Significance of Bacillus subtilis strain SJ-101 as a bioinoculant for concurrent plant growth promotion and nickel accumulation in Brassica juncea

Two cadmium-resistant bacteria Burkholderia contaminans HA09 and Arthrobacter humicola improve phytoremediation efficiency of cadmium in Ageratum conyzoides L

To investigate the impact and mechanism of Cd-tolerant bacteria in soil on promoting Cd accumulation in Ageratum conyzoides L., we verified the impact of inoculating two strains, B-1 (Burkholderia contaminans HA09) and B-7 (Arthrobacter humicola), on Cd accumulation in A. conyzoides through a pot experiment. Additionally, we investigated the dissolution of CdCO3 and nutrient elements, as well as the release of indoleacetic acid (IAA) by the two strains. The results showed that both strains can significantly improve the dissolution of CdCO3. Strains B-1 and B-7 had obvious effect of dissolving phosphorus, which was 5.63 and 2.76 times higher than that of the control group, respectively. Strain B-7 had significant effect of dissolution potassium, which was 1.79 times higher than that of the control group. Strains B-1 and B-7 had significant nitrogen fixation effect, which was 29.53 and 44.39 times higher than that of the control group, respectively. In addition, inoculating with strain B-1 and B-7 significantly increased the Cd extraction efficiency of A. conyzoides (by 114% and 45% respectively) through enhancing Cd accumulation and the biomass of A. conyzoides. Furthermore, the inoculation of strain B-1 and B-7 led to a significant increase in the activities of CAT and SOD, as well as the content of chlorophyll a and total chlorophyll in the leaves of A. conyzoides. To sum up, strain B-1 and B-7 can promote the phytoremediation efficiency of A. conyzoides on Cd by promoting the biomass and Cd accumulation of A. conyzoides.

Recent advances and mechanisms of microbial bioremediation of nickel from wastewater

The global concern over emerging pollutants, characterized by their low concentrations and high toxicity, necessitates effective remediation strategies. Among these pollutants, pharmaceutical and personal care products, pesticides, surfactants, and persistent organic pollutants have gained significant attention. These contaminants are extensively distributed within aquatic ecosystems, posing threats to both human and aquatic physiological systems. Nickel, a valuable metal renowned for its corrosion-resistant properties, is widely utilized in various industrial processes, leading to the generation of nickel-containing waste streams, including batteries, catalysts, wastewater, and electrolyte bleed-off. Contamination of soil, water, or air by these waste materials can have adverse effects on the environment and human health. This review article focuses on the recent advancements in environmental and economic implications associated with the removal of nickel from diverse waste sources. Physicochemical technologies employed for treating different nickel-containing effluents and wastewater are discussed, alongside bioremediation techniques and the underlying mechanisms by which microorganisms facilitate nickel removal. The recovery of nickel from waste materials holds paramount importance not only from an economic standpoint but also to mitigate environmental impacts.

Bacterial indole-3-acetic acid: A key regulator for plant growth, plant-microbe interactions, and agricultural adaptive resilience

Indole-3-acetic acid (IAA), a fundamental phytohormone categorized under auxins, not only influences plant growth and development but also plays a critical role in plant-microbe interactions. This study reviews the role of IAA in bacteria-plant communication, with a focus on its biosynthesis, regulation, and the subsequent effects on host plants. Bacteria synthesize IAA through multiple pathways, which include the indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), and several other routes, whose full mechanisms remain to be fully elucidated. The production of bacterial IAA affects root architecture, nutrient uptake, and resistance to various abiotic stresses such as drought, salinity, and heavy metal toxicity, enhancing plant resilience and thus offering promising routes to sustainable agriculture. Bacterial IAA synthesis is regulated through complex gene networks responsive to environmental cues, impacting plant hormonal balances and symbiotic relationships. Pathogenic bacteria have adapted mechanisms to manipulate the host's IAA dynamics, influencing disease outcomes. On the other hand, beneficial bacteria utilize IAA to promote plant growth and mitigate abiotic stresses, thereby enhancing nutrient use efficiency and reducing dependency on chemical fertilizers. Advancements in analytical methods, such as liquid chromatography-tandem mass spectrometry, have improved the quantification of bacterial IAA, enabling accurate measurement and analysis. Future research focusing on molecular interactions between IAA-producing bacteria and host plants could facilitate the development of biotechnological applications that integrate beneficial bacteria to improve crop performance, which is essential for addressing the challenges posed by climate change and ensuring global food security. This integration of bacterial IAA producers into agricultural practice promises to revolutionize crop management strategies by enhancing growth, fostering resilience, and reducing environmental impact.

Role of Plant-Growth-Promoting Rhizobacteria in Plant Machinery for Soil Heavy Metal Detoxification

Heavy metals migrate easily and are difficult to degrade in the soil environment, which causes serious harm to the ecological environment and human health. Thus, soil heavy metal pollution has become one of the main environmental issues of global concern. Plant-growth-promoting rhizobacteria (PGPR) is a kind of microorganism that grows around the rhizosphere and can promote plant growth and increase crop yield. PGPR can change the bioavailability of heavy metals in the rhizosphere microenvironment, increase heavy metal uptake by phytoremediation plants, and enhance the phytoremediation efficiency of heavy-metal-contaminated soils. In recent years, the number of studies on the phytoremediation efficiency of heavy-metal-contaminated soil enhanced by PGPR has increased rapidly. This paper systematically reviews the mechanisms of PGPR that promote plant growth (including nitrogen fixation, phosphorus solubilization, potassium solubilization, iron solubilization, and plant hormone secretion) and the mechanisms of PGPR that enhance plant-heavy metal interactions (including chelation, the induction of systemic resistance, and the improvement of bioavailability). Future research on PGPR should address the challenges in heavy metal removal by PGPR-assisted phytoremediation.

Unraveling lipid peroxidation-mediated regulation of redox homeostasis for sustaining plant health

Lipid peroxidation (LPO) is a complex process that, depending on the context, can either result in oxidative injury or promote redox homeostasis. LPO is a series of reactions in which polyunsaturated fatty acids are attacked by free radicals that result in the synthesis of lipid peroxides. LPO can alter membrane fluidity and operation and produce secondary products that amplify oxidative stress. LPO can activate cellular signaling pathways that promote antioxidant defense mechanisms that provide oxidative stress protection by elevating antioxidant enzyme action potentials. Enzymatic and nonenzymatic mechanisms tightly regulate LPO to prevent excessive LPO and its adverse consequences. This article emphasizes the dual nature of LPO as a mechanism that can both damage cells and regulate redox homeostasis. In addition, it also highlights the major enzymatic and nonenzymatic mechanisms that tightly regulate LPO to prevent excessive oxidative damage. More importantly, it emphasizes the importance of understanding the cellular and biochemical complexity of LPO for developing strategies targeting this process for efficient management of plant stress.

The Viable but Non-Culturable (VBNC) State, a Poorly Explored Aspect of Beneficial Bacteria

Many bacteria have the ability to survive in challenging environments; however, they cannot all grow on standard culture media, a phenomenon known as the viable but non-culturable (VBNC) state. Bacteria commonly enter the VBNC state under nutrient-poor environments or under stressful conditions. This review explores the concept of the VBNC state, providing insights into the beneficial bacteria known to employ this strategy. The investigation covers different chemical and physical factors that can induce the latency state, cell features, and gene expression observed in cells in the VBNC state. The review also covers the significance and applications of beneficial bacteria, methods of evaluating bacterial viability, the ability of bacteria to persist in environments associated with higher organisms, and the factors that facilitate the return to the culturable state. Knowledge about beneficial bacteria capable of entering the VBNC state remains limited; however, beneficial bacteria in this state could face adverse environmental conditions and return to a culturable state when the conditions become suitable and continue to exert their beneficial effects. Likewise, this unique feature positions them as potential candidates for healthcare applications, such as the use of probiotic bacteria to enhance human health, applications in industrial microbiology for the production of prebiotics and functional foods, and in the beer and wine industry. Moreover, their use in formulations to increase crop yields and for bacterial bioremediation offers an alternative pathway to harness their beneficial attributes.

