Mitigation of heavy metal stress in the soil through optimized interaction between plants and microbes

Flavonoid metabolism plays an important role in response to lead stress in maize at seedling stage

BACKGROUND

Pb stress, a toxic abiotic stress, critically affects maize production and food security. Although some progress has been made in understanding the damage caused by Pb stress and plant response strategies, the regulatory mechanisms and resistance genes involved in the response to lead stress in crops are largely unknown.

RESULTS

In this study, to uncover the response mechanism of maize to Pb stress phenotype, physiological and biochemical indexes, the transcriptome, and the metabolome under different concentrations of Pb stress were combined for comprehensive analysis. As a result, the development of seedlings and antioxidant system were significantly inhibited under Pb stress, especially under relatively high Pb concentrations. Transcriptome analysis revealed 3559 co-differentially expressed genes(co-DEG) under the four Pb concentration treatments (500 mg/L, 1000 mg/L, 2000 mg/L, and 3000 mg/L Pb(NO3)2), which were enriched mainly in the GO terms related to DNA-binding transcription factor activity, response to stress, response to reactive oxygen species, cell death, the plasma membrane and root epidermal cell differentiation. Metabolome analysis revealed 72 and 107 differentially expressed metabolites (DEMs) under T500 and T2000, respectively, and 36 co-DEMs. KEGG analysis of the DEMs and DEGs revealed a common metabolic pathway, namely, flavonoid biosynthesis. An association study between the flavonoid biosynthesis-related DEMs and DEGs revealed 20 genes associated with flavonoid-related metabolites, including 3 for genistin and 17 for calycosin.

CONCLUSION

In summary, the study reveals that flavonoid metabolism plays an important role in response to Pb stress in maize, which not only provides genetic resources for the genetic improvement of maize Pb tolerance in the future but also enriches the theoretical basis of the maize Pb stress response.

Biological Nano-Agrochemicals for Crop Production as an Emerging Way to Address Heat and Associated Stresses

Climate change is a global problem facing all aspects of the agricultural sector. Heat stress due to increasing atmospheric temperature is one of the most common climate change impacts on agriculture. Heat stress has direct effects on crop production, along with indirect effects through associated problems such as drought, salinity, and pathogenic stresses. Approaches reported to be effective to mitigate heat stress include nano-management. Nano-agrochemicals such as nanofertilizers and nanopesticides are emerging approaches that have shown promise against heat stress, particularly biogenic nano-sources. Nanomaterials are favorable for crop production due to their low toxicity and eco-friendly action. This review focuses on the different stresses associated with heat stress and their impacts on crop production. Nano-management of crops under heat stress, including the application of biogenic nanofertilizers and nanopesticides, are discussed. The potential and limitations of these biogenic nano-agrochemicals are reviewed. Potential nanotoxicity problems need more investigation at the local, national, and global levels, as well as additional studies into biogenic nano-agrochemicals and their effects on soil, plant, and microbial properties and processes.

Microbial mediated remediation of heavy metals toxicity: mechanisms and future prospects

Heavy metal pollution has become a serious concern across the globe due to their persistent nature, higher toxicity, and recalcitrance. These toxic metals threaten the stability of the environment and the health of all living beings. Heavy metals also enter the human food chain by eating contaminated foods and cause toxic effects on human health. Thus, remediation of HMs polluted soils is mandatory and it needs to be addressed at higher priority. The use of microbes is considered as a promising approach to combat the adverse impacts of HMs. Microbes aided in the restoration of deteriorated environments to their natural condition, with long-term environmental effects. Microbial remediation prevents the leaching and mobilization of HMs and they also make the extraction of HMs simple. Therefore, in this context recent technological advancement allowed to use of bioremediation as an imperative approach to remediate polluted soils. Microbes use different mechanisms including bio-sorption, bioaccumulation, bioleaching, bio-transformation, bio-volatilization and bio-mineralization to mitigate toxic the effects of HMs. Thus, keeping in the view toxic HMs here in this review explores the role of bacteria, fungi and algae in bioremediation of polluted soils. This review also discusses the various approaches that can be used to improve the efficiency of microbes to remediate HMs polluted soils. It also highlights different research gaps that must be solved in future study programs to improve bioremediation efficency.

