Mitigation of Nickel Toxicity and Growth Promotion in Sesame through the Application of a Bacterial Endophyte and Zeolite in Nickel Contaminated Soil

Divergent responses of phosphorus solubilizing bacteria with P-laden biochar for enhancing nutrient recovery, growth, and yield of canola (Brassica napus L.)

The growing global population has led to a heightened need for food production, and this rise in agricultural activity is closely tied to the application of phosphorus-based fertilizers, which contributes to the depletion of rock phosphate (RP) reserves. Considering the limited P reserves, different approaches were conducted previously for P removal from waste streams, while the adsorption of ions is a novel strategy with more applicability. In this study, a comprehensive method was employed to recover phosphorus from wastewater by utilizing biochar engineered with minerals such as calcium, magnesium, and iron. Elemental analysis of the wastewater following a batch experiment indicated the efficiency of the engineered biochar as an adsorbent. Subsequently, the phosphorus-enriched biochar, hereinafter (PL-BCsb), obtained from the wastewater, underwent further analysis through FTIR, XRD, and nutritional assessments. The results revealed that the PL-BCsb contained four times higher (1.82%) P contents which further reused as a fertilizer supplementation for Brassica napus L growth. PL-BCsb showed citric acid (34.03%), Olsen solution (10.99%), and water soluble (1.74%) P desorption. Additionally, phosphorous solubilizing bacteria (PSB) were incorporated with PL-BCsb along two P fertilizer levels P45 (45 kg ha-1) and P90 (90 kg ha-1) for evaluation of phosphorus reuse efficiency. Integrated application of PL-BCsb with half of the suggested amount of P45 (45 kg ha-1) and PSB increased growth, production, physiological, biochemical, and nutritional qualities of canola by almost two folds when compared to control. Similarly, it also improved soil microbial biomass carbon up to four times, alkaline and acid phosphatases activities both by one and half times respectively as compared to control P (0). Furthermore, this investigation demonstrated that waste-to-fertilizer technology enhanced the phosphorus fertilizer use efficiency by 55-60% while reducing phosphorus losses into water streams by 90%. These results have significant implications for reducing eutrophication, making it a promising approach for mitigating environmental pollution and addressing climate change.

Nickel (Ni) phytotoxicity and detoxification mechanisms: A review

Scientists studying the environment, physiology, and biology have been particularly interested in nickel (Ni) because of its dual effects (essentiality and toxicity) on terrestrial biota. It has been reported in some studies that without an adequate supply of Ni, plants are unable to finish their life cycle. The safest Ni limit for plants is 1.5 μg g^-1, while the limit for soil is between 75 and 150 μg g^-1. Ni at lethal levels harms plants by interfering with a variety of physiological functions, including enzyme activity, root development, photosynthesis, and mineral uptake. This review focuses on the occurrence and phytotoxicity of Ni with respect to growth, physiological and biochemical aspects. It also delves into advanced Ni detoxification mechanisms such as cellular modifications, organic acids, and chelation of Ni by plant roots, and emphasizes the role of genes involved in Ni detoxification. The discussion has been carried out on the current state of using soil amendments and plant-microbe interactions to successfully remediate Ni from contaminated sites. This review has identified potential drawbacks and difficulties of various strategies for Ni remediation, discussed the importance of these findings for environmental authorities and decision-makers, and concluded by noting the sustainability concerns and future research needs regarding Ni remediation.

Plant Growth Regulators with a Balanced Supply of Nutrients Enhance the Phytoextraction Efficiency of Parthenium hysterophorus for Cadmium in Contaminated Soil