Research progress and hotspots on microbial remediation of heavy metal-contaminated soil: a systematic review and future perspectives

Microbial remediation technology has received much attention as a green, ecological, and inexpensive technology, and there is great potential for the application of microbial remediation technology for heavy metals (HMs) contaminated soil alone and in conjunction with other technologies in environmental remediation. To gain an in-depth understanding of the latest research progress, research hotspots, and development trends on microbial remediation of HMs-contaminated soil, and to objectively reflect the scientific contributions and impacts of relevant countries/regions, institutions, and individuals of this field, in this manuscript, ISI Web of Knowledge's Web of Science™ core collection database, data visualization, and analysis software Bibliometrix, VOSviewer, and HistCite Pro were used to collect and analyze the relevant literature from 2000 to 2022, and 1409 publications were subjected to scientometric analyses. It involved 327 journals, 5150 authors, 75 countries/regions, and 2740 keywords. The current progress and hotspots on microbial remediation of HMs-contaminated soil since the twenty-first century were analyzed in terms of the top 10 most productive countries (regions), high-yielding authors, source journals, important research institutions, and hotspots of research directions. Over the past 22 years, China, India, and the USA have been the countries with the most articles. The institution and author with the most publications are the Chinese Acad Sci and Zhu YG, respectively. Journal of Hazardous Materials is the most productive journal. The keywords showed 6 co-occurrence clusters. These findings revealed the research hotspots, knowledge gaps, and future exploration trends related to microbial remediation of HMs-contaminated soil.

Biofilm-forming plant growth-promoting rhizobacterial consortia isolated from mines and dumpsites assist green remediation of toxic metal (Ni and Pb) using Brassica juncea

To study how biofilm-forming rhizobacteria isolated from mines and dumpsites improved the phytoremediation efficacy of B. juncea in metal-contaminated soil. Out of 91 isolates, six were chosen for research based on their tolerance to metals, and their efficient PGPR properties, and subjected to the design of a consortium. A compatibility study revealed no antagonistic interaction between rhizobacterial-consortiums. The results of the biofilm formation and FEG-SEM studies revealed that a consortium-BC8 formed a strong biofilm on the root surface of B. juncea seedlings. Based on results obtained with the phytoextraction efficiency of B. juncea in consortium-BC8 (SMHMZ46 and SMHMP23), they were identified as Klebsiella variicola and Pseudomonas otitidis, respectively, and submitted to NCBI GenBank with accession numbers MZ145092 and OK560623. This rhizobacteria is the first to be reported as assisting Ni and Pb phytoremediation by employing B. juncea. Soil inoculation with consortium-BC8 increased the amount of soluble Ni and Pb by 13.25-fold and 10.69-fold, respectively, when compared to the control. These consortiums-BC8 significantly increased vegetative growth and metal accumulation in root and shoot with a translocation-factor of 1.58 for Ni and soil to root with a bioconcentration-factor of 1.3 for Pb in B. juncea grown in individual soil contamination with 96.05 mg/kg NiCl2 and 89.63 mg/kg Pb(NO3)2, which are significantly higher than other consortium treatments and the non-inoculated control. B. juncea amendments with a biofilm-forming consortium-BC8 having TF, BCF, and BAC > 1 for Ni, whereas BCF > 1, TF, and BAC < 1 for Pb, are appropriate for green remediation of Ni and phytostabilization of Pb.

Plant growth-promoting and heavy metal-resistant Priestia and Bacillus strains associated with pioneer plants from mine tailings

Open mine tailings dams are extreme artificial environments containing sizeable potentially toxic elements (PTEs), including heavy metals (HMs), transition metals, and metalloids. Furthermore, these tailings have nutritional deficiencies, including assimilable phosphorus sources, organic carbon, and combined nitrogen, preventing plant colonization. Bacteria, that colonize these environments, have mechanisms to tolerate the selective pressures of PTEs. In this work, several Priestia megaterium (formerly Bacillus megaterium), Bacillus mojavensis, and Bacillus subtilis strains were isolated from bulk tailings, anthills, rhizosphere, and endosphere of pioneer plants from abandoned mine tailings in Zacatecas, Mexico. Bacillus spp. tolerated moderate HMs concentrations, produced siderophores and indole-3-acetic acid (IAA), solubilized phosphates, and reduced acetylene in the presence of HMs. The strains harbored different PIB-type ATPase genes encoding for efflux pumps and Cation Diffusion Facilitator (CDF) genes. Moreover, nifH and nifD nitrogenase genes were detected in P. megaterium and B. mojavensis genomic DNA. They showed similarity with sequences of the beta-Proteobacteria species, which may represent likely horizontal transfer events. These Bacillus species precede the colonization of mine tailings by plants. Their phenotypic and genotypic features could be essential in the natural recovery of the sites by reducing the oxidative stress of HMs, fixing nitrogen, solubilizing phosphate, and accumulating organic carbon. These traits of the strains reflect the adaptations of Bacillus species to the mine tailings environment and could contribute to the success of phytoremediation efforts.

Mesorhizobium improves chickpea growth under chromium stress and alleviates chromium contamination of soil

Environmental pollution has become a transnational issue that impacts ecosystems, soil, water, and air and is directly related to human health and well-being. Chromium pollution decreases the development of plant and microbial populations. It warrants the need to remediate chromium-contaminated soil. Decontaminating chromium-stressed soils via phytoremediation is a cost-effective and environmentally benign method. Using multifunctional plant growth-promoting rhizobacteria (PGPR) lower chromium levels and facilitates chromium removal. PGPR work by altering root architecture, secreting chemicals that bind metals in the rhizosphere, and reducing phytotoxicity brought on by chromium. The present study aimed to investigate the chromium bioremediation capacity of metal-tolerant PGPR isolate while promoting the growth of chickpeas in the presence of varying levels of chromium (15.13, 30.26, and 60.52 mg/kg of chromium). The isolate, Mesorhizobium strain RC3, substantially reduced chromium content (60.52 mg/kg) in the soil. It enhanced the root length by 10.87%, the shoot length by 12.38%, the number of nodules by 6.64%, and nodule dry weight by 13.77% at 90 days. After 135 days of sowing, more improvement in the root length (18.05), shoot length (21.60%)the chlorophyll content (6.83%), leghaemoglobin content (9.47%), and the highest growth in the crop seed yield (27.45%) and crop protein content (16.83%)The isolate reduced chromium accumulation in roots, shoots, and grains chickpea. Due to chromium bioremediation and its plant growth-promoting and chromium-attenuating qualities, Mesorhizobium strain RC3 could be used as a green bioinoculant for plant growth promotion under chromium stress.

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.

Endophytic Plant Growth-Promoting Bacterium Bacillus subtilis Reduces the Toxic Effect of Cadmium on Wheat Plants

Heavy metal ions, in particular cadmium (Cd), have a negative impact on the growth and productivity of major crops, including wheat. The use of environmentally friendly approaches, in particular, bacteria that have a growth-stimulating and protective effect, can increase the resistance of plants. The effects of the pre-sowing seed treatment with the plant growth-promoting endophyte Bacillus subtilis 10-4 (BS) on cadmium acetate (Cd)-stressed Triticum aestivum L. (wheat) growth, photosynthetic pigments, oxidative stress parameters, roots' lignin content, and Cd ions accumulation in plants were analyzed. The results showed that the tested Cd-tolerant BS improved the ability of wheat seeds to germinate in the presence of different Cd concentrations (0, 0.1, 0.5, and 1 mM). In addition, the bacterial treatment significantly decreased the damaging effects of Cd stress (1 mM) on seedlings' linear dimensions (lengths of roots and shoots), biomass, as well as on the integrity and permeability of the cell walls (i.e., lipid peroxidation and electrolyte leakage) and resulted in reduced H₂O₂ generation. The pretreatment with BS prevented the Cd-induced degradation of the leaf photosynthetic pigments chlorophyll (Chl) a, Chl b, and carotenoids. Moreover, the bacterial treatment intensified the lignin deposition in the roots under normal and, especially, Cd stress conditions, thereby enhancing the barrier properties of the cell wall. This manifested in a reduced Cd ions accumulation in the roots and in the restriction of its translocation to the aboveground parts (shoots) of the bacterized plants under Cd stress in comparison with non-bacterized controls. Thus, the pre-sowing seed treatment with the endophyte BS may serve as an eco-friendly approach to improve wheat production in Cd-contaminated areas.

Isolation and characterization of a plant growth‑promoting rhizobacterium strain MD36 that promotes barley seedlings and growth under heavy metals stress

Plant growth, promoting, bacteria, (PGPB) can improve plant germination and growth in heavy metal-contaminated land and enhance heavy metal removal efficiency. In this study, we isolated PGPR bacterial strains which can withstand heavy metal pollution and tested their ability to improve barley germination under heavy metal stress. Out of 16 multi-resistant heavy metal isolates, strain MD36 was identified by 16S rRNA sequencing and shared close relation to different species of the genus Glutamicibacter. The new isolated strain showed other important PGPR activities, mainly IAA production and salt tolerance. The effect of adding the strain MD36 to barley grains under heavy metal stress enhanced their germination up to 100%, while the percentage of germination ranged between 0 and 20% for non-inoculated grains. In addition, in these conditions, MD36 can significantly enhance barley growth by reducing the heavy metal effect. This study strongly recommends the use of MD36 as seed coatings trials in the field to enhance growth and yield in soils contaminated with heavy metals, as well as in bioremediation of HM-contaminated salt-containing soils and water.

