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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.

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AIM: The aim of this study was to explore the decolourization and bioremediation ability of non-encapsulated and encapsulated Pseudomonas aeruginosa (strain KBN 12) against the azo dye brilliant blue (BB). METHODS AND RESULTS: Six efficient BB dye-decolourizing bacteria were isolated from textile dye effluent. The most efficient free cells of P. aeruginosa KBN 12 along with the optimized conditions such as carbon source (maltose: 5 g L-1), and nitrogen source (ammonium chloride: 4 g L-1) at pH 6 at 37°C decolourized 72.69% of BB dye aerobically after 9 days of incubation under static conditions. Encapsulated (calcium alginate) P. aeruginosa KBN 12 decolourized 87.67% of BB dye aerobically after 9 days of incubation under the same optimized conditions. Fourier-transform infrared spectroscopy (FTIR) and gas chromatography (GC) analysis of the chemical structure of BB dye after decolourization found changes in functional and chemical groups. Phytotoxicity and soil respiration enzyme assays revealed that the decolourized dye or dye products were less toxic than the pure BB dye. CONCLUSION: The encapsulation of P. aeruginosa KBN 12 proved to be an effective method for BB dye decolourization or remediation.

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Xanthomonas oryzae causes tremendous damage in rice plants (Oryza sativa L). Therefore, this study is focused on siderophore-producing Bacillus albus (CWTS 10) for managing BLB disease caused by X. oryzae. Both B. albus and its crude siderophore (methanolic and diethyl ether) extracts inhibited X. oryzae (10-12 mm). Fourier transform infrared spectroscopy (FTIR) analysis of the extracts indicated the presence of catecholate siderophore functional groups. Liquid chromatography-mass spectrometry (LC-MS) analysis revealed the presence of antimicrobial compounds such as 2-deoxystreptamine, miserotoxin, fumitremorgin C, pipercide, pipernonaline, gingerone A, and deoxyvasicinone. Complete genome sequencing revealed the gene clusters for antibiotic, siderophore, antibacterial, antifungal, and secondary metabolite production. An in vivo study revealed that bacteria (CWTS 10) and their siderophore extracts effectively inhibited X. oryzae. The mode of application of bacterial or siderophore extracts in terms of DI and DSI percentage was as follows: soak method > inoculation method > spray method. In addition to providing enhanced antagonistic activity, there was a significant increase in root and shoot length and weight (wet and dry) of treated plants compared to control plants challenged with X. oryzae. Thus, the results clearly indicate that siderophore-producing B. albus and its siderophore extracts strongly inhibited X. oryzae. However, further field experiments are required before being formulated to protect rice crops from X. oryzae.

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AIM: The aims of this study were to explore the antagonistic potential of siderophore-producing Bacillus subtilis (CWTS 5) for the suppression of Ralstonia solanacearum and to explore the mechanisms of inhibition by FTIR, LC-MS, and whole genome analysis. METHODS AND RESULTS: A siderophore-producing B. subtilis (CWTS 5) possessing several plant growth-promoting properties such as IAA and ACC deaminase production, phosphate solubilization, and nitrogen fixation was assessed for its inhibitory effect against R. solanacearum, and its mechanisms were explored by in vitro and in vivo analyses. The active secondary metabolites in the siderophore extracts were identified as 2-deoxystreptamine, miserotoxin, fumitremorgin C, pipercide, pipernonaline, gingerone A, and deoxyvasicinone by LC-MS analysis. The Arnow's test and antiSMASH analysis confirmed the presence of catecholate siderophores, and the functional groups determined by FTIR spectroscopy confirmed the presence of secondary metabolites in the siderophore extract possessing antagonistic effect. The complete genome sequence of CWTS 5 revealed the gene clusters responsible for siderophore, antibiotics, secondary metabolite production, and antibacterial and antifungal metabolites. Furthermore, the evaluation of CWTS 5 against R. solanacearum in pot studies demonstrated 40.0% reduced disease severity index (DSI) by CWTS 5, methanolic extract (DSI-26.6%), ethyl acetate extract (DSI-20.0%), and increased plant growth such as root and shoot length, wet weight and dry weight of Solanum lycopersicum L. owing to its antagonistic potential. This genomic insight will support future studies on the application of B. subtilis as a plant growth promoter and biocontrol agent against R. solanacearum for bacterial wilt management. CONCLUSION: The results of this study revealed that B. subtilis (CWTS 5) possesses multiple mechanisms that control R. solanacearum, reduce disease incidence, and improve S. lycopersicum growth.

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Soil is an important residence under various biotic and abiotic conditions. Contamination of soil by various means has hazardous effects on both plants and humans. Soil contamination by heavy metals occurs due to various man-made activities, including improper industrial and agricultural practices. Among the heavy metals, after arsenic, lead (Pb) was found to be the second most toxic metal and potent pollutants that accumulate in sediments and soils. Pb is not considered an essential element for promoting plant growth but is readily absorbed and accumulated in different plant parts. Many parameters such as pH, root exudation, soil particle size, cation exchange capacity, and other physicochemical parameters are involved in Pb uptake in plants. Excess amounts of Pb pose a threat to plant growth and cause toxicity such as chlorosis, blackening of the root system, and stunted growth. Pb toxicity may inhibit photosynthesis, disturb water balance and mineral nutrition, and alter the hormonal status, structure, and membrane permeability of plants. Therefore, this review addresses the effects of Pb toxicity and its impact on plant growth, including the morphological, physiological, and biological effects of Pb toxicity, the mechanisms behind different strategies promoting plant growth, and in combating Pb-induced stress. The bioremediation strategy for Pb removal from Pb-contaminated soil also plays an important role in combating Pb toxicity using bacterial community. Pb-contaminated soil may be remediated using different technologies such as rhizofiltration and phytoremediation, which tend to have a great capacity to curb Pb-contamination within the soil.

