Amelioration effect of chromium-tolerant bacteria on growth, physiological properties and chromium mobilization in chickpea (Cicer arietinum) under chromium stress

Combined application of earthworms and plant growth promoting rhizobacteria improve metal uptake, photosynthetic efficiency and modulate secondary metabolites levels under chromium metal toxicity in Brassica juncea L

Chromium (Cr) toxicity impairs essential morphological and metabolic activities in plants. The present investigation was carried out to evaluate the beneficial role of plant growth promoting rhizobacterial strains namely Pseudomonas aeruginosa (M1), Burkholderia gladioli (M2) and earthworms (Eisenia fetida) in alleviating Cr toxicity in 10 days old Brassica juncea L. The findings delineated that addition of earthworms and PGPR restored growth, boosted Cr uptake and showed upregulation of metal transporter genes (SULTR 1-4). Supplementation of rhizospheric amendments reinstated Cr induced impairment in photosynthetic attributes. Gaseous exchange attributes, the efficiency of PS II, the content of total phenols, anthocyanin and flavonoids was enhanced with application of earthworms along with PGPR. Confocal imaging of primary photosynthetic pigment (chlorophyll), accessory photosynthetic pigment (carotenoids) and total phenols showed maximum fluorescence with combined inoculation of earthworms and both microbial strains (M1M2). The gene expression analysis revealed that Phyotene synthase (PSY), Photosystem II core protein psb A, psb B were down regulated in Cr stressed seedlings which upon supplementation with earthworms and PGPR were upregulated. Further, Phenylalanine ammonialyase (PAL), chalcone synthase (CHS) were upregulated with addition of earthworms and PGPR. Increased nitric oxide content, enhanced activity and upregulation of nitrate reductase (NR) gene was observed with addition of PGPR and earthworms.

Zinc-lysine and iron-lysine mitigate chromium toxicity in pearl millet (Pennisetum glaucum) through modulating photosynthetic and antioxidant system and inhibiting chromium uptake and translocation

Chromium (Cr) is an ever-present abiotic stress that negatively affects crop cultivation and production worldwide. High rhizospheric Cr concentrations inhibit nutrients uptake and their translocation to aboveground parts, thus can affect the growth and development of crop plants. This experiment was designed to evaluate the effects of sole and combined zinc-lysine and iron-lysine applications on photosynthetic efficacy, antioxidative defense, oxidative stress, and nutrient uptake and translocation under Cr stress. Chromium stress exhibited toxic effects on the growth, physiological, and biochemical indices of pearl millet. The combined application of zinc-lysine and iron-lysine significantly decreased malondialdehyde (MDA; 25%) and hydrogen peroxide (H2O2; 22.44%), while increased superoxide dismutase (SOD; 19.75%), catalase (CAT; 26.16%), peroxidase (POD; 19.62%), and ascorbate peroxidase (APX; 23.52%) activities under Cr toxicity compared to the control treatment. In addition, the combined application of zinc-lysine and iron-lysine effectively improved net photosynthesis (43.63%), stomatal conductance (20.05%), transpiration rate (20.14%), internal CO2 concentration (34.28%), total chlorophyll (43.12%), relative water content (23.95%), membrane stability index (32.77%), and proline content (25.53%) under stress condition and compared with control. Our results also indicated that the combined application of zinc-lysine and iron-lysine decreased Cr uptake in both shoot and root by 31.25% and 32%, and increased zinc and iron uptake by 39.28% and 36.67%, respectively, over the control, under Cr stress conditions. Moreover, under stress conditions, combined zinc-lysine and iron-lysine effectively improved growth traits particularly shoot and root dry weights, by 8% and 36.84%, respectively, over the control treatment. Overall, our results demonstrated that combined zinc-lysine and iron-lysine was more effective in mitigating Cr toxicity in pearl millet compared with the sole application of these treatments or the control.

Microbes mediated alleviation of chromium (Cr VI) stress for improved phytoextraction in fodder maize (Zea mays L.) cultivar

