotypes, 16 exhibited tolerance to NaCl and PEG-6000. These included 8 PSB, 4 NFB, and 4 osmotolerant-PGPR groups, distributed across 14 different genera. Biochemical characterization showcased diverse PGP capabilities, particularly in P solubilization. The dynamic responses of drought-tolerant PSB to salt and PEG-6000-induced drought stress involved variations in organic acid (OA) secretion, with specific acids, including palmitic, lactic, and stearic, playing crucial roles in enhancing available P fractions. Inoculation with rhizobacteria significantly increased both shoot (SDW) and root (RDW) dry weights of wheat plants, as well as rhizosphere available P. PSB11 (Arthrobacter oryzae) emerged as the most effective strain, plausibly due to its positive impact on root morphological traits (length, surface, and volume). Other isolates, PSB10 (Priestia flexa), PSB13 (Bacillus haynesii), and particularly PGPR2 (Arthrobacter pascens) significantly increased shoot P content (up to 68.91 %), with a 2-fold increase in chlorophyll content. The correlation analysis highlighted positive associations between SDW, shoot P content, chlorophyll content index (CCI), and leaf area. Additionally, a negative correlation emerged between microbial biomass P and root morphophysiological parameters. This pattern could be explained by reduced competition between plants and rhizobacteria for accessible P, as indicated by low microbial biomass P and strong plant growth. Our investigation reveals the potential of drought-tolerant rhizobacteria in enhancing wheat resilience to moderate drought and low-P conditions. This is demonstrated through exceptional performance in influencing root architecture, P utilization efficiency, and overall plant physiological parameters. Beyond these outcomes, the innovative isolation procedure employed, targeting rhizobacteria from diverse groups, opens new avenues for targeted isolation techniques. This unique approach contributes to the novelty of our study, offering promising prospects for targeted bioinoculants in mitigating the challenges of drought and P deficiency in wheat cultivation.

A comprehensive scenario of heavy metals pollution in the rivers of Bangladesh during the last two decades

For decades, rivers have been used for transporting pollutants loaded with heavy metals (HMs) causing severe pollution in downstream. The current study aimed to review the levels and sources of 10 HMs, viz. As, Pb, Cd, Cr, Fe, Mn, Cu, Co, Ni, and Zn in the surface water of the rivers in Bangladesh. The PRISMA criteria were used to conduct a systematic review of the available literature published between 2001 and 2020, and thus a total of 55 documents were finally selected for review. The mean concentration of each HM exceeding the threshold limits as per World Health Organization (WHO), the United States Environmental Protection Agency (USEPA), and Department of Environment (DoE), Bangladesh standards were higher in the last decade (2011-2020) than in the previous one (2001-2010). Most HM concentrations in water were found above the threshold limits in three divisions (Dhaka, Rajshahi, and Chattogram). The Buriganga River in Dhaka has been the top polluted river in Bangladesh. Among the 10 HMs, six metals (As, Pb, Cd, Cr, Fe, and Mn) exceeded the limit set by WHO, USEPA, and DoE in all three seasons, where mean values of most of the HMs were found to be the highest in the summer season. Statistical analyses identified possible sources of HMs such as natural weathering, electroplating, fertilizers and pesticides, mining and manufacturing, textiles, coal mining and burning, batteries, and paint industries. Strong legislations and regulations, awareness programs, continuous monitoring, and comprehensive research are urgently needed to control riverine HMs pollution in Bangladesh.

Diving into freshwater microbial metabolites: Pioneering research and future prospects

In practically every facet of life, especially nutrition, agriculture, and healthcare, microorganisms offer a prospective origin for abundant natural substances and products. Among these microorganisms, bacteria also possess the capability to rapidly acclimate to diverse environments, utilize varied resources, and effectively respond to environmental fluctuations, including those influenced by human activities like pollution and climate change. The ever-changing environment of freshwater bodies influences bacterial communities, offering opportunities for improving health and environmental conservation that remain unexplored. Herein, the study discusses the bacterial taxa along with specialised metabolites with antioxidant, antibacterial, and anticancer activity that have been identified from freshwater environments, thus achieving Sustainable Development Goals addressing health and wellbeing (SDG-3), economic growth (SDG-8) along with industrial development (SDG-9). The present review is intended as a compendium for research teams working in the fields of medicinal chemistry, organic chemistry, clinical research, and natural product chemistry.

Exploring the Potential of Halotolerant Actinomycetes from Rann of Kutch, India: A Study on the Synthesis, Characterization, and Biomedical Applications of Silver Nanoparticles

A tremendous increase in the green synthesis of metallic nanoparticles has been noticed in the last decades, which is due to their unique properties at the nano dimension. The present research work deals with synthesis mediated by the actinomycete Streptomyces tendae of silver nanoparticles (AgNPs), isolated from Little and Greater Rann of Kutch, India. The confirmation of the formation of AgNPs by the actinomycetes was carried out by using a UV-Vis spectrophotometer where an absorbance peak was obtained at 420 nm. The X-ray diffraction pattern demonstrated five characteristic diffraction peaks indexed at the lattice plane (111), (200), (231), (222), and (220). Fourier transform infrared showed typical bands at 531 to 1635, 2111, and 3328 cm-1. Scanning electron microscopy shows that the spherical-shaped AgNPs particles have diameters in the range of 40 to 90 nm. The particle size analysis displayed the mean particle size of AgNPs in aqueous medium, which was about 55 nm (±27 nm), bearing a negative charge on their surfaces. The potential of the S. tendae-mediated synthesized AgNPs was evaluated for their antimicrobial, anti-methicillin-resistant Staphylococcus aureus (MRSA), anti-biofilm, and anti-oxidant activity. The maximum inhibitory effect was observed against Pseudomonas aeruginosa at (8 µg/mL), followed by Escherichia coli and Aspergillus niger at (32 µg/mL), and against Candida albicans (64 µg/mL), whereas Bacillus subtilis (128 µg/mL) and Staphylococcus aureus (256 µg/mL) were much less sensitive to AgNPs. The biosynthesized AgNPs displayed activity against MRSA, and the free radical scavenging activity was observed with an increase in the dosage of AgNPs from 25 to 200 µg/mL. AgNPs in combination with ampicillin displayed inhibition of the development of biofilm in Pseudomonas aeruginosa and Streptococcus pneumoniae at 98% and 83%, respectively. AgNPs were also successfully coated on the surface of cotton to prepare antimicrobial surgical cotton, which demonstrated inhibitory action against Bacillus subtilis (15 mm) and Escherichia coli (12 mm). The present research integrates microbiology, nanotechnology, and biomedical science to formulate environmentally friendly antimicrobial materials using halotolerant actinomycetes, evolving green nanotechnology in the biomedical field. Moreover, this study broadens the understanding of halotolerant actinomycetes and their potential and opens possibilities for formulating new antimicrobial products and therapies.

Aspergillus niger as an eco-friendly agent for potassium release from K- bearing minerals: Isolation, screening and culture medium optimization using Plackett-Burman design and response surface methodology

The potential of Aspergillus niger, to enhance non-exchangeable potassium (K+) release from mineral structures were investigated as a cost-effective and environmentally friendly alternative to traditional chemical fertilizers. Optimizing the culture medium for maximum K+ release, alongside identifying potential mechanisms of action of the A. niger including the production of various organic acids and pH reduction in the minerals feldspar and phlogopite, were among the primary objectives of the present study. K+ dissolution from feldspar and phlogopite in the presence of Aspergillus niger were examined through a two-step experiment; impact of different carbon sources (glucose, sucrose, and fructose) on K+ release using the Plackett-Burman design (PBD) with 12 experimental runs and effect of other independent variables including pH (ranging from 5 to 10), carbon concentration (3-12.3 g l-1), and incubation time (5-18 days) on K+ release using the central composite design (CCD). Our results indicated that the PBD demonstrated a strong predictive capacity (RMSE = 0.012-0.018 g l-1 and R2 = 0.85-0.89) for K+ release. According to the CCD model, pH exerted a significant positive influence on increasing soluble K+ release (P < 0.001). The highest levels of K+ release (157.8 and 175.3 mg l-1 in feldspar and phlogopite, respectively) were observed at the central levels (0) of time and carbon source, and at the +α level (+1.68) of pH. Furthermore, based on the CCD model, the optimal conditions for achieving high K+ release from feldspar and phlogopite in a medium were pHs of 10.36 and 10.31, sucrose concentrations of 11.23 and 11.32 g l-1, and incubation times of 15 and 18 days, respectively. The determination coefficients of the CCD model indicated that 89.5% and 92.6% of the changes in soluble K+ for feldspar and phlogopite, could be explained by this model, respectively. In the current study, the production of organic acids and the resulting pH reduction, along with the reduction in mineral particle size in feldspar and phlogopite, were identified as potential mechanisms influencing the enhancement of potassium solubility. The predominant acids in both feldspar and phlogopite were lactic acid (70.9 and 69.15 mg l-1) and citric acid (40.48 and 22.93 mg l-1), although the production levels of organic acids differed in the two minerals. Overall, our findings highlight the potential of A. niger to proficiently release non-exchangeable potassium from mineral matrices, indicating its promising potential in agricultural applications.