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The biofortification approach has been widely used to enhance mineral nutrients in staple foods such as rice (Oryza sativa). In the present study, iron-solubilizing plant growth-promoting bacteria (PGPB) were evaluated for iron fortification of rice grains and NPK via field experiments. Inoculation of iron-solubilizing bacteria showed significant improvements in growth parameters, such as plant height, root and shoot dry weight, panicle length, grain yield, and nitrogen, potassium, phosphorus, and iron uptake. The mobilization of iron was ranged from 53.88% to 89.05% in rice grains compared to the uninoculated plants. The present study results revealed that application of PGPB strains is vital approach to combat the problem of iron deficiency in rice and subsequently in humans.

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Climate change is considered a natural disaster that causes the ecosystem to fluctuate and increase temperature, as well as the amount of UV radiation (UV-A and UV-B) on the Earth's surface. Consequently, greenhouse gases such as chlorofluorocarbons, methane, nitrogen oxide, and carbon dioxide have become obstacles to the development of sustainable agriculture. To overcome environmental stress such as phytopathogens, drought, salinity, heavy metals, and high-low temperatures, the utilization of microorganisms is a viable option. The synthesis of secondary metabolites by methylotrophic bacteria improves plant metabolism, enhances tolerance, and facilitates growth. The genus Methylobacterium is a pink-pigmented facultative methylotrophs which abundantly colonizes plants, especially young leaves, owing to the availability of methanol. Secondary metabolites such as amino acids, carotenoids, hormones, antimicrobial compounds, and other compounds produced by methylotrophic bacteria enhance plant metabolism under stress conditions. Therefore, in this review, we discuss the role of secondary metabolites produced by methylotrophic bacteria and their role in promoting plant growth under stress.

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Salinity is one of the significant abiotic stresses that exert harmful effects on plant growth and crop production. It has been reported that the harmfulness of salinity can be mitigated by the use of salt-tolerant plant growth-promoting (PGP) bacteria. In this study, four bacteria were selected from a total of 30 cultures, based on salt-tolerant and PGP properties. The isolates were found to produce indole acetic acid (8.49-19.42 µg/ml), siderophore (36.04-61.77%), and solubilize potassium and inorganic phosphate. Identification based on 16S rRNA gene sequencing revealed that the isolates belonged to Cronobacter (two isolates) and Enterobacter (two isolates). Inoculation of PGP bacteria under 2 and 10% salinity stress showed enhanced plant growth parameters in Vigna radiata compared to both salinity and non-salinity control plants. The rate of germination (113.32-206.64%), root length (128.79-525.31%), shoot length (34.09-50.32%), fresh weight, and dry weight were 3-fold higher in bacteria-treated seeds than control plants. The estimation of chlorophyll (1-5-fold), carotenoids (1-4-fold), and proline content (3.65-14.45%) was also higher compared to control plants. Further, the bacterized seeds showed enhanced nitrogen and phosphorous uptake and mobilized sodium ions from roots to leaves. Overall the strains SS4 and SS5 performed well in both 2 and 10% salt-amended soils. These strains could be formulated as a bioinoculant to mitigate the salinity stress in salinized soils.

Salinity severely affects the growth and productivity of Vigna radiate (mung bean) worldwide. Approximately 50 mM concentration of NaCl can cause >60% yield loss of mung bean. In this study, inoculation of salt-tolerant root nodule-associated plant growth-promoting bacteria showed 2­3-fold enhancements in mung bean plant growth, biomass, and physiology even at 2 and 10% salinity stress. Further, the inoculated mung bean plants showed an increment in the uptake of nitrogen and phosphorous in the salinized conditions and mobilized the Na⁺ ions from root to shoot to reduce the toxicity posed by Na⁺ ions. Therefore the strains identified in this study could be formulated to mitigate the salinity stress and improve the mung bean growth in salinized soils.

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Plants are immobile and are exposed to various biotic and abiotic stresses, including heat, cold, drought, flooding, nutrient deficiency, heavy metal exposure, phytopathogens, and pest attacks. The stressors significantly affect agricultural productivity when exceed a certain threshold. It has been reported that most of the stressed plants are reported to have increased ethylene synthesis from its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene is a plant hormone that plays a vital role in the regulation of various physiological processes, such as respiration, nitrogen fixation, and photosynthesis. The increment in the plant hormone ethylene would reduce plant growth and development, and if the ethylene level increased beyond the limit, it could also result in plant death. Therefore, plant growth-promoting bacteria (PGPB) possessing ACC deaminase activity play an essential role in the management of biotic and abiotic stresses by hydrolysing 1-aminocyclopropane-1-carboxylic acid using ACC deaminase. In this review, the importance of ACC deaminase-producing bacteria in promoting plant growth under various abiotic stressors is discussed.

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