This study investigates the potential of chromium (VI) resistant bacterial isolates to alleviate heavy metal stress in fodder maize plants and enhance phytoremediation. Twenty-one bacterial strains were isolated from contaminated water, with five strains; Bacillus thuringiensis (BHR1), Bacillus cereus (BHR2), Enterobacter cloacae (BHR4), Bacillus pumilus (BHR5), and Bacillus altitudinis (BHR6) selected based on their significant plant-growth promoting (PGP) traits and heavy metal tolerance. Under chromium (Cr VI) stress, the BHR1 strain significantly improved seed germination, seedling length and vigor index of fodder maize variety (J 1007) especially at 150 mg/L Cr (VI), where these parameters increased by 3.75, 3.23 and 6.44 folds, respectively. After 60 days, BHR1 also enhanced shoot and root lengths by 4.91 and 4.06 folds, respectively and increase fresh and dry biomass, especially at higher Cr (VI) concentrations. Photosynthetic pigments, chlorophyll a and b, were also elevated by 3.04 and 2.26 times, respectively. Additionally, BHR1 reduced oxidative stress markers, including proline and malondialdehyde (MDA), and decreased electrolyte leakage, thus improving membrane stability. The strain further increased antioxidant enzyme activities and chromium uptake in root and shoot tissues, enhancing the translocation factor by 95 %. This suggests that BHR1 can significantly promote fodder maize growth and accelerate chromium removal from contaminated soil, offering valuable insights into plant-microbe interactions under Cr (VI) stress.

Genome-wide analysis of miR172-mediated response to heavy metal stress in chickpea (Cicer arietinum L.): physiological, biochemical, and molecular insights

BACKGROUND

Chickpea (Cicer arietinum L.), a critical diploid legume in the Fabaceae family, is a rich source of protein, vitamins, and minerals. However, heavy metal toxicity severely affects its growth, yield, and quality. MicroRNAs (miRNAs) play a crucial role in regulating plant responses to both abiotic and biotic stress, including heavy metal exposure, by suppressing the expression of target genes. Plants respond to heavy metal stress through miRNA-mediated regulatory mechanisms at multiple physiological, biochemical, and molecular levels. Although the Fabaceae family is well represented in miRNA studies, chickpeas have been notably underrepresented. This study aimed to investigate the effects of heavy metal-induced stress, particularly from 100 µM concentrations of cadmium (Cd), chromium (Cr), nickel (Ni), lead (Pb), and 30 µM arsenic (As), on two chickpea varieties: ILC 482 (sensitive) and Azkan (tolerant). The assessment focused on physiological, biochemical, and molecular parameters. Furthermore, a systematic characterization of the miR172 gene family in the chickpea genome was conducted to better understand the plant's molecular response to heavy metal stress.

RESULTS

Variance analysis indicated significant effects of genotype (G), treatment (T), and genotype-by-treatment (GxT) interactions on plant growth, physiological, and biochemical parameters. Heavy metal stress negatively impacted plant growth in chickpea genotypes ILC 482 and Azkan. A reduction in chlorophyll content and relative leaf water content was observed, along with increased cell membrane damage. In ILC 482, the highest hydrogen peroxide (H₂O₂) levels in shoot tissue were recorded under As, Cd, and Ni treatments, while in Azkan, peak levels were observed with Pb treatment. Malondialdehyde (MDA) levels in root tissue were highest in ILC 482 under Cd and Ni exposure and in Azkan under As, Cr, and Cd treatments. Antioxidant enzyme activities, including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX), were significantly elevated under heavy metal stress in both genotypes. Gene expression analysis revealed upregulation of essential antioxidant enzyme genes, such as SOD, CAT, and APX, with APX showing notable increases in both shoot and root tissues compared to the control. Additionally, seven miR172 genes (miR172a, miR172b, miR172c, miR172d, miR172e, miR172f, and miR172g) were identified in the chickpea genome, distributed across five chromosomes. All genes exhibited conserved hairpin structures essential for miRNA functionality. Phylogenetic analysis grouped these miR172 genes into three clades, suggesting strong evolutionary conservation with other plant species. The expression analysis of miR172 and its target genes under heavy metal stress showed varied expression patterns, indicating their role in enhancing heavy metal tolerance in chickpea.

CONCLUSIONS

Heavy metal stress significantly impaired plant growth and physiological and biochemical parameters in chickpea genotypes, except for cell membrane damage. The findings underscore the critical role of miR172 and its target genes in modulating chickpea's response to heavy metal stress. These insights provide a foundational understanding for developing stress-tolerant chickpea varieties through miRNA-based genetic engineering approaches.

