The Effects of the Endophytic Bacterium Pseudomonas fluorescens Sasm05 and IAA on the Plant Growth and Cadmium Uptake of Sedum alfredii Hance

Enhancing the phytoextraction efficiency of heavy metals in acidic and alkaline soils by Sedum alfredii Hance: A study on the synergistic effect of plant growth regulator and plant growth-promoting bacteria

Plant growth regulators (PGR) and plant growth-promoting bacteria (PGPB) have the potential in phytoremediation of heavy metals (HMs) contaminated soils. However, their sole application may not yield the optimal results, thus necessitating the combined application. The present study aimed to enhance the phytoremediation efficiency of Sedum alfredii Hance (S. alfredii) in acidic and alkaline soils through the combination of PGR (Brassinolide, BR) and PGPB (Pseudomonas fluorescens, P. fluorescens). The combination of BR and P. fluorescens (BRB treatment) effectively increased the removal efficiency of S. alfredii for Cd, Pb, and Zn by 355.2 and 155.3 %, 470.1 and 128.9 %, and 408.4 and 209.6 %, in acidic and alkaline soils, respectively. Moreover, BRB treatment led to a substantial increase in photosynthetic pigments contents and antioxidant enzymes activities, resulting in a remarkable increase in biomass (86.71 and 47.22 %) and dry mass (101.49 and 42.29 %) of plants grown in acidic and alkaline soils, respectively. Similarly, BRB treatment significantly elevated the Cd (109.4 and 71.36 %), Pb (174.9 and 48.03 %), and Zn levels (142.8 and 104.3 %) in S. alfredii shoots, along with cumulative accumulation of Cd (122.7 and 79.47 %), Pb (183.8 and 60.49 %), and Zn (150.7 and 117.9 %), respectively. In addition, the BRB treatment lowered the soil pH and DTPA-HMs contents, while augmenting soil enzymatic activities, thereby contributing soil microecology and facilitating the HMs absorption and translocation by S. alfredii to over-ground tissues. Furthermore, the evaluation of microbial community structure in phyllosphere and rhizosphere after remediation revealed the shift in microbial abundance. The combined treatment altered the principal effects on S. alfredii HMs accumulation from bacterial diversity to the soil HMs availability. In summary, our findings demonstrated that synergistic application of BR and P. fluorescens represents a viable approach to strengthen the phytoextraction efficacy of S. alfredii in varying soils.

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

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

Factors influencing cadmium accumulation in plants after inoculation with rhizobacteria: A meta-analysis

Rhizobacteria have the potential to enhance phytoremediation by generating substances that stimulate plant development and influence the effectiveness of cadmium (Cd) remediation by adjusting Cd availability via metal solubilization. Furthermore, rhizobacterial inoculation affects plants' metal tolerance and uptake by controlling the expression of several metal transporters, channels, and metal chelator genes. A meta-analysis was conducted to quantitatively assess the effects of rhizobacteria on Cd accumulation in plants using 207 individual observations from 47 articles. This meta-analysis showed an average Cd concentration increase of 8.09 % in plant cells under rhizobacteria treatment. The effects of different plant-microbial interactions on the bioaccumulation of Cd in plants varied. Selecting the proper rhizobacteria-plant association is essential to affect Cd buildup in plant roots and shoots. A more extended planting period (>30 days) and a suitable soil pH (<6, 7-8) would aid in the phytoextraction of Cd from the soil. This study comprehensively and quantitatively investigated the effects of plants, rhizobacteria, soil pH, planting period, experimental sites, and plant organs on plant Cd accumulation. According to the analysis of explanatory factors, plant species, planting period, soil pH, and rhizobacteria species have a more decisive influence on Cd accumulation than other factors. The results provide information for future research on the successful remediation of soils contaminated with Cd. More investigations are required to elucidate the intricate interactions between plant roots and microorganisms.

Plant growth-promoting rhizobacteria: A good companion for heavy metal phytoremediation

Phytoremediation is an environment-friendly approach regarded as a potential candidate for remediating heavy metal (HM)-contaminated soils. However, the low efficacy of phytoremediation is a major limitation that hampers its large-scale application. Therefore, developing strategies to enhance phytoremediation efficacy for contaminated soils is crucial. Plant growth-promoting rhizobacteria (PGPR) considerably contribute to phytoremediation intensification. To improve the efficiency of plant-microbe symbiosis for remediation, the mechanisms underlying PGPR-stimulated HM accumulation and tolerance in plants should be comprehensively understood. This review focuses on hyperaccumulators, PGPR, and the mechanisms by which PGPR enhance phytoremediation from four aspects: providing nutrients to plants, secreting plant hormones and specific enzymes, inducing systemic resistance, and altering the bioavailability of HMs in soils. It also provides a theoretical and technical basis for future research on PGPR synergism in promoting the phytoextraction efficiency in HM-contaminated soils.

