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.

Methylobacterium spp. mitigation of UV stress in mung bean (Vigna radiata L.)

Methylotrophs are a diverse group of bacteria that abundantly colonize the phyllosphere and have great potential to withstand UV irradiation because of their pigmented nature and ability to promote plant growth through various mechanisms. The present study investigated the effects of UVB radiation on plant growth-promoting (PGP) properties of methylotrophic bacteria and the growth of Vigna radiata L. A total of 55 methylotrophic bacteria were isolated from desert plants, and 15 methylotrophs were resistant to UVB radiation for 4 h. All UVB-resistant methylotrophs possess a methyldehydrogenase gene. Identification based on 16S rRNA gene sequencing revealed that all 15 UVB-resistant methylotrophs belonged to the genera Methylorubrum (07), Methylobacterium (07), and Rhodococcus (01). Screening of methylotrophs for PGP activity in the presence and absence of UVB radiation revealed that all isolates showed ACC deaminase activity and growth on a nitrogen-free medium. Furthermore, the production of IAA-like substances ranged from 8.62 to 85.76 µg/mL, siderophore production increased from 3.47 to 65.75% compared to the control. Seed germination assay with V. radiata L. (mung bean) exposed to UVB radiation revealed that methylotrophs improved seed germination, root length, and shoot length compared to the control. The present findings revealed that the isolates SD3, SD2, KD1, KD5, UK1, and UK3 reduced the deleterious effects of UVB radiation on mung bean plants and can be used to protect seedlings from UVB radiation for sustainable agriculture.

Microbial volatile compounds (MVCs): an eco-friendly tool to manage abiotic stress in plants

Microbial volatile compounds (MVCs) are produced during the metabolism of microorganisms, are widely distributed in nature, and have significant applications in various fields. To date, several MVCs have been identified. Microbial groups such as bacteria and fungi release many organic and inorganic volatile compounds. They are typically small odorous compounds with low molecular masses, low boiling points, and lipophilic moieties with high vapor pressures. The physicochemical properties of MVCs help them to diffuse more readily in nature and allow dispersal to a more profound distance than other microbial non-volatile metabolites. In natural environments, plants communicate with several microorganisms and respond differently to MVCs. Here, we review the following points: (1) MVCs produced by various microbes including bacteria, fungi, viruses, yeasts, and algae; (2) How MVCs are effective, simple, efficient, and can modulate plant growth and developmental processes; and (3) how MVCs improve photosynthesis and increase plant resistance to various abiotic stressors.

Encapsulation of Pseudomonas aeruginosa strain KBN12 decolourizes and bioremediates brilliant blue dye mediated toxicity in mung bean (Vigna radiata L.)

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.

Predation pressure regulates plant growth promoting (PGP) attributes of bacterial species

AIM

The present study aimed to investigate the effect of bacterivorous soil protists on plant growth promoting (PGP) attributes of bacterial species and their co-inoculative impact on rice seedling growth.

METHODS AND RESULTS

The effect of protists on the PGP attributes of bacteria was tested using standard protocols. The results revealed that the plant-beneficial properties of plant growth promoting bacteria (PGPB) were altered in the presence of various protist species. A significant increase in the production of siderophore units (86.66%), ammonia (34.80 µmol mL-1), and phosphate solubilization index (PSI) (5.6) was observed when Bacillus cereus (Bc) and Pseudomonas fluorescens (Ps) were co-inoculated with unidentified species belonging to the family Kreyellidae (C5). In the case of Enterobacter cloacae co-inoculated with C5 (Kreyellidae), a higher amount of siderophore (51.33%), ammonia (25.18 µmol mL-1), and indole-3-acetic acid (IAA)-like substance (28.59 µg mL-1) production were observed. The biofilm-forming ability of B. cereus is enhanced in the presence of Tetrahymena sp. (C2Bc), unidentified Kreyellidae (C5Bc), and Colpoda elliotti (C12Bc), whereas E. cloacae showed higher biofilm formation in the presence of Tetrahymena sp. alone Although IAA production decreased under predation pressure, a significant increase in shoot length (64.24%) and primary root length (98.18%) in co-inoculative treatments (C12Bc and C5Bc) compared to bacteria alone (25% and 61.50% for shoots and roots respectively) was observed. The results of enhanced PGP attributes and rice seedlings growth under predation pressure correlated with the enhanced bacterial activity under predation pressure and protist involvement in plant growth development.