Deciphering the alleviation potential of nitric oxide, for low temperature and chromium stress via maintaining photosynthetic capacity, antioxidant defence, and redox homeostasis in rice (Oryza sativa)

Sodium nitroprusside (SNP) is a potent nitric oxide (NO) donor that enhances plant tolerance to various abiotic stresses. This research aims to assess the effect of SNP application on rice seedlings subjected to individual and combined exposure to two abiotic stresses viz., low-temperature (LT) and chromium (Cr). Exposure to LT, Cr, and LT+Cr caused severe oxidative damage by stimulating greater production and accumulation of reactive oxygen species (ROS) leading to lipid peroxidation and cell membrane instability. The combined LT+CR stress more intensly increased the cellular oxidative stress and excessive Cr uptake that in turn deteriorated the chlorophyll pigments and photosynthesis, as well as effected the level of tetrapyrrole biosynthesis in rice plants. The reduction in rice seedling growth was more obvious under LT+Cr treatment than their individual effects. The exogenous application of SNP diminished the toxic impact of LT and Cr stress. This was attributed to the positive role of SNP in regulating the endogenous NO levels, free amino acids (FAAs) contents, tetrapyrrole biosynthesis and antioxidants. Consequently, SNP-induced NO decreased photorespiration, ROS generation, lipid peroxidation, and electrolyte leakage. Moreover, exogenous SNP diminished the Cr uptake and accumulation by modulating the ionic homeostasis and strengthening the heavy metals detoxification mechanism, thus improving plant height, biomass and photosynthetic indexes. Essentially, SNP boosts plant tolerance to LT and Cr stress by regulating antioxidants, detoxification mechanism, and the plant's physio-biochemical. Hence, applying SNP is an effective method for boosting rice plant resilience and productivity in the face of escalating environmental stresses and pollutants.

Isolation and identification of a new Bacillus glycinifermentans strain from date palm rhizosphere and its effect on barley seeds under heavy metal stress

Soil contamination by heavy metals is one of the major problems that adversely decrease plant growth and biomass production. Inoculation with the plant growth-promoting rhizobacteria (PGPR) can attenuate the toxicity of heavy metals and enhancing the plant growth. In this study, we evaluated the potential of a novel extremotolerant strain (IS-2 T) isolated from date palm rhizosphere to improve barley seedling growth under heavy metal stress. The species-level identification was carried out using morphological and biochemical methods combined with whole genome sequencing. The bacterial strain was then used in vitro for inoculating Hordeum vulgare L. exposed to three different Cr, Zn, and Ni concentrations (0.5, 1, and 2 mM) in petri dishes and different morphological parameters were assessed. The strain was identified as Bacillus glycinifermentans species. This strain showed high tolerance to pH (6-11), salt stress (0.2-2 M), and heavy metals. Indeed, the minimum inhibitory concentrations at which bacterium was unable to grow were 4 mM for nickel, 3 mM for zinc, more than 8 mM for copper, and 40 mM for chromium, respectively. It was observed that inoculation of Hordeum vulgare L. under metal stress conditions with Bacillus glycinifermentans IS-2 T stain improved considerably the growth parameters. The capacity of the IS-2 T strain to withstand a range of abiotic stresses and improve barley seedling development under lab conditions makes it a promising candidate for use as a PGPR in zinc, nickel, copper, and chromium bioremediation.

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.

New roles for Bacillus thuringiensis in the removal of environmental pollutants

For a long time, the well-known Gram-positive bacterium Bacillus thuringiensis (Bt) has been extensively studied and developed as a biological insecticide for Lepidoptera and Coleoptera pests due to its ability to secrete a large number of specific insecticidal proteins. In recent years, studies have found that Bt strains can also potentially biodegrade residual pollutants in the environment. Many researchers have isolated Bt strains from multiple sites polluted by exogenous compounds and characterized and identified their xenobiotic-degrading potential. Furthermore, its pathway for degradation was also investigated at molecular level, and a number of major genes/enzymes responsible for degradation have been explored. At present, a variety of xenobiotics involved in degradation in Bt have been reported, including inorganic pollutants (used in the field of heavy metal biosorption and recovery and precious metal recovery and regeneration), pesticides (chlorpyrifos, cypermethrin, 2,2-dichloropropionic acid, etc.), organic tin, petroleum and polycyclic aromatic hydrocarbons, reactive dyes (congo red, methyl orange, methyl blue, etc.), and ibuprofen, among others. In this paper, the biodegrading ability of Bt is reviewed according to the categories of related pollutants, so as to emphasize that Bt is a powerful agent for removing environmental pollutants.

