Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils

Integrated biochemical and transcriptomic analysis reveals the effects of Burkholderia sp. SRB-1 on cadmium accumulating in Chrysopogon zizanioides L. under Cd stress

Application of plant growth-promoting rhizobacteria plays a vital role in enhancing phytoremediation efficiency. In this study, multiple approaches were employed to investigate the underlying mechanisms of Burkholderia sp. SRB-1 (SRB-1) on elevating Cd uptake and accumulation. Inoculation experiment indicated that SRB-1 could facilitate plant growth and Cd tolerance, as evidenced by the enhanced plant biomass and antioxidative enzymes activities. Cd content in plant shoots and roots increased about 36.56%-39.66% and 25.97%-130.47% assisted with SRB-1 when compared with control. Transcriptomics analysis revealed that SRB-1 upregulated expression of amiE, AAO1-2 and GA2-ox related to auxin and gibberellin biosynthesis in roots. Auxin and gibberellin, as hormone signals, regulated plant Cd tolerance and growth through activating hormone signal transduction pathways, which might also contribute to 67.94% increase of dry weight. The higher expression levels of ATP-binding cassette transporter subfamilies (ABCB, ABCC, ABCD and ABCG) in Chrysopogon zizanioides roots contributed to higher Cd uptake in Cd15 B (323.83 mg kg^-1) than Cd15 (136.28 mg kg^-1). Further, SRB-1 facilitated Cd migration from roots to shoots via upregulating the expression of Nramp, ZIP and HMA families. Our integrative analysis provided a molecular-scale perspective on Burkholderia sp. SRB-1 contributing to C. zizanioides performance.

Signaling and crosstalk of rhizobacterial and plant hormones that mediate abiotic stress tolerance in plants

Agricultural areas exhibiting numerous abiotic stressors, such as elevated water stress, temperatures, and salinity, have grown as a result of climate change. As such, abiotic stresses are some of the most pressing issues in contemporary agricultural production. Understanding plant responses to abiotic stressors is important for global food security, climate change adaptation, and improving crop resilience for sustainable agriculture, Over the decades, explorations have been made concerning plant tolerance to these environmental stresses. Plant growth-promoting rhizobacteria (PGPR) and their phytohormones are some of the players involved in developing resistance to abiotic stress in plants. Several studies have investigated the part of phytohormones in the ability of plants to withstand and adapt to non-living environmental factors, but very few have focused on rhizobacterial hormonal signaling and crosstalk that mediate abiotic stress tolerance in plants. The main objective of this review is to evaluate the functions of PGPR phytohormones in plant abiotic stress tolerance and outline the current research on rhizobacterial hormonal communication and crosstalk that govern plant abiotic stress responses. The review also includes the gene networks and regulation under diverse abiotic stressors. The review is important for understanding plant responses to abiotic stresses using PGPR phytohormones and hormonal signaling. It is envisaged that PGPR offer a useful approach to increasing plant tolerance to various abiotic stresses. However, further studies can reveal the unclear patterns of hormonal interactions between plants and rhizobacteria that mediate abiotic stress tolerance.

Deciphering endemic rhizosphere microbiome community's structure towards the host-derived heavy metals tolerance and plant growth promotion functions in serpentine geo-ecosystem

Environmental microbes in rhizosphere soil and surrounding plants have the potential to alter ecosystem functions. We investigated the microbial communities inhabiting the rhizosphere soils of both serpentine and non-serpentine rhizosphere zones to evaluate their heavy metal tolerance and ability to promote plant growth, utilizing 16S rRNA metabarcoding. The Biolog-EcoPlate technique was employed to determine how abiotic stress factors affect carbon utilization capacity by rhizospheric microbial communities in the serpentine geo-ecosystem. The phyla Proteobacteria, Acidobacteria, Bacteroidetes, and Nitrospirae colonized in the roots of Miscanthus sp., Biden sp., and Oryza sp. showed noticeable differences in different rhizosphere zones. The PICRUSt2-based analysis identified chromium/iron resistance genes (ceuE, chrA) and arsenic resistance genes (arsR, acr3, arsC) abundant in all the studied rhizosphere soils. Notably, nickel resistance genes (nikA, nikD, nikE, and nikR) from Arthrobacter, Microbacterium, and Streptomyces strongly correlate with functions related to solubilization of nickel and an increase in siderophore and IAA production. The abundance of Arthrobacter, Clostridium, Geobacter, Dechloromonas, Pseudomonas, and Flavobacterium was positively correlated with chromium and nickel but negatively correlated with the calcium/magnesium ratio. Our results contribute to a better understanding of the functions of plant-tolerant PGPR interaction in the heavy metal-contaminated rhizosphere and eco-physiological responses from long-term biological weathering.

Endophytic fungi mediates production of bioactive secondary metabolites via modulation of genes involved in key metabolic pathways and their contribution in different biotechnological sector

Endophytic fungi stimulate the production of an enormous number of bioactive metabolites in medicinal plants and affect the different steps of biosynthetic pathways of these secondary metabolites. Endophytic fungi possess a number of biosynthetic gene clusters that possess genes for various enzymes, transcription factors, etc., in their genome responsible for the production of secondary metabolites. Additionally, endophytic fungi also modulate the expression of various genes responsible for the synthesis of key enzymes involved in metabolic pathways of such as HMGR, DXR, etc. involved in the production of a large number of phenolic compounds as well as regulate the expression of genes involved in the production of alkaloids and terpenoids in different plants. This review aims to provide a comprehensive overview of gene expression related to endophytes and their impact on metabolic pathways. Additionally, this review will emphasize the studies done to isolate these secondary metabolites from endophytic fungi in large quantities and assess their bioactivity. Due to ease in synthesis of secondary metabolites and their huge application in the medical industry, these bioactive metabolites are now being extracted from strains of these endophytic fungi commercially. Apart from their application in the pharmaceutical industry, most of these metabolites extracted from endophytic fungi also possess plant growth-promoting ability, bioremediation potential, novel bio control agents, sources of anti-oxidants, etc. The review will comprehensively shed a light on the biotechnological application of these fungal metabolites at the industrial level.

Metal-Resistant PGPR Strain Azospirillum brasilense EMCC1454 Enhances Growth and Chromium Stress Tolerance of Chickpea (Cicer arietinum L.) by Modulating Redox Potential, Osmolytes, Antioxidants, and Stress-Related Gene Expression

Heavy metal stress, including from chromium, has detrimental effects on crop growth and yields worldwide. Plant growth-promoting rhizobacteria (PGPR) have demonstrated great efficiency in mitigating these adverse effects. The present study investigated the potential of the PGPR strain Azospirillum brasilense EMCC1454 as a useful bio-inoculant for boosting the growth, performance and chromium stress tolerance of chickpea (Cicer arietinum L.) plants exposed to varying levels of chromium stress (0, 130 and 260 µM K₂Cr₂O₇). The results revealed that A. brasilense EMCC1454 could tolerate chromium stress up to 260 µM and exhibited various plant growth-promoting (PGP) activities, including nitrogen fixation, phosphate solubilization, and generation of siderophore, trehalose, exopolysaccharide, ACC deaminase, indole acetic acid, and hydrolytic enzymes. Chromium stress doses induced the formation of PGP substances and antioxidants in A. brasilense EMCC1454. In addition, plant growth experiments showed that chromium stress significantly inhibited the growth, minerals acquisition, leaf relative water content, biosynthesis of photosynthetic pigments, gas exchange traits, and levels of phenolics and flavonoids of chickpea plants. Contrarily, it increased the concentrations of proline, glycine betaine, soluble sugars, proteins, oxidative stress markers, and enzymatic (CAT, APX, SOD, and POD) and non-enzymatic (ascorbic acid and glutathione) antioxidants in plants. On the other hand, A. brasilense EMCC1454 application alleviated oxidative stress markers and significantly boosted the growth traits, gas exchange characteristics, nutrient acquisition, osmolyte formation, and enzymatic and non-enzymatic antioxidants in chromium-stressed plants. Moreover, this bacterial inoculation upregulated the expression of genes related to stress tolerance (CAT, SOD, APX, CHS, DREB2A, CHI, and PAL). Overall, the current study demonstrated the effectiveness of A. brasilense EMCC1454 in enhancing plant growth and mitigating chromium toxicity impacts on chickpea plants grown under chromium stress circumstances by modulating the antioxidant machinery, photosynthesis, osmolyte production, and stress-related gene expression.