Elucidating potential bioindicators from insights in the diversity and assembly processes of prokaryotic and eukaryotic communities in the Mekong River

Drivers for spatio-temporal distribution patterns of overall planktonic prokaryotes and eukaryotes in riverine ecosystems are generally not fully understood. This study employed amplicon metabarcoding to investigate the distributions and assembly mechanisms of bacterial and eukaryotic communities in the Mekong River. The prevailing bacteria taxa were found to be Betaproteobacteria, Actinobacteria, and Bacteroidetes, while the dominant eukaryotic organisms were cryptophytes, chlorophytes, and diatoms. The community assemblages were influenced by a combination of stochastic and deterministic processes. Drift (DR) and dispersal limitation (DL), signifying the stochastic mechanism, were the main processes shaping the overall prokaryotic and eukaryotic communities. However, homogeneous selection (HoS), indicating deterministic mechanism, played a major role in the assembly process of core prokaryotic communities, especially in the wet season. In contrast, the core eukaryotic communities including Opisthokonta, Sar, and Chlorophyta were dominated by stochastic processes. The significance of HoS within prokaryotic communities was also found to exhibit a decreasing trend from the upstream sampling sites (Chiang Saen and Chiang Khan, Nong Khai) towards the downstream sites (Mukdahan, and Khong Chiam) of the Mekong River. The environmental gradients resulting from the site-specific variations and the gradual decrease in elevation along the river may have a potential influence on the role of HoS in community assembly. Crucial environmental factors that shape the phylogenetic structure within distinct bins of the core prokaryotic communities including water depth, temperature, chloride, sodium, and sulphate were identified, as inferred by their correlation with the beta Net Relatedness Index (betaNRI) during the wet season. Overall, these findings enhance understanding of the complex mechanisms governing the spatio-temporal dynamics of prokaryotic and eukaryotic communities in the Mekong River. Finally, insights gained from this study could provide information on further use of specific core bacteria as microbial-based bioindicators that are effective for the assessment and conservation of the Mekong River ecosystem.

Siderophore-producing bacteria mitigate cobalt stress in black gram (Vigna mungo L.), and the mitigation strategies are associated with iron concentration

Cobalt (Co) is considered an essential element in agriculture as it is an important constituent of vitamin B12. Due to natural and anthropogenic factors, heavy metals, especially Co, accumulate in agricultural fields, but their high exposure produces ramifications in crop plants, thereby reducing crop yield and biomass. Excessive Co in plants causes oxidative stress, and as the stress progresses, Co competes with iron (Fe) thereby decreasing chlorophyll content and resulting in Fe deficiency in plants. A major concern is to counter the Co toxicity. Therefore, the current study aimed to mitigate Co-stress or Co-toxicity by using siderophore producing microbes and simultaneously mobilize Co and iron (Fe) in required amounts. In this study, 250 bacteria were isolated from agricultural and non-agricultural soils and screened for siderophore production. Initial siderophore screening revealed that 28.8% of the isolates produced siderophore. Subsequent screening for Co-tolerance showed that 16 isolates were tolerant to up to 20,000 ppm of Co and produced ACC deaminase, siderophore (96.82-99.67%), indole-3-acetic acid (15.15-70.55 µg/mL) and phosphate solubilisation (39.33-142.67 µg/mL). A plate assay (200 mM Co stress) revealed that four isolates (KSBTS 12, SBTS 12, CWTS 5 and CWTS 10) enhanced the growth of black gram (Vigna mungo L.). Furthermore, evaluation in pot studies (2000 ppm Co stress) revealed enhanced root (60.69-174.24%) and shoot length (3.27-143.96%) compared to the control. Inoculated plants also enhanced the uptake of nitrogen (37.33-42.36 mg/g) and phosphorous (3.12-3.92 mg/g), chlorophyll content (7.60-22.97 mg/g), siderophore quantity in the soils (282.41-331.53%) and the soil respiration activity such as hydrolysis of fluorescein diacetate (11.33-24.88 µg/g), dehydrogenase enzyme (109.76-197.26 µg/g) and alkaline phosphatase (631.53-918.20 µg/g). In conclusion, CWTS 5 (Bacillus subtilis) and CWTS 10 (Bacillus albus) can be used to mitigate Co-stress and mobilize Co and Fe in plants.

Biodegradation of carbofuran by Pseudomonas aeruginosa S07: biosurfactant production, plant growth promotion, and metal tolerance

Pesticides are an indispensable part of modern farming as it aids in controlling pests and hence increase crop yield. But, unmanaged use of pesticides is a growing concern for safety and conservation of the environment. In the present study, a novel biosurfactant-producing bacterium, Pseudomonas aeruginosa S07, was utilized to degrade carbofuran pesticide, and it was obtained at 150 mg/L concentration; 89.2% degradation was achieved on the 5th day of incubation in in vitro culture condition. GC-MS (gas chromatography and mass spectrometry) and LC-MS (liquid chromatography and mass spectrometry) analyses revealed the presence of several degradation intermediates such as hydroxycarbofurnan, ketocarbofuran, and hydroxybenzofuran, in the degradation process. The bacterium was found to exhibit tolerance towards several heavy metals: Cu, Co, Zn, Ni, and Cd, where maximum and least tolerance were obtained against Co and Ni, respectively. Additionally, the bacterium also possesses plant growth-promoting activity showing positive results in nitrogen fixation, phosphate solubilising, ammonia production, and potassium solubilizing assays. Thus, from the study, it can be assumed that the bacterium can be useful in the production of bioformulation for remediation and rejuvenation of pesticide-contaminated sites in the coming days.

Stenotrophomonas in diversified cropping systems: friend or foe?

In the current scenario, the use of synthetic fertilizers is at its peak, which is an expensive affair, possesses harmful effects to the environment, negatively affecting soil fertility and beneficial soil microfauna as well as human health. Because of this, the demand for natural, chemical-free, and organic foods is increasing day by day. Therefore, in the present circumstances use of biofertilizers for plant growth-promotion and microbe-based biopesticides against biotic stresses are alternative options to reduce the risk of both synthetic fertilizers and pesticides. The plant growth promoting rhizobacteria (PGPR) and microbial biocontrol agents are ecologically safe and effective. Owning their beneficial properties on plant systems without harming the ecosystem, they are catching the widespread interest of researchers, agriculturists, and industrialists. In this context, the genus Stenotrophomonas is an emerging potential source of both biofertilizer and biopesticide. This genus is particularly known for producing osmoprotective substances which play a key role in cellular functions, i.e., DNA replication, DNA-protein interactions, and cellular metabolism to regulate the osmotic balance, and also acts as effective stabilizers of enzymes. Moreover, few species of this genus are disease causing agents in humans that is why; it has become an emerging field of research in the present scenario. In the past, many studies were conducted on exploring the different applications of Stenotrophomonas in various fields, however, further researches are required to explore the various functions of Stenotrophomonas in plant growth promotion and management of pests and diseases under diverse growth conditions and to demonstrate its interaction with plant and soil systems. The present review discusses various plant growth and biocontrol attributes of the genus Stenotrophomonas in various food crops along with knowledge gaps. Additionally, the potential risks and challenges associated with the use of Stenotrophomonas in agriculture systems have also been discussed along with a call for further research in this area.

Isolation, Identification, and Characterization of Phosphate-Solubilizing Bacteria from Tunisian Soils

Soil microorganisms play an important role in maintaining natural ecological balance through active participation in carbon, nitrogen, sulfur, and phosphorous cycles. Phosphate-solubilizing bacteria (PSB) are of high importance in the rhizosphere, enhancing the solubilization of inorganic phosphorus complexes into soluble forms available for plant nutrition. The investigation of this species of bacteria is of major interest in agriculture, as they can be used as biofertilizers for crops. In the present study, 28 isolates of PSB were obtained after the phosphate enrichment of soil samples from five Tunisian regions. Five PSB species were identified by 16S rRNA gene sequencing including Pseudomonas fluorescens, P. putida, and P. taiwanensis, Stenotrophomonas maltophilia, and Pantoea agglomerans. Solid and liquid Pikovskaya’s (PVK) and National Botanical Research Institute’s (NBRIP) media containing insoluble tricalcium phosphate were used for the evaluation of the phosphate solubilization ability of the bacterial isolates by two methods: visual evaluation of the solubilization zone around colonies (halo) and determination of solubilized phosphates in liquid medium by the colorimetric method of the vanado-molybdate yellow. Based on the results of the halo method, the isolate of each species that showed the higher phosphate solubilization index was selected for evaluation of phosphate solubilization by the colorimetric method. In the liquid media, the bacterial isolates showed phosphate solubilization ranging from 535.70 to 618.57 µg mL−1 in the NBRIP medium, and 374.20 to 544.28 µg mL−1 in the PVK medium, with the highest values produced by P. fluorescens. The best phosphate solubilization ability and higher reduction in broth pH, which indicates higher organic acid production, were achieved in NBRIP broth for most of the PSB. Strong correlations were observed between the average capability of PSB to solubilize phosphates and both the pH and total phosphorous content in the soil. The production of the hormone indole acetic acid (IAA), which can promote plant growth, was observed for all five PSB species. Among them, P. fluorescens obtained from the forest soil of northern Tunisia showed the highest production of IAA (50.4 ± 0.9 µg mL−1).