Epigenetic Control of Plant Response to Heavy Metals

Plants are sessile organisms that must adapt to environmental conditions, such as soil characteristics, by adjusting their development during their entire life cycle. In case of low-distance seed dispersal, the new generations are challenged with the same abiotic stress encountered by the parents. Epigenetic modification is an effective option that allows plants to face an environmental constraint and to share the same adaptative strategy with their progeny through transgenerational inheritance. This is the topic of the presented review that reports the scientific progress, up to date, gained in unravelling the epigenetic response of plants to soil contamination by heavy metals and metalloids, collectively known as potentially toxic elements. The effect of the microbial community inhabiting the rhizosphere is also considered, as the evidence of a transgenerational transfer of the epigenetic status that contributes to the activation in plants of response mechanisms to soil pollution.

Bacterial Communities Associated with the Roots of Typha spp. and Its Relationship in Phytoremediation Processes

Heavy metal pollution is a severe concern worldwide, owing to its harmful effects on ecosystems. Phytoremediation has been applied to remove heavy metals from water, soils, and sediments by using plants and associated microorganisms to restore contaminated sites. The Typha genus is one of the most important genera used in phytoremediation strategies because of its rapid growth rate, high biomass production, and the accumulation of heavy metals in its roots. Plant growth-promoting rhizobacteria have attracted much attention because they exert biochemical activities that improve plant growth, tolerance, and the accumulation of heavy metals in plant tissues. Because of their beneficial effects on plants, some studies have identified bacterial communities associated with the roots of Typha species growing in the presence of heavy metals. This review describes in detail the phytoremediation process and highlights the application of Typha species. Then, it describes bacterial communities associated with roots of Typha growing in natural ecosystems and wetlands contaminated with heavy metals. Data indicated that bacteria from the phylum Proteobacteria are the primary colonizers of the rhizosphere and root-endosphere of Typha species growing in contaminated and non-contaminated environments. Proteobacteria include bacteria that can grow in different environments due to their ability to use various carbon sources. Some bacterial species exert biochemical activities that contribute to plant growth and tolerance to heavy metals and enhance phytoremediation.

Biocontrol Efficacy of Endophyte Pseudomonas poae to Alleviate Fusarium Seedling Blight by Refining the Morpho-Physiological Attributes of Wheat

Some endophyte bacteria can improve plant growth and suppress plant diseases. However, little is known about the potential of endophytes bacteria to promote wheat growth and suppress the Fusarium seedling blight pathogen Fusarium graminearum. This study was conducted to isolate and identify endophytic bacteria and evaluate their efficacy for the plant growth promotion and disease suppression of Fusarium seedling blight (FSB) in wheat. The Pseudomonas poae strain CO showed strong antifungal activity in vitro and under greenhouse conditions against F. graminearum strain PH-1. The cell-free supernatants (CFSs) of P. poae strain CO were able to inhibit the mycelium growth, the number of colonies forming, spore germination, germ tube length, and the mycotoxin production of FSB with an inhibition rate of 87.00, 62.25, 51.33, 69.29, and 71.08%, respectively, with the highest concentration of CFSs. The results indicated that P. poae exhibited multifarious antifungal properties, such as the production of hydrolytic enzymes, siderophores, and lipopeptides. In addition, compared to untreated seeds, wheat plants treated with the strain showed significant growth rates, where root and shoot length increased by about 33% and the weight of fresh roots, fresh shoots, dry roots, and dry shoots by 50%. In addition, the strain produced high levels of indole-3-acetic acid, phosphate solubilization, and nitrogen fixation. Finally, the strain demonstrated strong antagonistic properties as well as a variety of plant growth-promoting properties. Thus, this result suggest that this strain could be used as an alternate to synthetic chemicals, which can serve as an effective method of protecting wheat from fungal infection.

Cadmium stimulated cooperation between bacterial endophytes and plant intrinsic detoxification mechanism in Lonicera japonica thunb

Due to the intimate association between plant physiology and metabolism, the internal colonizing microbe (endophytes) community must be adjusted to support plant productivity in response to cell damage in plants under stress. However, how endophytes coordinate their activities with plant intrinsic mechanisms such as antioxidative systems and detoxification pathways during Cd accumulation remains unknown. In this hydroponic pot study, we investigated how exposure of Lonicera japonica. thunb. to different levels of Cd (0.5, 2.5, 5, 10, and 20 mg kg^-1) affected plant growth, metabolic pathways, and endophyte community structure and function. Although Cd accumulation increased at 5 mg kg^-1 Cd, the biomass and height of L. japonica increased in association with elevated endophyte-involved plant detoxification activities. Endophytes, such as Sphingomonas, Klenkia, and Modestobacter, expressed major antioxidative regulators (superoxide dismutase and ascorbate acid) to detoxify Cd in L. japonica. Furthermore, L. japonica and its endophytes synergistically regulated the toxic effects of Cd accumulation via multiple plant metabolic defensive pathways to increase resistance to metal-induced stress.