Harnessing plant-beneficial bacterial encapsulation: A sustainable strategy for facilitating cadmium bioaccumulation in Medicago sativa

Plant-beneficial bacteria (PBB) have emerged as a promising approach for assisting phytoremediation of heavy metal (HM)-contaminated soils. However, their colonization efficiency is often challenged by complex soil environments. In this study, we screened one rhizobacterium (Klebsiella variicola Y38) and one endophytic bacterium (Serratia surfactantfaciens Y15) isolated from HM-contaminated soils and plants for their high resistance to Cd and strong growth-promoting abilities. These strains were encapsulated individually or in combination with alginate and applied with Medicago sativa in Cd-contaminated soil pot experiments. The effectiveness of different bacterial formulations in promoting plant growth and enhancing Cd bioconcentration in M. sativa was evaluated. Results showed that PBB application enhanced plant growth and antioxidant capacity while reducing oxidative damage. Encapsulated formulations outperformed unencapsulated ones, with combined formulations yielding superior results to individual applications. Quantitative PCR indicated enhanced PBB colonization in Cd-contaminated soils with alginate encapsulation, potentially explaining the higher efficacy of alginate-encapsulated PBB. Additionally, the bacterial agents modified Cd speciation in soils, resulting in increased Cd bioaccumulation in M. sativa by 217-337 %. The alginate-encapsulated mixed bacterial agent demonstrated optimal effectiveness, increasing the Cd transfer coefficient by 3.2-fold. Structural equation modeling and correlation analysis elucidated that K. variicola Y38 promoted Cd bioaccumulation in M. sativa roots by reducing oxidative damage and enhancing root growth, while S. surfactantfaciens Y15 facilitated Cd translocation to shoots, promoting shoot growth. The combined application of these bacteria leveraged the benefits of both strains. These findings contribute to diversifying strategies for effectively and sustainably remediating Cd-contaminated soils, while laying a foundation for future investigations into bacteria-assisted phytoremediation.

The general external exposome and the development or progression of chronic kidney disease: A systematic review and meta-analyses

The impact of environmental risk factors on chronic kidney disease (CKD) remains unclear. This systematic review aims to provide an overview of the literature on the association between the general external exposome and CKD development or progression. We searched MEDLINE and EMBASE for case-control or cohort studies, that investigated the association of the general external exposome with a change in eGFR or albuminuria, diagnosis or progression of CKD, or CKD-related mortality. The risk of bias of included studies was assessed using the Newcastle-Ottawa Scale. Summary effect estimates were calculated using random-effects meta-analyses. Most of the 66 included studies focused on air pollution (n = 33), e.g. particulate matter (PM) and nitric oxides (NOx), and heavy metals (n = 21) e.g. lead and cadmium. Few studies investigated chemicals (n = 7) or built environmental factors (n = 5). No articles on other environment factors such as noise, food supply, or urbanization were found. PM2.5 exposure was associated with an increased CKD and end-stage kidney disease incidence, but not with CKD-related mortality. There was mixed evidence regarding the association of NO2 and PM10 on CKD incidence. Exposure to heavy metals might be associated with an increased risk of adverse kidney outcomes, however, evidence was inconsistent. Studies on effects of chemicals or built environment on kidney outcomes were inconclusive. In conclusion, prolonged exposure to PM2.5 is associated with an increased risk of CKD incidence and progression to kidney failure. Current studies predominantly investigate the exposure to air pollution and heavy metals, whereas chemicals and the built environment remains understudied. Substantial heterogeneity and mixed evidence were found across studies. Therefore, long-term high-quality studies are needed to elucidate the impact of exposure to chemicals or other (built) environmental factors and CKD.

Cadmium phytoremediation potential of Houttuynia cordata: Insights from growth, uptake, and rhizosphere mechanisms