Phytoremediation as an Effective Remedy for Removing Trace Elements from Ecosystems

The pollution of soil by trace elements is a global problem. Conventional methods of soil remediation are often inapplicable, so it is necessary to search intensively for innovative and environment-friendly techniques for cleaning up ecosystems, such as phytoremediation. Basic research methods, their strengths and weaknesses, and the effects of microorganisms on metallophytes and plant endophytes resistant to trace elements (TEs) were summarised and described in this manuscript. Prospectively, bio-combined phytoremediation with microorganisms appears to be an ideal, economically viable and environmentally sound solution. The novelty of the work is the description of the potential of "green roofs" to contribute to the capture and accumulation of many metal-bearing and suspended dust and other toxic compounds resulting from anthropopressure. Attention was drawn to the great potential of using phytoremediation on less contaminated soils located along traffic routes and urban parks and green spaces. It also focused on the supportive treatments for phytoremediation using genetic engineering, sorbents, phytohormones, microbiota, microalgae or nanoparticles and highlighted the important role of energy crops in phytoremediation. Perceptions of phytoremediation on different continents are also presented, and new international perspectives are presented. Further development of phytoremediation requires much more funding and increased interdisciplinary research in this direction.

Biotechnological potential of Bacillus sp. S26 for alleviation of abiotic and biotic stresses in vine

Grapevine (Vitis spp.) is one of the most cultivated fruit plants in the world. Vineyard growers apply copper-based products in these crops to prevent fungal diseases, generating worries about Cu contamination in soils and food. In this context, this study identifies prokaryotic communities associated with grapevine plants grown under different levels of Cu-contaminated soils. Moreover, the study isolates new bacteria to improve Cu resistance in plants. Soil Cu content correlated inversely with operational taxonomic units (OTUs) belonging to the groups Acidobacteria (SubGroup 2), Latescibacteria, Pedosphaeraceae, and Candidatus Udaeobacter. A total of 14 new bacterial isolates were obtained from copper-contaminated soils. These isolates produced Indolic Compounds (IC) in a range of 25 to 96 µg mL^- 1, highlighting bacterial strains S20 and S26 as the highest producers. These new bacteria also produced siderophores, highlighting strains S19 and S26, which removed 58 and 59% of Fe ions from the CAS complex, respectively. From the in vitro antagonistic activity against Colletotrichum spp. strains, the authors identified some bacterial strains that inhibited phytopathogen growth. Bacterial strain Bacillus sp. S26 was chosen for inoculation experiments in grapevine plants. This bacterial isolate improved the growth of grapevine plants in Cu-contaminated soils. However, growth promotion did not occur in unstressed plants. More studies are necessary for developing a new bioinoculant containing S26 cells aiming to reduce biotic and abiotic stresses in grapevine.

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.

Exopolysaccharide-producing bacteria enhanced Pb immobilization and influenced the microbiome composition in rhizosphere soil of pakchoi (Brassica chinensis L.)

Lead (Pb) contamination of planting soils is increasingly serious, leading to harmful effects on soil microflora and food safety. Exopolysaccharides (EPSs) are carbohydrate polymers produced and secreted by microorganisms, which are efficient biosorbent materials and has been widely used in wastewater treatment to remove heavy metals. However, the effects and underlying mechanism of EPS-producing marine bacteria on soil metal immobilization, plant growth and health remain unclear. The potential of Pseudoalteromonas agarivorans Hao 2018, a high EPS-producing marine bacterium, to produce EPS in soil filtrate, immobilize Pb, and inhibit its uptake by pakchoi (Brassica chinensis L.) was studied in this work. The effects of strain Hao 2018 on the biomass, quality, and rhizospheric soil bacterial community of pakchoi in Pb-contaminated soil were further investigated. The results showed that Hao 2018 reduced the Pb concentration in soil filtrate (16%–75%), and its EPS production increased in the presence of Pb2+. When compared to the control, Hao 2018 remarkably enhanced pakchoi biomass (10.3%–14.3%), decreased Pb content in edible tissues (14.5%–39.2%) and roots (41.3%–41.9%), and reduced the available Pb content (34.8%–38.1%) in the Pb-contaminated soil. Inoculation with Hao 2018 raised the pH of the soil, the activity of several enzymes (alkaline phosphatase, urease, and dehydrogenase), the nitrogen content (NH4+-N and NO3−-N), and the pakchoi quality (Vc and soluble protein content), while also raising the relative abundance of bacteria that promote plant growth and immobilize metals, such as Streptomyces and Sphingomonas. In conclusion, Hao 2018 reduced the available Pb in soil and pakchoi Pb absorption by increasing the pH and activity of multiple enzymes and regulating microbiome composition in rhizospheric soil.

Challenges and opportunities for improving the environmental quality of cadmium-contaminated soil in China

Considering the soil cadmium pollution problem, the Chinese government proposed to estimate the costs and practicality "to completely improve the soil quality by the middle of this century". This study analyzed the challenges in achieving this goal using biophysical data from 10 typical demonstration soil phytoextraction projects. The current annual phytoextraction efficiency was determined as 14.8-490 g ha^-1 a^-1 at 319 RMB g^-1 cadmium. A total of 798 billion RMB and 5 years were required for remediation of cadmium contamination, which was 22 times the investment in soil remediation during 2016-2022. The break-even point of phytoextraction projects was 29 years. The heavy financial burden was considered the primary challenge in improving the environmental quality of such soil. The cost could be reduced by 5.5-35.3 % through optimization measures such as resourcefulness of hyperaccumulator harvests, large-scale breeding, and mechanized management. The break-even point could be shortened to 6-15 years by intercropping/rotating crops, contributing to the goal. Active exploration of phytoextraction efficiency-more efficient accumulators, optimized agronomic measures-is worth practicing.

Compost and microbial biostimulant applications improve plant growth and soil biological fertility of a grass-based phytostabilization system

In this work, a grass-based phytoremediation system integrated with an organic amendment and biostimulants was evaluated for remediating contaminated sites. Plant growth and biological fertility were monitored to assess the efficacy of a vegetative cap used as a safety measure to reduce sanitary and environmental risks of industrially contaminated soils and soil-washing sludges. Both matrices were potentially contaminated with Pb and Zn with an ecological risk index from low to moderate. According to potentially toxic elements (PTEs) bioaccessibility tests, the exposure to the released fine particulate matter may cause serious risks to human beings, in particular to children. The grass mixture was well adapted to both the substrates and a low PTEs mobility was detected, thus, reducing the leaching risk to ground water sources. Compost addition augmented significantly nitrogenase reductase (nifH) and ammonia monooxygenase (amoA) gene expression abundance in both substrates. Furthermore, a positive interaction between compost fertilization and a Trichoderma-based biostimulant inoculation was recorded in sludges resulting in a significant stimulation of nitrogen-fixing and ammonia-oxidizing bacteria. The application of compost and biostimulant increased soil fertility and plant growth. Furthermore, there was a slight reduction in PTE bioaccessibility, thus, improving the efficiency of the phytostabilization, limiting the resuspension and dispersion of the health-risk soil particulate.

Regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi

Endophytic fungi exist widely in plants and play an important role in the growth and adaptation of plants. They could be used in phytoremediation techniques against heavy metal contaminated soil since beneficial microbial symbionts can endow plants with resistance to external heavy metal stresses. This review summarized the regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi. Potential endophytic fungi in enhancing plant's adaption to heavy metal stresses include arbuscular mycorrhizal fungi, dark septate endophytic fungi, plant growth promoting endophytic fungi. The mechanisms involve coevolution strategy, immune regulation and detoxification transport to improve the ability of plants to adapt to heavy metal stress. They can increase the synthesis of host hormones and maintaining the balance of endogenous hormones, strengthen osmotic regulation, regulate carbon and nitrogen metabolism, and increase immune activity, antioxidant enzyme and glutathione activity. They also help to improve the detoxification transport and heavy metal emission capacity of the host by significantly producing iron carrier, metallothionein and 1-aminocyclopropane-1-carboxylic acid deaminase. The combination of endophytic fungi and hyperaccumulation plants provides a promising technology for the ecological restoration of heavy metal contaminated soil. Endophytic fungi reserves further development on enhancing host plant's adaptability to heavy metal stresses.

Vanadium-resistant endophytes modulate multiple strategies to facilitate vanadium detoxification and phytoremediation in Pteris vittata

Vanadium (V) contamination of soils poses potential risks to humans and ecosystems. This study was conducted to evaluate the effects of endophyte-assisted phytoremediation and to determine the mechanisms involved in V detoxification and plant growth promotion. Results showed that the endophytic bacterium Serratia marcescens PRE01 could successfully colonize the roots and increase the total V uptake of Pteris vittata by 25.4 %, with higher plant biomass and V accumulation in roots. Endophyte inoculation significantly improved the secretion of phytic, malic, and oxalic acids and accelerated FeVO₄ dissolution and subsequent Fe and V uptake in the rhizosphere. Under V stress without inoculation, V removed by shoot uptake, root uptake, and root surface adsorption accounted for 21.76 %, 42.14 %, and 30.93 % of the total V removal efficiency, respectively. To detoxify excess V, PRE01 effectively strengthened the adsorption of V on the root surface, with an increase in its contribution to the total V removal efficiency from 30.93 % to 38.10 %. Furthermore, beneficial endophytes could alleviate oxidative damage caused by V stress by reinforcing the plant antioxidant system and promoting V(V) reduction in root tissues. These findings clearly reveal that inoculation with endophytes is a promising method for modulating multiple strategies to enhance the phytoremediation of V-contaminated soils.