CONCLUSIONS

Protists may act as regulators of the bacterial activities involved in plant growth promotion and thus enhance plant growth.

Complete genome sequencing of Bacillus subtilis (CWTS 5), a siderophore-producing bacterium triggers antagonistic potential against Ralstonia solanacearum

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 solubilisation, 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 (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.

Current status and future prospect of managing lead (Pb) stress through microbes for sustainable agriculture

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.

Secondary metabolites from methylotrophic bacteria: their role in improving plant growth under a stressed environment

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.

Zinc solubilizing Bacillus sp (SS9) and Enterobacter sp (SS7) promote mung bean (Vigna radiata L.) growth, nutrient uptake and physiological profiles

Zinc (Zn) is a crucial micronutrient required for optimum plant growth. Zn-solubilizing bacteria (ZSB) are potential alternatives for Zn supplementation and convert applied inorganic Zn to available forms. In this study, ZSB were isolated from the root nodules of wild legumes. From a set of 17 bacteria, the isolates SS9 and SS7 were found to be efficient in tolerating 1 g (w/v) Zn. The isolates were identified as Bacillus sp (SS9, MW642183) and Enterobacter sp (SS7, MW624528) based on morphology and 16S rRNA gene sequencing. The screening of PGP bacterial properties revealed that both isolates possessed production of indole acetic acid (50.9 and 70.8 µgmL-1), siderophore (40.2% and 28.0%), and solubilization of phosphate and potassium. The pot study experiment in the presence and absence of Zn revealed that the Bacillus sp and Enterobacter sp inoculated plants showed enhanced mung bean plant growth (45.0% to 61.0% increment in shoot length and 26.9 to 30.9% in root length) and biomass compared to the control. The isolates also enhanced photosynthetic pigments such as total chlorophyll (1.5 to 6.0-fold) and carotenoids (0.5 to 3.0-fold) and 1-2-fold increase in Zn, phosphorous (P), and nitrogen (N) uptake compared to the Zn-stressed control. The present results indicated that the inoculation of Bacillus sp (SS9) and Enterobacter sp(SS7) reduced the toxicity of Zn and, in turn, enhanced the plant growth and mobilization of Zn, N, and P to the plant parts.

Augmenting the bioavailability of iron in rice grains from field soils through the application of iron-solubilizing bacteria

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.

Assessment of copper (Cu) nanoparticle for their biocontrol activity against Xanthomonas oryzae pv. oryzae, growth promotion, and physiology of rice (Oryza sativa L.) plants

Among the various biotic factors that disrupt crop yield, Xanthomonas oryzae pv oryzae (Xoo) is the most ruinous microbe of rice and causes bacterial leaf blight (BLB) disease. The present study focused on the utilization of copper nanoparticles (Cu-NPs) to control BLB. The copper nanosuspension (259.7 nm) prepared using Na-CMC, CuSO4·7H2O, and NaOH showed effectively inhibited Xoo (65.0 μg/ml). The performance of Cu-NPs in vivo showed enhanced plant attributes (127.9% root length and 53.9% shoot length) compared to the control and CuSO4 treated seedling. Furthermore, Cu-NPs treated seedlings showed 23.01% disease incidence (DI) compared to CuSO4 (85.71%) treated and control plants (91.83%). In addition to enhancing the growth parameters and reducing DI, seed priming with Cu-NPs improved the total chlorophyll content to 36.0% compared to the control. The assessment of antioxidant enzymes such as superoxide dismutase (1.9 U), polyphenol oxidase, peroxidase, and phenylalanine ammonia-lyase (two- to three-fold) in roots and shoots of rice plants revealed significant enhancement in Cu-NPs treated seedlings (P < 0.05). The present study suggests that Cu-NPs can be used to control Xoo and enhance rice growth.