Biosorption and Symbiotic Potential of Horse Gram Rhizobia in Soils Contaminated with Cobalt

The current study aims evaluation of biosorption and symbiotic potential of horse gram plants associated with rhizobia inspite of Cobalt (Co) metal stress, and these rhizobia strains play a pivotal role in the phytoremediation of Co heavy metal-contaminated soils. Horse gram rhizobial isolates HGR-4, HGR-6, HGR-13 and HGR-25 were able to tolerate 1000 µg g^-1 Co supplemented in culture media and also 100 µg g^-1 in Co supplemented soil. The plants nodulated with the isolates from the study have shown higher nodulation, nitrogen and leghaemoglobin content in the potted experiment on par with the control plants. Atomic absorption spectroscopic analysis of Co content in horse gram plants inoculated with these four isolates showed maximum biosorption of Co among the bacterial root nodules. Application of these strains can be potentially aid the phytoextraction of Co from contaminated soils on association with horse gram plants.

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.

Bioremediation of Heavy Metals by the Genus Bacillus

Environmental contamination with heavy metals is one of the major problems caused by human activity. Bioremediation is an effective and eco-friendly approach that can reduce heavy metal contamination in the environment. Bioremediation agents include bacteria of the genus Bacillus, among others. The best-described species in terms of the bioremediation potential of Bacillus spp. Are B. subtilis, B. cereus, or B. thuringiensis. This bacterial genus has several bioremediation strategies, including biosorption, extracellular polymeric substance (EPS)-mediated biosorption, bioaccumulation, or bioprecipitation. Due to the above-mentioned strategies, Bacillus spp. strains can reduce the amounts of metals such as lead, cadmium, mercury, chromium, arsenic or nickel in the environment. Moreover, strains of the genus Bacillus can also assist phytoremediation by stimulating plant growth and bioaccumulation of heavy metals in the soil. Therefore, Bacillus spp. is one of the best sustainable solutions for reducing heavy metals from various environments, especially soil.

Synthesis and characterization of copper oxide nanoparticles: its influence on corn (Z. mays) and wheat (Triticum aestivum) plants by inoculation of Bacillus subtilis

Nanotechnology is now playing an emerging role in green synthesis in agriculture as nanoparticles (NPs) are used for various applications in plant growth and development. Copper is a plant micronutrient; the amount of copper oxide nanoparticles (CuONPs) in the soil determines whether it has positive or adverse effects. CuONPs can be used to grow corn and wheat plants by combining Bacillus subtilis. In this research, CuONPs were synthesized by precipitation method using different precursors such as sodium hydroxide (0.1 M) and copper nitrate (Cu(NO₃)₂) having 0.1 M concentration with a post-annealing method. The NPs were characterized through X-ray diffraction (XRD), scanning electron microscope (SEM), and ultraviolet (UV) visible spectroscopy. Bacillus subtilis is used as a potential growth promoter for microbial inoculation due to its prototrophic nature. The JAR experiment was conducted, and the growth parameter of corn (Z. mays) and wheat (Triticum aestivum) was recorded after 5 days. The lab assay evaluated the germination in JARs with and without microbial inoculation under CuONP stress at different concentrations (25 and 50 mg). The present study aimed to synthesize CuONPs and systematically investigate the particle size effects of copper (II) oxide (CuONPs) (< 50 nm) on Triticum aestivum and Z. mays. In our results, the XRD pattern of CuONPs at 500 °C calcination temperature with monoclinic phase is observed, with XRD peak intensity slightly increasing. The XRD patterns showed that the prepared CuONPs were extremely natural, crystal-like, and nano-shaped. We used Scherrer's formula to calculate the average size of the particle, indicated as 23 nm. The X-ray diffraction spectrum of synthesized materials and SEM analysis show that the particles of CuONPs were spherical in nature. The results revealed that the synthesized CuONPs combined with Bacillus subtilis used in a field study provided an excellent result, where growth parameters of Z. Mays and Triticum aestivum such as root length, shoot length, and plant biomass was improved as compared to the control group.

Sustainable Applications of Endophytic Bacteria and Their Physiological/Biochemical Roles on Medicinal and Herbal Plants: Review

Bacterial endophytes reside within the tissues of living plant species without causing any harm or disease to their hosts. These endophytes can be isolated, identified, characterized, and used as biofertilizers. Moreover, bacterial endophytes increase the plants' resistance against diseases, pests, and parasites, and are a promising source of pharmaceutically important bioactives. For instance, the production of antibiotics, auxins, biosurfactants, cytokinin's, ethylene, enzymes, gibberellins, nitric oxide organic acids, osmolytes, and siderophores is accredited to the existence of various bacterial strains. Thus, this manuscript intends to review the sustainable applications of endophytic bacteria to promote the growth, development, and chemical integrity of medicinal and herbal plants, as well as their role in plant physiology. The study of the importance of bacterial endophytes in the suppression of diseases in medicinal and herbal plants is crucial and a promising area of future investigation.

Plant Growth Promoting Rhizobacteria in Plant Health: A Perspective Study of the Underground Interaction

Plants are affected by various environmental stresses such as high or low temperatures, drought, and high salt levels, which can disrupt their normal cellular functioning and impact their growth and productivity. These stressors offer a major constraint to the morphological, physiological, and biochemical parameters; thereby attributing serious complications in the growth of crops such as rice, wheat, and corn. Considering the strategic and intricate association of soil microbiota, known as plant growth-promoting rhizobacteria (PGPR), with the plant roots, PGPR helps plants to adapt and survive under changing environmental conditions and become more resilient to stress. They aid in nutrient acquisition and regulation of water content in the soil and also play a role in regulating osmotic balance and ion homeostasis. Boosting key physiological processes, they contribute significantly to the alleviation of stress and promoting the growth and development of plants. This review examines the use of PGPR in increasing plant tolerance to different stresses, focusing on their impact on water uptake, nutrient acquisition, ion homeostasis, and osmotic balance, as well as their effects on crop yield and food security.

Screening of Azotobacter, Bacillus and Pseudomonas Species as Plant Growth-Promoting Bacteria

In this study, bacteria from the genus of Azotobacter, Bacillus and Pseudomonas were isolated from the roots of Phaseolus vulgaris and used as plant growth-promoting bacteria for Sinapis alba L., Brassica napus L., Amaranthus retroflexus L., Linum usitatissimum L., Panicum miliaceum L. and Rumex patientia L. plants. The results showed that all three bacteria had different effects on plants growth considering both sterile and non-sterile soil. Bacillus sp. induced the greatest influence in terms of the root length of Sinapis alba L. grown in sterile soil (with 28%), while considering non-sterile soil, Pseudomonas sp. increased the root and shoot length by 11.43% and 25.15%, respectively, compared to the blank sample. Azotobacter sp. exerted the highest beneficial influence on Brassica napus L. growth in non-sterile soil, since the root and shoot lengths were stimulated with 27.64% and 52.60%, respectively, compared to uninoculated plants. Bacillus sp. had a positive effect on the growth of the shoot length of Amaranthus retroflexus L. (with 30.30% in sterile soil and 3.69% in non-sterile soil compared to the control). Azotobacter sp. stimulated the growth of the root length of Rumex patientia L. with 35.29% in sterile soil and also the shoot length of Panicum miliaceum L. in non-sterile soil by 20.51% compared to the control. Further, the roots and shoots of Linum usitatissimum L. grown in non-sterile soil and in the presence of Pseudomonas sp. increased by 178.38% and 15.08%, respectively, compared to the flax grown in sterile soil. Statistically, according to Tukey’s Honestly Significant Difference (HSD) test results, not all observed differences in plants grown with the selected bacteria are significantly different compared to the control.

Analysis of the Propionate Metabolism in Bacillus subtilis during 3-Indolacetic Production

The genera Bacillus belongs to the group of microorganisms that are known as plant growth-promoting bacteria, their metabolism has evolved to produce molecules that benefit the growth of the plant, and the production of 3-indole acetic acid (IAA) is part of its secondary metabolism. In this work, Bacillus subtilis was cultivated in a bioreactor to produce IAA using propionate and glucose as carbon sources in an M9-modified media; in both cases, tryptophan was added as a co-substrate. The yield of IAA using propionate is 17% higher compared to glucose. After 48 h of cultivation, the final concentration was 310 mg IAA/L using propionate and 230 mg IAA/L using glucose, with a concentration of 500 mg Trp/L. To gain more insight into propionate metabolism and its advantages, the genome-scale metabolic model of B. subtilis (iBSU 1147) and computational analysis were used to calculate flux distribution and evaluate the metabolic capabilities to produce IAA using propionate. The metabolic fluxes demonstrate that propionate uptake favors the production of precursors needed for the synthesis of the hormone, and the sensitivity analysis shows that the control of a specific growth rate has a positive impact on the production of IAA.