Potential of Miscanthus floridulus associated with endophytic bacterium Bacillus cereus BL4 to remediate cadmium contaminated soil

Phytoremediation assisted by endophytic bacteria is promising to efficiently remediate cadmium (Cd) contaminated soil. Bacillus cereus BL4, isolated from Miscanthus floridulus growing around a pyrite mine, exhibited high Cd tolerance and plant growth-promoting traits and could improve Cd bioavailability in soil. As a result of the pot experiment, after inoculation with strain BL4, the fresh weight, height, and Cd accumulation of Miscanthus floridulus shoots increased by 19.08-32.26 %, 6.02-16.60 %, and 23.67 %-24.88 %, respectively, and roots increased by 49.38-56.41 %, 22.87-33.93 %, and 28.51 %-42.37 %, respectively. Under Cd stress, the chlorophyll content, photosynthetic rate, and root activity of Miscanthus floridulus increased, while the membrane permeability and malonaldehyde (MDA) content significantly decreased after the inoculation of BL4, which indicated the alleviation of the cytotoxicity of Cd. Accordingly, the glutathione (GSH) content increased, and the activities of antioxidant enzymes presented downward trends after BL4 inoculation. Cd bioavailability in soil increased after BL4 inoculation, accompanied by increases in the activities of soil enzymes (invertase, urease, alkaline phosphatase, dehydrogenase, FDA hydrolase, and catalase) as well as the richness and diversity of soil bacteria. Our findings revealed that strain BL4 might strengthen the phytoremediation of Cd by Miscanthus floridulus through its effects on plant physio-biochemistry and soil microecology, which provided a basis for the relative application to Cd-contaminated soil.

Optimistic contributions of plant growth-promoting bacteria for sustainable agriculture and climate stress alleviation

Global climate change is the major cause of abiotic and biotic stresses that have adverse effects on agricultural productivity to an irreversible level, thus threatening to limit gains in production and imperil sustainable agriculture. These climate change-induced abiotic stresses, especially saline, drought, extreme temperature, and so on affect plant morphological, physiological, biochemical, and metabolic characteristics through various pathways and mechanisms, ultimately hindering plant growth, development, and productivity. However, overuse and other inappropriate uses of agrochemicals are not conducive to the protection of natural resources and the environment, thus hampering sustainable agricultural development. With the vigorous development of modern agriculture, the application of plant growth-promoting bacteria (PGPB) can better ensure sustainable agriculture, due to their ability to improve soil properties and confer stress tolerance in plants. This review deciphered the underlying mechanisms of PGPB involved in enhancing plant stress tolerance and performance under various abiotic and biotic stresses. Moreover, the recent advancements in PGPB inoculation techniques, the commercialization of PGPB-based technology and the current applications of PGPB in sustainable agriculture were extensively discussed. Finally, an outlook on the future directions of microbe-aided agriculture was pointed out. Providing insights into plant-PGPB interactions under biotic and abiotic stresses and offering evidence and strategies for PGPB better commercialization and implementation can inspire the development of innovative solutions exploiting PGPB under climatological conditions.

Research progress and hotspot analysis of rhizosphere microorganisms based on bibliometrics from 2012 to 2021

Rhizosphere microorganisms are important organisms for plant growth promotion and bio-control. To understand the research hot topics and frontier trends of rhizosphere microorganisms comprehensively and systematically, we collected 6,056 publications on rhizosphere microorganisms from Web of Science and performed a bibliometric analysis by CiteSpace 6.1.3 and R 5.3.1. The results showed that the total number of references issued in this field has been on the rise in the past decades. China, India, and Pakistan are the top three countries in terms of the number of articles issued, while Germany, the United States, and Spain were the countries with the highest number of co-published papers with other countries. The core research content in this field were the bio-control, bacterial community, ACC deaminase, phytoremediation, induced systematic resistance, and plant growth promotion. Seeding growth, Bacillus velezensis, plant-growth, and biological-control were currently and may be the highlights in the field of rhizosphere microorganisms research for a long time in the future. The above study results quantitatively, objectively, and scientifically described the research status and research focus of rhizosphere microorganisms from 2012 to 2021 from the perspective of referred papers, with a view to promoting in-depth research in this field and providing reference information for scholars in related fields to refine research trends and scientific issues.

Application of Salvinia sps. in remediation of reactive mixed azo dyes and Cr (VI) - Its pathway elucidation

The emerging industrialization has resulted in the rapid growth of textile industries across the globe. The presence of xenobiotic pollutants in textile wastewater threatens the ecosystem. Applying different microbes (bacteria, fungi & algae) has paved the way for phytoremediation - the eco-friendly, cost-effective method. The present study focuses on the phytoremediation of reactive dyes - Reactive red, Reactive Brown & Reactive Black and Cr (VI) in synthetic textile wastewater using Salvinia sps. The mixed azo dyes of each 100 mg/L showed decolourization of 75 ± 0.5% and 82 ± 0.5% of removal of 20 mg/L of Cr (VI) after eight days of incubation in a phytoreactor setup. Chlorophyll analysis revealed the gradual decrease in the photosynthetic pigments during the remediation. The degraded metabolites were analyzed using FT-IR and showed the presence of aromatic amines on day zero, which were converted to aliphatic amines on day four. The GC-MS analysis revealed the disruption of -NN- bond, rupture of -CN- bond, scission of -N-N-bond, and loss of -SO₃H from the Reactive Black dye leading to the formation of an intermediate p-Hydroxy phenylhydrazinyl. The rupture of Reactive red dye resulted in the formation of p-Hydrazinyl toluene sulphonic acid, Naphthyl amine -3,6-disulphonic acid and 8-Hydroxy Naphthyl amine -3,6-disulphonic acid. Decarboxylation, desulphonation, deoxygenation and deamination of Reactive Brown dye showed the presence of different metabolites and metabolic pathways were proposed for the reactive azo dyes which were phytoremediated.

Effects of vegetation restoration on distribution characteristics of heavy metals in soil in Karst plateau area of Guizhou

In southwest China, vegetation restoration is widely used in karst rocky desertification control projects. This technology can effectively fix the easily lost soil, gradually restore the plant community and improve soil fertility. However, the change law of soil heavy metals in the restoration process remains to be further studied. Therefore, in this work, Guizhou Caohai Nature Reserve as a typical karst area was taken as the research object to investigate the influence of vegetation restoration technology on repairing soil heavy metal pollution. The spatial distribution characteristics of soil heavy metals (chromium, nickel, arsenic, zinc, lead) before and after vegetation restoration in karst area were studied by comparative analysis and linear stepwise regression analysis. The main influencing factors and spatial distribution characteristics of heavy metals in karst area were further discussed. The results showed that: (1) heavy metals in karst soils are affected by surface vegetation, root exudates, microorganisms and leaching. Only heavy metals nickel (Ni) and lead (Pb) showed the tendency of surface enrichment and bottom precipitation enrichment in non-karst soils. Path analysis suggested that non-metallic soil factors such as soil bulk density (BD), total nitrogen (TN) and ammonium nitrogen (NH₄ ⁺-N) had direct effect on the content of heavy metals in soil. (2) The proportion of 0.25-2 mm aggregates in the surface soil of vegetation restoration belt was more than 40%, and the proportion of surface soil ≤2 mm aggregates in this increased to 83% and 88%, respectively, which could improve the soil structure and properties effectively. (3) Vegetation restoration effectively restored the nutrient elements such as carbon and nitrogen in the soil, and enhanced the soil material circulation. Furthermore the content of heavy metals in the surface soil higher than that in the 10-20 cm soil layer. Plant absorption, biosorption mechanism of microorganisms, coupling of root exudates, dissolution of soil soluble organic carbon and pH make the contents of heavy metals Cr, Ni and Pb in vegetation restoration belt slightly lower than those in karst soil. At the same time, affected by vegetation coverage, residual heavy metals in soil are further leached by surface runoff. Therefore, the content of heavy metals in soil could reduce combined heavy metal enrichment plants for extraction with remediation. This study elucidates the advantages and remedy mechanism of vegetation restoration in the remediation of heavy metal contaminated soils in Caohai area of Guizhou, and this plant activation and enrichment extraction remediation technology would be popularized and applied in the remediation of heavy metal contaminated soils in other karst areas.

Transcriptome profiling of genes regulated by phosphate-solubilizing bacteria Bacillus megaterium P68 in potato (Solanum tuberosum L.)