Bacteria isolated from e-waste soil enhance plant growth and mobilize trace metals in e-waste-amended soils

Worldwide accumulation of e-waste poses a major threat to environmental health. However, printed circuit boards contain precious metals, such as gold, and silver, and also contain micronutrient metal elements, such as Fe, Cu, Zn, etc. Therefore, the present study investigated the effects of e-waste-tolerant bacteria (ETB) on promoting plant growth in e-waste-amended soils and mobilizing trace metals into the plants. For this, a total of 18 bacteria were isolated and screened for e-waste tolerance. Screening for plant growth-promoting properties revealed the production of indole-3-acetic acid-like compounds, siderophore production, and phosphate solubilization. Identification based on 16S rRNA gene sequencing revealed that all isolates belonged to the genus Bacillus. Pot experiment revealed that the treated seeds showed the enhancement of chili plants root growth ranging from 106.55 to 208.07% compared to control plants (e-waste) and 0.0 to 47.90% (without e-waste). A similar enhancement was also observed in the shoot length, and size of the leaf compared to e-waste amended control plants. Inoculation of ETB significantly (p < 0.05) mobilized Fe, Zn, Cu, and Ni into chili plants. The identified ETB could be used to mitigate the toxicity posed by the e-waste, enhancing plant growth and mobilization of micronutrients into plants from e-waste.

In silico analysis of Typha domingensis Pers. phytocompounds against wound healing biomarkers and ascertaining through in vitro cell migration assay

Typha domingensis Pers. is known for its medicinal properties. Although traditionally T. domingensis Pers. has been used for wound healing, yet scientific investigations reporting its ability to heal wounds are lacking. Phytochemical profiling of T. domingensis Pers. inflorescence crude extract was carried out by LC-MS analysis. Ten phytochemicals were selected for in silico analysis based on retention time, mass-to-charge ratio and resolution of mass spectrum. Molecular docking of all ten compounds was done against selected wound healing biomarkers viz., interleukin 6(IL-6), interleukin β (IL-β), insulin-like growth factor tyrosine kinase receptor (IGF-1R) and transformation growth factor β (TGF-β). Based on this, catechin, mesalazine and piperazine were subjected for in vitro cell migration assay (3T3 L1 mouse fibroblast cell line) to assess their wound healing potentials. Molecular docking revealed that mesalazine, catechin, and piperazine have potential ligands based on lowest docking energy (ranging from - 4.1587 to - 0.972), Glide E score (ranging from - 26.929 to - 57.882), Glide G score (ranging from - 4.16 to - 7.972) and numbers of hydrogen bonds compared to other compounds studied. The migration assay revealed that, compared to control (52.5%), T. domingensis Pers. inflorescence crude extract showed maximum wound healing potential (80%) followed by Catechin (66.8%) Mesalazine (58.3%) and Piperazine (51.2%). The combined in silico and in vitro approach opens new dimension for designing innovative therapeutics to manage different types of wounds.

Interference of bio-control Trichoderma to enhance physical and physiological strength of sugarcane during Pokkah boeng infection