Cadmium (Cd) pollutes 7.0 % of China's land area. This study examined the potential of Houttuynia cordata for Cd phytoremediation because of its ability to accumulate Cd in its growth matrix. H. cordata were planted in plastic pots filled with paddy field soils having low (LCd), medium (MCd), and high (HCd) Cd levels of 0.19, 0.69, and 2.91 mg/kg, respectively. After six months of growth, harvested plant parts were evaluated for Cd uptake and tolerance mechanisms. Metabolomics and metagenomics approaches were employed to investigate the soil rhizosphere mechanism. Results showed that the average plant biomass increased as soil Cd increased. The biomass Cd contents surpassed the allowable Cd limits for food (≤ 0.2 mg/kg) and medicinal uses (≤ 0.3 mg/kg). Cd contents were higher in H. cordata roots (30.59-86.27 mg/kg) than in other plant parts (0.63-2.90 mg/kg), with significantly increasing values as Cd soil level increased. Phenolic acids, lipids, amino acids and derivatives, organic acids, and alkaloids comprised the majority (69 in MCd vs HCd and 73 % in LCd vs HCd) of the shared upregulated metabolites. In addition, 13 metabolites specific to H. cordata root exudates were significantly increased. The top two principal metabolic pathways were arginine and proline metabolism, and beta-alanine metabolism. H. cordata increased the abundance of Firmicutes and Glomeromycota across all three Cd levels, and also stimulated the growth of Patescibacteria, Rozellomycota, and Claroideoglomus in HCd. Accordingly, H. cordata demonstrated potential for remediation of Cd-contaminated soils, and safety measures for its production and food use must be highly considered.

Advances in amelioration of air pollution using plants and associated microbes: An outlook on phytoremediation and other plant-based technologies

Globally, air pollution is an unfortunate aftermath of rapid industrialization and urbanization. Although the best strategy is to prevent air pollution, it is not always feasible. This makes it imperative to devise and implement techniques that can clean the air continuously. Plants and microbes have a natural potential to transform or degrade pollutants. Hence, strategies that use this potential of living biomass to remediate air pollution seem to be promising. The simplest future trend can be planting suitable plant-microbe species capable of removing air pollutants like SO2, CO2, CO, NOX and particulate matter (PM) along roadsides and inside the buildings. Established wastewater treatment strategies such as microbial fuel cells (MFC) and constructed wetlands (CW) can be suitably modified to ameliorate air pollution. Green architecture involving green walls and green roofs is facile and aesthetic, providing urban ecosystem services. Certain microbe-based bioreactors such as bioscrubbers and biofilters may be useful in small confined spaces. Several generative models have been developed to assist with planning and managing green spaces in urban locales. The physiological limitations of using living organisms can be circumvent by applying biotechnology and transgenics to improve their potential. This review provides a comprehensive update on not just the plants and associated microbes for the mitigation of air pollution, but also lists the technologies that are available and/or can be modified and used for air pollution control. The article also gives a detailed analysis of this topic in the form of strengths-weaknesses-opportunities-challenges (SWOC). The strategies mentioned in this review would help to attain corporate Environmental Social and Governance (ESG) and Sustainable Development Goals (SDGs), while reducing carbon footprint in the urban scenario. The review aims to emphasise that urbanization is possible while tackling air pollution using facile, green techniques involving plants and associated microbes.

Rhizosphere microbiome of plants used in phytoremediation of mine tailing dams

Rhizospheric microbial communities improve the effectiveness of hyperaccumulators in the phytoremediation of heavy metals. However, limited access to tailing dams and inadequate assessment of plants' phytoremediation potential limit the characterization of native accumulators, hindering the effectiveness of local remediation efforts. This study evaluates the heavy metal sequestration potentials of Pennisetum purpureum, Leucaena leucocephala, and Pteris vittata and their associated rhizospheric microbial communities at the Marlu and Pompora tailing dams in Ghana. The results indicate shoot hyperaccumulation of Cd (334.5 ± 6.3 mg/kg) and Fe (10,647.0 ± 12.6 mg/kg) in P. purpureum and L. leucocephala, respectively. Analysis of rhizospheric bacterial communities revealed the impact of heavy metal contamination on bacterial community composition, associating Fe and Cd hyperaccumulation with Bacillus, Arthrobacter, and Sphingomonas species. This study reports the hyperaccumulation potentials of L. leucocephala and P. purpureum enhanced by associated rhizosphere bacterial communities, suggesting their potential application as an environmentally friendly remediation process of heavy metals contaminated lands.

Preparation and characterization of green silicon nanoparticles and their effects on growth and lead (Pb) accumulation in maize (Zea mays L.)