The insoluble phosphorus in the soil is extremely difficult to be absorbed and used directly through the potato root system. Although many studies have reported that phosphorus-solubilizing bacteria (PSB) can promote plant growth and uptake of phosphorus, the molecular mechanism of phosphorus uptake and growth by PSB has not been investigated yet. In the present study, PSB were isolated from rhizosphere soil in soybean. The data of potato yield and quality revealed that the strain P68 was the most effective In the present study, PSB identification, potato field experiment, pot experiment and transcriptome profiling to explored the role of PSB on potato growth and related molecular mechanisms. The results showed that the P68 strain (P68) was identified as Bacillus megaterium by sequencing, with a P-solubilizing ability of 461.86 mg·L^-1 after 7-day incubation in National Botanical Research Institute's Phosphate (NBRIP) medium. Compared with the control group (CK), P68 significantly increased the yield of potato commercial tubers by 17.02% and P accumulation by 27.31% in the field. Similarly, pot trials showed that the application of P68 significantly increased the biomass, total phosphorus content of the potato plants, and available phosphorus of the soil up by 32.33, 37.50, and 29.15%, respectively. Furthermore, the transcriptome profiling results of the pot potato roots revealed that the total number of bases was about 6G, and Q30 (%) was 92.35-94.8%. Compared with the CK, there were a total of 784 differential genes (DEGs) regulated when treated with P68, which 439 genes were upregulated and 345 genes were downregulated. Interestingly, most of the DEGs were mainly related to cellular carbohydrate metabolic process, photosynthesis, and cellular carbohydrate biosynthesis process. According to the KEGG pathway analysis, a total of 46 categorical metabolic pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database were annotated to 101 DEGs found in potato roots. Compared with the CK, most of the DEGs were mainly enriched in glyoxylate and dicarboxylate metabolism (sot00630), nitrogen metabolism (sot00910), tryptophan metabolism (sot00380), and plant hormone signal transduction (sot04075), and these DEGs might be involved in the interactions between Bacillus megaterium P68 and potato growth. The qRT-PCR analysis of differentially expressed genes showed that inoculated treatments P68 significantly upregulated expression of the phosphate transport, nitrate transport, glutamine synthesis, and abscisic acid regulatory pathways, respectively, and the data from qRT-PCR were consistent with that obtained from RNA-seq. In summary, PSB may be involved in the regulation of nitrogen and phosphorus nutrition, glutaminase synthesis, and abscisic acid-related metabolic pathways. This research would provide a new perspective for studying the molecular mechanism of potato growth promotion by PSB in the level of gene expression and related metabolic pathways in potato roots under the application of Bacillus megaterium P68.

Culturable bacteria diversity in stem liquid and resina from Populus euphratica and screening of plant growth-promoting bacteria

BACKGROUND

Populus euphratica Olivier is a kind of tree capable of growing in extremely arid desert and semi-desert environments. In this study, a culture-dependent method was used to analyze the bacterial diversity of stem liquid of P. euphratica and resina of P. euphratica, and to further evaluate plant growth promoting (PGP) activity.

RESULTS

A total of 434 bacteria were isolated from stem fluid and resina of P. euphratica in Ebinur Lake Wetland Nature Reserve and Mulei Primitive forest. The results of taxonomic composition analysis shows that Gammaproteobacteria, Firmicutes, and Actinobacteria_c are the three dominant groups in all the communities, and the representative genera are Bacillus, Nesterenkonia and Halomonas. The diversity analysis shows that the culturable bacterial community diversity of P. euphratica in Ebinur Lake Wetland Nature Reserve is higher than that in Mulei Primitive forest, and the bacterial community diversity of P. euphratica stem fluid is higher than that of resina. According to PGP activity evaluation, 158 functional bacteria with plant growth promoting potential were screened. Among them, 61 strains havephosphorus solubilizing abilities, 80 strains have potassium solubilizing abilities, 32 strains have nitrogen fixation abilities, and 151 strains have iron ammonia salt utilization abilities. The germination rate, plant height, and dry weight of the maize seedlings treated with strains BB33-1, TC10 and RC6 are significantly higher than those of the control group.

CONCLUSION

In this study, a large number of culturable bacteria were isolated from P. euphratica, which provides new functional bacteria sources for promoting plant growth.

Inoculation of Indigenous Arbuscular Mycorrhizal Fungi as a Strategy for the Recovery of Long-Term Heavy Metal-Contaminated Soils in a Mine-Spill Area

Symbiotic associations with arbuscular mycorrhizal fungi (AMF) offer an effective indirect mechanism to reduce heavy metal (HM) stress; however, it is still not clear which AMF species are more efficient as bioremediating agents. We selected different species of AMF: Rhizoglomus custos (Custos); Rhizoglomus sp. (Aznalcollar); and Rhizophagus irregularis (Intraradices), in order to study their inoculation in wheat grown in two soils contaminated with two levels of HMs; we tested the phytoprotection potential of the different AMF symbioses, as well as the physiological responses of the plants to HM stress. Plants inoculated with indigenous Aznalcollar fungus exhibited higher levels of accumulation, mainly in the shoots of most of the HM analyzed in heavily contaminated soil. However, the plants inoculated with the non-indigenous Custos and Intraradices showed depletion of some of the HM. In the less-contaminated soil, the Custos and Intraradices fungi exhibited the greatest bioaccumulation capacity. Interestingly, soil enzymatic activity and the enzymatic antioxidant systems of the plant increased in all AMF treatments tested in the soils with both degrees of contamination. Our results highlight the different AMF strategies with similar effectiveness, whereby Aznalcollar improves phytoremediation, while both Custos and Intraradices enhance the bioprotection of wheat in HM-contaminated environments.

Potential utilization of vitamin C industrial effluents in agriculture: Soil fertility and bacterial community composition

The potential of industrial effluents from vitamin C (VC) production was assessed for agricultural applications by monitoring plant growth, soil properties, and microbial community structure. The results demonstrated that two types of effluents-residue after evaporation (RAE) and concentrated bacterial solution after ultrafiltration (CBS)-had positive effects on the yield and VC content of pak choi. The highest yield and VC content were achieved with a combined RAE-CBS treatment (55.82 % and 265.01 % increase, respectively). The soil fertility was also enhanced by the application of RAE and CBS. Nitrate nitrogen and organic carbon contents in the soil were positively correlated with the RAE addition, while ammonium nitrogen and available phosphorus were positively correlated with the CBS addition. The diversity of bulk and rhizosphere soil bacterial communities increased significantly after the addition of RAE-CBS. The abundance of Sphingomonas and Rhizobium significantly increased after the RAE-CBS treatment, which affected aromatic compound hydrolysis and nitrogen fixation positively. Changes in plant growth and soil fertility were closely related to the upregulation of functional gene expression related to C, N, and P cycling. RAE and CBS application exerted various positive synergistic effects on plant growth, soil fertility, and bacterial community structure. Consequently, the study results confirmed the potential of RAE and CBS application in agriculture. This study provides an innovative solution for utilizing VC industrial wastewater in agriculture in a resourceful and economically beneficial manner while alleviating the corresponding environmental burden.

Indigenous rhizosphere microbial community characteristics of the phytostabilizer Athyrium wardii (Hook.) grown in a Pb/Zn mine tailing

Plant rhizosphere microbiome usually changes dramatically in adaptation to the mine environment to endure high heavy metal concentration, which in turn improves the process of revegetation and phytostabilization of mine tailing and deserves deep investigation. A field study was conducted to investigate the indigenous microbial community of a mining ecotype (ME) of the phytostabilizer Athyrium wardii (Hook.) grown in a Pb/Zn mine tailing and a corresponding non-mining ecotype (NME) grown in an uncontaminated adjacent site. Our study found a slight difference in microbial α-diversity between the ME and NME, and no significant difference between the rhizosphere and bulk soil. Both bacterial and fungal community compositions differed between the ME and NME, for which the differences were mainly driven by pH and metal contaminants. The ME harbored a unique microbial community in the rhizosphere soils different from the bulk soil and NME counterparts. The dominant phyla in the ME rhizosphere were Proteobacteria, Chloroflexi, Actinobacteria, Nitrospirae, and Ascomycota. Several genera from Proteobacteria, Acidobacteria, and Ascomycota were more abundant in the ME rhizosphere than in the NME rhizosphere. Network analysis revealed that keystone taxa were different in the two sites. Some keystone taxa from Gemmatimonadaceae, and Burkholderiaceae and Ascomycota played a critical role in microbial interactions within the mine tailing network. The unique microbial community with high tolerance in the rhizosphere soils of ME may show great benefit for plant growth and metal tolerance of the ME and thereby contributing to the process of revegetation and phytostabilization of mine tailings.