Antifouling Potential of Ethyl Acetate Extract of Marine Bacteria Pseudomonas aeruginosa Strain RLimb

Biofouling is defined as the excessive colonization process of epibiotic organisms, ranging from microfoulers to macrofoulers, on any submerged surface in water. Previous research has attempted to explore the antifouling activity of bacterial isolates due to the biofouling problems occurring worldwide. One solution is to inhibit the early stage of fouling using secondary metabolites produced by marine bacteria. This study aims to determine the antifouling activities of the marine microorganism P. aeruginosa and to characterize the bacteria isolated as a potential anti-biofouling agent. The bacterial isolate was cultured and isolated on a media culture. The bacteria culture extract was extracted using ethyl acetate and concentrated prior to the bioassay method. It was screened for antibacterial activities against Gram-positive and Gram-negative bacteria, such as Bacillus cereus, Streptococcus uberis, Pseudomonas sp., and Vibrio parahaemolyticus, using the disk diffusion technique. The extract was investigated to verify its bioactivity in the prevention of biofilm formation following the crystal violet assay and aquarium test. The results indicated the inhibition of activity through biofilm formation, with the highest percentage at 83% of biofilm inhibition at a concentration of 0.1563 mg/mL. The bacterial isolate at a concentration of 5% showed the highest reduction in bacteria colonies in the aquarium test (161.8 × 103 CFU/mL compared to 722.5 × 103 CFU/mL for the blank sample). The bacterial isolate was characterized through phenotypic and genotypic tests for species identification. It was identified as a Gram-stain-negative, aerobic, and long-rod-shaped bacteria, designated as RLimb. Based on the 16S rDNA gene sequencing analysis, RLimb was identified as Pseudomonas aeruginosa (accession number: OP522351), exhibiting a similarity of 100% to the described neighbor P. aeruginosa strain DSM 50071. These results indicated that these isolated bacteria can potentially be used as a substitute for toxic antifoulants to prevent the formation of microfoulers.

Genomics assisted characterization of plant growth-promoting and metabolite producing psychrotolerant Himalayan Chryseobacterium cucumeris PCH239

Here, we report the first complete genome of a psychrotolerant and yellow-pigmented rhizobacteria Chryseobacterium cucumeris PCH239. It was obtained from the rhizospheric soil of the Himalayan plant Bergenia ciliata. The genome consists of a single contig (5.098 Mb), 36.3% G + C content, and 4899 genes. The cold adaptation, stress response, and DNA repair genes promote survivability in a high-altitude environment. PCH239 grows in temperature (10-37 °C), pH (6.0-8.0), and NaCl (2.0%). The genome derived plant growth-promoting activities of siderophore production (siderophore units 53 ± 0.6), phosphate metabolism (PSI 5.0 ± 0.8), protease, indole acetic acid production (17.3 ± 0.5 µg/ml), and ammonia (2.89 ± 0.4 µmoles) were experimentally validated. Interestingly, PCH239 treatment of Arabidopsis seeds significantly enhances germination, primary, and hairy root growth. In contrast, Vigna radiata and Cicer arietinum seeds had healthy radicle and plumule elongation, suggesting varied plant growth-promotion effects. Our findings suggested the potential of PCH239 as a bio-fertilizer and biocontrol agent in the challenging conditions of cold and hilly regions.

Replacement of water yam (Dioscorea alata L.) indigenous root endophytes and rhizosphere bacterial communities via inoculation with a synthetic bacterial community of dominant nitrogen-fixing bacteria

Biofertilizers containing high-density plant growth-promoting bacteria are gaining interest as a sustainable solution to environmental problems caused by eutrophication. However, owing to the limitations of current investigative techniques, the selected microorganisms are not always preferred by the host plant, preventing recruitment into the native microbiota or failing to induce plant growth-promoting effects. To address this, five nitrogen-fixing bacteria previously isolated from water yam (Dioscorea alata L.) plants and showing dominant abundance of 1% or more in the water yam microbiota were selected for analysis of their plant growth-promoting activities when used as a synthetic bacterial inoculant. Water yam cv. A-19 plants were inoculated twice at 10 and 12 weeks after planting under greenhouse conditions. Bacterial communities in root, rhizosphere, and bulk soil samples were characterized using high-throughput 16S rRNA amplicon sequencing. Compared with non-inoculated plants, all bacterial communities were significantly altered by inoculation, mainly at the genus level. The inoculation effects were apparently found in the root communities at 16 weeks after planting, with all inoculated genera showing dominance (in the top 35 genera) compared with the control samples. However, no significant differences in any of the growth parameters or nitrogen contents were observed between treatments. At 20 weeks after planting, the dominance of Stenotrophomonas in the inoculated roots decreased, indicating a decline in the inoculation effects. Interestingly, only the Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium clade was dominant (>1% relative abundance) across all samples, suggesting that bacteria related to this clade are essential core bacteria for water yam growth. This is the first report on addition of a synthetic nitrogen-fixing bacterial community in water yam plants showing that native bacterial communities can be replaced by a synthetic bacterial community, with declining in the effects of Stenotrophomonas on the modified communities several weeks after inoculation.

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

Introduction

Maize is a major staple cereal crop grown and consumed globally. However, due to climate change, extreme heat and drought stresses are greatly affecting its production especially in sub-Saharan Africa. The use of a bio-based approach to mitigate drought stress is therefore suggested using plant growth-promoting rhizobacteria (PGPR).

Methods

This study investigated the abilities of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing PGPR Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13 and Bacillus sp. MRBP10 isolated from maize rhizosphere soil, to ameliorate the effect of drought stress in maize genotypes MR44 and S0/8/W/I137TNW//CML550 under two water regimes; mild drought stress (50% FC) and well-watered conditions (100% FC). The rhizobacterial strains were identified by 16S rRNA sequencing and biochemical tests, and evaluated for plant growth-promoting and abiotic stress tolerance traits.

Results and discussion

The synergistic effect of the bacterial strains had a highly significant (p &lt; 0.001) effect on the total soluble sugar, soil moisture content and relative water content, which were enhanced under water-stress in the inoculated plants. Relative water content was significantly highest (p &lt; 0.001) in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 (60.55%). Total chlorophyll content was significantly enhanced in maize seedlings sole inoculated with Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13, and co-inoculated with Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 by 15.91%, 14.99% and 15.75% respectively, over the un-inoculated control. Soil moisture content increased by 28.67% and 30.71% compared to the un-inoculated control when plants were inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 and Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 respectively. The interactive effect of genotype × bacteria significantly enhanced biomass production. Leaf area was highest in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Pseudomonas sp. MRBP13 (212.45 ± 0.87 cm2) under drought stress. Treatment of maize seeds with Pseudomonas sp. MRBP 4 + Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 significantly increased the root length (10.32 ± 0.48 cm) which enhanced survival of the maize seedlings. Bioinoculation of maize seeds with these strains could boost maize production cultivated in arid regions.

Molecular Characterization and Mineralizing Potential of Phosphorus Solubilizing Bacteria Colonizing Common Bean (Phaseolus vulgaris L.) Rhizosphere in Western Kenya

Phosphorus solubilizing bacteria (PSB) are a category of microbes that transform insoluble phosphates in soil into soluble forms that crops can utilize. Phosphorus in natural soils is abundant but poorly soluble. Hence, introducing PSB is a safer way of improving its solubility. The aim of this study was to genetically characterize and determine the mineralization capability of selected PSB colonizing rhizospheres of common beans in Western Kenya. Seven potential phosphorus solubilizing bacteria (PSB) were isolated from various subregions of Western Kenya. 16S ribosomal RNA gene sequencing and National Center for Biotechnology Information (NCBI), Basic Local Alignment Search Tool (BLAST) identified the isolates. The phosphate solubilization potential of the isolates was evaluated under agar and broth medium of National Botanical Research Institute's phosphate (NBRIP) supplemented with tricalcium calcium phosphate (TCP). Identified isolates were as follows: KK3 as Enterobacter mori, B5 (KB5) as Pseudomonas kribbensis, KV1 as Enterobacter asburiae, KB3 as Enterobacter mori, KK1 as Enterobacter cloacae, KBU as Enterobacter tabaci, and KB2 as Enterobacter bugandensis. The strains B5 and KV1 were the most effective phosphorus solubilizers with 4.16 and 3.64 indices, respectively. The microbes converted total soluble phosphate concentration in broth medium which was 1395 and 1471 P μg/mL, respectively. The least performing isolate was KBU with a 2.34 solubility index. Significant (p ≤ 0.05) differences in plant biomass for Rose coco and Mwitemania bean varieties were observed under inoculation with isolates B5 and KV1. PSB isolates found in common bean rhizospheres exhibited molecular variations and isolates B5 and KV1 are the potential in solving the insufficiency of phosphorus for sustainable crop production.