Detrimental effects of Cd and temperature on rice and functions of microbial community in paddy soils

Heavy metal (HM) contamination and high environmental temperature (HT) are caused by anthropogenic activities that negatively impact soil microbial communities and agricultural productivity. Although HM contaminations have deleterious effects on microbes and plants; there are hardly any reports on the combined effects of HM and HT. Here, we reported that HT coupled with cadmium (Cd) accumulation in soil and irrigated water could seriously affect crop growth and productivity, alternatively influencing the microbial community and nutrient cycles of paddy soils in rice fields. We analyzed different mechanisms of plants and microflora in the rhizospheric region, such as plant rhizospheric nitrification, endophytes colonization, nutrient uptake, and physiology of temperature-sensitive (IR64) and temperature-resistant Huanghuazhan (HZ) rice cultivars against different Cd levels (2, 5 and 10 mg kg^-1) with rice plants grown under 25 °C and 40 °C temperatures. Consequently, an increment in Cd accumulation was observed with rising temperature leading to enhanced expression of OsNTRs. In contrast, a greater decline in the microbial community was detected in IR64 cultivar than HZ. Similarly, ammonium oxidation, root-IAA, shoot-ABA production, and 16S rRNA gene abundance in the rhizosphere and endosphere were significantly influenced by HT and Cd levels, resulting in a significant decrease in the colonization of endophytes and the surface area of roots, leading to a decreased N uptake from the soil. Overall, the outcomes of this study unveiled the novel effects of Cd, temperature, and their combined effect on rice growth and functions of the microbial community. These results provide effective strategies to overcome Cd-phytotoxicity on the health of endophytes and rhizospheric bacteria in Cd-contaminated soil by using temperature-tolerant rice cultivars.

Development of a Multicomponent Microbiological Soil Inoculant and Its Performance in Sweet Potato Cultivation

The cultivation and consumption of sweet potato (Ipomoea batatas) are increasing globally. As the usage of chemical fertilizers and pest control agents during its cultivation may lead to soil, water and air pollution, there is an emerging need for environment-friendly, biological solutions enabling increased amounts of healthy crop and efficient disease management. Microbiological agents for agricultural purposes gained increasing importance in the past few decades. Our goal was to develop an agricultural soil inoculant from multiple microorganisms and test its application potential in sweet potato cultivation. Two Trichoderma strains were selected: Trichoderma ghanense strain SZMC 25217 based on its extracellular enzyme activities for the biodegradation of plant residues, and Trichoderma afroharzianum strain SZMC 25231 for biocontrol purposes against fungal plant pathogens. The Bacillus velezensis strain SZMC 24986 proved to be the best growth inhibitor of most of the nine tested strains of fungal species known as plant pathogens, therefore it was also selected for biocontrol purposes against fungal plant pathogens. Arthrobacter globiformis strain SZMC 25081, showing the fastest growth on nitrogen-free medium, was selected as a component with possible nitrogen-fixing potential. A Pseudomonas resinovorans strain, SZMC 25872, was selected for its ability to produce indole-3-acetic acid, which is among the important traits of potential plant growth-promoting rhizobacteria (PGPR). A series of experiments were performed to test the selected strains for their tolerance to abiotic stress factors such as pH, temperature, water activity and fungicides, influencing the survivability in agricultural environments. The selected strains were used to treat sweet potato in two separate field experiments. Yield increase was observed for the plants treated with the selected microbial consortium (synthetic community) in comparison with the control group in both cases. Our results suggest that the developed microbial inoculant has the potential to be used in sweet potato plantations. To the best of our knowledge, this is the first report about the successful application of a fungal-bacterial consortium in sweet potato cultivation.

Auxin alleviates cadmium toxicity by increasing vacuolar compartmentalization and decreasing long-distance translocation of cadmium in Poa pratensis