Effects of Bacillus subtilis and Pseudomonas fluorescens as the soil amendment

The application of soil beneficial bacteria (SBB) in agriculture is steadily increasing as it provides a promising way to replace chemical fertilisers and other supplements. Although the role of SBB as a biofertiliser is well understood, little is known about the response of soil physiochemical properties via the change in soil enzymatic activities with SBB growth. In this study, sterilised bulk soil was inoculated with Bacillus subtilis (BS) and Pseudomonas fluorescens (PF), which exhibit excellent characteristics in vitro for potentially improving soil quality. It is found that the contents of bioavailable nitrogen and ammonium in soil inoculated with SBB increased significantly, up to 34% and 57% relative to a control. This resulted from the enhancement of soil urease activity with BS and PF treatments by approximately 90% and 70%, respectively. The increased soil urease activity can be explained by the increased microorganism activity evident from the larger population size of BS (0.78–0.97 CFU mL⁻¹/CFU mL⁻¹) than PF (0.55–0.79 CFU mL⁻¹/CFU mL⁻¹) (p < 0.05). Results of principal component analysis also reinforce the interaction apparent in the significant relationship between soil urease activity and microbial biomass carbon (p < 0.05). Therefore, it can be concluded that the enhancement of soil enzymatic activities induced bulk soil fertility upregulation because of bacterial growth. These results demonstrate the application of SBB to be a promising strategy for bulk soil amendment, particularly nutrient restoration.

Induction of resilience strategies against biochemical deteriorations prompted by severe cadmium stress in sunflower plant when Trichoderma and bacterial inoculation were used as biofertilizers

Background

Cadmium (Cd) is a highly toxic heavy metal. Its emission is suspected to be further increased due to the dramatic application of ash to agricultural soils and newly reclaimed ones. Thereby, Cd stress encountered by plants will exacerbate. Acute and chronic exposure to Cd can upset plant growth and development and ultimately causes plant death. Microorganisms as agriculturally important biofertilizers have constantly been arising as eco-friendly practices owing to their ability to built-in durability and adaptability mechanisms of plants. However, applying microbes as a biofertilizer agent necessitates the elucidation of the different mechanisms of microbe protection and stabilization of plants against toxic elements in the soil. A greenhouse experiment was performed using Trichoderma harzianum and plant growth-promoting (PGP) bacteria (Azotobacter chroococcum and Bacillus subtilis) individually and integrally to differentiate their potentiality in underpinning various resilience mechanisms versus various Cd levels (0, 50, 100, and 150 mg/kg of soil). Microorganisms were analyzed for Cd tolerance and biosorption capacity, indoleacetic acid production, and phosphate and potassium solubilization in vitro. Plant growth parameters, water relations, physiological and biochemical analysis, stress markers and membrane damage traits, and nutritional composition were estimated.

Results

Unequivocal inversion from a state of downregulation to upregulation was distinct under microbial inoculations. Inoculating soil with T. harzianum and PGPB markedly enhanced the plant parameters under Cd stress (150 mg/kg) compared with control plants by 4.9% and 13.9%, 5.6% and 11.1%, 55.6% and 5.7%, and 9.1% and 4.6% for plant fresh weight, dry weight, net assimilation rate, and transpiration rate, respectively; by 2.3% and 34.9%, 26.3% and 69.0%, 26.3% and 232.4%, 135.3% and 446.2%, 500% and 95.6%, and 60% and 300% for some metabolites such as starch, amino acids, phenolics, flavonoids, anthocyanin, and proline, respectively; by 134.0% and 604.6% for antioxidants including reduced glutathione; and by 64.8% and 91.2%, 21.9% and 72.7%, and 76.7% and 166.7% for enzymes activity including ascorbate peroxidase, glutathione peroxidase, and phenylalanine ammonia-lyase, respectively. Whereas a hampering effect mediated by PGP bacterial inoculation was registered on levels of superoxide anion, hydroxyl radical, electrolyte leakage, and polyphenol oxidase activity, with a decrease of 0.53%, 14.12%, 2.70%, and 5.70%, respectively, under a highest Cd level (150 mg/kg) compared with control plants. The available soil and plant Cd concentrations were decreased by 11.5% and 47.5%, and 3.8% and 45.0% with T. harzianum and PGP bacterial inoculation, respectively, compared with non-inoculated Cd-stressed plants. Whereas, non-significant alternation in antioxidant capacity of sunflower mediated by T. harzianum action even with elevated soil Cd concentrations indicates stable oxidative status. The uptake of nutrients, viz., K, Ca, Mg, Fe, nitrate, and phosphorus, was interestingly increased (34.0, 4.4, 3.3, 9.2, 30.0, and 1.0 mg/g dry weight, respectively) owing to the synergic inoculation in the presence of 150 mg of Cd/kg.

Conclusions

However, strategies of microbe-induced resilience are largely exclusive and divergent. Biofertilizing potential of T. harzianum showed that, owing to its Cd biosorption capability, a resilience strategy was induced via reducing Cd bioavailability to be in the range that turned its effect from toxicity to essentiality posing well-known low-dose stimulation phenomena (hormetic effect), whereas using Azotobacter chroococcum and Bacillus subtilis, owing to their PGP traits, manifested a resilience strategy by neutralizing the potential side effects of Cd toxicity. The synergistic use of fungi and bacteria proved the highest efficiency in imparting sunflower adaptability under Cd stress.

Herbal Arsenal against Skin Ailments: A Review Supported by In Silico Molecular Docking Studies

Maintaining healthy skin is important for a healthy body. At present, skin diseases are numerous, representing a major health problem affecting all ages from neonates to the elderly worldwide. Many people may develop diseases that affect the skin, including cancer, herpes, and cellulitis. Long-term conventional treatment creates complicated disorders in vital organs of the body. It also imposes socioeconomic burdens on patients. Natural treatment is cheap and claimed to be safe. The use of plants is as old as mankind. Many medicinal plants and their parts are frequently used to treat these diseases, and they are also suitable raw materials for the production of new synthetic agents. A review of some plant families, viz., Fabaceae, Asteraceae, Lamiaceae, etc., used in the treatment of skin diseases is provided with their most common compounds and in silico studies that summarize the recent data that have been collected in this area.

Mitigating abiotic stress: microbiome engineering for improving agricultural production and environmental sustainability

The responses of plants to different abiotic stresses and mechanisms involved in their mitigation are discussed. Production of osmoprotectants, antioxidants, enzymes and other metabolites by beneficial microorganisms and their bioengineering ameliorates environmental stresses to improve food production. Progressive intensification of global agriculture, injudicious use of agrochemicals and change in climate conditions have deteriorated soil health, diminished the microbial biodiversity and resulted in environment pollution along with increase in biotic and abiotic stresses. Extreme weather conditions and erratic rains have further imposed additional stress for the growth and development of plants. Dominant abiotic stresses comprise drought, temperature, increased salinity, acidity, metal toxicity and nutrient starvation in soil, which severely limit crop production. For promoting sustainable crop production in environmentally challenging environments, use of beneficial microbes has emerged as a safer and sustainable means for mitigation of abiotic stresses resulting in improved crop productivity. These stress-tolerant microorganisms play an effective role against abiotic stresses by enhancing the antioxidant potential, improving nutrient acquisition, regulating the production of plant hormones, ACC deaminase, siderophore and exopolysaccharides and accumulating osmoprotectants and, thus, stimulating plant biomass and crop yield. In addition, bioengineering of beneficial microorganisms provides an innovative approach to enhance stress tolerance in plants. The use of genetically engineered stress-tolerant microbes as inoculants of crop plants may facilitate their use for enhanced nutrient cycling along with amelioration of abiotic stresses to improve food production for the ever-increasing population. In this chapter, an overview is provided about the current understanding of plant-bacterial interactions that help in alleviating abiotic stress in different crop systems in the face of climate change. This review largely focuses on the importance and need of sustainable and environmentally friendly approaches using beneficial microbes for ameliorating the environmental stresses in our agricultural systems.