Melatonin and strigolactone mitigate chromium toxicity through modulation of ascorbate-glutathione pathway and gene expression in tomato

Chromium (Cr) is considered one of the most hazardous metal contaminant reducing crop production and putting human health at risk. Phytohormones are known to regulate chromium stress, however, the function of melatonin and strigolactones in Chromium stress tolerance in tomato is rarely investigated. Here we investigated the potential role of melatonin (ML) and strigolactone (SL) on mitigating Chromium toxicity in tomato. With exposure to 300 μM Cr stress a remarkable decline in growth (63.01%), biomass yield (50.25)%, Pigment content (24.32%), photosynthesis, gas exchange and Physico-biochemical attributes of tomato was observed. Cr treatment also resulted in oxidative stress closely associated with higher H2O2 generation (215.66%), Lipid peroxidation (50.29%), electrolyte leakage (440.01%) and accumulation of osmolytes like proline and glycine betine. Moreover, Cr toxicity up-regulated the transcriptional expression profiles of antioxidant, stress related and metal transporter genes and down-regulated the genes related to photosynthesis. The application of ML and SL alleviated the Cr induced phytotoxic effects on photosynthetic pigments, gas exchange parameters and restored growth of tomato plants. ML and SL supplementation induced plant defense system via enhanced regulation of antioxidant enzymes, ascorbate and glutathione pool and transcriptional regulation of several genes. The coordinated regulation of antioxidant and glyoxalase systems expressively suppressed the oxidative stress. Hence, ML and SL application might be considered as an effective approach for minimizing Cr uptake and its detrimental effects in tomato plants grown in contaminated soils. The study may also provide new insights into the role of transcriptional regulation in the protection against heavy metal toxicity.

Microbe-Plant Interactions Targeting Metal Stress: New Dimensions for Bioremediation Applications

In the age of industrialization, numerous non-biodegradable pollutants like plastics, HMs, polychlorinated biphenyls, and various agrochemicals are a serious concern. These harmful toxic compounds pose a serious threat to food security because they enter the food chain through agricultural land and water. Physical and chemical techniques are used to remove HMs from contaminated soil. Microbial-metal interaction, a novel but underutilized strategy, might be used to lessen the stress caused by metals on plants. For reclaiming areas with high levels of heavy metal contamination, bioremediation is effective and environmentally friendly. In this study, the mechanism of action of endophytic bacteria that promote plant growth and survival in polluted soils-known as heavy metal-tolerant plant growth-promoting (HMT-PGP) microorganisms-and their function in the control of plant metal stress are examined. Numerous bacterial species, such as Arthrobacter, Bacillus, Burkholderia, Pseudomonas, and Stenotrophomonas, as well as a few fungi, such as Mucor, Talaromyces, Trichoderma, and Archaea, such as Natrialba and Haloferax, have also been identified as potent bioresources for biological clean-up. In this study, we additionally emphasize the role of plant growth-promoting bacteria (PGPB) in supporting the economical and environmentally friendly bioremediation of heavy hazardous metals. This study also emphasizes future potential and constraints, integrated metabolomics approaches, and the use of nanoparticles in microbial bioremediation for HMs.

Versatility of Stenotrophomonas maltophilia: Ecological roles of RND efflux pumps

S. maltophilia is a widely distributed bacterium found in natural, anthropized and clinical environments. The genome of this opportunistic pathogen of environmental origin includes a large number of genes encoding RND efflux pumps independently of the clinical or environmental origin of the strains. These pumps have been historically associated with the uptake of antibiotics and clinically relevant molecules because they confer resistance to many antibiotics. However, considering the environmental origin of S. maltophilia, the ecological role of these pumps needs to be clarified. RND efflux systems are highly conserved within bacteria and encountered both in pathogenic and non-pathogenic species. Moreover, their evolutionary origin, conservation and multiple copies in bacterial genomes suggest a primordial role in cellular functions and environmental adaptation. This review is aimed at elucidating the ecological role of S. maltophilia RND efflux pumps in the environmental context and providing an exhaustive description of the environmental niches of S. maltophilia. By looking at the substrates and functions of the pumps, we propose different involvements and roles according to the adaptation of the bacterium to various niches. We highlight that i°) regulatory mechanisms and inducer molecules help to understand the conditions leading to their expression, and ii°) association and functional redundancy of RND pumps and other efflux systems demonstrate their complex role within S. maltophilia cells. These observations emphasize that RND efflux pumps play a role in the versatility of S. maltophilia.

Chromium (VI) bioremoval from contaminated wastewater using Pseudomonas aeruginosa ATHA23 producing biofilm supported on clinoptilolite