Heavy metal contamination in soil, such as cadmium (Cd), poses a serious threat to global food security and human health. It must be managed using environmentally friendly and cost-effective technologies. Plants with high resistance to Cd stress and high biomass production could be potential candidates for the phytoremediation of Cd-contaminated soils to improve Cd phytoextraction. In this regard, the present study was carried out to determine the effect of gibberellic acid (GA₃), indole acetic acid (IAA), and fertilizers (N, P, and K) on Parthenium hysterophorus growth and biomass production as well as Cd phytoextraction capabilities. A pot experiment was conducted with various combinations of PGRs and fertilizers, with treatments arranged in five replicates using a completely randomized design. After harvesting, each plant was divided into various parts such as stems, roots, and leaves, and different growth, physiological, and biochemical parameters were recorded. Results showed that under Cd stress, growth, physiological, and biochemical parameters were all significantly decreased. With the combined application of plant growth regulators (GA₃ and IAA) and nutrients, Cd stress was alleviated and all parameters significantly improved. In comparison to the control treatment, the combined application of N + P + K + GA₃ + IAA resulted in the highest fresh and dry biomass production of the root (12.31 and 5.11 g pot^-1), shoot (19. 69 and 6.99 g pot^-1), leaves (16.56 and 7.09 g pot^-1), and entire plant (48.56 and 19.19 g pot^-1). Similarly, the same treatment resulted in higher chlorophyll a and b and total chlorophyll contents under Cd stress, which were 2.19, 2.03, and 3.21 times higher than the control, which was Cd stress without any treatment. The combination of N + P + K + GA₃ + IAA also resulted in the highest proline and phenolic contents. In the case of different enzyme activities, the combined application of N + P + K + GA₃ + IAA under Cd stress led to a high increase in catalase (2.5 times), superoxide (3.5 times), and peroxidase (3.7 times) compared to the control. With the combined application of N+ P+ K + GA₃ + IAA, the maximum values of BCF (8.25), BAC (2.6), and RF (5.14%) were measured for phytoextraction potential. On the basis of these findings, it is concluded that P. hysterophorus has a high potential to grow, produce the most biomass, and act as a Cd hyperaccumulator in Cd-contaminated soil.

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

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

Physiological and Biochemical Responses of Solanum lycopersicum L. to Benzo[a]pyrene Contaminated Soils

Benzo[a]pyrene (BaP) is noted as one of the main cancer-causing pollutants in human beings and may damage the development of crop plants. The present work was designed to explore more insights into the toxic effects of BaP on Solanum lycopersicum L. at various doses (20, 40, and 60 MPC) spiked in Haplic Chernozem. A dose-dependent response in phytotoxicity were noted, especially in the biomass of the roots and shoots, at doses of 40 and 60 MPC BaP and the accumulation of BaP in S. lycopersicum tissues. Physiological and biochemical response indices were severely damaged based on applied doses of BaP. During the histochemical analysis of the localization of superoxide in the leaves of S. lycopersicum, formazan spots were detected in the area near the leaf's veins. The results of a significant increase in malondialdehyde (MDA) from 2.7 to 5.1 times, proline 1.12- to 2.62-folds, however, a decrease in catalase (CAT) activity was recorded by 1.8 to 1.1 times. The activity of superoxide dismutase (SOD) increased from 1.4 to 2, peroxidase (PRX) from 2.3 to 5.25, ascorbate peroxidase (APOX) by 5.8 to 11.5, glutathione peroxidase (GP) from 3.8 to 7 times, respectively. The structure of the tissues of the roots and leaves of S. lycopersicum in the variants with BaP changed depending on the dose: it increased the intercellular space, cortical layer, and the epidermis, and the structure of the leaf tissues became looser.

Sustainable Soil Management: Effects of Clinoptilolite and Organic Compost Soil Application on Eco-Physiology, Quercitin, and Hydroxylated, Methoxylated Anthocyanins on Vitis vinifera

Climate change and compostinS1g methods have an important junction on the phenological and ripening grapevine phases. Moreover, the optimization of these composting methods in closed-loop corporate chains can skillfully address the waste problem (pomace, stalks, and pruning residues) in viticultural areas. Owing to the ongoing global warming, in many wine-growing regions, there has been unbalanced ripening, with tricky harvests. Excessive temperatures in fact impoverish the anthocyanin amount of the must while the serious water deficits do not allow a correct development of the berry, stopping its growth processes. This experiment was created to improve the soil management and the quality of the grapes, through the application of a new land conditioner (Zeowine) to the soil, derived from the compost processes of industrial wine, waste, and zeolite. Three treatments on a Sangiovese vineyard were conducted: Zeowine (ZW) (30 tons per ha), Zeolite (Z) (10 tons per ha), and Compost (C) (20 tons per ha). During the two seasons (2021-2022), measurements were made of single-leaf gas exchange and leaf midday water potential, as well as chlorophyll fluorescence. In addition, the parameters of plant yield, yeast assimilable nitrogen, technological maturity, fractionation of anthocyanins (Cyanidin-3-glucoside, Delphinidin-3-glucoside, Malvidin-3-acetylglucoside, Malvidin-3-cumarylglucoside, Malvidin-3-glucoside, Peonidin-3-acetylglucoside, Peonidin-3-cumarylglucoside, Peonidin-3-glucoside, and Petunidin-3-glucoside), Caffeic Acid, Coumaric Acid, Gallic Acid, Ferulic Acid, Kaempferol-3-O-glucoside, Quercetin-3-O-rutinoside, Quercetin-3-O-glucoside, Quercetin-3-O-galactoside, and Quercetin-3-O-glucuronide were analyzed. The Zeowine and zeolite showed less negative water potential, higher photosynthesis, and lower leaf temperature. Furthermore, they showed higher levels of anthocyanin accumulation and a lower level of quercetin. Finally, the interaction of the beneficial results of Zeowine (soil and grapevines) was evidenced by the embellishment of the nutritional and water efficiency, the minimizing of the need for fertilizers, the closure of the production cycle of waste material from the supply chain, and the improvement of the quality of the wines.