Nitrogen and Phosphorus Fertilizer Increases the Uptake of Soil Heavy Metal Pollutants by Plant Community

Soil heavy metal pollution is widespread around the world. Compared with hyperaccumulation plants, non-hyperaccumulator plant communities have many advantages in the remediation of heavy metals pollution in soil. The application of nitrogen (N) and phosphorus (P) is inexpensive and convenient, which can promote the growth of plant. N and P fertilizer might increase plant community remediation of heavy metal polluted soils. In our study, the effects of N and P fertilizer on remediation of soil Cd, Cu, Pb pollution by plant community were studied through a greenhouse experiment. Our results indicated that addition of N, P and N + P fertilizer increased plant community aboveground biomass. Simultaneously, addition of N and P fertilizer increased the accumulation of heavy metals in aboveground of the plant community and accelerated plants absorption soil heavy metals. Among them, N fertilizer had the best effect. Our results provide an inexpensive method for remediation heavy metal pollution of contaminated farmland, abandoned land and mine tailings, etc.

Bioremediation of a saline-alkali soil polluted with Zn using ryegrass associated with Fusariumincarnatum

Biotechnological strategies have become effective in the remediation of polluted soils as they are cost-effective and do not present a risk of secondary pollution. However, using a single bioremediation technique (microorganism or plant) is not suitable for achieving a high remediation rate of polluted saline-alkali soils with heavy metals. Therefore, the present study aims to assess the effects and mechanisms of combined ryegrass and Fusarium incarnatum on the zinc (Zn)-polluted saline-alkali soil over 45 days. According to the obtained results, the combined Fusarium incarnatum-ryegrass showed the highest remediation rate of 49.35% after 45 days, resulting in a significantly lower soil Zn concentration than that observed in the control group. In addition, the inoculation of Fusarium incarnatum showed a positive effect on the soil EPS secretion. The soil protein contents ranged from 0.035 to 0.055 mg/kg, while the soil polysaccharide contents increased from 0.25 to 0.61 mg/g. The soil microbial flora and ryegrass showed resistance to saline and alkaline stresses through the secretion of extracellular polysaccharides. The three-dimensional fluorescence spectrum (3D-EEM) confirmed that EPS in the soil was mainly a fulvic acid-like substance. The fluorescein diacetate (FDA) hydrolase activity in the saline-alkali soil was first increased due to the effect of Fusarium incarnatum and then decreased to a minimum value of 96 μg/(g·h). In addition, the Fusarium incarnatum inoculation improved the diversity and richness of soil fungi. Although the Fusarium incarnatum inoculation had a slight effect on the germination of ryegrass, it increased the biomass and enrichment coefficient. The results revealed a translocation factor (TF) value of 0.316 at 45 days after ryegrass sowing, showing significant enrichment of the soil Zn heavy metal zinc in the ryegrass roots.

Assessing the effects of nickel on, e.g., Medicago sativa L. nodules using multidisciplinary approach

Industrial wastes and fertilizers can introduce excessive levels of nickel (Ni) into the environment, potentially causing threats to plants, animals, as well as human beings. However, the number of studies on the effects of Ni toxicity on nodules is fairly limited. To address this issue, the effects of increasing Ni concentration on alfalfa nodules were assessed at chemical, biochemical, and transcriptomic levels. For this purpose, plants were grown in soils supplied with Ni (control, 0 mg/kg; C1, 50 mg/kg; C2, 150 mg/kg; C3, 250 mg/kg; and C4, 500 mg/kg) for 90 days. Ni loads in leaves, roots, and nodules were monitored after the exposure period. A set of biochemical biomarkers of oxidative stress was determined in nodules including antioxidants and metal homeostasis as well as lipid peroxidation. Gene expression levels of the main targets involved in oxidative stress and metal homeostasis were assessed. Our data indicated a high concentration of Ni in leaves, roots, and nodules where values reached 25.64 ± 3.04 mg/kg, 83.23 ± 5.16 mg/kg, and 125.71 ± 4.53 mg/kg in dry weight, respectively. Moreover, a significant increase in nodule biomass was observed in plants exposed to C4 in comparison to control treatment and percentage increased by 63%. Then, lipid peroxidation increased with a rate of 95% in nodules exposed to C4. Enzymatic activities were enhanced remarkably, suggesting the occurrence of oxidative stress, with increased superoxide dismutase (SOD), glutathione reductase (GR), and ascorbate peroxidase (APX). Our results showed also a significant upregulation of SOD, GR and APX genes in nodules. Nodule homoglutathione (HGSH) levels increased with the different Ni concentrations, with a remarkable decrease of glutathione S-transferase (GST) activity and glutathione (GSH) content for the highest Ni concentration with 43% and 52% reduction, respectively. The phytochelatin (PC) and metallothionein (MT) concentrations increased in nodules, which implied the triggering of a cellular protection mechanism for coping with Ni toxicity. The results suggested that Ni promotes a drastic oxidative stress in alfalfa nodules, yet the expression of MT and PC to reduce Ni toxicity could be used as Ni stress bioindicators. Our findings provide new insights into the central role of alfalfa nodules in limiting the harmful effects of soil pollution. Therefore, nodules co-expressing antioxidant enzymes may have high phytoremediation potential.

Potential application of enhanced phytoremediation for heavy metals treatment in Nepal

Heavy metals contamination in soil and water resources is a great threat to developing countries because of the lack of waste treatment facilities. A majority of wastewater treatment methods are known to be expensive and out of reach for municipalities and small pollution treatment enterprises. Phytotechnology is a promising, sustainable, environment-friendly, and cost-effective technique for domestic and industrial wastewater treatment in places where land is available. However, interest in conventional remediation methods and the lack of information on recent advances in a significant portion of the society in developing countries have restrained the applications of phytoremediation. This review discusses the concept of phytoremediation, mechanisms of heavy metals removal by plants, and the potential application of enhanced phytoremediation technologies in developing countries like Nepal. The authors also review the commercially viable hyperaccumulator species with their native distribution, heavy metals intake capacity, and their availability in Nepal. Those native plants can be utilized locally or introduced strategically in other parts/countries as well. Thus, for a flora-rich country like Nepal, this study holds great potential and presents enhanced phytoremediation as an effective and sustainable strategy for the future.

Application for Ecological Restoration of Contaminated Soil: Phytoremediation

Nowadays, with the rapid development of industry and agriculture, heavy metal pollution is becoming more and more serious, mainly deriving from natural and man-made sources [...].

The rhizosphere microbiome improves the adaptive capabilities of plants under high soil cadmium conditions

Cadmium (Cd) contamination of agricultural soils poses a potential public health issue for humans. Phytoremediation-based accumulating plants are an effective and sustainable technology for Cadmium remediation of contaminated agricultural soil. The rhizosphere microbiome can promote the growth and Cadmium accumulation in hyperaccumulators, but its taxonomic and functional traits remain elusive. The present study used two ecotypes of Sedum alfredii, an accumulating ecotype (AE) and a non-accumulating ecotype (NAE), as model plants to investigate the rhizosphere microbiome assemblages and influence on plant growth under high cadmium conditions. Our results showed that distinct root microbiomes assembled in association with both ecotypes of S. alfredii and that the assemblages were based largely on the lifestyles of the two ecotypes. In addition, we demonstrated that the functions of the microbes inhabiting the rhizosphere soils were closely associated with root-microbe interactions in both ecotypes of S. alfredii. Importantly, our results also demonstrated that the rhizosphere microbiome assembled in the AE rhizosphere soils contributed to plant growth and cadmium uptake under high cadmium conditions through functions such as nitrogen fixation, phosphorus solubilization, indole acetic acid (IAA) synthesis, and siderophore metabolism. However, this phenomenon was not clearly observed in the NAE. Our results suggest that the rhizosphere microbiome plays important roles in biogeochemical nutrient and metal cycling that can contribute to host plant fitness.

Isolation of bacterial strains from compost teas and screening of their PGPR properties on potato plants

The beneficial effect of compost and compost tea on plant growth and protection is mainly associated with the microbial diversity and the presence of bacteria with plant growth-promoting effect. PGPR are considered as eco-friendly bio-fertilizers that may reduce the use of chemical pesticides and fertilizers. Three composts (AT, A10, and A30) were previously prepared from industrial wastes (olive mill wastewater, olive pomace, coffee ground, and phosphogypsum). In the present study, we isolated three bacterial strains from the compost teas. The phylogenetic identification of these bacterial strains (B.AT, B.A10, and B.A30) showed that they correspond to Serratia liquefaciens (B.AT and B.A10) and Achromobacter spanius (B.A30) species. A further characterization of the PGPR traits of these bacteria showed that they produce siderophore, exopolysaccharides, and IAA. Their effect on potato plant growth, yields, and tuber quality was performed under field culture conditions. Results showed that these strains can be characterized as PGPR, the best effect on potato plant growth was observed with Serratia liquefaciens (B.AT), the best yield and tuber quality was observed with Serratia liquefaciens (B.A10) while bacterial treatment with Achromobacter spanius (B.A30) is a Cd-tolerant PGPR.