Tremendous benefits have been derived from the use of fungicides but excessive use of chemical fungicides not only posing threat to human and animal life but also contaminates the prevailing environment. Damage by pathogenic fungi alone causes significant damage to crops like maize, rice, wheat, soybeans, and potatoes. Therefore, it becomes imperative that these diseases are checked and controlled, for which chemical pesticides are being sprayed on plants extensively. Considering the devastating damage and toxicity, the global focus has taken a drift from synthetic chemicals to nature-friendly biological control agents. The present study focuses on the use of biological control agents particularly Trichoderma in sugarcane during Pokkah boeng infection. In the present experiment, twenty promising Trichoderma strains were evaluated for plant growth promotion, lytic enzymes, and physiological and biocontrol activity. Out of the twenty, four potential Trichoderma strains were assessed in the pot experiment viz. T. harzianum strain T28, T41 and T49 and T. aureoviride strain T38. The T. harzianum (T28) showed efficient plant growth-promoting traits as it produced IAA (20.67 µg/ml), phosphorus solubilization (18.57 µg/ml), and cell wall degrading enzymes such as chitinase (24.98 µg/ml) and β-glucanase (29.98 µg/ml). The interference of biocontrol agent T. harzianum (T28) controlled the disease by 73.55%. Apart from this, the inoculation of Trichoderma (T28) enhanced growth attributes including germination percentage (26.61%), mean tiller number (8.28 tiller/pot), individual cane length (241.5 cm), single cane weight (1.13 kg) and the number of milleable canes (6.00 cane/pot). Improvements in physiological activities at different growth stages of the sugarcane crop were observed based on the photosynthetically active radiation (PAR) on the leaf surface, transpiration rate, stomatal conductance, and photosynthetic rate. Further, improvement in juice quality parameters was also observed as it recorded the highest ⁰brix, sucrose, and commercial cane sugar by 21.26%, 19.28%, and 13.50%, respectively, by applying T. harzianum strain T28. Thus, results proved that T. harzianum strain T28 may be an effective eco-friendly biocontrol tool for managing Pokkah boeng disease in sugarcane. This is the first report of the biocontrol potential of Trichoderma spp. against Fusarium proliferatum causing Pokkah boeng disease in sugarcane.

Salt-tolerant bacteria enhance the growth of mung bean (Vigna radiata L.) and uptake of nutrients, and mobilize sodium ions under salt stress condition

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.

Role of ACC deaminase producing bacteria for abiotic stress management and sustainable agriculture production

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.

Predatory activity of Acanthamoeba sp genotype T4 on different plant growth-promoting bacteria and their combined effect on rice seedling growth

Heterotrophic protists play a crucial role in plant growth promotion via nutrient cycling and shift in microbial community composition in the soil ecosystem. Selective predation pressure by protists contributes to the evaluation of plant beneficial traits in rhizospheric bacteria. However, not always all plant growth-promoting bacterial (PGPB) strains are benefitted by predation. This study aimed to examine the predatory effect of Acanthamoeba sp genotype T4 on a range of PGPB strains and their combined impact on early rice seedling growth. Acanthamoeba sp isolated from rice rhizosphere soils were used to assess predation against several PGPB such as Pseudomonas, Bacillus, Enterobacter, Morganella, Stenotrophomonas, Providencia, and Lysinibacillus on Nutrient Yeast Extract agar (NYE) plate. The controlled experiment on the germinated rice seeds (Oryza sativa L.) grown in Petri dishes containing each PGPB strain and Acanthamoeba sp was performed to evaluate the combined impact on plant performance. The PGPB-Acanthamoeba combined treatments in Petri dishes showed significant rice seedling growth compared to PGPB alone, non-PGPB and control. Our results indicated the positive but different impact of Acanthamoeba sp with different PGPB species on early rice plant growth. Further in-depth research should be carried out with diverse protists and PGPB species to assess which protist species can be linked to enhancement of indigenous soil PGPB for improved plant growth.

Effect of zinc-resistant Lysinibacillus species inoculation on growth, physiological properties, and zinc uptake in maize (Zea mays L.)