Synergistic impact of two autochthonous saprobic fungi (A. niger and T. pseudokoningii) on the growth, ionic contents, and metals uptake in Brassica juncea L. and Vigna radiata L. under tannery solid waste contaminated soil

Unrestricted disposal of tannery solid waste (TSW) into agricultural soils has resulted in the contamination of heavy metals (HMs) such as chromium (Cr) cadmium (Cd), Copper (Cu), and Zinc (Zn) along with the severe potential to degrade the environmental quality around the world. In the present study, a combined phyto- and myco-remediation strategy was evaluated to enhance the growth, ionic contents, and phytoextraction potential of Brassica juncea and Vigna radiata for HMs from TSW-contaminated soil. A pot experiment was conducted in the greenhouse using single or combined inoculation of Trichoderma pseudokoningii (Tp) and Aspergillus niger (An) in B. juncea and V. radiata under TSW-contaminated soil at different doses (0, 50, and 100%). The results showed that the growth parameters of both B. juncea and V. radiata were severely affected under 50 and 100% TSW treatment. The combined inoculation of both the fungal species ameliorated the positive impacts of 50 and 100% TSW application on growth and ionic contents accumulation in B. juncea and V. radiata. The combined application of An + Tp at 100% TSW enhanced the shoot length (87.8, 157.2%), root length (123.9, 120.6%), number of leaves (184.2, 175.0%), number of roots (104.7, 438.9%), and dry weight (179.4, 144.8%) of B. juncea and V. radiata, respectively as compared to control with any fungal treatment at 100% TSW. A single application of An at different doses of TSW enhanced the metal concentration in B. juncea, whereas Tp increased the concentration of the metals in V. radiata. The concentration of Cr in roots (196.2, 263.8%), shoots (342.4, 182.2%), Cu in roots (187.6, 137.0%), shoots (26.6, 76.0%), Cd in roots (245.2, 184.6%), shoots (142.1, 73.4%), Zn in roots (73.4, 57.5%), shoots (62.9, 57.6%), in B. juncea were increased by the application of An at 50 and 100% treatment levels of TSW, respectively compared to control (C). Moreover, the HMs (Cr, Cu, Cd, and Zn) uptake was also improved under 50 and 100% TSW with the combined inoculation of Tp + An in both B. juncea and V. radiata. In conclusion, the combined inoculation of Tp + An was more effective in metal removal from TSW-treated soil.NOVELTY STATEMENTLimited studies have been conducted on filamentous fungi systematically under metal-contaminated sites for their diversity, metal tolerance, and their potential in enhancing the phytoremediation potential of different crop plants.In the present study, single and/or combined inoculation of fungal strains was found effective in alleviating different metals stress in tannery solid waste contaminated soil by improving defense mechanisms and plant growth due to the association between fungal strains and plants.The combined application of both fungal strains had an additive effect in enhancing the bioaccumulation capacity of B. juncea and V. radiata compared to their single inoculation.

Assessment of Pb and Ni and potential health risks associated with the consumption of vegetables grown on the roadside soils in District Swat, Khyber Pakhtunkhwa, Pakistan