More has yet to be investigated on the increased efficiency of microbes for the removal of heavy metals from industrial wastewaters. The objective was to determine the Cr (VI) bioabsorption and bioreduction ability of biofilm-producing bacteria supported on clinoptilolite from contaminated aqueous solutions. Chromium (VI)-tolerant bacteria, namely Pseudomonas aeruginosa ATHA23, were identified by biochemical methods and 16S rDNA sequencing and were deposited in NCBI (accession number: KF680991). Preparation of clinoptilolite, bacterial growth and isolation, biofilm production including extracellular polysaccharides (EPS) and Cr (VI) removal efficiency, affected by the experimental treatments, were investigated. The use of FTIR characterized clinoptilolite properties with and without biofilm in the presence and absence of Cr (IV). Higher Cr (VI) levels in the bacterial growth medium, increased EPS production with the highest value (0.171 mg L^-1), produced 18 h after treating the bacteria with Cr (VI) (100 mg L^-1). However, in the absence of Cr (VI), EPS significantly decreased to 0.117 mg L^-1. Plackett-Burman and Taguchi statistical analyses were used to optimize the experimental treatments affecting the removal efficiency of Cr (VI). Among the anions (nitrate, sulfate, and chloride), sulfate decreased Cr removal efficiency. The absorption data were best fitted to the pseudo-second order, and the data of Cr adsorption by clinoptilolite-biofilm were also better fitted to Freundlich isotherm model. The Cr (VI) bioremediation potential of P. aeruginosa ATHA23 by the production of biofilm supported on clinoptilolite has been shown for the first time, which is of significance for the environment and the industry.

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.

Mitigation of Negative Effects of Chromium (VI) Toxicity in Faba Bean (Vicia faba) Plants through the Supplementation of Kinetin (KN) and Gibberellic Acid (GA3)

The present study was carried out to explore the possible role of kinetin and gibberellic acid (GA3) on faba bean under chromium (Cr) stress. Cr treatment negatively affected growth and biomass production, reduced photosynthetic pigments, and inhibited photosynthesis, gas exchange parameters, antioxidant enzymes, and the glyoxylase cycle. Moreover, Cr stress enhanced the production of malondialdehyde (MDA, 216.11%) and hydrogen peroxide (H₂O₂, 230.16%), electrolyte leakage (EL, 293.30%), and the accumulation of proline and glycine betaine. Exogenous application of kinetin and GA3 increased growth and biomass, improved pigment contents and photosynthesis, as well as up-regulated the antioxidant system by improving the antioxidant enzyme activities and the content of nonenzymatic components, and the glyoxylase cycle. Additionally, kinetin and GA3 application displayed a considerable enhancement in proline (602.61%) and glycine betaine (423.72), which help the plants to maintain water balance under stress. Furthermore, a decline in Cr uptake was also observed due to kinetin and GA3 application. Exogenous application of kinetin and GA3 ameliorated the toxic effects of Cr in faba bean plants, up-shooting the tolerance mechanisms, including osmolyte metabolism and the antioxidant system.

The potential of Bacillus subtilis and phosphorus in improving the growth of wheat under chromium stress

AIM

Hexavalent chromium (Cr⁺⁶ ) is one of the most toxic heavy metals that have deteriorating effects on the growth and quality of the end product of wheat. Consequently, this research was designed to evaluate the role of Bacillus subtilis and phosphorus fertilizer on wheat facing Cr⁺⁶ stress.

METHODS AND RESULTS

The soil was incubated with Bacillus subtilis and phosphorus fertilizer before sowing. The statistical analysis of the data showed that the co-application of B. subtilis and phosphorus yielded considerably more significant (p < 0.05) results compared with an individual application of the respective treatments. The co-treatment improved the morphological, physiological and biochemical parameters of plants compared with untreated controls. The increase in shoot length, root length, shoot fresh weight and root fresh weight was 38.17%, 29.31%, 47.89% and 45.85%, respectively, compared with untreated stress-facing plants. The application of B. subtilis and phosphorus enhanced osmolytes content (proline 39.98% and sugar 41.30%), relative water content and stability maintenance of proteins (86.65%) and cell membranes (66.66%). Furthermore, augmented production of antioxidants by 67.71% (superoxide dismutase), 95.39% (ascorbate peroxidase) and 60.88% (catalase), respectively, were observed in the Cr⁺⁶ - stressed plants after co-application of B. subtilis and phosphorus.

CONCLUSION

It was observed that the accumulation of Cr⁺⁶ was reduced by 54.24%, 59.19% and 90.26% in the shoot, root and wheat grains, respectively. Thus, the combined application of B. subtilis and phosphorus has the potential to reduce the heavy metal toxicity in crops.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study explored the usefulness of Bacillus subtilis and phosphorus application on wheat in heavy metal stress. It is a step toward the combinatorial use of plant growth-promoting rhizobacteria with nutrients to improve the ecosystems' health.