Complete genome analysis of sugarcane root associated endophytic diazotroph Pseudomonas aeruginosa DJ06 revealing versatile molecular mechanism involved in sugarcane development

Sugarcane is an important sugar and bioenergy source and a significant component of the economy in various countries in arid and semiarid. It requires more synthetic fertilizers and fungicides during growth and development. However, the excess use of synthetic fertilizers and fungicides causes environmental pollution and affects cane quality and productivity. Plant growth-promoting bacteria (PGPB) indirectly or directly promote plant growth in various ways. In this study, 22 PGPB strains were isolated from the roots of the sugarcane variety GT42. After screening of plant growth-promoting (PGP) traits, it was found that the DJ06 strain had the most potent PGP activity, which was identified as Pseudomonas aeruginosa by 16S rRNA gene sequencing. Scanning electron microscopy (SEM) and green fluorescent protein (GFP) labeling technology confirmed that the DJ06 strain successfully colonized sugarcane tissues. The complete genome sequencing of the DJ06 strain was performed using Nanopore and Illumina sequencing platforms. The results showed that the DJ06 strain genome size was 64,90,034 bp with a G+C content of 66.34%, including 5,912 protein-coding genes (CDSs) and 12 rRNA genes. A series of genes related to plant growth promotion was observed, such as nitrogen fixation, ammonia assimilation, siderophore, 1-aminocyclopropane-1-carboxylic acid (ACC), deaminase, indole-3-acetic acid (IAA) production, auxin biosynthesis, phosphate metabolism, hydrolase, biocontrol, and tolerance to abiotic stresses. In addition, the effect of the DJ06 strain was also evaluated by inoculation in two sugarcane varieties GT11 and B8. The length of the plant was increased significantly by 32.43 and 12.66% and fresh weight by 89.87 and 135.71% in sugarcane GT11 and B8 at 60 days after inoculation. The photosynthetic leaf gas exchange also increased significantly compared with the control plants. The content of indole-3-acetic acid (IAA) was enhanced and gibberellins (GA) and abscisic acid (ABA) were reduced in response to inoculation of the DJ06 strain as compared with control in two sugarcane varieties. The enzymatic activities of oxidative, nitrogen metabolism, and hydrolases were also changed dramatically in both sugarcane varieties with inoculation of the DJ06 strain. These findings provide better insights into the interactive action mechanisms of the P. aeruginosa DJ06 strain and sugarcane plant development.

Use of beneficial bacterial endophytes: A practical strategy to achieve sustainable agriculture

Beneficial endophytic bacteria influence their host plant to grow and resist pathogens. Despite the advantages of endophytic bacteria to their host, their application in agriculture has been low. Furthermore, many plant growers improperly use synthetic chemicals due to having no or little knowledge of the role of endophytic bacteria in plant growth, the prevention and control of pathogens and poor access to endobacterial bioproducts. These synthetic chemicals have caused soil infertility, environmental contamination, disruption to ecological cycles and the emergence of resistant pests and pathogens. There is more that needs to be done to explore alternative ways of achieving sustainable plant production while maintaining environmental health. In recent years, the use of beneficial endophytic bacteria has been noted to be a promising tool in promoting plant growth and the biocontrol of pathogens. Therefore, this review discusses the roles of endophytic bacteria in plant growth and the biocontrol of plant pathogens. Several mechanisms that endophytic bacteria use to alleviate plant biotic and abiotic stresses by helping their host plants acquire nutrients, enhance plant growth and development and suppress pathogens are explained. The review also indicates that there is a gap between research and general field applications of endophytic bacteria and suggests a need for collaborative efforts between growers at all levels. Furthermore, the presence of scientific and regulatory frameworks that promote advanced biotechnological tools and bioinoculants represents major opportunities in the applications of endophytic bacteria. The review provides a basis for future research in areas related to understanding the interactions between plants and beneficial endophytic microorganisms, especially bacteria.

Optimization of soluble phosphate and IAA production using response surface methodology and ANN approach

Phosphorus (P) is often found inaccessible to plants, as it forms precipitates with cations and can be converted to accessible forms by using Phosphate solubilizing bacteria (PSB). In the present study, isolation and characterization of PSB from rhizospheric soil of coffee plants were performed. The influence of four independent variables (incubation temperature, incubation time, pH, and inoculum size) was investigated and optimized using an artificial neural network and response surface methodology on the solubility of phosphate and indole acetic acid production. The bacterium that can dissolve phosphate were isolated in Pikovskaya's agar containing insoluble tricalcium phosphate. Total, six Phosphate Solubilizing Bacteria were isolated and three of them (PSB1, PSB3, and PSB4) were found to be effectively solubilizing phosphate. Based on phosphate solubilizing index results Pseudomonas bacteria (PSB1) was selected for modeling. The results showed that both models performed reasonably well, but properly trained artificial neural networks have the more powerful modeling capability compared to the response surface method. The optimum conditions were found to be incubation temperature of 37.5 °C, incubation time of 9 days, pH of 7.2, and inoculum size of 1.89 OD. Under these conditions, the model predicted solubility of phosphate of 260.69 μg/ml and production of IAA of 80.00 μg/ml with a desirability value of 0.947. In general, the isolated Pseudomonas is expected to have phosphorus-degrading ability that promotes plant growth, and further field experimental work is required to use this bacterial strain as biofertilizer, as an alternative to synthetic fertilizer.

Genetic Determinants of Antagonistic Interactions and the Response of New Endophytic Strain Serratia quinivorans KP32 to Fungal Phytopathogens

Fungal phytopathogens are challenging to control due to their penetration into plant tissues. Therefore, plant-colonizing bacteria could serve as an excellent weapon in fighting fungal infections. In this study, we aim to determine the biocontrol potential of the new endophytic strain Serratia quinivorans KP32, isolated from the roots of Petroselinum crispum L.; identify the related mechanisms; and understand the basis of its antagonistic interaction with taxonomically diverse fungi at the molecular level. The KP32 strain presented biological activity against Rhizoctonia solani, Colletotrichum dematium, Fusarium avenaceum, and Sclerotinia sclerotiorum, and its ability to inhibit the growth of the phytopathogens was found to be mediated by a broad spectrum of biocontrol features, such as the production of a number of lytic enzymes (amylases, chitinases, and proteases), siderophores, volatile organic and inorganic compounds, salicylic acid, and N-acyl-homoserine lactones. The higher expression of chitinase (chiA) and genes involved in the biosynthesis of hydrogen cyanide (hcnC), enterobactin (entB), and acetoin (budA) in bacteria exposed to fungal filtrates confirmed that these factors could act in combination, leading to a synergistic inhibitory effect of the strain against phytopathogens. We also confirm the active movement, self-aggregation, exopolysaccharide production, and biofilm formation abilities of the KP32 strain, which are essential for effective plant colonization. Its biological activity and colonization potential indicate that KP32 holds tremendous potential for use as an active biopesticide and plant growth promoter.

Single-cell Raman-activated sorting and cultivation (scRACS-Culture) for assessing and mining in situ phosphate-solubilizing microbes from nature

Due to the challenges in detecting in situ activity and cultivating the not-yet-cultured, functional assessment and mining of living microbes from nature has typically followed a 'culture-first' paradigm. Here, employing phosphate-solubilizing microbes (PSM) as model, we introduce a 'screen-first' strategy that is underpinned by a precisely one-cell-resolution, complete workflow of single-cell Raman-activated Sorting and Cultivation (scRACS-Culture). Directly from domestic sewage, individual cells were screened for in-situ organic-phosphate-solubilizing activity via D2O intake rate, sorted by the function via Raman-activated Gravity-driven Encapsulation (RAGE), and then cultivated from precisely one cell. By scRACS-Culture, pure cultures of strong organic PSM including Comamonas spp., Acinetobacter spp., Enterobacter spp. and Citrobacter spp., were derived, whose phosphate-solubilizing activities in situ are 90-200% higher than in pure culture, underscoring the importance of 'screen-first' strategy. Moreover, employing scRACS-Seq for post-RACS cells that remain uncultured, we discovered a previously unknown, low-abundance, strong organic-PSM of Cutibacterium spp. that employs secretary metallophosphoesterase (MPP), cell-wall-anchored 5'-nucleotidase (encoded by ushA) and periplasmic-membrane located PstSCAB-PhoU transporter system for efficient solubilization and scavenging of extracellular phosphate in sewage. Therefore, scRACS-Culture and scRACS-Seq provide an in situ function-based, 'screen-first' approach for assessing and mining microbes directly from the environment.

Halotolerant rhizobacteria isolated from a mangrove forest alleviate saline stress in Musa acuminata cv. Berangan

Saline soils resulting from anthropogenic activity and climate change present a challenge to future food security. Towards addressing this, we isolated and characterized halotolerant bacteria from a Malaysian mangrove forest, and explored their effect on morpho-physiological and biochemical parameters of banana plantlets under salt stress. A total of 88 rhizobacterial and 16 endophytic bacterial isolates collected from the roots and rhizosphere of Rhizophora apiculata, Avicennia alba and Sonneratia alba, were found to tolerate up to 400 mM of sea salt. Based on best performance in multiple plant growth traits, three rhizobacterial strains RB1, RB3 and RB4 and three endophytic bacterial strains EB1, EB2 and EB3 were used for further analysis. The rhizobacterial strains were identified as Bacillus sp. and endophytic bacteria as Pseudomonas sp. based on 16 S rRNA gene sequence. SEM observation confirmed colonization of each strain on banana plantlet roots. When colonized plantlets were subjected to 90 mM salt and compared to uninoculated (control) and mock inoculated plants, improved plant growth was observed with each of the strains, especially with bacterial strains EB3 and RB3. Biochemical analysis of plantlets revealed that root colonization with EB3 and RB3 enhanced levels of plant chlorophyll (> 5-fold), carotenoid (> 2.85-fold) and proline (2.6-fold and 2.3-fold), while plantlets also showed reduced MDA content (0.45-fold and 0.51-fold), significantly reduced generation of ROS (0.23-fold and 0.47-fold) and lower levels of electrolyte leakage (0.77 and 0.51-fold). Antioxidant enzymes also showed enhanced activity with EB3 and RB3. Our results indicate that these halotolerant Bacillus and Pseudomonas strains from the mangrove have multifunctional plant growth promoting activity and can reduce salt stress in bananas. This data provides a reference for exploring halotolerant microbes from hypersaline environments to overcome salt stress in plants.