Vegetables cultivated near roads absorb toxic metals from polluted soil, which enter the human body through the food chain and cause serious health problems to humans. The present study investigated the concentration of lead (Pb) and nickel (Ni) in soils and vegetables grown along the roadside of District Swat, Pakistan, and the health risks associated with the consumption of the tested vegetables. In results, Pb concentration was higher in plants located at the distance between 0-10 m away from the roadside than the WHO permissible limit. In such plants, Pb concentration was higher than Ni. Rumex dentatus contained the highest concentration of Pb (75.63 mg kg^-1 DW) among the tested vegetables while Ni concentration (27.57 mg kg^-1 DW) was highest in Trachyspermum ammi as compared to other plants. Concentration and accumulation of both the metals decreased in soil and plants with increasing distance from the road. Similarly, target hazard quotient values noted for Pb (up to 3.37) were greater than unity, which shows that there is a potential risk associated with the consumption of tested vegetables near the road. Moreover, the values of target cancer risk (up to 0.8413) were greater than 0.0001, which shows that there is a risk of cancer with the consumption of tested vegetables. In conclusion, the consumption of tested vegetables was very dangerous as it may lead to higher risks of cancer. Strict regulatory control is recommended on the cultivation of these vegetables along the roadside to avoid any contamination due to roadside exhaust.

Plant-endophyte mediated improvement in physiological and bio-protective abilities of marigold (Tagetes patula)

Endophytic bacteria improve the growth, physiology, and metabolite profile of plants. They are known as potential biocontrol agents of soil-borne diseases. This study evaluated the effects of endophytic bacterial strains on growth, vase life, biochemical attributes, and antioxidant and nematicidal activities of French marigold (Tagetes patula). French marigold seeds were sole and consortium inoculated with three promising endophytic bacterial strains, Burkholderia phytofirmans (PsJN), Enterobacter sp. (MN17), and Bacillus sp. (MN54). The vase life of French marigold was promoted by 66.6% in the individual application of PsJN and 100% in plants treated with consortium compared to the uninoculated control. The shoot and root fresh weights were also increased by 65.9 and 68.7%, with the combined application of all three strains. The total phenolics, flavonoid, and protein contents were higher in consortium treatment with an increase of up to 38.0, 55.9, and 65.9%, respectively, compared to the uninoculated control. Furthermore, combined application of endophytic bacterial strains promoted DPPH radical scavenging, mortality of plant-parasitic nematodes, and ferric reducing antioxidant power activities with increase of up to 278.0, 103.8, and 178.0%, respectively, compared to uninoculated control. An increase in antioxidant activities of ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX), and superoxide dismutase (SOD) were observed up to 77.3, 86.0, 91.6, and 102.9%, respectively by combined application of endophytic bacterial strains. So, given the economic importance of floriculture crops, endophytic bacterial isolates studied here have shown a great potential for improving the productivity of cultivated ornamental French marigold.

Alleviation of Cadmium and Nickel Toxicity and Phyto-Stimulation of Tomato Plant L. by Endophytic Micrococcus luteus and Enterobacter cloacae

Cadmium (Cd) and nickel (Ni) are two of the most toxic metals, wreaking havoc on human health and agricultural output. Furthermore, high levels of Cd and Ni in the soil environment, particularly in the root zone, may slow plant development, resulting in lower plant biomass. On the other hand, endophytic bacteria offer great promise for reducing Cd and Ni. Moreover, they boost plants’ resistance to heavy metal stress. Different bacterium strains were isolated from tomato roots. These isolates were identified as Micrococcus luteus and Enterobacter cloacae using 16SrDNA and were utilized to investigate their involvement in mitigating the detrimental effects of heavy metal stress. The two bacterial strains can solubilize phosphorus and create phytohormones as well as siderophores. Therefore, the objective of this study was to see how endophytic bacteria (Micrococcus luteus and Enterobactercloacae) affected the mitigation of stress from Cd and Ni in tomato plants grown in 50 μM Cd or Ni-contaminated soil. According to the findings, Cd and Ni considerably lowered growth, biomass, chlorophyll (Chl) content, and photosynthetic properties. Furthermore, the content of proline, phenol, malondialdehyde (MDA), H₂O₂, OH, O₂, the antioxidant defense system, and heavy metal (HM) contents were significantly raised under HM-stress conditions. However, endophytic bacteria greatly improved the resistance of tomato plants to HM stress by boosting enzymatic antioxidant defenses (i.e., catalase, peroxidase, superoxide dismutase, glutathione reductase, ascorbate peroxidase, lipoxygenase activity, and nitrate reductase), antioxidant, non-enzymatic defenses, and osmolyte substances such as proline, mineral content, and specific regulatory defense genes. Moreover, the plants treated had a higher value for bioconcentration factor (BCF) and translocation factor (TF) due to more extensive loss of Cd and Ni content from the soil. To summarize, the promotion of endophytic bacterium-induced HM resistance in tomato plants is essentially dependent on the influence of endophytic bacteria on antioxidant capacity and osmoregulation.