Soil contamination by heavy metals is one of the major abiotic stresses that cause retarded plant growth and low productivity. Among the heavy metals, excessive accumulations of zinc (Zn) cause toxicity to plants. The toxicity caused by Zn could be managed by application of Zn-tolerant plant growth-promoting (PGP) bacteria. In this study, five Zn-tolerant bacteria (100-400 mg^-1 Zn resistant) were selected and identified as Lysinibacillus spp. based on 16S rRNA gene sequencing. The PGP properties of the Lysinibacillus spp. showed the production of indole acetic acid (60.0-84.0 μg/ml) and siderophore, as well as solubilization of potassium. Furthermore, the isolates were evaluated under greenhouse condition with 2 g kg^-1 Zn stress and without Zn stress along with control on Zea mays. The results showed that Lysinibacillus spp. coated seeds enhanced plant growth attributes and biomass yield in both conditions compared with control plants. The enhancement of root growth ranged from 49.2 to 148.6% and shoot length from 83.3 to 111.7% under Zn-stressed soils. Also, the inoculated seedlings substantially enhanced chlorophyll a and b, proline, total phenol, and ascorbic acid. The uptake of Zn by maize root ranged from 31.5 to 210.0% compared with control plants. Therefore, this study suggested that the tested Zn-tolerant Lysinibacillus spp. may be used for cultivation of Z. mays in Zn-contaminated agricultural lands.

Isolation and characterization of Lactococcus garvieae from the fish gut for in vitro fermentation with carbohydrates from agro-industrial waste

This study focused on agro-industrial waste such as fruit peels by extracting prebiotics as a carbon source for lactic acid bacteria (LAB). Four strains of LAB were selected from Oreochromis niloticus (B2 and B3) and Nemipterus japonicas (R4 and R5), and identified as Lactococcus garvieae through 16S rRNA gene sequencing. The analysis of probiotic characteristics revealed that all four strains were able to tolerate sodium chloride (up to 7 %), bile salt (up to 3 %), and broad range of pH (2-9). Further, analysis of polysaccharide contents in the agro-industrial waste materials such as peels of pineapple, orange, lemon, sugarcane, pomegranate, and sweet lemon revealed that the concentration ranged from 3.91-163.85 mg/g. It was observed that orange peels (20.38-140.99 mg/g), sweet lemon peels (22.03-161.93 mg/g), and pomegranate peels (38.19-163.85 mg/g) yielded maximum indigestible polysaccharide. Evaluation of synbiotic combination of probiotic and prebiotic revealed that L. garvieae strains had better fermentation efficiency with orange, sweet lemon, and pineapple compared to lemon, sugarcane, and pomegranate. In nutshell, different types of agro-industrial waste evaluated in this research were found to be a cheap and fermentable carbon sources for LAB. Further study should be conducted to analyze this symbiotic combination as feed supplements for fish in aquaculture as well as various fermentation industries.

Effect of copper-resistant Stenotrophomonas maltophilia on maize (Zea mays) growth, physiological properties, and copper accumulation: potential for phytoremediation into biofortification

In this study, Cu-tolerant PGP bacteria were isolated from the contaminated soils of Tapi (Surat, Gujarat, India). From a set of 118 bacteria isolated from the contaminated soil, the isolate RBTS7 was found to be efficient in tolerating 0.3 g (w/v) Cu. The isolate was identified as Stenotrophomonas maltophilia, based on biochemical and 16S rRNA gene sequencing. Further, the isolate was also found to produce indole acetic acid (140 µg/ml) and siderophore, and solubilize potassium. Inoculation study was carried out in the presence and absence of Cu in the greenhouse. The results revealed that S. maltophilia enhanced plant growth and biomasses compared to control. In addition to plant growth attributes, the isolate also enhanced chlorophyll a and b (434.1 and 496.7%) contents and antioxidant properties such as proline (168.2%), total phenolic compounds (33.5%), and ascorbic acid oxidase (62.3%) compared to control with Cu and without Cu. Inoculation of S. maltophilia + Cu enhanced the uptake of Cu in maize root (77.4%) and stem (112.0%) compared to Cu-stressed control. The results clearly indicated the inoculation of S. maltophilia reduced the toxicity of Cu and in turn enhanced the plant growth and mobilization of Cu to the plant parts.