Prospects for Using Phosphate-Solubilizing Microorganisms as Natural Fertilizers in Agriculture

Phosphates are known to be essential for plant growth and development, with phosphorus compounds being involved in various physiological and biochemical reactions. Phosphates are known as one of the most important factors limiting crop yields. The problem of phosphorus deficiency in the soil has traditionally been solved by applying phosphate fertilizers. However, chemical phosphate fertilizers are considered ineffective compared to the organic fertilizers manure and compost. Therefore, increasing the bioavailability of phosphates for plants is one of the primary goals of sustainable agriculture. Phosphate-solubilizing soil microorganisms can make soil-insoluble phosphate bioavailable for plants through solubilization and mineralization. These microorganisms are currently in the focus of interest due to their advantages, such as environmental friendliness, low cost, and high biological efficiency. In this regard, the solubilization of phosphates by soil microorganisms holds strong potential in research, and inoculation of soils or crops with phosphate-solubilizing bacteria is a promising strategy to improve plant phosphate uptake. In this review, we analyze all the species of phosphate-solubilizing bacteria described in the literature to date. We discuss key mechanisms of solubilization of mineral phosphates and mineralization of organic phosphate-containing compounds: organic acids secreted by bacteria for the mobilization of insoluble inorganic phosphates, and the enzymes hydrolyzing phosphorus-containing organic compounds. We demonstrate that phosphate-solubilizing microorganisms have enormous potency as biofertilizers since they increase phosphorus bioavailability for the plant, promote sustainable agriculture, improve soil fertility, and raise crop yields. The use of phosphate-solubilizing microbes is regarded as a new frontier in increasing plant productivity.

Development of miniaturized agar based assays in 96-well microplates applicable to high-throughput screening of industrially valuable microorganisms

High-throughput screening (HTS) is a present-day approach for assaying thousands of cultures in parallel. This miniaturization allows rapid screening of large number of microorganims capable of producing bio-based materials thereby meeting the demands of the ever evolving food, pharmaceutical and cosmetic industry. In this study, agar-based assays for phosphate solubilization, cellulose degradation and lactic acid production were developed in 96-well microplates using Biomek FX^P Automated Liquid Handling system. Techno-economic analysis from this study reveals the lower overall cost per assay using HTS as compared to conventional Petri plate assays. Though automated liquid handling workstations have been used to perform liquid-based assays, there are very few studies which report their use for agar-based microplate assays. These findings thus corroborate the establishment of rapid and efficient miniaturized, qualitative agar-based screening methods for identifying microorganisms with potential for commercial application.

Synergistic Plant-Microbe Interactions between Endophytic Actinobacteria and Their Role in Plant Growth Promotion and Biological Control of Cotton under Salt Stress

Bacterial endophytes are well-acknowledged inoculants to promote plant growth and enhance their resistance toward various pathogens and environmental stresses. In the present study, 71 endophytic strains associated with the medicinal plant Thymus roseus were screened for their plant growth promotion (PGP), and the applicability of potent strains as bioinoculant has been evaluated. Regarding PGP traits, the percentage of strains were positive for the siderophore production (84%), auxin synthesis (69%), diazotrophs (76%), phosphate solubilization (79%), and production of lytic enzymes (i.e., cellulase (64%), lipase (62%), protease (61%), chitinase (34%), and displayed antagonistic activity against Verticillium dahliae (74%) in vitro. The inoculation of strain XIEG05 and XIEG12 enhanced plant tolerance to salt stress significantly (p < 0.05) through the promotion of shoot, root development, and reduced the activities of antioxidant enzymes (SOD, POD, and CAT), compared with uninoculated controls in vivo. Furthermore, inoculation of strain XIEG57 was capable of reducing cotton disease incidence (DI) symptoms caused by V. dahliae at all tested salt concentrations. The GC-MS analysis showed that many compounds are known to have antimicrobial and antifungal activity. Our findings provide valuable information for applying strains XIEG05 and XIEG12 as bioinoculant fertilizers and biological control agent of cotton under saline soil conditions.

Bioprospecting Plant Growth Promoting Rhizobacteria for Enhancing the Biological Properties and Phytochemical Composition of Medicinally Important Crops

Traditionally, medicinal plants have long been used as a natural therapy. Plant-derived extracts or phytochemicals have been exploited as food additives and for curing many health-related ailments. The secondary metabolites produced by many plants have become an integral part of human health and have strengthened the value of plant extracts as herbal medicines. To fulfil the demand of health care systems, food and pharmaceutical industries, interest in the cultivation of precious medicinal plants to harvest bio-active compounds has increased considerably worldwide. To achieve maximum biomass and yield, growers generally apply chemical fertilizers which have detrimental impacts on the growth, development and phytoconstituents of such therapeutically important plants. Application of beneficial rhizosphere microbiota is an alternative strategy to enhance the production of valuable medicinal plants under both conventional and stressed conditions due to its low cost, environmentally friendly behaviour and non-destructive impact on fertility of soil, plants and human health. The microbiological approach improves plant growth by various direct and indirect mechanisms involving the abatement of various abiotic stresses. Given the negative impacts of fertilizers and multiple benefits of microbiological resources, the role of plant growth promoting rhizobacteria (PGPR) in the production of biomass and their impact on the quality of bio-active compounds (phytochemicals) and mitigation of abiotic stress to herbal plants have been described in this review. The PGPR based enhancement in the herbal products has potential for use as a low cost phytomedicine which can be used to improve health care systems.

Metallophores production by bacteria isolated from heavy metal-contaminated soil and sediment at Lerma-Chapala Basin

Environmental pollution as a result of heavy metals (HMs) is a worldwide problem and the implementation of eco-friendly remediation technologies is thus required. Metallophores, low molecular weight compounds, could have important biotechnological applications in the fields of agriculture, medicine, and bioremediation. This study aimed to isolate HM-resistant bacteria from soils and sediments of the Lerma-Chapala Basin and evaluated their abilities to produce metallophores and to promote plant growth. Bacteria from the Lerma-Chapala Basin produced metallophores for all the tested metal ions, presented a greater production of As³⁺ metallophores, and showed high HM resistance especially to Zn²⁺, As⁵⁺, and Ni²⁺. A total of 320 bacteria were isolated with 170 strains showing siderophores synthesis. Members of the Delftia and Pseudomonas genera showed above 92 percent siderophore units (psu) during siderophores production and hydroxamate proved to be the most common functional group among the analyzed siderophores. Our results provided evidence that Lerma-Chapala Basin bacteria and their metallophores could potentially be employed in bioremediation processes or may even have potential for applications in other biotechnological fields.

Implication of plant growth-promoting rhizobacteria of Bacillus spp. as biocontrol agents against wilt disease caused by Fusarium oxysporum Schlecht. in Vicia faba L

Out of seven Fusarium spp. isolated from infected faba bean roots, two Fusarium oxysporum were selected and showed faba bean-wilt disease severity with percentages of 68% and 47% under greenhouse conditions. The F. oxysporum showed the highest wilt disease was selected to complete the current study. Three rhizobacterial strains were isolated and identified as Bacillus velezensis Vb1, B. paramycoides Vb3, and B. paramycoides Vb6. These strains showed the highest in-vitro antagonistic activity by the dual-culture method against selected F. oxysporum with inhibition percentages of 59±0.2, 46±0.3, and 52±0.3% for Vb1, Vb3, and Vb6, respectively. These rhizobacterial strains exhibit varied activity for nitrogen-fixing and phosphate-solubilizing. Moreover, these strains showed positive results for ammonia, HCN, and siderophores production. The phytohormones production (indole-3-acetic acid, ABA, benzyl, kinten, ziaten, and GA₃) and secretion of various lytic enzymes were recorded by these strains with varying degrees. Under greenhouse conditions, the rhizobacterial strains Vb1, Vb3, Vb6, and their consortium can protect faba bean from wilt caused by F. oxysporum with percentages of 70, 60, 65, and 82%, respectively. Under field conditions, the inoculation with the rhizobacterial consortium (Vb1+Vb3+Vb6) significantly increases the growth performance of the F. oxysporum-infected faba bean plant and recorded the highest wilt protection (83.3%).