Soil Risk Assessment in the Surrounding Area of Hulene-B Waste Dump, Maputo (Mozambique)

Soil contamination in areas close to unplanned dumpsites represents an increasing risk to the ecosystems and human health. This study aimed to evaluate soil quality in the area surrounding the Hulene-B waste dump, Maputo, Mozambique, and to estimate potential ecological and human health risks. A total of 71 surface soil samples were collected in the surrounding area of the dump, along with 10 samples in areas considered not impacted by the dump. Chemical and mineralogical analyses were performed using XRF and XRD. Quartz was the most abundant mineral phase, followed by feldspars, carbonates, clay minerals, and Fe oxides/hydroxides. Results showed a significant contribution to ecological degradation by PTE enrichment, ranked as Zn >> Cu > Cr > Zr > Pb > Ni > Mn. Carcinogenic risk for both children and adults was significant due to Pb soil content. Soil sample concentrations of Cr, Cu, Mn, Ni, Pb, Zn, and Zr, posing a risk especially in children, suggested the need for continuous monitoring, as well as the definition and implementation of mitigation measures.

Exogenous Caffeine (1,3,7-Trimethylxanthine) Application Diminishes Cadmium Toxicity by Modulating Physio-Biochemical Attributes and Improving the Growth of Spinach (Spinacia oleracea L.)

Leafy vegetables usually absorb and retain heavy metals more readily than most of the other crop plants, and thus contribute ≥70% of the total cadmium (Cd) intake of humans. Caffeine mediates plant growth and has proved to be beneficial against pathogens and insects. Therefore, it was hypothesized that foliar applications of caffeine could alter metabolism and reduce Cd toxicity in spinach (Spinacia oleracea L.). Seven-day old spinach seedlings were provided with Cd (0, 50, and 100 µM) stress. Caffeine (0, 5, or 10 mM) foliar spray was given twice at after 20 days of seeds germination with an interval of one week. In results, Cd stress reduced photosynthetic pigments biosynthesis, increased oxidative stress, imbalanced nutrient retention, and inhibited plant growth. On the other hand, the caffeine-treated spinach plants showed better growth owing to the enhanced biosynthesis of chlorophylls, better oxidative defense systems, and lower accumulation and transport of Cd within the plant tissues. Furthermore, caffeine application enhanced the accumulation of the proline and ascorbic acid, but reduced MDA and H2O2 contents and Cd in plant leaves, and ultimately improved mineral nutrition of spinach plants exposed to different Cd regimes. In conclusion, exogenous application of caffeine significantly diminishes Cd stress by modulating physiological, biochemical, and growth attributes of spinach (Spinacia oleracea L.)

The Impact of Biotic and Abiotic Stress Factors on Development of European Ash Tissue Cultures

Fraxinus excelsior L. is threatened by a variety of environmental factors causing a decline of the species. The most important biotic factors negatively affecting the condition of the F. excelsior population are fungi such as the pathogen Hymenoscyphus fraxineus. Abiotic factors with potentially harmful effect to the F. excelsior population are the accumulation of heavy metals and salinity in soils. Thus, the aim of this study was to investigate the impact of selected biotic and abiotic stress factors to determine which of them pose a threat to European ash. The study was conducted using in vitro techniques based on callus and seedlings regenerated via indirect organogenesis. Tissue cultures exclude the influence of other factors, including the environmental impact on ash extinction. The results confirmed very strong pathogenic potential of H. fraxineus in which after 14 days the callus tissue cells died as the tissue failed to activate its defense mechanisms. Experiments showed the high toxicity of cadmium in concentration of 0.027 mmol/L. Salinity caused the activity of oxidation enzymes to vary among seedlings and calluses in the control suggesting the enzymes play a role in controlling the morphogenetic development of tissue cultures.

Nickel stress-tolerance in plant-bacterial associations

Nickel (Ni) is an essential element for plant growth and is a constituent of several metalloenzymes, such as urease, Ni-Fe hydrogenase, Ni-superoxide dismutase. However, in high concentrations, Ni is toxic and hazardous to plants, humans and animals. High levels of Ni inhibit plant germination, reduce chlorophyll content, and cause osmotic imbalance and oxidative stress. Sustainable plant-bacterial native associations are formed under Ni-stress, such as Ni hyperaccumulator plants and rhizobacteria showed tolerance to high levels of Ni. Both partners (plants and bacteria) are capable to reduce the Ni toxicity and developed different mechanisms and strategies which they manifest in plant-bacterial associations. In addition to physical barriers, such as plants cell walls, thick cuticles and trichomes, which reduce the elevated levels of Ni entrance, plants are mitigating the Ni toxicity using their own antioxidant defense mechanisms including enzymes and other antioxidants. Bacteria in its turn effectively protect plants from Ni stress and can be used in phytoremediation. PGPR (plant growth promotion rhizobacteria) possess various mechanisms of biological protection of plants at both whole population and single cell levels. In this review, we highlighted the current understanding of the bacterial induced protective mechanisms in plant-bacterial associations under Ni stress.