Amelioration of Chromium-Induced Oxidative Stress by Combined Treatment of Selected Plant-Growth-Promoting Rhizobacteria and Earthworms via Modulating the Expression of Genes Related to Reactive Oxygen Species Metabolism in Brassica juncea

Chromium (Cr) toxicity leads to the enhanced production of reactive oxygen species (ROS), which are extremely toxic to the plant and must be minimized to protect the plant from oxidative stress. The potential of plant-growth-promoting rhizobacteria (PGPR) and earthworms in plant growth and development has been extensively studied. The present study was aimed at investigating the effect of two PGPR (Pseudomonas aeruginosa and Burkholderia gladioli) along with earthworms (Eisenia fetida) on the antioxidant defense system in Brassica juncea seedlings under Cr stress. The Cr toxicity reduced the fresh and dry weights of seedlings, enhanced the levels of superoxide anion (O₂•^-), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and electrolyte leakage (EL), which lead to membrane as well as the nuclear damage and reduced cellular viability in B. juncea seedlings. The activities of the antioxidant enzymes, viz., superoxide dismutase (SOD), guaiacol peroxidase (POD), ascorbate peroxidase (APOX), glutathione peroxidase (GPOX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were increased; however, a reduction was observed in the activity of catalase (CAT) in the seedlings under Cr stress. Inoculation of the PGPR and the addition of earthworms enhanced the activities of all other antioxidant enzymes except GPOX, in which a reduction of the activity was observed. For total lipid- and water-soluble antioxidants and the non-enzymatic antioxidants, viz., ascorbic acid and glutathione, an enhance accumulation was observed upon the inoculation with PGPR and earthworms. The supplementation of PGPR with earthworms (combined treatment) reduced both the reactive oxygen species (ROS) and the MDA content by modulating the defense system of the plant. The histochemical studies also corroborated that the combined application of PGPR and earthworms reduced O₂•^-, H₂O₂, lipid peroxidation, and membrane and nuclear damage and improved cell viability. The expression of key antioxidant enzyme genes, viz., SOD, CAT, POD, APOX, GR, DHAR, and GST showed the upregulation of these genes at post-transcriptional level upon the combined treatment of the PGPR and earthworms, thereby corresponding to the improved plant biomass. However, a reduced expression of RBOH1 gene was noticed in seedlings supplemented under the effect of PGPR and earthworms grown under Cr stress. The results provided sufficient evidence regarding the role of PGPR and earthworms in the amelioration of Cr-induced oxidative stress in B. juncea.

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.

Cadmium-Tolerant Plant Growth-Promoting Bacteria Curtobacterium oceanosedimentum Improves Growth Attributes and Strengthens Antioxidant System in Chili (Capsicum frutescens)

The remediation of potentially toxic element-polluted soils can be accomplished through the use of microbial and plant-assisted bioremediation. A total of 32 bacteria were isolated from soil samples contaminated with potentially toxic elements. The isolated bacterial strain DG-20 showed high tolerance to cadmium (up to 18 mM) and also showed bioaccumulative Cd removal properties, as demonstrated by atomic absorption spectroscopy studies. By sequencing the 16S rRNA gene, this strain was identified as Curtobacterium oceanosedimentum. Under stress and normal conditions, isolate DG-20 also produced a wide range of plant growth promoting traits, including ammonia production (51–73 µg/mL) and IAA production (116–183 µg/mL), alongside siderophore production and phosphate solubilization. Additionally, pot experiments were conducted to determine whether the strain could promote Chili growth when Cd salts are present. Over the control, bacterial colonization increased root and shoot lengths significantly up to 58% and 60%, respectively. Following inoculation with the Cd-tolerant strain, the plants also increased in both fresh and dry weight. In both the control and inoculated plants, Cd was accumulated more in roots than in shoots, indicating that Chili was phytostabilizing Cd levels. Besides improving the plant attributes, Cd-tolerant bacteria were also found to increase the amount of total chlorophyll, proline, total phenol, and ascorbic acid in the soil when added to the soil. These results suggest that the inoculant provides protection to plants from negative effects. The results of the present study predict that the combined properties of the tested strain in terms of Cd tolerance and plant growth promotion can be exploited for the purpose of the bioremediation of Cd, and for the improvement of Chili cultivation in soils contaminated with Cd.

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.