Bioinoculants-Natural Biological Resources for Sustainable Plant Production

Agricultural sustainability is of foremost importance for maintaining high food production. Irresponsible resource use not only negatively affects agroecology, but also reduces the economic profitability of the production system. Among different resources, soil is one of the most vital resources of agriculture. Soil fertility is the key to achieve high crop productivity. Maintaining soil fertility and soil health requires conscious management effort to avoid excessive nutrient loss, sustain organic carbon content, and minimize soil contamination. Though the use of chemical fertilizers have successfully improved crop production, its integration with organic manures and other bioinoculants helps in improving nutrient use efficiency, improves soil health and to some extent ameliorates some of the constraints associated with excessive fertilizer application. In addition to nutrient supplementation, bioinoculants have other beneficial effects such as plant growth-promoting activity, nutrient mobilization and solubilization, soil decontamination and/or detoxification, etc. During the present time, high energy based chemical inputs also caused havoc to agriculture because of the ill effects of global warming and climate change. Under the consequences of climate change, the use of bioinputs may be considered as a suitable mitigation option. Bioinoculants, as a concept, is not something new to agricultural science, however; it is one of the areas where consistent innovations have been made. Understanding the role of bioinoculants, the scope of their use, and analysing their performance in various environments are key to the successful adaptation of this technology in agriculture.

Genomic and Metabolomic Insights into Secondary Metabolites of the Novel Bacillus halotolerans Hil4, an Endophyte with Promising Antagonistic Activity against Gray Mold and Plant Growth Promoting Potential

The endophytic bacterial strain Hil4 was isolated from leaves of the medicinal plant Hypericum hircinum. It exhibited antifungal activity against Botrytis cinerea and a plethora of plant growth promoting traits in vitro. Whole genome sequencing revealed that it belongs to Bacillus halotolerans and possesses numerous secondary metabolite biosynthetic gene clusters and genes involved in plant growth promotion, colonization, and plant defense elicitation. The Mojavensin cluster was present in the genome, making this strain novel among plant-associated B. halotolerans strains. Extracts of secreted agar-diffusible compounds from single culture secretome extracts and dual cultures with B. cinerea were bioactive and had the same antifungal pattern on TLC plates after bioautography. UHPLC-HRMS analysis of the single culture secretome extract putatively annotated the consecutively produced antimicrobial substances and ISR elicitors. The isolate also proved efficient in minimizing the severity of gray mold post-harvest disease on table grape berries, as well as cherry tomatoes. Finally, it positively influenced the growth of Arabidopsis thaliana Col-0 and Solanum lycopersicum var. Chondrokatsari Messinias after seed biopriming in vitro. Overall, these results indicate that the B. halotolerans strain Hil4 is a promising novel plant growth promoting and biocontrol agent, and can be used in future research for the development of biostimulants and/or biological control agents.

Differential Root Exudation and Architecture for Improved Growth of Wheat Mediated by Phosphate Solubilizing Bacteria

Phosphorous (P) deficiency is a major challenge faced by global agriculture. Phosphate-solubilizing bacteria (PSB) provide a sustainable approach to supply available phosphates to plants with improved crop productivity through synergistic interaction with plant roots. The present study demonstrates an insight into this synergistic P-solubilizing mechanism of PSB isolated from rhizosphere soils of major wheat-growing agro-ecological zones of Pakistan. Seven isolates were the efficient P solubilizers based on in vitro P-solubilizing activity (233-365 μg ml-1) with a concomitant decrease in pH (up to 3.5) by the production of organic acids, predominantly acetic acid (∼182 μg ml-1) and gluconic acid (∼117 μg ml-1). Amplification and phylogenetic analysis of gcd, pqqE, and phy genes of Enterobacter sp. ZW32, Ochrobactrum sp. SSR, and Pantoea sp. S1 showed the potential of these PSB to release orthophosphate from recalcitrant forms of phosphorus. Principal component analysis indicates the inoculation response of PSB consortia on the differential composition of root exudation (amino acids, sugars, and organic acids) with subsequently modified root architecture of three wheat varieties grown hydroponically. Rhizoscanning showed a significant increase in root parameters, i.e., root tips, diameter, and surface area of PSB-inoculated plants as compared to uninoculated controls. Efficiency of PSB consortia was validated by significant increase in plant P and oxidative stress management under P-deficient conditions. Reactive oxygen species (ROS)-induced oxidative damages mainly indicated by elevated levels of malondialdehyde (MDA) and H2O2 contents were significantly reduced in inoculated plants by the production of antioxidant enzymes, i.e., superoxide dismutase, catalase, and peroxidase. Furthermore, the inoculation response of these PSB on respective wheat varieties grown in native soils under greenhouse conditions was positively correlated with improved plant growth and soil P contents. Additionally, grain yield (8%) and seed P (14%) were significantly increased in inoculated wheat plants with 20% reduced application of diammonium phosphate (DAP) fertilizer under net house conditions. Thus, PSB capable of such synergistic strategies can confer P biofortification in wheat by modulating root morphophysiology and root exudation and can alleviate oxidative stress under P deficit conditions.

Isolation, Characterization, and Evaluation of Native Rhizobacterial Consortia Developed From the Rhizosphere of Rice Grown in Organic State Sikkim, India, and Their Effect on Plant Growth

Eight rhizospheric bacteria were isolated from the organic paddy fields of Sikkim, India, and identified as Pseudomonas kribbensis KSB, Burkholderia cenocepacia SRD, Kosakonia oryzendophytica YMA7, Pseudomonas rhodesiae SRB, Bacillus sp. ARA, Paenibacillus polymyxa COW3, Bacillus aryabhattai PSB2, and Bacillus megaterium PSB1. They showed plant growth-promoting attributes in rice and have bio-control potential against phytopathogen Colletotrichum gloeosporioides of large cardamom (Amomum subulatum). Burkholderia cenocepacia SRD showed production of indole acetic acid and ammonia and solubilization of phosphate and potassium and also possessed nitrogen fixation potential. It showed antagonistic activity against two other plant pathogens of large cardamom, viz., Curvularia eragrostidis and Pestalotiopsis sp., under in vitro conditions. The liquid bacterial consortium was prepared using the bacterial strains SRB, PSB1, and COW3 (Consortia-1); PSB2, SRD, and COW3 (Consortia-2); and COW3, KSB, and YMA7 (Consortia-3) to increase the growth and yield of rice plants under organic farming conditions. Greenhouse and field studies showed that the Consortia-3 had the highest plant growth-promoting activity. Consortia-3 demonstrated better agronomic performance in terms of root length (9.5 cm),number of leaflets per plant (5.3), grains per panicle (110.6), test grain weight (27.4 g), dry root weight per plant (0.73 g), and total dry biomass per plant (8.26 g).

Benefits of phosphate solubilizing bacteria on belowground crop performance for improved crop acquisition of phosphorus

Although research on plant growth promoting bacteria began in the 1950s, basic and applied research on bacteria improving use of phosphorus (P) continues to be a priority among many agricultural research institutions. Ultimately, identifying agriculturally beneficial microbes, notably P solubilizing bacteria (PSB), that enhance the efficient use of P supports more sustainable cropping systems and the judicious use of mineral nutrients. In parallel, there is more attention on improving crop root P acquisition of existing soil P pools as well as by increasing the proportion of fertilizer P that is taken up by crops. Today, new lines of research are emerging to investigate the co-optimization of PSB-fertilizer-crop root processes for improved P efficiency and agricultural performance. In this review, we compile and summarize available findings on the beneficial effects of PSB on crop production with a focus on crop P acquisition via root system responses at the structural, functional and transcriptional levels. We discuss the current state of knowledge on the mechanisms of PSB-mediated P availability, both soil- and root-associated, as well as crop uptake via P solubilization, mineralization and mobilization, mainly through the production of organic acids and P-hydrolyzing enzymes, and effects on phytohormone signaling for crop root developement. The systematic changes caused by PSB on crop roots are discussed and contextualized within promising functional trait-based frameworks. We also detail agronomic profitability of P (mineral and organic) and PSB co-application, in amended soils and inoculated crops, establishing the connection between the influence of PSB on agroecosystem production and the impact of P fertilization on microbial diversity and crop functional traits for P acquisition.

PLANT GROWTH POTENTIAL OF SALT TOLERANT ENDOPHYTE Pseudomonas Sihuiensis ISOLATED FROM CHICKPEA

Bacterial endosymbionts are well characterized for plant growth promotion. In this study, the root, nodules, and stem of the Cicer arietinum crop planted in a semi-arid zone were used as a source to isolate potential plant growth bacteria. The ability to grow under salt stress was determined, and the potential isolate was screened for plant growth promotion traits. The selected isolate was identified by the 16S rDNA method. Pot trials were conducted to know the ability of the isolate to promote plant growth in-vivo. Among various isolates obtained, a bacterial isolate obtained from root showed the ability to grow in the presence of 10 % Sodium fluoride (NaF). The isolate produced Indole Acetic acid in an amount of 72 mg per liter in production medium. The bacteria solubilized phosphate and produce exopolysaccharide (2.12 g per liter). The isolate was identified as Pseudomonas sihuiensis. The result of pot trials reveals that the endophyte promotes plant growth under stress conditions and may be used as a bio-fertilizer.