The excessive accumulation of heavy metals, particularly lead (Pb) in agricultural soils, is a growing problem worldwide and needs urgent attention. This study aimed to prepare green silicon (Si) NPs using extract of Chenopodium quinoa leaves and evaluated their effects on Pb uptake and growth of maize (Zea mays L.). The results indicated that Pb exposure negatively affected the growth and chlorophyll contents of maize varieties, while SiNPs positively affected these attributes. Pb alone increased the electrolyte-leakage (EL), hydrogen-peroxide (H2O2) and selected antioxidant enzyme activities in leaves, whereas SiNPs decreased EL and H2O2 concentrations and further enhanced the enzyme activities as compared to their respective treatments without SiNPs. Pb-only treatments led to an increase in Pb concentrations and total Pb uptake in both shoots and roots. In contrast, SiNPs resulted in reduced Pb concentrations, with a concurrent decrease in total Pb uptake in shoots compared to the control treatment. The findings demonstrated that foliar application of SiNPs can mitigate the toxic effects of Pb in maize plants by triggering the antioxidant enzyme system and reducing the oxidative stress. Taken together, SiNPs have the potential to enhance maize production in Pb-contaminated soils. However, future research and application efforts should prioritize key aspects such as optimizing NPs synthesis, understanding positive mechanisms of green-synthesized NPs, and conducting multiple crop tests and real-world field trials.

Effects of three plant growth-promoting bacterial symbiosis with ryegrass for remediation of Cd, Pb, and Zn soil in a mining area

The quality of soil containing heavy metals (HMs) around nonferrous metal mining areas is often not favorable for plant growth. Three types of plant growth promoting rhizobacteria (PGPR)-assisted ryegrass were examined here to treat Cd, Pb, and Zn contaminated soil collected from a nonferrous metal smelting facility. The effects of PGPR-assisted plants on soil quality, plant growth, and the migration and transformation of HMs were evaluated. Results showed that inter-root inoculation of PGPR to ryegrass increased soil redox potential, urease, sucrase and acid phosphatase activities, microbial calorimetry, and bioavailable P, Si, and K content. Inoculation with PGPR also increased aboveground parts and root length, P, Si, and K contents, and antioxidant enzyme activities. The most significant effect was that the simultaneous inoculation of all three PGPRs increased the ryegrass extraction (%) of Cd (59.04-79.02), Pb (105.56-157.13), and Zn (27.71-40.79), compared to CK control (without fungi). Correspondingly, the inter-root soil contents (%) of total Cd (39.94-57.52), Pb (37.59-42.17), and Zn (34.05-37.28) were decreased compared to the CK1 control (without fungi and plants), whereas their bioavailability was increased. Results suggest that PGPR can improve soil quality in mining areas, promote plant growth, transform the fraction of HMs in soil, and increase the extraction of Cd, Pb, and Zn by ryegrass. PGPR is a promising microbe-assisted phytoremediation strategy that can promote the re-greening of vegetation in the mining area while remediating HMs pollution.

Ecological features of trace elements tolerant microbes isolated from sewage sludge of urban wastewater treatment plant

Worldwide wastewater treatment plants generate enormous amounts of sewage sludge, and their further disposal depends on the treatment technologies applied and spontaneously occurring microbiological processes. From different ages urban sewage sludge, 12 strains of bacteria with simultaneous tolerance to two or more trace elements: Co, Ni, Cu, Zn, Cd and Pb at concentration of 3-5 mmol were isolated and identified by PCR of target genes and Sanger sequencing methods. The isloated metal(loids) tolerant strains belong to the species, i.e., Serratia fonticola, Rhodococcus qingshengii, Pseudomonas fragi, Pseudomonas extremaustralis, Pseudomonas cedrina, Stenotrophomonas maltophilia, Serratia liquefaciens and Citrobacter freundii. The ecological features of the isolated strains were studied. The optimal growth temperatures for most strains was 15-30°C at pH range of 5-9, although some strains grew at 7°C (Pseudomonas fragi SS0-4, Serratia fonticola SS0-9 and Serratia fonticola SS12-11). Satisfactory growth of two strains (Serratia fonticola SS0-1and Citrobacter freundii SS60-12) was noted in an acidic medium at pH 4. Most of the strains grew in the NaCl concentration range of 1-5%. The isolated bacteria resistant to high concentrations of trace elements can be used for the effective mineralization of sewage sludge and for the decontamination of wastewater.