Synergistic Interaction between Symbiotic N2 Fixing Bacteria and Bacillus strains to Improve Growth, Physiological Parameters, Antioxidant Enzymes and Ni Accumulation in Faba Bean Plants (Vicia faba) under Nickel Stress

Several activities in the agriculture sector lead to the accumulation of Nickel (Ni) in soil. Therefore, effective and economical ways to reduce soil bioavailability of Ni must be identified. Five isolates of Rhizobium leguminosarum biovar Viceae (ICARDA 441, ICARDA 36, ICARDA 39, TAL−1148, and ARC−207) and three bacterial strains (Bacillus subtilis, B. circulance, and B. coagulans) were evaluated for tolerance and biosorption of different levels of Ni (0, 20, 40, 60, and 80 mg L−1). Pot experiments were conducted during the 2019/2020 and 2020/2021 seasons using four inoculation treatments (inoculation with the most tolerant Rhizobium (TAL−1148), inoculation with the most tolerant Rhizobium (TAL−1148) + B. subtilis, inoculation with the most tolerant Rhizobium (TAL−1148) + B. circulance, and inoculation with the most tolerant Rhizobium (TAL−1148) + B. coagulans) under different levels of Ni (0, 200, 400, and 600 mg kg−1), and their effects on growth, physiological characteristics, antioxidant enzymes, and Ni accumulation in faba bean plants (Vicia faba C.V. Nobaria 1) were determined. The results showed that Rhizobium (TAL−1148) and B. subtilis were the most tolerant of Ni. In pot trials, inoculation with the most tolerant Rhizobium TAL−1148 + B. subtilis treatment was shown to be more effective in terms of growth parameters (dry weight of plant, plant height, number of nodules, and N2 content), and this was reflected in physiological characteristics and antioxidant enzymes under 600 mg kg−1 Ni compared to the other treatments in the 2019/2020 season. In the second season, 2020/2021, a similar pattern was observed. Additionally, lower concentrations of Ni were found in faba bean plants (roots and shoots). Therefore, a combination of the most tolerant Rhizobium (TAL−1148) + B. subtilis treatment might be used to reduce Ni toxicity.

Minerals solubilizing and mobilizing microbiomes: A sustainable approaches for managing minerals deficiency in agricultural soil

Agriculture faces challenges to fulfill the rising food demand due to shortage of arable land and various environmental stressors. Traditional farming technologies help in fulfilling food demand but they are harmful to humans and environmental sustainability. The food production along with agro-environmental sustainability could be achieved by encouraging farmers to use agro-environmental sustainable products such as biofertilizers and biopesticides consisting of live microbes or plant extract instead of chemical-based inputs. The ecofriendly formulations play a significant role in plant growth promotion, crop yield, and repairing degraded soil texture and fertility sustainably. Mineral solubilizing microbes that provide vital nutrients like phosphorus, potassium, zinc, and selenium are essential for plant growth and development and could be developed as biofertilizers. These microbes could be plant-associated (rhizospheric, endophytic, and phyllospheric) or inhabits the bulk soil, and diverse extreme habitats. Mineral solubilizing microbes from soil, extreme environments, surface and internal parts of the plant belong to diverse phyla such as Ascomycota, Actinobacteria, Basidiomycota, Bacteroidetes, Chlorobi, Cyanobacteria, Chlorophyta, Euryarchaeota, Firmicutes, Gemmatimonadetes, Mucoromycota, Proteobacteria, and Tenericutes. Mineral solubilizing microbes (MSMs) directly or indirectly stimulate plant growth and development either by releasing plant growth regulators; solubilizing phosphorus, potassium, zinc, selenium, and silicon; biological nitrogen fixation; and production of siderophores, ammonia, hydrogen cyanide, hydrolytic enzymes, and bioactive compound/secondary metabolites. Biofertilizer developed using mineral solubilizing microbes is an eco-friendly solution to the sustainable food production system in many countries worldwide. The present review deals with the biodiversity of mineral solubilizing microbes, and potential roles in crop improvement and soil well-being for agricultural sustainability.

Alleviation of cadmium stress in rice by inoculation of Bacillus cereus

Heavy metal resistant bacteria are of great importance because they play a crucial role in bioremediation. In the present study, 11 bacterial strains isolated from industrial waste were screened under different concentrations of cadmium (Cd) (100 µM and 200 µM). Among 11 strains, the Cd tolerant Bacillus cereus (S₆D_1-105) strain was selected for in vitro and in vivo studies. B. cereus was able to solubilize potassium, and phosphate as well as produce protease and siderophores during plate essays. Moreover, we observed the response of hydroponically grown rice plants, inoculated with B. cereus which was able to promote plant growth, by increasing plant biomass, chlorophyll contents, relative water content, different antioxidant enzymatic activity such as catalase, superoxide dismutase, ascorbate peroxidase, polyphenol oxidase and phenylalanine ammonia-lyase and reducing malondialdehyde content in both roots and leaves of rice plants under Cd stress. Our results showed that the B. cereus can be used as a biofertilizer which might be beneficial for rice cultivation in Cd contaminated soils.

Native Heavy Metal-Tolerant Plant Growth Promoting Rhizobacteria Improves Sulla spinosissima (L.) Growth in Post-Mining Contaminated Soils

The potential of rhizobacteria in assisting plants used in the phytostabilization or re-vegetation of soils contaminated by heavy metals is gaining interest all around the world. In this context, six rhizobacterial strains isolated from highly heavy metal-contaminated soils situated in abandoned mining sites around the Oujda region (Morocco) were tested with Sulla spinosissima (L.), a native leguminous plant expanding in this area. The strains used were multi-resistant to heavy metals and possessed multiple plant growth-promoting traits. Potential beneficial effects of the strains were also evaluated in planta by measuring various growth and physiological parameters of inoculated Sulla plants grown in sterilized sand. Inoculation with the Rhodococcus qingshengii strain LMR340 boosted plant biomass (39% to 83% increase compared to uninoculated plants), chlorophyll and carotenoid content (up to 29%), and antioxidant enzyme activities (15% to 80% increase). Based on these interesting findings, selected strains were inoculated into plants growing in a heavy metal, multi-polluted, and poor soil. Under these conditions, non-inoculated plants and those inoculated with the strain LMR250 were unable to grow, while the other five bacterial inoculants restored plant growth. The best performing strain, Pseudarthrobacter oxydans LMR291, could be considered as a good biofertilizer and/or biostimulant candidate to be used for promoting the growth of selected plants in re-vegetation and/or phytostabilization programs of degraded and contaminated soils.

Plant microbe based remediation approaches in dye removal: A review

Increased industrialization demand using synthetic dyes in the newspaper, cosmetics, textiles, food, and leather industries. As a consequence, harmful chemicals from dye industries are released into water reservoirs with numerous structural components of synthetic dyes, which are hazardous to the ecosystem, plants and humans. The discharge of synthetic dye into various aquatic environments has a detrimental effect on the balance and integrity of ecological systems. Moreover, numerous inorganic dyes exhibit tolerance to degradation and repair by natural and conventional processes. So, the present condition requires the development of efficient and effective waste management systems that do not exacerbate environmental stress or endanger other living forms. Numerous biological systems, including microbes and plants, have been studied for their ability to metabolize dyestuffs. To minimize environmental impact, bioremediation uses endophytic bacteria, which are plant beneficial bacteria that dwell within plants and may improve plant development in both normal and stressful environments. Moreover, Phytoremediation is suitable for treating dye contaminants produced from a wide range of sources. This review article proves a comprehensive evaluation of the most frequently utilized plant and microbes as dye removal technologies from dye-containing industrial effluents. Furthermore, this study examines current existing technologies and proposes a more efficient, cost-effective method for dye removal and decolorization on a big scale. This study also aims to focus on advanced degradation techniques combined with biological approaches, well regarded as extremely effective treatments for recalcitrant wastewater, with the greatest industrial potential.

An Evaluation of Aluminum Tolerant Pseudomonas aeruginosa A7 for In Vivo Suppression of Fusarium Wilt of Chickpea Caused by Fusarium oxysporum f. sp. ciceris and Growth Promotion of Chickpea

Chickpea wilt, caused by Fusarium oxysporum f. sp. ciceris, is a disease that decreases chickpea productivity and quality and can reduce its yield by as much as 15%. A newly isolated, moss rhizoid-associated Pseudomonas aeruginosa strain A7, demonstrated strong inhibition of Fusarium oxysporum f. sp. ciceris growth. An in vitro antimicrobial assay revealed A7 to suppress the growth of several fungal and bacterial plant pathogens by secreting secondary metabolites and by producing volatile compounds. In an in vivo pot experiment with Fusarium wilt infection in chickpea, the antagonist A7 exhibited a disease reduction by 77 ± 1.5%, and significantly reduced the disease incidence and severity indexes. Furthermore, A7 promoted chickpea growth in terms of root and shoot length and dry biomass during pot assay. The strain exhibited several traits associated with plant growth promotion, extracellular enzymatic production, and stress tolerance. Under aluminum stress conditions, in vitro growth of chickpea plants by A7 resulted in a significant increase in root length and plant biomass production. Additionally, hallmark genes for antibiotics production were identified in A7. The methanol extract of strain A7 demonstrated antimicrobial activity, leading to the identification of various antimicrobial compounds based on retention time and molecular weight. These findings strongly suggest that the strain’s significant biocontrol potential and plant growth enhancement could be a potential environmentally friendly process in agricultural crop production.