Core Microbiota in the Rhizosphere of Heavy Metal Accumulators and Its Contribution to Plant Performance

Persistent microbial symbioses can confer greater fitness to their host under unfavorable conditions, but manipulating such beneficial interactions necessitates a mechanistic understanding of the consistently important microbiomes for the plant. Here, we examined the phylogenetic profiles and plant-beneficial traits of the core microbiota that consistently inhabits the rhizosphere of four divergent Cd hyperaccumulators and an accumulator. We evidenced the existence of a conserved core rhizosphere microbiota in each plant distinct from that in the non-hyperaccumulating plant. Members of Burkholderiaceae and Sphingomonas were the shared cores across hyperaccumulators and accumulators. Several keystone taxa in the rhizosphere networks were part of the core microbiota, the abundance of which was an important predictor of plant Cd accumulation. Furthermore, an inoculation experiment with synthetic communities comprising isolates belonging to the shared cores indicated that core microorganisms could facilitate plant growth and metal tolerance. Using RNA-based stable isotope probing, we discovered that abundant core taxa overlapped with active rhizobacteria utilizing root exudates, implying that the core rhizosphere microbiota assimilating plant-derived carbon may provide benefits to plant growth and host phenotype such as Cd accumulation. Our study suggests common principles underpinning hyperaccumulator-microbiome interactions, where plants consistently interact with a core set of microbes contributing to host fitness and plant performance. These findings lay the foundation for harnessing the persistent root microbiomes to accelerate the restoration of metal-disturbed soils.

Characterization of soil microbial community activity and structure for reducing available Cd by rice straw biochar and Bacillus cereus RC-1

The combination of biochar and specific bacteria has been widely applied to remediate Cadmium-contaminated soil. But little is known about how such composites affect the dynamic distribution of metal fractions. This process is accompanied by the alternations of soil properties and microbial community structures. Composite of rice straw biochar and Bacillus cereus RC-1 were applied to investigate its impacts on Cd alleviation and soil microbial diversity and structure. The bacterial/biochar composite treatment decreased the fraction of HOAc-extractable Cd by 38.82%, and increased residual Cd by 23.95% compared to the untreated control. Moreover, compared with the untreated control, the composite treatment significantly increased the soil pH by about 1.5 units, and the activities of catalase, urease and invertase enzymes were increased by 42.39%, 30.50% and 31.20%, respectively. Composite treatment increased soil bacterial and fungal alpha diversity, the relative abundance of Bacillus, Streptomyces, Arthrobacter, and Aspergillus species were also increased. Mantel test and correlation analysis indicated that the effects associated with fungal communities in influencing soil properties were lower than that those of bacterial communities by different treatment. Aggregated boosted tree (ABT) models analysis showed that soil chemical proprieties (as determined by SOM, CEC, AN, etc.,) contributed over 50% of the changes in bacterial and fungal communities by the composite treatment. The co-occurrence network results showed that all treatments enhanced the correlation between OUT groups and improved the possible relationships in the bacterial and fungal communities, especially the interrelationships between bacteria and fungi after the Cd fractions stabilized. These findings provide a new insight to optimal strategies for the remediation of Cd-contaminated soil.

Plant growth-promoting bacteria in phytoremediation of metal-polluted soils: Current knowledge and future directions

Soil metal contamination is a major concern due to the ever-rising number of areas afflicted worldwide and the detrimental effects of metals to the environment and human health. Due to their non-biodegradability and toxicity, it is paramount to prevent further metal contamination and remediate the thousands of contaminated sites across the planet. Yet, conventional reclamation based on physical and chemical methods is often expensive, impractical, and triggers secondary pollution issues. Hence, microbe-aided phytoremediation has been gaining significant traction due to its environment-friendly character, cost-effectiveness, and the breakthroughs achieved during the past few decades. Microorganisms are an essential part of natural ecosystems and play a crucial role in their restoration. Indeed, plant-microbe associations in metal-polluted soils are pivotal for plants to tolerate metal toxicity and thrive in these harsh environments. Therefore, improving the understanding of this intricate relationship is invaluable for boosting phytoremediation. In this review, we focus on the potential of plant growth promoting bacteria (PGPB) for enhancing phytoremediation of metal-polluted soils. We discuss the mechanisms employed by microbes to promote plant growth and assist the removal or immobilization of metals in soil, thereby enhancing phytoextraction and phytostabilization, respectively. Microbe-mediated metal removal and detoxification through processes entailing adsorption, chelation, transformation, and precipitation, to list but a few, are also critically examined. Moreover, this work covers the direct and indirect mechanisms used by PGPB to facilitate plant acquisition of nutrients like nitrogen and phosphorus, supply and regulate phytohormones, and exert control over antagonistic microorganisms. Lastly, we provide an outlook on the future directions of microbe-aided phytoremediation and phytomining. Clearly, to fully validate and comprehend the potential of PGPB-aided phytoremediation, a considerable shift from bench-scale to field research is necessary. What's more, it is envisaged that recent advancements in genetic engineering may soon help furthering the efficiency of microbe-assisted phytoremediation.

Reactive effects of pre-sowing magnetic field exposure on morphological characteristics and antioxidant ability of Brassica juncea in phytoextraction

As magnetic fields constantly act on living and biochemical processes, it is reasonable to hypothesize that magnetic field treatment of plant seeds would enhance the uptake capacity of non-essential elements. To verify this hypothesis, seeds of Brassica juncea were treated with 50, 100, 150, 200, and 400 mT fields, and the dry weight, Cd uptake capacity, ferritin content, antioxidant enzyme activity, and phytoremediation effects of the plant were compared at the end of the experiment. Relative to the control, low- and moderate-intensity fields (50-200 mT) enhanced the dry weight of plant leaves by 15.1%, 24.5%, 35.8%, and 49.1%, respectively, whereas the high-intensity field (400 mT) decreased the biomass yield by 18.9%. The content of Cd in the above-ground tissues of B. juncea enhanced with the increasing field intensity, accompanied by an increase in oxidative damage. The activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased with exposure to low (50 and 100 mT) and moderate (150 and 200 mT) intensities, followed by a reduction at a high intensity (400 mT). Catalase activity (CAT) and ferritin content exhibited an increasing trend with increasing intensity. The Cd decontamination index of B. juncea increased with the increasing magnetic field intensity until it reached a peak at 150 mT, after which the values remained constant. Considering the phytoremediation effect and energy consumption, 150 mT was the optimal scheme for magnetic-field-assisted phytoremediation using B. juncea. This study suggests that a suitable magnetic field can be regarded as an ecologically friendly physical trigger to improve the phytoextraction effect of B. juncea.

Genome Organization and Comparative Evolutionary Mitochondriomics of Brown Planthopper, Nilaparvata lugens Biotype 4 Using Next Generation Sequencing (NGS)

Nilaparvata lugens is the main rice pest in India. Until now, the Indian N. lugens mitochondrial genome has not been sequenced, which is a very important basis for population genetics and phylogenetic evolution studies. An attempt was made to sequence two examples of the whole mitochondrial genome of N. lugens biotype 4 from the Indian population for the first time. The mitogenomes of N. lugens are 16,072 and 16,081 bp long with 77.50% and 77.45% A + T contents, respectively, for both of the samples. The mitochondrial genome of N. lugens contains 37 genes, including 13 protein-coding genes (PCGs) (cox1-3, atp6, atp8, nad1-6, nad4l, and cob), 22 transfer RNA genes, and two ribosomal RNA (rrnS and rrnL) subunits genes, which are typical of metazoan mitogenomes. However, both samples of N. lugens mitogenome in the present study retained one extra copy of the trnC gene. Additionally, we also found 93 bp lengths for the atp8 gene in both of the samples, which were 60–70 bp less than that of the other sequenced mitogenomes of hemipteran insects. The phylogenetic analysis of the 19 delphacids mitogenome dataset yielded two identical topologies when rooted with Ugyops sp. in one clade, and the remaining species formed another clade with P. maidis and M. muiri being sisters to the remaining species. Further, the genus Nilaparvata formed a separate subclade with the other genera (Sogatella, Laodelphax, Changeondelphax, and Unkanodes) of Delphacidae. Additionally, the relationship among the biotypes of N. lugens was recovered as the present study samples (biotype-4) were separated from the three biotypes reported earlier. The present study provides the reference mitogenome for N. lugens biotype 4 that may be utilized for biotype differentiation and molecular-aspect-based future studies of N. lugens.

Is Endophytic Colonization of Host Plants a Method of Alleviating Drought Stress? Conceptualizing the Hidden World of Endophytes

In the wake of changing climatic conditions, plants are frequently exposed to a wide range of biotic and abiotic stresses at various stages of their development, all of which negatively affect their growth, development, and productivity. Drought is one of the most devastating abiotic stresses for most cultivated crops, particularly in arid and semiarid environments. Conventional breeding and biotechnological approaches are used to generate drought-tolerant crop plants. However, these techniques are costly and time-consuming. Plant-colonizing microbes, notably, endophytic fungi, have received increasing attention in recent years since they can boost plant growth and yield and can strengthen plant responses to abiotic stress. In this review, we describe these microorganisms and their relationship with host plants, summarize the current knowledge on how they “reprogram” the plants to promote their growth, productivity, and drought tolerance, and explain why they are promising agents in modern agriculture.