Processed animal manure improves morpho-physiological and biochemical characteristics of Brassica napus L. under nickel and salinity stress

Soil contamination with readily soluble salts and heavy metals is a major challenge concerning sustainable crop production. The use of organic wastes in agriculture not only helps in waste reduction but also acts as a soil conditioner and bio-stimulant for enhancing crop growth. In this regard, a pot experiment was conducted to investigate the effect of raw and processed animal manure (AM) on the growth, yield, and physicochemical parameters of Brassica napus L. developed under salinity and Ni stress. The experiment comprised two salinity levels (1.05 and 8 dS m^-1), two Ni levels (0 and 50 mg kg^-1), and two types of AMs (raw and processed at a rate of 2% w/w). A control treatment without AM incorporation was also included. In results, the application of AM markedly increased the growth and yield of B. napus under Ni and salinity stress; at the same time, it improved the physiological and chemical parameters of the said crop. Similarly, incorporation of processed AM significantly improved nutrient uptake and decreased Na/K ratios in the shoot and grain under the different stress conditions, as compared to the control. Likewise, Ni uptake in the grain, shoot, and root samples was also significantly reduced under the AM treatment. Also, the application of AM significantly reduced the daily intake of metal (DIM) index and the health risk index (HRI) values under the different stress conditions, as compared to the control. In conclusion, the application of processed AM constitutes an effective agricultural strategy to alleviate the adverse effects of Ni and salinity stress on growth, physiology, and yield of B. napus, thus resulting in enhanced productivity, as well as reduced risks associated with human health.

Changes in the Composition of the Soil Bacterial Community in Heavy Metal-Contaminated Farmland

The structural changes of microorganisms in soil are the focus of soil indicators research. The purpose of this study was to investigate the changes in the composition of the soil bacterial community in heavy metal-contaminated soil. A total of six soil samples (two sampling times) were collected from contaminated farmland at three different depths (surface, middle, and deep layer). The pH value was measured. The concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd, and Pb) and the soil bacterial community were analyzed using ICP-OES and 16S rRNA gene sequencing. The results of the two samplings showed that the pH value in the deep layer decreased from 6.88 to 6.23, and the concentrations of Cu, Zn, Cd, and Pb, with a smaller ion radius, increased by 16-28%, and Shannon, Chao1 increased by ~13%. The bacteria community composition at the three depths changed, but Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla. In the copper and zinc tolerance test, the isolated bacterium that was able to tolerate copper and zinc was Bacillus sp. We found that, the longer the heavy metal pollution was of concern, the higher the tolerance. These results can be used as references for the microbial remediation of heavy metal-contaminated farmland.

Plant growth regulators and EDTA improve phytoremediation potential and antioxidant response of Dysphania ambrosioides (L.) Mosyakin & Clemants in a Cd-spiked soil

Soil pollution due to potentially toxic elements is a worldwide challenge for health and food security. Chelate-assisted phytoextraction along with the application of plant growth regulators (PGRs) could increase the phytoremediation efficiency of metal-contaminated soils. The present study was conducted to investigate the effect of different PGRs [Gibberellic acid (GA3) and indole acetic acid (IAA)] and synthetic chelator (EDTA) on growth parameters and Cd phytoextraction potential of Dysphania ambrosioides (L.) Mosyakin & Clemants grown under Cd-spiked soil. GA3 (10-7 M) and IAA (10-5 M) were applied four times with an interval of 10 days through a foliar spray, while EDTA (40 mg kg-1 soil) was once added to the soil. The results showed that Cd stress significantly decreased fresh biomass, dry biomass, total water contents, and photosynthetic pigments as compared to control. Application of PGRs significantly enhanced plant growth and Cd phytoextraction. The combined application of GA3 and IAA with EDTA significantly increased Cd accumulation (6.72 mg pot-1 dry biomass) and bioconcentration factor (15.21) as compared to C1 (Cd only). The same treatment significantly increased chlorophyll, proline, phenolic contents, and antioxidant activities (CAT, SOD, and POD) while MDA contents were reduced. In roots, Cd accumulation showed a statistically significant and positive correlation with proline, phenolics, fresh biomass, and dry biomass. Similarly, Cd accumulation showed a positive correlation with antioxidant enzyme activities in leaves. D. ambrosioides showed hyperaccumulation potential for Cd, based on bioconcentration factor (BCF) > 1. In conclusion, exogenous application of GA3 and IAA reduces Cd stress while EDTA application enhances Cd phytoextraction and ultimately the phytoremediation potential of D. ambrosioides.