The Root Microbiome of Salicornia ramosissima as a Seedbank for Plant-Growth Promoting Halotolerant Bacteria

Root−associated microbial communities play important roles in the process of adaptation of plant hosts to environment stressors, and in this perspective, the microbiome of halophytes represents a valuable model for understanding the contribution of microorganisms to plant tolerance to salt. Although considered as the most promising halophyte candidate to crop cultivation, Salicornia ramosissima is one of the least-studied species in terms of microbiome composition and the effect of sediment properties on the diversity of plant-growth promoting bacteria associated with the roots. In this work, we aimed at isolating and characterizing halotolerant bacteria associated with the rhizosphere and root tissues of S. ramosissima, envisaging their application in saline agriculture. Endophytic and rhizosphere bacteria were isolated from wild and crop cultivated plants, growing in different estuarine conditions. Isolates were identified based on 16S rRNA sequences and screened for plant-growth promotion traits. The subsets of isolates from different sampling sites were very different in terms of composition but consistent in terms of the plant-growth promoting traits represented. Bacillus was the most represented genus and expressed the wider range of extracellular enzymatic activities. Halotolerant strains of Salinicola, Pseudomonas, Oceanobacillus, Halomonas, Providencia, Bacillus, Psychrobacter and Brevibacterium also exhibited several plant-growth promotion traits (e.g., 3-indole acetic acid (IAA), 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, siderophores, phosphate solubilization). Considering the taxonomic diversity and the plant-growth promotion potential of the isolates, the collection represents a valuable resource that can be used to optimize the crop cultivation of Salicornia under different environmental conditions and for the attenuation of salt stress in non-halophytes, considering the global threat of arable soil salinization.

Dual Microbial Inoculation, a Game Changer? - Bacterial Biostimulants With Multifunctional Growth Promoting Traits to Mitigate Salinity Stress in Spring Mungbean

Soil microbes play a vital role in improving plant growth, soil health, ameliorate biotic/abiotic stress and enhance crop productivity. The present study was aimed to investigate a coordinated effect of compatible consortium [salt tolerating Rhizobium and rhizobacterium with 1-aminocyclopropane-1-carboxylate (ACC) deaminase] in enhancing plant growth promoting (PGP) traits, symbiotic efficiency, nutrient acquisition, anti-oxidative enzymes, grain yield and associated profitability in spring mungbean. We identified a non-pathogenic compatible Rhizobium sp. LSMR-32 (MH644039.1) and Enterococcus mundtii LSMRS-3 (MH644178.1) from salt affected areas of Punjab, India and the same were assessed to develop consortium biofertilizer based on salt tolerance, multifarious PGP traits, antagonistic defense activities and presence of nifH, acds, pqq, and ipdc genes. Indole Acetic acid (IAA), P-solubilization, biofilm formation, exo-polysaccharides, siderophore, salt tolerance, ACC deaminase activities were all found highly significant in dual inoculant (LSMR-32 + LSMRS-3) treatment compared to LSMR-32 alone. Under saline soil conditions, dual inoculant showed a higher seed germination, plant height, biomass, chlorophyll content and macro and micro-nutrient uptake, than un-inoculated control. However, symbiotic (nodulation, nodule biomass and leghaemoglobin content) and soil quality parameters (phosphatase and soil dehydrogenase enzymes) increased numerically with LSMR-32 + LSMRS-3 over Rhizobium sp. LSMR-32 alone. Dual bacterial inoculation (LSMR-32 + LSMRS-3) increased the proline content (2.05 fold), anti-oxidative enzymes viz., superoxide dismutase (1.50 fold), catalase (1.43 fold) and peroxidase (3.88 folds) in contrast to control treatment. Decreased Na⁺ accumulation and increased K⁺ uptake resulted in favorable K⁺/Na⁺ ratio through ion homeostasis. Co-inoculation of Rhizobium sp. LSMR-32 and Enterococcus mundtii LSMRS-3 significantly improved the grain yield by 8.92% and led to superior B: C ratio over Rhizobium sp. alone under salt stress. To best of our knowledge this is perhaps the first field report from Indian soils that largely describes dual inoculation of Rhizobium sp. LSMR-32 and Enterococcus mundtii LSMRS-3 and the same can be considered as a game-changer approach to simultaneously induce salt tolerance and improve productivity in spring mungbean under saline stress conditions.

Heavy-metal resistance mechanisms developed by bacteria from Lerma-Chapala basin

Heavy-metal (HM) contamination is a huge environmental problem in many countries including Mexico. Currently, microorganisms with multiple heavy-metal resistance and/or plant-promoting characteristics have been widely used for bioremediation of HM-contaminated soils. The aim of the study was isolated bacteria with multiple heavy-metal resistance and to determinate the resistance mechanism developed by these organisms. A total of 138 aerobic bacteria were isolated from soil and sediments surrounding the Lerma-Chapala basin located in the boundary of the States of Michoacán and Jalisco states of Mexico. One hundred and eight strains showed at least 1 plant growth-promoting features. The Lerma-Chapala basin bacteria were also resistant to high concentrations of HMs including the metalloid arsenic. Sequence analysis of 16S RNA genes reveled that these bacteria were mainly affiliated to the phyla Proteobacteria (38%), Firmicutes (31%) and Actinobacteria (25%), covering 21 genera with Bacillus as the most abundant one. Among them, at least 27 putative novel species were detected in the genera Acinetobacter, Arthrobacter, Bacillus, Agrobacterium, Dyadobacter, Enterobacter, Exiguobacterium, Kluyvera, Micrococcus, Microbacterium and Psychrobacter. In addition, these bacteria developed various heavy-metal-resistance mechanisms, such as biosorption/bioaccumulation, immobilization and detoxification. Therefore, the bacteria isolated from soils and sediments of Lerma-Chapala basin could be used in bioremediation strategies.

Alcaligenes aquatilis GTE53: Phosphate solubilising and bioremediation bacterium isolated from new biotope "phosphate sludge enriched-compost"

The isolation and identification of beneficial bacteria from the active phase of composting is considered to be a key bio-quality parameter for the assessment of the process. The aim of this work was the selection and identification of beneficial bacteria from a co-composting experiment of vegetable waste (VW), olive oil mill waste (O₂MW), and phosphate sludge (PS). Phosphate-solubilizing strains were isolated from the thermophilic phase using Pikovskaya (PVK) solid medium supplemented with tricalcium phosphate Ca3(PO4) (TCP) as the sole source of phosphorus (P). Therefore, the selected isolate Alcaligenes aquatilis GTE53 was tested to tolerate abiotic stresses (different levels of temperature, variable pH, high salinity and water stress). The isolate was also assessed for indole acetic acid (IAA) and siderophores synthesis, nitrogen fixation, phenol degradation and pathogens inactivation. The quality of the co-composting process was also investigated by monitoring the physico-chemical parameters. The obtained results showed that A. aquatilis GTE53 displayed a higher solubilization index of 2.4 and was efficiently dissolved, up to 162.8 and 247.4 mg·mL^-1 of inorganic phosphate from PS and phosphate rock (PR), respectively. A. aquatilis GTE53 exhibited siderophores and IAA release, along with atmospheric nitrogen fixation. In addition to that, A. aquatilis GTE53 showed a high resistance to heat and tolerance to acidic and alkaline pH, high salinity and water stress. Moreover, A. aquatilis GTE53 could degrade 99.2% of phenol from a high-concentrated medium (1100 mg·L^-1 of phenol) and can inactivate the most abundant pathogens in industrial wastes: Escherichia coli, Streptococcus sp., Salmonella sp., and Fusarium oxysporum albedinis. Analysis of temperature, pH, electrical conductivity, carbon/nitrogen (C/N) ratio, indicated successful co-composting. An efficient transformation of P to the available form and a great abatement of polyphenols, were also recorded during the process. The findings of this study will help to advance the understanding of A. aquatilis GTE53 functions and will facilitate its application to promote beneficial microbial organisms during composting, thus obtaining a high-quality product.

Effect of Plant Growth Promoting Rhizobacteria (PGPR) and Arbuscular Mycorrhizal Fungi (AMF) on Salt Stress Tolerance of Casuarina obesa (Miq.)

Salinity is one of the main abiotic stresses limiting plant growth and development. However, the use of salt-tolerant plants combined with beneficial soil microorganisms could improve the effectiveness of biological methods for saline soil recovery. The aim of this study is to identify the Casuarina obesa/ Arbuscular Mycorrhizal fungi (AMF)/Plant Growth Promoting Rhizobacteria (PGPR) association that could be used in salt-land rehabilitation programs. Thus, the plants were grown under greenhouse on sandy soil, inoculated either with PGPR (Pantoea agglomerans and Bacillus sp.), or with AMF (Rhizophagus fasciculatus and Rhizophagus aggregatum) or co inoculated with PGPR and AMF and watered with a saline solution (0, 150, and 300 mM). After 4 months of cultivation, the plants were harvested and the results obtained showed that inoculation improves the survival rate, height and biomass of the plants compared to the control plants. The results also showed that inoculation increases the total amount of chlorophyll and the accumulation of plant proline at all levels of salt concentration. However, P. agglomerans and Bacillus sp. strains alone or in combination with R. fasciculatus increased plant growth. This study showed that these strains of PGPR, whether or not associated with AMF, could be biological tools to improve C. obesa performance under saline stress conditions.