The auxin-producing Bacillus thuringiensis RZ2MS9 promotes the growth and modifies the root architecture of tomato (Solanum lycopersicum cv. Micro-Tom)

Strains of Bacillus thuringiensis (Bt) are commonly commercialized as bioinoculants for insect pest control, but their benefits go beyond their insecticidal property: they can act as plant growth-promoters. Auxins play a major role in the plant growth promotion. However, the mechanism of auxin production by the Bacilli group, and more specifically by Bt strains, is unclear. In previous work, the plant growth-promoting rhizobacterium (PGPR) B. thuringiensis strain RZ2MS9 increased the corn roots. This drew our attention to the strain's auxin production trait, earlier detected in vitro. Here, we demonstrate that in its genome, RZ2MS9 harbours the complete set of genes required in two pathways that are used for Indole acetic acid (IAA) production. We also detected that the strain produces almost five times more IAA during the stationary phase. The bacterial application increased the shoot dry weight of the Micro-Tom (MT) tomato by 24%. The application also modified MT root architecture, with an increase of 26% in the average lateral root length and inhibition of the axial root. At the cellular level, RZ2MS9-treated MT plants presented elongated root cortical cells with intensified mitotic activity. Altogether, these are the best characterized auxin-associated phenotypes. Besides that, no growth alteration was detected in the auxin-insensitive diageotropic (dgt) plants either with or without the RZ2MS9 inoculation. Our results suggest that auxins play an important role in the ability of B. thuringiensis RZ2MS9 to promote MT growth and provide a better understanding of the auxin production mechanism by a Bt strain.

Elsayed E, El Enshasy HA. Role of Bacillus cereus in Improving the Growth and Phytoextractability of Brassica nigra (L.) K. Koch in Chromium Contaminated Soil

Plant growth-promoting rhizobacteria (PGPR) mediate heavy metal tolerance and improve phytoextraction potential in plants. The present research was conducted to find the potential of bacterial strains in improving the growth and phytoextraction abilities of Brassica nigra (L.) K. Koch. in chromium contaminated soil. In this study, a total of 15 bacterial strains were isolated from heavy metal polluted soil and were screened for their heavy metal tolerance and plant growth promotion potential. The most efficient strain was identified by 16S rRNA gene sequencing and was identified as Bacillus cereus. The isolate also showed the potential to solubilize phosphate and synthesize siderophore, phytohormones (indole acetic acid, cytokinin, and abscisic acid), and osmolyte (proline and sugar) in chromium (Cr+3) supplemented medium. The results of the present study showed that chromium stress has negative effects on seed germination and plant growth in B. nigra while inoculation of B. cereus improved plant growth and reduced chromium toxicity. The increase in seed germination percentage, shoot length, and root length was 28.07%, 35.86%, 19.11% while the fresh and dry biomass of the plant increased by 48.00% and 62.16%, respectively, as compared to the uninoculated/control plants. The photosynthetic pigments were also improved by bacterial inoculation as compared to untreated stress-exposed plants, i.e., increase in chlorophyll a, chlorophyll b, chlorophyll a + b, and carotenoid was d 25.94%, 10.65%, 20.35%, and 44.30%, respectively. Bacterial inoculation also resulted in osmotic adjustment (proline 8.76% and sugar 28.71%) and maintained the membrane stability (51.39%) which was also indicated by reduced malondialdehyde content (59.53% decrease). The antioxidant enzyme activities were also improved to 35.90% (superoxide dismutase), 59.61% (peroxide), and 33.33% (catalase) in inoculated stress-exposed plants as compared to the control plants. B. cereus inoculation also improved the uptake, bioaccumulation, and translocation of Cr in the plant. Data showed that B. cereus also increased Cr content in the root (2.71-fold) and shoot (4.01-fold), its bioaccumulation (2.71-fold in root and 4.03-fold in the shoot) and translocation (40%) was also high in B. nigra. The data revealed that B. cereus is a multifarious PGPR that efficiently tolerates heavy metal ions (Cr+3) and it can be used to enhance the growth and phytoextraction potential of B. nigra in heavy metal contaminated soil.

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.

Chromium binding Bacillus cereus VITSH1-a promising candidate for heavy metal clean up

Bacteria survive metal stress by several mechanisms and metal binding is one such mechanism which has been screened in the present study to investigate the survival strategies of metal resistant bacteria. The production of siderophores, a metal chelating agent, was detected by chrome azurol S agar assay. The changes in cell wall studied by analysing the peptidoglycan and teichoic acid content indicated an increase in the cell wall content. Evaluation of morphological and physiological alterations like cell size, granularity analysed by SEM and flow cytometry analysis revealed an increase in cell size and granularity respectively. The transformation of phosphates monitored by ³¹ P NMR analysis indicated the presence of inorganic phosphate. Based on the cell wall changes and the ³¹ P NMR analysis, the surface charge of the organism was studied by zeta potential which displayed a difference at pH7.