Phytoextraction of iron from contaminated soils by inoculation of iron-tolerant plant growth-promoting bacteria in Brassica juncea L. Czern

Iron (Fe) is one of the essential micronutrients for all living organisms. Despite its abundance in most of the contaminated soil, it is usually in unavailable forms. The unavailable form of Fe could be mobilized to plants by the use of microorganisms. This study was carried out to show that the Fe-contaminated field soils could be used to accumulate Fe in the plant parts using bacterial inoculation. For this, from a set of bacterial isolates, four Fe-tolerant bacteria were selected and identified based on 16S rRNA gene sequencing. The Fe-tolerant bacteria belonged to the genus Bacillus toyonensis (MG430287), Rhodococcus hoagii (MG432495), Lysinibacillus mangiferihumi (MG432492), and Lysinibacillus fusiformis (MG430290). Screening of plant growth-promoting properties of these isolates revealed that all isolates were able to produce indole acetic acid (50.0-84.0 μg/ml), siderophore, and potassium solubilization (except R. hoagii). Pot assay using Fe-contaminated ((8.07-8.35 g kg^-1) soils River Directorate of India) revealed that Fe-tolerant bacteria enhanced the growth of Brassica juncea and its biomass. Besides the improved plant growth, the inoculated plants also showed an overall percentage increase in the uptake of iron in root, stem, and leaf (57.91-128.31%) compared with uninoculated plants. In addition to enhanced plant growth attributes, the isolates also improved the total chlorophyll content and antioxidant properties such as total phenol, proline, and ascorbic acid oxidase. Thus, the results clearly indicated that these isolates could be used as a bioinoculant to improve the sequestration of Fe from the contaminated soils and alleviation of Fe stress in plants.

Biocontrol and plant growth-promoting ability of plant-associated bacteria from tomato (Lycopersicum esculentum) under field condition

Forty-five bacterial isolates recovered from surface-sterilized root, stem and leaf tissues of tomato were studied for their antifungal activity against phytopathogens, and plant growth-promoting (PGP) and biocontrol traits. Six plant-associated bacteria suppressed all the pathogens tested under in vitro plate assay and also shown PGP and biocontrol traits. The six isolates showing PGP and biocontrol properties were identified as Bacillus spp., based on the microbial identification system (Biolog) and partial sequence analysis of 16S rDNA. Two independent field trials were conducted with biocontrol bacteria along with chemical control (Thiram+Fytolan) and control (Without treatment). The averaged results of two field trails revealed that tomato plants inoculated with BETS11 (11.73 t/ha) and BETR11 (11.24 t/ha) strains showed significantly higher yield and disease reduction on par with chemical control (11.81 t/ha). However, there was an increase in the yield with respect to uninoculated control except the isolate BETS5 (9.09 t/ha). Therefore, the isolates BETS11 and BETR11 may be used as efficient biofertilizer and bio-control agent for tomato production in the Island agricultural ecosystem.

Characterization of medicinal plant-associated biocontrol Bacillus subtilis (SSL2) by liquid chromatography-mass spectrometry and evaluation of compounds by in silico and in vitro methods

This study explores the antimicrobial properties of bioactive secondary metabolites extracted from the medicinal plant (Solanum surattense)-associated Bacillus subtilis strain SSL2. The secondary metabolites were extracted from B. subtilis (SSL2) using ethyl acetate, acetone, butanol, chloroform and methanol solvents. The crude extract was tested against two wilt causing pathogens: Ralstonia solanacearum and Fusarium oxysporum. The results revealed that the ethyl acetate extract has maximum inhibition against both the pathogens tested in this study. Furthermore, liquid chromatography-mass spectrometry (LC-MS) analysis of ethyl acetate extract identified 80 different compounds based on mass-to-charge ratio, database difference, resolution of mass spectrum and so on. Among the 80 compounds, citrulline (m/z = 158.0917), chloramphenicol (m/z = 195.075) and carnitine (m/z 162.11) were further selected based on m/z ratio for in silico and in vitro analyses. The in silico analysis revealed that citrulline, chloramphenicol and carnitine inhibited the virulent genes phcA (R. solanacearum) and ste12 (F. oxysporum). Further, under in vitro condition, citrulline and chloramphenicol were found to inhibit the growth of R. solanacearum and F. oxysporum. On the basis of the biocontrol activity of B. subtilis (SSL2) in in silico and in vitro conditions, the bacteria could be used as a biocontrol agent against both bacterial and fungal wilt-causing pathogens. However, this needs to be tested in pot studies or field conditions before being used as biocontrol agents.Communicated by Ramaswamy H. Sarma.