Characteristics changes on Applications of Antibiotics and Current Approaches to Enhance Productivity with Soil Microbiome

The contamination of environmental sully with antibiotics is regarded as a major problem today and predictable to attain more recognition in near future. However, human intervention resulting in antibiotic consumption is being enhancing all around the world. Our review of literature revealed the role of microbiome in sully and how antibiotic resistant genes raised. The structure of antibiotics basically influenced by natural components such as biotic and abiotic push which shifts based on different soils. Therefore, management of microbiome in soil and their expression studies were distinctively revealed. The assessment of antibiotic resistance genes with help of next generation sequencing provided a clear comprehension on genome and transcriptome of the bacterial genes. Thus, interaction of microbiome with soil can also be well understood. The current findings in our study will guide every researcher to follow logical protocol in analyzing microbiota composition is covered as well and also to understand its metagenomic and sequenced with next-generation sequencer which helps to comprehend the diverse micro-flora present in soil and its operation. Finally, later progresses in bioinformatics computer program, flow of work, and applications for analyzing metagenomic information are put in a nutshell.

A potent cadmium bioaccumulating Enterobacter cloacae strain displays phytobeneficial property in Cd-exposed rice seedlings

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Cd-resistant and halotolerant PGPR enterobacter cloacae AS10 was isolated.

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AS10 showed IAA, HCN production, P-solubilization, N₂ fixation, ACCD activity.

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AAS-TEM-EDAX-XRD-XRF-FTIR studies confirmed Cd bioaccumulation by AS10.

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AS10 reduced oxidative stress, Cd uptake and improved rice seedling growth in vitro.

In agricultural soil, cadmium (Cd) pollution compromises soil health, reduces crop yield, and produces Cd-contaminated crops. Bio-based approaches are necessary as an eco-friendly and sustainable solution to mitigate Cd-polluted areas. A heavy metal-resistant rhizobacterial strain (AS10) has been isolated from a heavy metal-defiled rice field. The 16S rDNA sequence and MALDI-TOF MS analyses of ribosomal protein reveal its identity closely similar to Enterobacter cloacae. The strain was found to withstand up to 4000 μg/ml Cd²⁺, 3312 µg/ml Pb²⁺ and 1500 µg/ml As³⁺. The Cd²⁺ removal efficiency was recorded as high as 72.11% when grown in 4000 μg/ml Cd²⁺. The strain's Cd-accumulation efficiency was also apprehended by TEM-EDAX followed by XRD-XRF-FTIR analyses. Besides, the strain showed solubilization of inorganic phosphate, ACC deaminase activity, nitrogen fixation and IAA production ability. Added further, the strain, as an efficient bioinoculant, significantly improved rice plant growth at the seedling stage through Cd immobilization. It prevented the surge of stress ethylene and oxidative stress in rice seedlings, resulting in overall plant growth improvement. Hence, the strain AS10 as potent plant growth-promoting rhizobacteria (PGPR) may be beneficial, especially in heavy metal-contaminated crop fields.

Characterization of plant growth-promoting traits of Enterobacter sp. and its ability to promote cadmium/lead accumulation in Centella asiatica L

In the present study, we characterized the plant growth-promoting traits of Enterobacter sp. FM-1 (FM-1) and investigated its ability to promote growth and increase IAA, P, and Fe concentrations as well as Cd and Pb accumulation in Centella asiatica L. (C. asiatica L.) in upstream area (UA) soil and downstream area (DA) soil that we collected from Siding mine. The results demonstrated that FM-1 secreted IAA, produced siderophores, and had P-solubilization ability even under Cd exposure. IAA secretion reached a maximum of 108.3 ± 1.3 mg L^-1 under Cd exposure at 25 mg L^-1. Siderophore production reached a maximum of 0.94 ± 0.01 under Cd exposure at 50 mg L^-1. Pot experiments indicated that FM-1 successfully colonized the roots of C. asiatica L. In both soils, inoculation with FM-1 decreased the pH in rhizosphere soil and increased the bioavailability of both Cd and Pb. In addition, inoculation with FM-1 increased the IAA, P, and Fe concentrations and simultaneously promoted both Cd and Pb accumulation in C. asiatica L. The Cd and Pb concentrations in leaves increased 1.73- and 1.07-fold in the UA soil and 1.25- and 1.11-fold in the DA soil, respectively. Thus, the Cd-resistant strain FM-1 presented excellent PGP traits and could facilitate Cd and Pb phytoremediation by C. asiatica L.

Phytoremediation of Toxic Metals: A Sustainable Green Solution for Clean Environment

Contamination of aquatic ecosystems by various sources has become a major worry all over the world. Pollutants can enter the human body through the food chain from aquatic and soil habitats. These pollutants can cause various chronic diseases in humans and mortality if they collect in the body over an extended period. Although the phytoremediation technique cannot completely remove harmful materials, it is an environmentally benign, cost-effective, and natural process that has no negative effects on the environment. The main types of phytoremediation, their mechanisms, and strategies to raise the remediation rate and the use of genetically altered plants, phytoremediation plant prospects, economics, and usable plants are reviewed in this review. Several factors influence the phytoremediation process, including types of contaminants, pollutant characteristics, and plant species selection, climate considerations, flooding and aging, the effect of salt, soil parameters, and redox potential. Phytoremediation’s environmental and economic efficiency, use, and relevance are depicted in our work. Multiple recent breakthroughs in phytoremediation technologies are also mentioned in this review.

Insights into soil bacterial and physicochemical properties of annual ryegrass-maize rotation (ARMR) system in southern China

The popularized application of annual ryegrass-maize rotation (ARMR) in southern China has been proposed to fully utilize the farmlands and to increase forage yield and quality. Herein, one growth cycle of ARMR was conducted and soil bacteria were analyzed by 16S rRNA sequencing for control (CK), after the preceding crop (monoculture, or mixed sowing of annual ryegrass and oat) and the successive crop (maize). Our results indicated that the α-diversity of soil bacteria was changed in the ARMR system, which was related to the activity of urease and available phosphatase. The mixed sowing of annual ryegrass and oat in preceding crop could improve the yield and quality, while it was accompanied by unbalanced soil community. With the increased sowing proportion of oat to annual ryegrass, the soil pH increased while the soil available phosphatase decreased. The ARMR system was found to benefit the soil microenvironment by increasing the beneficial soil bacteria and enzyme activity or decreasing the harmful soil bacteria. Considering the soil bacteria α-diversity index and physicochemical properties comprehensively, the recommended sowing regime is the mixed sowing of M2 (22.5 kg·hm-2 annual ryegrass with 75 kg·hm-2 oat).

Improved chromium tolerance of Medicago sativa by plant growth-promoting rhizobacteria (PGPR)

Background

Soil pollution by heavy metals increases the bioavailability of metals like hexavalent chromium (Cr (VI)), subsequently limiting plant growth and reducing the efficiency of phytoremediation. Plant growth-promoting rhizobacteria (PGPR) have substantial potential to enhance plant growth as well as plant tolerance to metal stress. The aim of this research was to investigate Cr (VI) phytoremediation enhancement by PGPR.

Results

The results showed that the 27 rhizobacterial isolates studied were confirmed as Cr (VI)-resistant PGPR, by using classical biochemical tests (phosphate solubilization, nitrogen fixation, indole acetic acid, exopolysaccharides, hydrogen cyanide, siderophores, ammonia, cellulase, pectinase, and chitinase production) and showed variable levels of Cr (VI) resistance (300–600 mg/L). The best four selected Cr (VI)-resistant PGPR (NT15, NT19, NT20, and NT27) retained most of the PGP traits in the presence of 100–200 mg/L concentrations of Cr (VI). The inoculation of Medicago sativa with any of these four isolates improved the shoot and root dry weight. The NT27 isolate identified using 16S rDNA gene sequence analyses as a strain of Pseudomonas sp. was most effective in terms of plant growth promotion and stress level decrease. It increased shoot and root dry weights of M. sativa by 97.6 and 95.4%, respectively, in the presence of Cr (VI) when compared to non-inoculated control plants. It also greatly increased chlorophyll content and decreased the levels of stress markers, malondialdehyde, hydrogen peroxide, and proline. The results of the effect of Pseudomonas sp. on Cr content and bioaccumulation factor (BAF) of the shoots and roots of M. sativa plants showed the increase of plant biomass concomitantly with the increase of Cr root concentration in inoculated plants. This would lead to a higher potential of Cr (VI) phytostabilization.