Phosphate-solubilizing Pseudomonas sp., and Serratia sp., co-culture for Allium cepa L. growth promotion

Different genus of bacteria has been reported with the capacity to solubilize phosphorus from phosphate rock (PR). Pseudomonas sp., (A18) and Serratia sp., (C7) isolated from soils at the “Departamento de Boyacá” Colombia, where Allium cepa is cultivated. Bacteria were cultured in MT11B media and evaluated as a bio-fertilizer for A. cepa germination and growth during two months at greenhouse scale. Pseudomonas sp., and Serratia sp., cultured at 30 °C, 48 h in SMRS1 agar modified with PR, (as an inorganic source of phosphorus), presented a phosphate solubilization index (SI) of 2.1 ± 0.2 and 2.0 ± 0.3 mm, respectively. During interaction assays no inhibition halos were observed, demonstrating there was no antagonism between them. In MT11B media growth curve (12 h) demonstrated that co-culture can grow in the presence of PR and glucose concentrations 7.5-fold, lower than in SMRS1 media and brewer's yeast hydrolysate; producing phosphatase enzymes with a volumetric activity of 1.3 ± 0.03 PU at 6 h of culture and 0.8 ± 0.04 PU at 12 h. Moreover, co-culture released soluble phosphorus at a rate of 58.1 ± 0.28 mg L⁻¹ at 8 h and 88.1 ± 0.32 mg L⁻¹ at 12 h. After five days of evaluation it was observed that germination percentage was greater than 90 % of total evaluated seeds, when placing them in contact with the co-culture in a concentration of 1 × 10⁸ CFU mL⁻¹. Furthermore, it was demonstrated that co-culture application (10 mL per experimental unit to complete 160 mL in two months) at 8.0 Log₁₀ CFU mL⁻¹ twice a week for two months increased A. cepa total dry weight (69 ± 13 mg) compared with total dry weight (38 ± 5.0 mg) obtained with the control with water.

Isolation and Characterisation of Endophytic Bacteria from Holostemma ada-kodien Schult

Plants with medical properties are often enriched with endophytes that have the potential to produce important bioactive compounds. Endophytes after entering the plant tissue may either colonize a particular tissue or may spread throughout the host plant without causing damage. The possession of pharmaceutical and biological properties has made the Holostemma ada-kodien Schult as one of the widely used plants of medicinal importance in India. Following the direct cut method three endophytic bacterial strains (UC H1, UC H4 and UC H7) were isolated, identified and characterized from the healthy looking rhizome of H. ada-kodien. Among these isolates, UC H1 and UC H4 were found to have many properties like antibacterial compounds, hydrolytic enzymes and plant growth promoting traits. The isolate UC H4 have ability for Indole-3-Acetic Acid (IAA) production of 513.54 U/ml and very good protease and pectinase activities of 20.65 U/ml and 16.09 U/ml respectively. So far no reports are available on the endophytic microflora of H. ada-kodien.

Isolated Phosphate-Solubilizing Soil Bacteria Promotes In vitro Growth of Solanum tuberosum L

The capacity of four bacterial strains isolated from productive soil potato fields to solubilize tricalcium phosphate on Pikovskaya agar or in a liquid medium was evaluated. A bacterial strain was selected to evaluate in vitro capacity of plant-growth promotion on Solanum tuberosum L. culture. Bacterial strain A3 showed the highest value of phosphate solubilization, reaching a 20 mm-diameter halo and a concentration of 350 mg/l on agar and in a liquid medium, respectively. Bacterial strain A3 was identified by 16S rDNA analysis as Bacillus pumilus with 98% identity; therefore, it is the first report for Bacillus pumilus as phosphate solubilizer. Plant-growth promotion assayed by in vitro culture of potato microplants showed that the addition of bacterial strain A3 increased root and stems length after 28 days. It significantly increased stem length by 79.3%, and duplicated the fresh weight of control microplants. In this paper, results reported regarding phosphorus solubilization and growth promotion under in vitro conditions represent a step forward in the use of innocuous bacterial strain biofertilizer on potato field cultures.

Plant growth-promoting endophytic bacteria augment growth and salinity tolerance in rice plants

Salt stress negatively affects growth and development of plants. However, it is hypothesized that plant growth-promoting endophytic bacteria can greatly alleviate the adverse effects of salinity and can promote growth and development of plants. In the present research, we aimed to isolate endophytic bacteria from halotolerant plants and evaluate their capacity for promoting crop plant growth. The bacterial endophytes were isolated from selected plants inhabiting sand dunes at Pohang beach, screened for plant growth-promoting traits and applied to rice seedlings under salt stress (NaCl; 150 mm). Out of 59 endophytic bacterial isolates, only six isolates, i.e. Curtobacterium oceanosedimentum SAK1, Curtobacterium luteum SAK2, Enterobacter ludwigii SAK5, Bacillus cereus SA1, Micrococcus yunnanensis SA2, Enterobacter tabaci SA3, resulted in a significant increase in the growth of Waito-C rice. The cultural filtrates of bacterial endophytes were tested for phytohormones, including indole-3-acetic acid, gibberellins and organic acids. Inoculation of the selected strains considerably reduced the amount of endogenous ABA in rice plants under NaCl stress, however, they increased GSH and sugar content. Similarly, these strains augmented the expression of flavin monooxygenase (OsYUCCA1) and auxin efflux carrier (OsPIN1) genes under salt stress. In conclusion, the pragmatic application of the above selected bacterial strains alleviated the adverse effects of NaCl stress and enhanced rice growth attributes by producing various phytohormones.

Rhizobium dioscoreae sp. nov., a plant growth-promoting bacterium isolated from yam (Dioscorea species)

This study investigated endophytic nitrogen-fixing bacteria isolated from two species of yam (water yam, Dioscorea alata L.; lesser yam, Dioscorea esculenta L.) grown in nutrient-poor alkaline soil conditions on Miyako Island, Okinawa, Japan. Two bacterial strains of the genus Rhizobium, S-93^T and S-62, were isolated. The phylogenetic tree, based on the almost-complete 16S rRNA gene sequences (1476 bp for each strain), placed them in a distinct clade, with Rhizobium miluonense CCBAU 41251^T, Rhizobium hainanense I66^T, Rhizobium multihospitium HAMBI 2975^T, Rhizobium freirei PRF 81^T and Rhizobium tropici CIAT 899^T being their closest species. Their bacterial fatty acid profile, with major components of C_19 : 0 cyclo ω8c and summed feature 8, as well as other phenotypic characteristics and DNA G+C content (59.65 mol%) indicated that the novel strains belong to the genus Rhizobium. Pairwise average nucleotide identity analyses separated the novel strains from their most closely related species with similarity values of 90.5, 88.9, 88.5, 84.5 and 84.4 % for R. multihospitium HAMBI 2975^T, R. tropici CIAT 899^T, R. hainanense CCBAU 57015^T, R. miluonense HAMBI 2971^T and R. freirei PRF 81^T, respectively; digital DNA–DNA hybridization values were in the range of 26–42 %. Considering the phenotypic characteristics as well as the genomic data, it is suggested that strains S-93^T and S-62 represent a new species, for which the name Rhizobium dioscoreae is proposed. The type strain is S-93^T (=NRIC 0988^T=NBRC 114257^T=DSM 110498^T).

Mitigation of drought stress in rice crop with plant growth-promoting abiotic stress-tolerant rice phyllosphere bacteria

In the search of effective drought-alleviating and growth-promoting phyllosphere bacteria, a total of 44 bacterial isolates were isolated from the leaf surface of drought-tolerant rice varieties, Mattaikar, Nootripattu, Anna R(4), and PMK3, and screened for their abiotic stress tolerance by subjecting their growth medium to temperature, salinity, and osmotic stress. Only eight isolates were found to grow and proliferate under different abiotic stress conditions. These isolates were identified using 16S ribosomal DNA gene sequence and submitted to the NCBI database. All the bacterial isolates were identified as Bacillus sp., except PB24, which was identified as Staphylococcus sp., and these isolates were further screened for plant growth-promoting (PGP) traits such as IAA production, GA production, ACC deaminase activity, and exopolysaccharide production under three different osmotic stress conditions adjusted using polyethylene glycol (PEG 6000). Additionally, mineral solubilization was measured under the normal condition. Bacillus endophyticus PB3, Bacillus altitudinis PB46, and Bacillus megaterium PB50 were found to have multifarious PGP traits. Consecutively, the performance of an individual strain to improve the plant growth was investigated under the osmotic stress (25% PEG 6000) and nonstress condition by inoculating them into rice seeds using hydroponics culture. Furthermore, the drought-alleviating potency of bacterial strains was assessed in the rice plants using pot experiment (-1.2 MPa) through bacterial foliar application during the reproductive stage. Finally, as a result of seed inoculation and foliar spray, the application of B. megaterium PB50 significantly improved the plant growth under osmotic stress, protected plants from physical drought through stomatal closure, and improved carotenoid, total soluble sugars, and total protein content. Metabolites of PB50 were profiled under both stress and nonstress conditions using gas chromatography-mass spectroscopy.