Plant growth-promoting actinobacterial inoculant assisted phytoremediation increases cadmium uptake in Sorghum bicolor under drought and heat stresses

In this study, two proficient Cadmium (Cd) resistant and plant growth-promoting actinobacterial strains were isolated from metal-polluted soils and identified as Streptomyces sp. strain RA04 and Nocardiopsis sp. strain RA07. Multiple abiotic stress tolerances were found in these two actinobacterial strains, including Cd stress (CdS), drought stress (DS) and high-temperature stress (HTS). Both actinobacterial strains exhibited multifarious plant growth-promoting (PGP) traits such as phosphate solubilization, and production of indole-3-acetic acid, siderophores and 1-aminocyclopropane-1-carboxylate deaminase under CdS, DS and HTS conditions. The inoculation of strains RA04 and RA07 significantly increased Sorghum bicolor growth and photosynthetic pigments under CdS, DS, HTS, CdS + DS and CdS + HTS conditions as compared to their respective uninoculated plants. The actinobacterial inoculants reduced malondialdehyde concentration and enhanced antioxidant enzymes in plants cultivated under various abiotic stress conditions, indicating that actinobacterial inoculants reduced oxidative damage. Furthermore, strains RA04 and RA07 enhanced the accumulation of Cd in plant tissues and the translocation of Cd from root to shoot under CdS, CdS + DS and CdS + HTS treatments as compared to their respective uninoculated plants. These findings suggest that RA04 and RA07 strains could be effective bio-inoculants to accelerate phytoremediation of Cd polluted soil even in DS and HTS conditions.

Bacterial Community with Plant Growth-Promoting Potential Associated to Pioneer Plants from an Active Mexican Volcanic Complex

Microorganisms in extreme volcanic environments play an important role in the development of plants on newly exposed substrates. In this work, we studied the structure and diversity of a bacterial community associated to Andropogon glomeratus and Cheilanthes aemula at El Chichón volcano. The genetic diversity of the strains was revealed by genomic fingerprints and by 16S rDNA gene sequencing. Furthermore, a metagenomic analysis of the rhizosphere samples was carried out for pioneer plants growing inside and outside the volcano. Multifunctional biochemical tests and plant inoculation assays were evaluated to determine their potential as plant growth-promoting bacteria (PGPB). Through metagenomic analysis, a total of 33 bacterial phyla were identified from A. glomeratus and C. aemula rhizosphere samples collected inside the volcano, and outside the volcano 23 bacterial phyla were identified. For both rhizosphere samples, proteobacteria was the most abundant phylum. With a cultivable approach, 174 bacterial strains were isolated from the rhizosphere and tissue of plants growing outside the volcanic complex. Isolates were classified within the genera Acinetobacter, Arthrobacter, Bacillus, Burkholderia, Cupriavidus, Enterobacter, Klebsiella, Lysinibacillus, Pantoea, Pseudomonas, Serratia, Stenotrophomonas and Pandoraea. The evaluated strains were able to produce indole compounds, solubilize phosphate, synthesize siderophores, showed ACC deaminase and nitrogenase activity, and they had a positive effect on the growth and development of Capsicum chinense. The wide diversity of bacteria associated to pioneer plants at El Chichón volcano with PGPB qualities represent an alternative for the recovery of eroded environments, and they can be used efficiently as biofertilizers for agricultural crops growing under adverse conditions.

Rhizosphere Microbial Communities and Geochemical Constraining Mechanism of Antimony Mine Waste-Adapted Plants in Southwestern China

Antimony (Sb) and arsenic (As) are two hazardous metalloid elements, and the biogeochemical cycle of Sb and As can be better understood by studying plant rhizosphere microorganisms associated with Sb mine waste. In the current study, samples of three types of mine waste—Sb mine tailing, waste rocks, and smelting slag—and associated rhizosphere microorganisms of adapted plants were collected from Qinglong Sb mine, southwest China. 16S rRNA was sequenced and used to study the composition of the mine waste microbial community. The most abundant phylum in all samples was Proteobacteria, followed by Bacteroidota, Acidobacteriota, and Actinobacteriota. The community composition varied among different mine waste types. Gammaproteobacteria was the most abundant microorganism in tailings, Actinobacteria was mainly distributed in waste rock, and Saccharimonadia, Acidobacteriae, and Ktedonobacteria were mainly present in slag. At the family level, the vast majority of Hydrogenophilaceae were found in tailings, Ktedonobacteraceae, Chthoniobacteraceae, and Acidobacteriaceae (Subgroup 1) were mostly found in slag, and Pseudomonadaceae and Micrococcaceae were mainly found in waste rock. Actinobacteriota and Arthrobacter are important taxa for reducing heavy metal(loid) mobility, vegetation restoration, and self-sustaining ecosystem construction on antimony mine waste. The high concentrations of Sb and As reduce microbial diversity.

Microbial Metabolites Beneficial to Plant Hosts Across Ecosystems

Plants are intimately connected with their associated microorganisms. Chemical interactions via natural products between plants and their microbial symbionts form an important aspect in host health and development, both in aquatic and terrestrial ecosystems. These interactions range from negative to beneficial for microbial symbionts as well as their hosts. Symbiotic microbes synchronize their metabolism with their hosts, thus suggesting a possible coevolution among them. Metabolites, synthesized from plants and microbes due to their association and coaction, supplement the already present metabolites, thus promoting plant growth, maintaining physiological status, and countering various biotic and abiotic stress factors. However, environmental changes, such as pollution and temperature variations, as well as anthropogenic-induced monoculture settings, have a significant influence on plant-associated microbial community and its interaction with the host. In this review, we put the prominent microbial metabolites participating in plant-microbe interactions in the natural terrestrial and aquatic ecosystems in a single perspective and have discussed commonalities and differences in these interactions for adaptation to surrounding environment and how environmental changes can alter the same. We also present the status and further possibilities of employing chemical interactions for environment remediation. Our review thus underlines the importance of ecosystem-driven functional adaptations of plant-microbe interactions in natural and anthropogenically influenced ecosystems and their possible applications.

The endophyte Stenotrophomonas maltophilia EPS modulates endogenous antioxidant defense in safflower (Carthamus tinctorius L.) under cadmium stress

Cadmium (Cd) pollution in agricultural soils induces oxidative stress in plants that in turn is the foremost limiting factor for agricultural productivity. In past few decades, plant-metal-microbe interaction is of great interest as an emerging environmentally friendly technology that can be exploited to alleviate metal stress in plants. Considering these, in the present study an endophytic bacterium strain EPS has been isolated from the roots of common bean. The present strain was identified as Stenotrophomonas maltophilia based on 16S rRNA gene sequence. The strain showed Cd tolerance and Cd-adsorption potentials. The inoculation of strain EPS in safflower seeds significantly enhanced the antioxidant defense of plants under Cd-stress conditions through increasing the levels of antioxidant molecules like phenolics, flavonoids and carotenoids as well as improving the activities of the antioxidative enzymes including guaiacol peroxidase (POX), ascorbate peroxidase (APX) and superoxide dismutase (SOD). The output of this study is that strain EPS inoculation mitigates Cd-induced oxidative stress and consequently it may be beneficial, especially in Cd-contaminated crop fields.

Response of Rhizobacterial strains and organic amendments on chickpea growth

Plant Growth Promoting Rhizobacteria (PGPR) are beneficial bacteria that colonize plant roots and promote plant growth through a variety of mechanisms such as phosphate solubilization, phytohormones production, antifungal activity and also improve plant growth and yield. Field experiment was carried out to investigate the residual effect of organic amendments plus soil microbes along with integrated nutrient management. (PGPR) are important soil organism that promotes plant growth and yield root colonization is an example of a direct and indirect mechanism. The treatments included control, (inorganic fertilizer and no organic fertilization).Five bacterial strains were identified morphologically and biochemically screened from the rhizospheres of chickpea, lentil, barseem, mungbean, and sesame. The experiment was conducted at the Arid Zone Research Center in D.I.Khan (Pakistan). The majority of isolates resulted in significant increase in shoot length, root length, and dry matter production of Cicer arietinum seedlings' shoot and root. The experiment represented that isolates treated plots with rhizobium strain inoculation resulted in greater plant height (35.000 cm) and nodule count (38.00) No of pods per plant-1 (44.66) when compared to the control treatment, While (Mesorhizobium cicero) along with organic amendments showed significant response the greater root length (50 cm) was observed in T4 treatment. The Performance of rhizobial strains on chickpea germination in an arid environment was found to significantly increase crop germination percentage. This combination thus increases nitrogen and phosphorus uptake in inoculation treated plots. The study found that plots with inoculation treatments yielded significantly higher than non-treated plots Treatment with Mesorhizobium Cicero and compost resulted in a higher grain yield (8%) as compared to the control. The greater grain yield was observed in Treatment T4 (183.67).The result showed that use of PGPR have the potential to increase nutrient absorption from soil while improved growth of chickpea.