Efficacy of Indole Acetic Acid and Exopolysaccharides-Producing Bacillus safensis Strain FN13 for Inducing Cd-Stress Tolerance and Plant Growth Promotion in Brassica juncea (L.)

Untreated wastewater used for irrigating crops is the major source of toxic heavy metals and other pollutants in soils. These heavy metals affect plant growth and deteriorate the quality of edible parts of growing plants. Phytohormone (IAA) and exopolysaccharides (EPS) producing plant growth-promoting rhizobacteria can reduce the toxicity of metals by stabilizing them in soil. The present experiment was conducted to evaluate the IAA and EPS-producing rhizobacterial strains for improving growth, physiology, and antioxidant activity of Brassica juncea (L.) under Cd-stress. Results showed that Cd-stress significantly decreased the growth and physiological parameters of mustard plants. Inoculation with Cd-tolerant, IAA and EPS-producing rhizobacterial strains, however, significantly retrieved the inhibitory effects of Cd-stress on mustard growth, and physiology by up regulating antioxidant enzyme activities. Higher Cd accumulation and proline content was observed in the roots and shoot tissues upon Cd-stress in mustard plants while reduced proline and Cd accumulation was recorded upon rhizobacterial strains inoculation. Maximum decrease in proline contents (12.4%) and Cd concentration in root (26.9%) and shoot (29%) in comparison to control plants was observed due to inoculation with Bacillus safensis strain FN13. The activity of antioxidant enzymes was increased due to Cd-stress; however, the inoculation with Cd-tolerant, IAA-producing rhizobacterial strains showed a non-significant impact in the case of the activity of superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT) in Brassica juncea (L.) plants under Cd-stress. Overall, Bacillus safensis strain FN13 was the most effective strain in improving the Brassica juncea (L.) growth and physiology under Cd-stress. It can be concluded, as the strain FN13 is a potential phytostabilizing biofertilizer for heavy metal contaminated soils, that it can be recommended to induce Cd-stress tolerance in crop plants.

Isolation and Characterization of Oil-Degrading Enterobacter sp. from Naturally Hydrocarbon-Contaminated Soils and Their Potential Use against the Bioremediation of Crude Oil

The contamination of crude oil in soil matrices is a persistent problem with negative repercussions because of the recalcitrant, hazardous, and mutagenic properties of its constituents. To mitigate the effect of crude oil contamination in soil, the use of microorganisms is a cheap and feasible option. In the current study, bacterial species from numerous polluted oil field surfaces were isolated and examined for their ability to degrade crude oil. Random soil samples polluted with hydrocarbons were collected and various bacterial isolates were isolated. Results revealed that 40% of total isolates had potential use for hydrocarbon biodegradation, the synthesis of exopolysaccharides and the solubilization of phosphorous. Following isolation and characterization to degrade crude oil, a pot trial was conducted using maize inoculated with the four best strains—i.e., S1 (PMEL-63), S2 (PMEL-67), S3 (PMEL-80), and S4 (PMEL-79)—in artificially hydrocarbon-polluted soil with concentrations of crude oil of 0, 1000, and 2000 ppm. Results revealed that S4 (PMEL-79) had significant potential to degrade hydrocarbon in polluted soils. The root length, shoot length, and fresh biomass of maize were increased by 65%, 45%, and 98%, respectively, in pots inoculated with S4 (PMEL-79) Enterobacter cloacae subsp., whereas the lowest root length was observed where no strain was added and the concentration of crude oil was at maximum. Moreover, S4 (PMEL-79) Enterobacter cloacae subsp. was found to be the most effective strain in degrading crude oil and increasing maize growth under polluted soil conditions. It was concluded that the isolation of microorganisms from oil-contaminated sites should be considered in order to identify the most effective microbial consortium for the biodegradation of naturally hydrocarbon-contaminated soils.