Beneficial features of plant growth-promoting rhizobacteria for improving plant growth and health in challenging conditions: A methodical review

New eco-friendly approaches are required to improve plant biomass production. Beneficial plant growth-promoting (PGP) bacteria may be exploited as excellent and efficient biotechnological tools to improve plant growth in various - including stressful - environments. We present an overview of bacterial mechanisms which contribute to plant health, growth, and development. Plant growth promoting rhizobacteria (PGPR) can interact with plants directly by increasing the availability of essential nutrients (e.g. nitrogen, phosphorus, iron), production and regulation of compounds involved in plant growth (e.g. phytohormones), and stress hormonal status (e.g. ethylene levels by ACC-deaminase). They can also indirectly affect plants by protecting them against diseases via competition with pathogens for highly limited nutrients, biocontrol of pathogens through production of aseptic-activity compounds, synthesis of fungal cell wall lysing enzymes, and induction of systemic responses in host plants. The potential of PGPR to facilitate plant growth is of fundamental importance, especially in case of abiotic stress, where bacteria can support plant fitness, stress tolerance, and/or even assist in remediation of pollutants. Providing additional evidence and better understanding of bacterial traits underlying plant growth-promotion can inspire and stir up the development of innovative solutions exploiting PGPR in times of highly variable environmental and climatological conditions.

Mining the Microbiome of Key Species from African Savanna Woodlands: Potential for Soil Health Improvement and Plant Growth Promotion

(1) Aims: Assessing bacterial diversity and plant-growth-promoting functions in the rhizosphere of the native African trees Colophospermum mopane and Combretum apiculatum in three landscapes of the Limpopo National Park (Mozambique), subjected to two fire regimes. (2) Methods: Bacterial communities were identified through Illumina Miseq sequencing of the 16S rRNA gene amplicons, followed by culture dependent methods to isolate plant growth-promoting bacteria (PGPB). Plant growth-promoting traits of the cultivable bacterial fraction were further analyzed. To screen for the presence of nitrogen-fixing bacteria, the promiscuous tropical legume Vigna unguiculata was used as a trap host. The taxonomy of all purified isolates was genetically verified by 16S rRNA gene Sanger sequencing. (3) Results: Bacterial community results indicated that fire did not drive major changes in bacterial abundance. However, culture-dependent methods allowed the differentiation of bacterial communities between the sampled sites, which were particularly enriched in Proteobacteria with a wide range of plant-beneficial traits, such as plant protection, plant nutrition, and plant growth. Bradyrhizobium was the most frequent symbiotic bacteria trapped in cowpea nodules coexisting with other endophytic bacteria. (4) Conclusion: Although the global analysis did not show significant differences between landscapes or sites with different fire regimes, probably due to the fast recovery of bacterial communities, the isolation of PGPB suggests that the rhizosphere bacteria are driven by the plant species, soil type, and fire regime, and are potentially associated with a wide range of agricultural, environmental, and industrial applications. Thus, the rhizosphere of African savannah ecosystems seems to be an untapped source of bacterial species and strains that should be further exploited for bio-based solutions.

Long-Chain Hydrocarbons (C21, C24, and C31) Released by Bacillus sp. MH778713 Break Dormancy of Mesquite Seeds Subjected to Chromium Stress

Volatile organic compounds (VOCs) produced by rhizobacteria have been proven to stimulate plant growth during germination and seedling stages. However, the modulating effect of bacterial volatiles on the germination of seeds subjected to heavy metal stress is scarcely studied. In this work, the ability of volatiles released by Bacillus sp. MH778713 to induce seed dormancy breakage in Prosopis laevigata and Arabidopsis thaliana seeds were examined. The minimal inhibitory concentration of chromium (Cr) VI that prevents seed germination of P. laevigata and A. thaliana on water-Cr-agar plates was 2500 and 100 mg L^–1, respectively. Remarkably, partitioned Petri-dish co-cultivation of Bacillus sp. MH778713 and plant seeds under Cr-stress showed the beneficial effect of volatiles emitted by Bacillus sp. MH778713, helping plant seeds to overcome Cr-stress. Among the metabolites emitted by Bacillus sp. MH778713, octadecane, heneicosane, 2,4-di-tert-butylphenol, hexadecane, eicosane, octacosane, and tetratriacontane were the most abundant. To confirm that these long-chain compounds produced by Bacillus sp. MH778713 could be responsible for the seed dormancy breakage, high pure organic compounds (2,4-di-tert-butylphenol, heneicosane, hentriacontane, and tetracosane) were used directly in germination assays of P. laevigata and A. thaliana seeds instead of volatiles emitted by Bacillus sp. MH778713. All organic compounds allowed Prosopis and Arabidopsis seeds to overcome Cr-toxicity and germinate. The results of this study provide new insight into the role of long-chain bacterial compounds produced by Bacillus sp. MH778713 as triggers of seed abiotic stress tolerance, surmounting chromium stress and stimulating seedling development.