Increase of P and Cd bioavailability in the rhizosphere by endophytes promoted phytoremediation efficiency of Phytolacca acinosa

The information about the spatial distribution of bioavailable phosphorus (P) and heavy metal (HM) in the rhizosphere could aid in the precise phytoremediation regulation. In this study, a rhizobox system was adapted to study soil-root interactions and used to access the endophyte inoculation variation on bioavailable P and cadmium (Cd) spatial distribution during phytoremediation of Cd contaminated soils. Results showed that endophyte PE31 Bacillus cereus inoculation enhanced Cd uptake of P. acinosa by 52.70% and 46.73% in low and high Cd contaminated soils, increasing the phytoremediation hotspot area from 45.78% and 15.29% to 60.97% and 21.80%, respectively. Available P and Cd significantly diminished because root activities depleted large amounts of bioavailable P and Cd concentrations. However, PE31 increased bioavailable P and Cd concentration in the rhizosphere soil. The bioavailable P enhancement in the rhizosphere was positively correlated to plant growth and Cd accumulation. Overall, endophyte inoculation compensated the diminution of bioavailable P and Cd in the rhizosphere to improve plant biomass and HM absorption, and thus promote phytoremediation efficiency. This study helped to better understand bioavailable P and Cd spatial distribution under endophyte inoculation, which could provide effective management strategies for the precise regulation of phytoremediation.

Enhancing the Phytoremediation of Heavy Metals by Combining Hyperaccumulator and Heavy Metal-Resistant Plant Growth-Promoting Bacteria

Heavy metals (HMs) have become a major environmental pollutant threatening ecosystems and human health. Although hyperaccumulators provide a viable alternative for the bioremediation of HMs, the potential of phytoremediation is often limited by the small biomass and slow growth rate of hyperaccumulators and HM toxicity to plants. Here, plant growth-promoting bacteria (PGPB)-assisted phytoremediation was used to enhance the phytoremediation of HM-contaminated soils. A PGPB with HM-tolerant (HMT-PGPB), Bacillus sp. PGP15 was isolated from the rhizosphere of a cadmium (Cd) hyperaccumulator, Solanum nigrum. Pot experiments demonstrated that inoculation with strain PGP15 could significantly increase the growth of S. nigrum. More importantly, strain PGP15 markedly improved Cd accumulation in S. nigrum while alleviating Cd-induced stress in S. nigrum. Specifically, PGP15 inoculation significantly decreased the contents of H₂O₂, MDA, and O 2 · - in S. nigrum, while the activities (per gram plant fresh weight) of SOD, APX, and CAT were significantly increased in the PGP15-inoculated plants compared with the control sample. These results suggested that the interactions between strain PGP15 and S. nigrum could overcome the limits of phytoremediation alone and highlighted the promising application potential of the PGPB-hyperaccumulator collaborative pattern in the bioremediation of HM-contaminated soils. Furthermore, the PGP15 genome was sequenced and compared with other strains to explore the mechanisms underlying plant growth promotion by HMT-PGPB. The results showed that core genes that define the fundamental metabolic capabilities of strain PGP15 might not be necessary for plant growth promotion. Meanwhile, PGP15-specific genes, including many transposable elements, played a crucial role in the adaptive evolution of HM resistance. Overall, our results improve the understanding of interactions between HMT-PGPB and plants and facilitate the application of HMT-PGPB in the phytoremediation of HM-contaminated soils.

Rhizobacteria of Bali With Obvious Growth-Promoting Properties on Corn (Zea mays L.)

Corn productivity in Indonesia is still relatively low compared with other countries. Therefore, it is necessary to increase the productivity of corn by using rhizobacteria, which have multiple traits. This study was conducted to obtain indigenous rhizobacteria of Bali that have multiple traits, can produce indoleacetic acid (IAA), fix nitrogen from atmosphere, produce siderophores, colonize roots, increase seed germination, and promote the growth of corn. Isolation of rhizobacteria was carried out from the rhizosphere of plants belonging to the Gramineae family that grows in Bali Island, Indonesia. Six isolates, namely, Sr3, Tb9, Rg1, Rg23, Al27, and Jg8, could produce IAA, fix nitrogen from the atmosphere, produce siderophores, and increased germination rate and vigor index of corn seedling. Among them, three isolates, namely, Rg1, Sr3, and Jg8, significantly (p <0.05) increased the germination rate of corn seeds, increased vigor index, increased root dry weight and shoot dry weight of corn at the age of 7 days, and were able to colonize corn roots. Compared with the control, the rhizobacteria treatment increased the germination of corn seeds ranging from 5.04 to 13.05%. Based on the analysis of the 16S rRNA gene, it was found that these rhizobacteria species were Glutamicibacter nicotianae strain Rg1 (accession number OM349119), Brevibacillus invocatus strain Sr3 (accession number OM327515), and Micrococcus luteus strain Jg8 (accession number OM362349). Under a greenhouse condition, all the three isolates significantly (p <0.05) increased nutrient uptake, the leaf chlorophyll content, net assimilation rate, and crop growth rate of corn when compared with control. These results suggested that these isolates of rhizobacteria obviously promoted the growth of corn and can be developed as biostimulant to promote the growth and increase the corn yield in Bali, Indonesia.

Integrating Broussonetia papyrifera and Two Bacillus Species to Repair Soil Antimony Pollutions

Heavy metal resistant bacteria play an important role in the metal biogeochemical cycle in soil, but the benefits of microbial oxidation for plants and soil have not been well-documented. The purpose of this study was to explore the contribution of two Bacillus spp. to alleviate the antimony (Sb) toxicity in plants, and, then, to propose a bioremediation method for Sb contaminated soil, which is characterized by environmental protection, high efficiency, and low cost. This study explored the effects of Bacillus cereus HM5 and Bacillus thuringiensis HM7 inoculation on Broussonetia papyrifera and soil were evaluated under controlled Sb stressed conditions (0 and 100 mmol/L, antimony slag) through a pot experiment. The results show that the total root length, root volume, tips, forks, crossings, and root activities of B. papyrifera with inoculation are higher than those of the control group, and the strains promote the plant absorption of Sb from the soil environment. Especially in the antimony slag treatment group, B. cereus HM5 had the most significant effect on root promotion and promoting the absorption of Sb by B. papyrifera. Compared with the control group, the total root length, root volume, tips, forks, crossings, and root activities increased by 64.54, 70.06, 70.04, 78.15, 97.73, and 12.95%, respectively. The absorption of Sb by root, stem, and leaf increased by 265.12, 250.00, and 211.54%, compared with the control group, respectively. Besides, both B. cereus HM5 and B. thuringiensis HM7 reduce the content of malondialdehyde, proline, and soluble sugars in plant leaves, keeping the antioxidant enzyme activity of B. papyrifera at a low level, and alleviating lipid peroxidation. Principal component analysis (PCA) shows that both B. cereus HM5 and B. thuringiensis HM7 are beneficial to the maintenance of plant root functions and the improvement of the soil environment, thereby alleviating the toxicity of Sb. Therefore, B. cereus HM5 and B. thuringiensis HM7 in phytoremediation with B. papyrifera is a promising inoculant used for bacteria-assisted phytoremediation on Sb contaminated sites.

Remediation for trace metals in polluted soils by turfgrass assisted with chemical reagents

Trace metal pollution in soils is one of the universal environmental problems in the world. Phytoremediation is a green, safe, ecological, and economic method to achieve continuous reduction of soil pollutants. Turfgrass is a plant with great landscape value and has considerable biomass when used for remediation of trace metal contaminated soil. However, its remediation ability needs to be improved in future application. The combined application of turfgrass, citric acid (CA) and auxin (gibberellin, GA₃) were applied in the phytoremediation of an artificial nutritive soil derived from sludge, and a field scale orthogonal experiment (L₉) was conducted to understand the interaction effect and obtain the optimum phytoremediation. Experimental results showed that the types and cultural patterns of turfgrass mainly determined plant height, root length and trace metal concentration in turfgrass, however CA treatment was prone to increase the aboveground biomass and the concentrations of most trace metals in turfgrasses, especially the concentration of Ni in turfgrass. GA₃ spraying significantly increased the concentration of Cd in turfgrass. The culture patterns of turfgrass played 42.4% influence on acid-extractable Cd, while CA applying had 53.8% influence on the acid-extractable Ni. The annual phytoextraction amount of trace metals based on five mowing a year were proposed to assess the remediation ability of treatments, which of the combination treatment (T3, intercropping Zoysia matrella and Lolium perenne, and applying 400 mg kg^-1 CA and 30 mg kg^-1 GA₃) were 1.6-2.1 times higher CK group. This research provides technical reference for intercropping turfgrass for remediation of trace metals in sludge-derived nutritive soil.