Conclusions

This study demonstrates that the association M. sativa-Pseudomonas sp. may be an efficient biological system for the bioremediation of Cr (VI)-contaminated soils.

Rhizosphere Bacteria in Plant Growth Promotion, Biocontrol, and Bioremediation of Contaminated Sites: A Comprehensive Review of Effects and Mechanisms

Agriculture in the 21st century is facing multiple challenges, such as those related to soil fertility, climatic fluctuations, environmental degradation, urbanization, and the increase in food demand for the increasing world population. In the meanwhile, the scientific community is facing key challenges in increasing crop production from the existing land base. In this regard, traditional farming has witnessed enhanced per acre crop yields due to irregular and injudicious use of agrochemicals, including pesticides and synthetic fertilizers, but at a substantial environmental cost. Another major concern in modern agriculture is that crop pests are developing pesticide resistance. Therefore, the future of sustainable crop production requires the use of alternative strategies that can enhance crop yields in an environmentally sound manner. The application of rhizobacteria, specifically, plant growth-promoting rhizobacteria (PGPR), as an alternative to chemical pesticides has gained much attention from the scientific community. These rhizobacteria harbor a number of mechanisms through which they promote plant growth, control plant pests, and induce resistance to various abiotic stresses. This review presents a comprehensive overview of the mechanisms of rhizobacteria involved in plant growth promotion, biocontrol of pests, and bioremediation of contaminated soils. It also focuses on the effects of PGPR inoculation on plant growth survival under environmental stress. Furthermore, the pros and cons of rhizobacterial application along with future directions for the sustainable use of rhizobacteria in agriculture are discussed in depth.

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.

Bacteria Isolated from Wastewater Irrigated Agricultural Soils Adapt to Heavy Metal Toxicity While Maintaining Their Plant Growth Promoting Traits

The present study explored the plant growth promotion and bioremediation potential of bacteria inhabiting wastewater irrigated agricultural soils. Thirty out of 75 bacterial isolates (40%), 29/75 (39%) and 28/75 (37%) solubilized Zn, K and PO4 during plate essays respectively. Fifty-six percent of the isolates produced siderophores, while 30% released protease in vitro. Seventy-four percent of bacteria resisted Pb, Ni and Cd at various concentrations added to the culture media plates. Sixteen out of 75 (26%) isolates were able to fix N in Nbf medium. Among these 16 N fixers, N fixing nifH, nifD and nifK genes was detected through PCR in 8, 7 and 1 strain respectively using gene specific primers designed in the study with Enterobacter sp. having all three (nifHKD) genes. Isolated bacteria showed resemblance to diverse genera such as Bacillus, Pseudomonas, Enterobacter, Citrobacter, Acinetobacter, Serratia, Klebsiella and Enterococcus based on 16S rRNA gene sequence analysis. In addition to showing the best mineral solubilization and metal resistance potential, Citrobacter sp. and Enterobacter sp. also removed 87%, 79% and 43% and 86%, 78% and 51% of Ni, Cd and Pb, respectively, from aqueous solution. These potent bacteria may be exploited both for bioremediation and biofertilization of wastewater irrigated soils leading to sustainable agriculture.

Nickel in terrestrial biota: Comprehensive review on contamination, toxicity, tolerance and its remediation approaches

Nickel (Ni) has been a subject of interest for environmental, physiological, biological scientists due to its dual effect (toxicity and essentiality) in terrestrial biota. In general, the safer limit of Ni is 1.5 μg g^-1 in plants and 75-150 μg g^-1 in soil. Litreature review indicates that Ni concentrations have been estimated up to 26 g kg^-1 in terrestrial, and 0.2 mg L^-1 in aquatic resources. In case of vegetables and fruits, mean Ni content has been reported in the range of 0.08-0.26 and 0.03-0.16 mg kg^-1. Considering, Ni toxicity and its potential health hazards, there is an urgent need to find out the suitable remedial approaches. Plant vascular (>80%) and cortical (<20%) tissues are the major sequestration site (cation exchange) of absorbed Ni. Deciphering molecular mechanisms in transgenic plants have immense potential for enhancing Ni phytoremediation and microbial remediation efficiency. Further, it has been suggested that integrated bioremediation approaches have a potential futuristic path for Ni decontamination in natural resources. This systematic review provides insight on Ni effects on terrestrial biota including human and further explores its transportation, bioaccumulation through food chain contamination, human health hazards, and possible Ni remediation approaches.

Insight Into the Role of PGPR in Sustainable Agriculture and Environment

A multitude of roles is played by microbes in food and agriculture that include nutrient cycling and management, organic matter decomposition and fermentation. Plant growth promoting rhizobacteria (PGPR), representing microbial groups and with ability of colonizing plant roots, influence plant growth through various indirect and direct modes in order to promote its growth and/or protect it from diseases or damage due to insect attack. Thus, PGPR research has received renewed interest worldwide. Increasing number of crop-specific PGPR are being commercialized these days. Approaches like seed-inoculation and soil application either alone or in combination with bacterial culture/product for increased nutrient availability through phosphate solubilisation, potassium solubilisation, sulfur oxidation, nitrogen fixation, iron, and copper chelation are gaining popularity. Arbuscular mycorrhizal fungi (AMF) are root fungal symbiont that improve management of abiotic stress such as phosphorus deficiency. PGPR involves roles like production of indole acetic acid (IAA), ammonia (NH3), hydrogen cyanide (HCN), catalase, etc. PGPR also improve nutrient uptake by altering the level of plant hormone that enhances root surface area by increasing its girth and shape, thereby helping in absorbing more nutrients. PGPR facilitate seed germination, seedling growth and crop yield. An array of microbes including Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes, Arthrobacter, Burkholderia, Bacillus, and Serratia enhance plant growth. Various Pseudomonas sp. have demonstrated significant increase in germination, seedling growth and yield in different agricultural crops, including wheat. Hence, developing a successful crop-specific PGPR formulation, the candidate should possess characteristics like high rhizosphere competence, extensive competitive saprophytic ability, growth enhancing ability, ease of mass production, broad-spectrum action, safety toward the environment and compatibility with other partnering organisms.

Investigation of arsenic-resistant, arsenite-oxidizing bacteria for plant growth promoting traits isolated from arsenic contaminated soils

The problem of arsenic (As) pollution being severe warrants opting for low-cost microbial remediation strategies. The present study of identifying suitable bacterial strains led to the isolation of eleven As-tolerant strains from the As-contaminated rhizosphere soils of West Bengal, India. They were found to oxidize/reduce 55-31.6% of 5 mM As(III) and 73-37.6% of 5 mM As(V) within 12 h. The four isolates (BcAl-1, JN 73, LAR-2, and AR-30) had a high level of As(III) oxidase activity along with a higher level of As(V) and As(III) resistance. The agar diffusion assay of the isolates further confirmed their ability to endure As stress. The presence of aoxB gene was observed in these four As(III) oxidizing isolates. Evaluation of plant growth-promoting characteristics revealed that BcAl-1 (Burkholderia cepacia), JN 73 (Burkholderia metallica), AR-30 (Burkholderia cenocepacia), and LAR-2 (Burkholderia sp.) had significant plant growth-promoting characteristics (PGP), including the ability to solubilize phosphate, siderophore production, indole acetic acid-like molecules production, ACC deaminase production, and nodule formation under As stressed condition. BcAl-1 and JN 73 emerged as the most promising traits in As removal as well as plant growth promotion.

Green remediation of toxic metals contaminated mining soil using bacterial consortium and Brassica juncea

Microorganism-assisted phytoremediation is being developed as an efficient green approach for management of toxic metals contaminated soils and mitigating the potential human health risk. The capability of plant growth promoting Actinobacteria (Streptomyces pactum Act12 - ACT) and Firmicutes (Bacillus subtilis and Bacillus licheniformis - BC) in mono- and co-applications (consortium) to improve soil properties and enhance phytoextraction of Cd, Cu, Pb, and Zn by Brassica juncea (L.) Czern. was studied here for the first time in both incubation and pot experiments. The predominant microbial taxa were Proteobacteria, Actinobacteria and Bacteroidetes, which are important lineages for maintaining soil ecological activities. The consortium improved the levels of alkaline phosphatase, β-D glucosidase, dehydrogenase, sucrase and urease (up to 33%) as compared to the control. The bacterial inoculum also triggered increases in plant fresh weight, pigments and antioxidants. The consortium application enhanced significantly the metals bioavailability (DTPA extractable) and mobilization (acid soluble fraction), relative to those in the unamended soil; therefore, significantly improved the metals uptake by roots and shoots. The phytoextraction indices indicated that B. juncea is an efficient accumulator of Cd and Zn. Overall, co-application of ACT and BC can be an effective solution for enhancing phytoremediation potential and thus reducing the potential human health risk from smelter-contaminated soil. Field studies may further credit the understanding of consortium interactions with soil and different plant systems in remediating multi-metal contaminated environments.