Influence of salt tolerant Trichoderma spp. on growth of maize (Zea mays) under different salinity conditions

In the present study, a total of 70 Trichoderma spp. were isolated from the rhizosphere soils of vegetable and spice crops that were grown in Andaman and Nicobar Islands, India. Initial screening of Trichoderma spp. for salt tolerant properties showed 32 isolates were able to tolerate 10% NaCl. Furthermore, these isolates were screened for their potential plant growth-promoting characteristics such as IAA production, phosphate solubilization, and siderophore production. Among 32 isolates, nine isolates were able to produce IAA, siderophore, and solubilize phosphate. Jar trial was carried out on maize under axenic conditions at 1.67, 6.25, 11.25, 17.2, and 22.9 dS m^-1 salt stress using the best nine isolates. Three isolates (TRC3, NRT2, and THB3) were effective in improving germination percentage, reducing reduction percentage of germination (RPG) and also in increasing the shoot and root length under axenic conditions. These three isolates were further tested under pot trial at 52 (sea water), 27, 15, 7, and 1.67 dS m^-1 . TRC3 was found to be the most effective isolate compared to the other isolates and significantly increased the physiological parameters like shoot, root length, leaf area, total biomass, and stem and leaf fresh weight at all stress levels. Similarly, total chlorophyll content also increased by TRC3 over control. All three isolates, NRT2, TRC3, and THB3 showed lower accumulation of malondialdehyde (MDA) content whereas, proline and phenol content were higher than the uninoculated control plants under both normal and saline conditions. The results suggest that these isolates could be utilized for the alleviation of salinity stress in maize.

Characterization of cadmium-resistant bacteria for its potential in promoting plant growth and cadmium accumulation in Sesbania bispinosa root

The cadmium (Cd) resistant bacteria were isolated from soils of Damanganga river, Vapi, and identified 11 potential Cd resistant bacteria based on 16S rDNA sequences. The Cd resistant bacteria belonged to four different genera: Providencia spp., Morganella sp., Stenotrophomonas sp., and Bacillus spp. The assessment of plant growth-promoting (PGP) parameters revealed that the Cd tolerant bacteria showed one or more PGP properties. Further, a pot experiment was conducted to elucidate the effects of Cd resistant bacteria on the plant growth and the uptake of Cd by Sesbania bispinosa. The bacterized seedlings recorded 36.0-74.8% and 21.2-32.9% higher root and shoot lengths, respectively, in Cd amended soil compared with control. The Cd mobilization in the root of S. bispinosa by microbial inoculants ranged from 0.02 ± 0.01 to 1.11 ± 0.06 ppm. The enhanced concentrations of Cd accumulation in S. bispinosa roots correspond to the effect of the bacterial strains on metal mobilization in soil. The present observations showed that the Cd resistant strains protect the plants against the inhibitory effects of Cd, probably due to the production of PGP properties. The present results provided a new insight into the phytoremediation of Cd contaminated soil.