Insights into conventional and recent technologies for arsenic bioremediation: A systematic review

Arsenic (As) bioremediation has been an economical and sustainable approach, being practiced widely under several As-contaminated environments. Bioremediation of As involves the use of bacteria, fungi, yeast, plants, and genetically modified organisms for detoxification/removal of As from the contaminated site. The understanding of multi-factorial biological components involved in these approaches is complex and more and more efforts are on their way to make As bioremediation economical and efficient. In this regard, we systematically reviewed the recent literature (n=200) from the last two decades regarding As bioremediation potential of conventional and recent technologies including genetically modified plants for phytoremediation and integrated approaches. Also, the responsible mechanisms behind different approaches have been identified. From the literature, it was found that As bioremediation through biosorption, bioaccumulation, phytoextraction, and volatilization involving As-resistant microbes has proved a very successful technology. However, there are various pathways of As tolerance of which the mechanisms have not been fully understood. Recently, phytosuction separation technology has been introduced and needs further exploration. Also, integrated approaches like phytobial, constructed wetlands using As-resistant bacteria with plant growth-promoting activities have not been extensively studied. It is speculated that the integrated bioremediation approaches with practical applicability and reliability would prove most promising for As remediation. Further technological advancements would help explore the identified research gaps in different approaches and lead us toward sustainability and perfection in As bioremediation.

Salicylic Acid Improves Boron Toxicity Tolerance by Modulating the Physio-Biochemical Characteristics of Maize (Zea mays L.) at an Early Growth Stage

The boron (B) concentration surpasses the plant need in arid and semi-arid regions of the world, resulting in phyto-toxicity. Salicylic acid (SA) is an endogenous signaling molecule responsible for stress tolerance in plants and is a potential candidate for ameliorating B toxicity. In this study, the effects of seed priming with SA (0, 50, 100 and 150 µM for 12 h) on the growth, pigmentation and mineral concentrations of maize (Zea mays L.) grown under B toxicity were investigated. One-week old seedlings were subjected to soil spiked with B (0, 15 and 30 mg kg−1 soil) as boric acid. Elevating concentrations of B reduced the root and shoot length, but these losses were significantly restored in plants raised from seeds primed with 100 µM of SA. The B application decreased the root and shoot fresh/dry biomasses significantly at 30 mg kg−1 soil. The chlorophyll and carotenoid contents decreased with increasing levels of B, while the contents of anthocyanin, H2O2, ascorbic acid (ASA) and glycinebetaine (GB) were enhanced. The root K and Ca contents were significantly increased, while a reduction in the shoot K contents was recorded. The nitrate concentration was significantly higher in the shoot as compared to the root under applied B toxic regimes. However, all of these B toxicity effects were diminished with 100 µM SA applications. The current study outcomes suggested that the exogenously applied SA modulates the response of plants grown under B toxic conditions, and hence could be used as a plant growth regulator to stimulate plant growth and enhance mineral nutrient uptake under B-stressed conditions.

Peptone-Induced Physio-Biochemical Modulations Reduce Cadmium Toxicity and Accumulation in Spinach (Spinacia oleracea L.)

The accumulation of cadmium (Cd) in edible plant parts and fertile lands is a worldwide problem. It negatively influences the growth and productivity of leafy vegetables (e.g., spinach, Spinacia oleracea L.), which have a high tendency to radially accumulate Cd. The present study investigated the influences of peptone application on the growth, biomass, chlorophyll content, gas exchange parameters, antioxidant enzymes activity, and Cd content of spinach plants grown under Cd stress. Cd toxicity negatively affected spinach growth, biomass, chlorophyll content, and gas exchange attributes. However, it increased malondialdehyde (MDA), hydrogen peroxide (H₂O₂), electrolyte leakage (EL), proline accumulation, ascorbic acid content, Cd content, and activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in spinach plants. The exogenous foliar application of peptone increased the growth, biomass, chlorophyll content, proline accumulation, and gas exchange attributes of spinach plants. Furthermore, the application of peptone decreased Cd uptake and levels of MDA, H₂O₂, and EL in spinach by increasing the activity of antioxidant enzymes. This enhancement in plant growth and photosynthesis might be due to the lower level of Cd accumulation, which in turn decreased the negative impacts of oxidative stress in plant tissues. Taken together, the findings of the study revealed that peptone is a promising plant growth regulator that represents an efficient approach for the phytoremediation of Cd-polluted soils and enhancement of spinach growth, yield, and tolerance under a Cd-dominant environment.