Diversity and functional characteristics of endophytic bacteria from two grass species growing on an oil-contaminated site in the Yellow River Delta, China

Phragmites australis and Chloris virgata are native, dominant, salt-tolerant grass species that grow in the Yellow River Delta, China, and have potential applications in the phytoremediation of petroleum-polluted saline soil. The characteristics of endophytic bacterial communities of Phragmites australis and Chloris virgata and their functions in hydrocarbon degradation and plant growth promotion have been studied using both high-throughput sequencing and conventional microbial techniques. Through 16S rRNA gene amplicon sequencing, we found five bacterial phyla that were dominant among the endophytic bacterial communities of the two grass species, including Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes, and Tenericutes. The phylum Proteobacteria was common among the endophytic bacterial communities of the two grass species. The diversity in the endophytic bacterial community of Chloris virgata was generally higher than that in the community of Phragmites australis. Thirty-eight hydrocarbon-degrading endophytic bacteria were isolated from the two grasses via culturing techniques. Based on phylogenetic analyses, the bacterial isolates were classified into the phyla Proteobacteria, Firmicutes, and Actinobacteria. The majority of strains belonged to the genera Bacillus and Pseudomonas. More than 70% of the isolates of hydrocarbon-degrading endophytes exhibited the ability to stimulate plant growth. These isolates mainly belonged to Bacillus sp., Pseudomonas sp., Beijerinckia sp., Serratia sp., Acinetobacter sp., Microbacterium sp., and Rhizobium sp. Altogether, the present study revealed that Phragmites australis and Chloris virgata growing on petroleum-polluted saline soil in the Yellow River Delta harbor several diverse species of endophytic bacteria and serve as novel sources of beneficial bacteria and hydrocarbon degradation.

Burkholderia gladioli MB39 an Antarctic Strain as a Biocontrol Agent

Bioprospecting sub-explored environments such as Antarctic locations leads to finding out diverse activities, reducing harmful chemical usage that affects both human health and the environment. In this study, ~ 7000 cold-adapted bacterial strains were isolated from samples around Melchior Antarctic Base at 5 °C and more than 13,000 at 15 °C. Out of them, 900 different colony morphotypes were evaluated for antimicrobial production, and 13 isolates demonstrated antibacterial and antifungal activities. One isolate, closely related to Burkholderia gladioli according to 16S rDNA (99.8%), gyrB (99.6%) and Cpn60 (99.4%) gene sequence analysis, showed a consistent, broad antimicrobial spectrum against both pathogenic and phytopathogenic bacteria. Its potent antifungal activity inhibits the growth of various plant pathogenic fungi, whereas it was mainly studied against Penicillium digitatum and Macrophomina phaseolina, the causal agents of blue mould in postharvest fruits and charcoal rot in soybean crops, respectively. The antibacterial compound exhibited low molecular weight (< 6000 Da), resistance to lytic enzymes and stability in a broad range of temperature and pHs. Observations of the B. gladioli MB39 antifungal effects over M. phaseolina mycelia by scanning electron microscopy showed alterations in hyphal structures, reduced hyphal extension, and severe cell morphology changes such as cytoplasmic leakage, flattened and empty mycelia. Here we report the isolation and identification of a cold-adapted B. gladioli strain. The results describe the effectiveness of the antarctic strain for bacterial and fungal phytopathogens biocontrol and its potential for crop protection plans.

Bacterial tolerance strategies against lead toxicity and their relevance in bioremediation application

Among heavy metals, lead (Pb) is a non-essential metal having a higher toxicity and without any crucial known biological functions. Being widespread, non-biodegradable and persistent in every sphere of soil, air and water, Pb is responsible for severe health and environmental issues, which need appropriate remediation measures. However, microbes inhabiting Pb-contaminated area are found to have evolved distinctive mechanisms to successfully thrive in the Pb-contaminated environment without exhibiting any negative effects on their growth and metabolism. The defensive strategies used by bacteria to ameliorate the toxic effects of lead comprise biosorption, efflux, production of metal chelators like siderophores and metallothioneins and synthesis of exopolysaccharides, extracellular sequestration and intracellular bioaccumulation. Lead remediation technologies by employing microbes may appear as potential advantageous alternatives to the conventional physical and chemical means due to specificity, suitability for applying in situ condition and feasibility to upgrade by genetic engineering. Developing strategies by designing transgenic bacterial strain having specific metal binding properties and metal chelating proteins or higher metal adsorption ability and using bacterial activity such as incorporating plant growth-promoting rhizobacteria for improved Pb resistance, exopolysaccharide and siderophores and metallothionein-mediated immobilization may prove highly effective for formulating bioremediation vis-a-vis phytoremediation strategies.

Nickel tolerance and biosorption potential of rhizobia associated with horse gram [Macrotyloma uniflorum (Lam.) Verdc.]

The rhizobia isolated from root nodules of horse gram were screened for nickel (Ni) tolerance in vitro. The strain HGR-4 could tolerate 1000 µg g^-1 of Ni. It was also observed that horse gram plants associated with HGR-4 have shown Ni stress tolerance in Ni amended soils up to a concentration of 100 µg g^-1. In another experiment, the plants associated with HGR-4 have shown higher nodulation, nitrogen level, and leghaemoglobin content at 80 µg g^-1 of Ni than control plants without HGR-4 inoculation. Analysis of biosorption potential of Ni in horse gram plants inoculated by the strain HGR-4 was done using atomic absorption spectroscopy revealed maximum biosorption in bacterial root nodules. Besides, there was a reduction in the content of the heavy metal in the soil samples which demonstrates a fair amount of heavy metal extraction and accumulation of Ni by rhizobia associated root nodules of the horse gram. This study demonstrates that the strain HGR-4 (GQ483457 Rhizobium sp. ATCC BAA-2335) could be a potential source for phytoextraction of Ni contaminated soils upon its association with horse gram. The study could be of use in phytoremediation of metal (Ni) contaminated soils in the future. Novelty statement: The phytoremediation of nickel (Ni) using of rhizobia associated with horse gram remains unevaluated till now. Horse gram associated with rhizobia could produce nodules and fix nitrogen even in Ni amended soils. The biosorption potential of the rhizobial strains was analyzed from both root nodules and soil. These findings imply that horse gram plants associated with these rhizobial strains could be used to remediate Ni metal in contaminated soils.

Evaluation of interaction among indigenous rhizobacteria and Vigna unguiculata on remediation of metal-containing abandoned magnesite mine tailing

Abandoned magnesite mine heap causing pollution to nearby farmland and water reservoir. Thus the intention of this research was to screening metal mobilizing and absorbing bacteria from the rhizosphere section of V. unguiculata from farmland nearby to magnesite mine. Further, studied their stimulus effect on growth, biomass, and phytoextraction prospective of V unguiculata in mine tailing. The results of the physicochemical properties of mine tailing shows that four metals (Pb, Mn, Cd, and Zn) were crossing the permissible limit. Out of 27 isolates, 2 isolates (MMS15 and MMS17) were identified with maximum metal tolerance for up to 700 mg L^-1 (MIC) and metal mobilization (Pb 5.5 and 5.87, Mn 6.6 and 4.88, Cd 1.99 and 2.59, and Zn 6.55 and 6.94 mg kg^-1) and biosorption efficiency as Pb 3.74 and 3.74, Mn 4.9 and 4.7, Cd 2.41 and 3.96, and Zn 4.3 and 4.9 mg g^-1. These two strains were identified as members of B. cereus and Kosakonia sp. using 16S rRNA technique and labelled strains NDRMN001 and MGR1, respectively. The Kosakonia sp. MGR1 effectively fixes the nitrogen in the rate of 81.94% and B. cereus NDRMN001 solubilizes 69.98 ± 2.31 mg L^-1 of soluble phosphate. The experimental group's study results show that the group C (Kosakonia sp. MGR1 and B. cereus NDRMN001) has effectively stimulate the growth, biomass, and phytoextraction potential of V. unguiculata. The results conclude that the optimistic interaction between these two bacteria could be more significant to minimize the metal pollution in magnesite mine tailing.