Isolation and Characterization of Trichoderma spp. for Antagonistic Activity Against Root Rot and Foliar Pathogens

Trichoderma, soil-borne filamentous fungi, are capable of parasitising several plant pathogenic fungi. Twelve isolates of Trichoderma spp. isolated from different locations of South Andaman were characterized for their cultural, morphological and antagonistic activity against soil borne and foliar borne pathogens. The sequencing of these isolates showed seven different species. The isolates revealed differential reaction patterns against the test pathogens viz., Sclerotium rolfsii, Colletotrichum gloeosporioides and C. capsici. However, the isolates, TND1, TWN1, TWC1, TGD1 and TSD1 were most effective in percentage inhibition of mycelial growth of test pathogens. Significant chitinase and β-1,3-glucanase activities of all Trichoderma isolates has been recorded in growth medium. T. viride was found with highest chitinase whereas T. harzianum was recorded with highest β-1,3-glucanase activities.

First Report of Anthracnose Disease of Black Pepper (Piper nigrum) Caused by an Unknown Species of Colletotrichum

Black pepper is cultivated in Andaman and Nicobar Islands, India as a spice crop. During a survey performed in June of 2007 in South Andaman, two kinds of leaf anthracnose symptoms were observed. The classic symptom, angular to irregular or circular brownish lesions with a chlorotic halo and pinhead size acervuli on the leaves, was consistent with the disease previously reported on pepper caused by Colletotrichum gloeosporioides (2). This symptom also caused splitting and the production of hollow berries. The new symptom was novel, with leaves initially exhibiting pale green or yellowish green lesions. As the disease progressed, lesion margins became brown to black with slightly raised areas containing numerous acervuli. Unlike the classic symptom, the new symptom was characterized as leaf lesions that rarely caused defoliation; berries showed no lesions. The foliar disease incidence was up to 15%, but direct economic loss of berries was not noticed. This new symptom was similar to symptoms caused by C. dracaenophilum, a species identified as a pathogen on lucky bamboo (Dracaena sanderiana) (1). A fungus was isolated in pure culture from the pale green lesions and cultured on potato dextrose agar (PDA). At room temperature (28 ± 2°C), fungal growth on PDA was slow and mycelium appeared whitish at the margin with pale pink centers. A pinkish color was observed on the reverse side of the plate, reflecting profuse sporulation. The conidia were hyaline, broadly clavate to cylindrical, and measured 12.5 to 15 × 5 to 7.5 μm (average 14 × 7.5 μm). The internal transcribed spacer (ITS) region of the fungal DNA was amplified, sequenced, and submitted to NCBI GenBank (Accession No. EU744584). The specimen was deposited in the MTCC of IMTECH, Chandigarh, India (Accession No. MTCC9344). Pathogenicity was tested in five replications on 15- to 20-day-old pepper plants and repeated twice. A 1-ml conidial suspension (10⁸ spores/ml) of the fungus was brushed on two intact leaves of each pepper plant and incubated for 2 weeks in a glasshouse at 28°C and 70% relative humidity with natural daylight conditions. Plants brushed with sterile water served as control. Similar pale green symptoms were observed only on treated leaves and the same organism was reisolated from lesions. BLAST searches of the GenBank using the ITS sequence revealed that this fungus was a member of the genus Colletotrichum, but a species level identification could not be made with these data. The fungus was most similar in sequence to unnamed endophytic strains of Colletotrichum (96% sequence identity) and phytopathogenic isolates of C. dracaenophilum (93% sequence similarity). Although the symptomatology and sequence data were most closely matched with those documented for C. dracaenophilum (1), the morphological and cultural characteristics of the black pepper anthracnose fungus differed from C. dracaenophilum and other known species of Colletotrichum (3). Together these morphological and molecular data suggest that this form of anthracnose disease on black pepper may be caused by a novel, undescribed species of Colletotrichum. Further investigations will be required to characterize this organism to the species level. References: (1) S. G. Bobev et al. Plant Dis. 92:173, 2008. (2) P. Santha Kumari and A. Sanker. J. Mycol. Plant Pathol. 33:329, 2003. (3) B. C. Sutton. In: Colletotrichum. Biology, Pathology and Control. CAB International, Wallingford, 1992.