Kentucky bluegrass (Poa pratensis L.) hyperaccumulates cadmium (Cd) and exhibits a hypertolerance. Thus, it has potential for the phytoremediation of Cd-containing soil. Auxin signaling is involved in the response to Cd stress. However, the mechanisms of auxin-mediated detoxification and Cd translocation in plants remain unclear. This study aimed to investigate the effects of exogenous application of indole-3-acetic acid (IAA) on the Cd translocation, subcellular Cd distribution, chemical forms of Cd, and transcriptional regulation of Kentucky bluegrass. The results showed that the exogenous application of IAA increased the amount of organelle-bound Cd and vacuole-compartmentalized Cd in root cells, reduced the Cd concentration in the leaf tissues (epidermis, mesophyll, and vascular bundle) and root tissues (rhizodermis and cortex) but increased in the stele, and alleviate Cd-induced leaf chlorosis and growth inhibition. The expression of genes associated with Cd transporters (ABCs, ZIPs, NASs, OPTs, and YSLs), phosphatases, oxalate decarboxylases and lignin biosynthesis were significantly regulated by exogenous IAA under Cd stress. A positive regulation of phosphatases and oxalate decarboxylases genes related to an increase in phosphate- and oxalate-bound Cd, as well as a decrease in pectate- and protein-bound Cd and inorganic Cd, thereby contributing to a decrease in Cd phytotoxicity. The significant regulation of Cd transporters associated with decreasing the long-distance translocation of Cd, and the activation of lignin biosynthesis may contribute to the development of root endodermal barriers and increase the deposition of undissolved Cd phosphates and oxalate-bound Cd in the stele. These results revealed the important role of auxin in Cd detoxification and translocation in Kentucky bluegrass and they provide a theoretical basis for the phytoremediation of Cd-containing soil.

Alleviation of metal stress in rape seedlings (Brassica napus L.) using the antimony-resistant plant growth-promoting rhizobacteria Cupriavidus sp. S-8-2

This study investigated an effective strategy for remediating antimony (Sb)-contaminated soil using the bacterial strain screened from Sb-contaminated fern rhizospheres due to its superior growth-promoting, heavy-metal(loid) resistant, and antibiotic-tolerant characteristics. The strain that belongs to Cupriavidus sp. was determined by 16S rRNA sequencing and showed no morphological changes when grown with high concentrations of Sb (608.8 mg/L). The strain showed prominent indole acetic acid (IAA), phosphate-solubilizing abilities, and ACC deaminase activity under Sb stress. Moreover, IAA and soluble phosphate levels increased in the presence of 608.8 mg/L Sb. Inoculation of rape seedlings with Cupriavidus sp. S-8-2 enhanced several morphological and biochemical growth features compared to untreated seedlings grown under Sb stress. Inoculation of Cupriavidus sp. S-8-2 increased root weight by more than four-fold for fresh weight and over two-fold for dry weight, despite high environmental Sb. The strain also reduced Sb-mediated oxidative stress and malondialdehyde contents by reducing Sb absorption, thus alleviating Sb-induced toxicity. Environmental Scanning Electron Microscope (ESEM) imaging and dilution plating technique revealed Cupriavidus sp. S-8-2 is localized on the surface of roots. Identifying the Sb-resistant plant growth-promoting bacterium suggested its usefulness in the remediation of contaminated agricultural soil and for the promotion of crop growth. We highly recommend the strain for further implementation in field experiments.

Metabolic profile and molecular characterization of endophytic bacteria isolated from Pinus sylvestris L. with growth-promoting effect on sunflower

Endophytic bacteria inhabit plant tissues such as roots, stems, leaves, fruits, and seeds and can multiply inside plant tissue without damaging them. This study involves the isolation, characterization, metabolic profiling, and effect of endophytic bacteria isolated from the roots of Scots pine (Pinus sylvestris), on the growth of sunflower. In the current study, fifteen isolates of endophytic bacteria were obtained from the roots of Scots pine, and their molecular characterization was performed using 16 s rRNA ribotyping. The molecular characterization revealed that the strains belonged to Bacillus spp., Pseudomonas spp., Micrococcus sp., Serratia sp., Enterobacter sp., Pantoea sp., Staphylococcus sp., and Microbacterium sp. Among the isolated strains, 9 strains showed positive results for ammonium production, 12 strains for calcium solubilization, 11 strains for magnesium solubilization, 5 strains for zinc solubilization, 12 strains for phosphate solubilization, 8 strains for potassium solubilization, 10 strains for indole acetic acid (IAA) production, 9 strains for siderophore, and 6 strains for hydrogen cyanide (HCN) production. The greenhouse experiment results demonstrated that all isolated endophytic bacteria improved the shoot length, dry weight, and chlorophyll content of sunflower, whereas a significant increase was observed by PS-3 (Bacillus cereus), PS-6 (Serratia marcescens), and PS-8 (Pseudomonas putida). Besides, the concentration of nitrogen, phosphorus, and potassium were also measured in sunflower shoots, and results asserted that bacterial inoculation increased the bioavailability of these essential nutrients to plants compared to uninoculated control. Thus, these endophytic bacteria could be used as an encouraging option to improve plant growth and performance.

Dark septate endophyte Exophiala pisciphila promotes maize growth and alleviates cadmium toxicity

Dark septate endophytes (DSE) are typical root endophytes with the ability to enhance plant growth and tolerance to heavy metals, but the underlying mechanisms are unclear. Here, the physiological and molecular mechanisms of a DSE strain, Exophiala pisciphila, in mitigating cadmium (Cd, 20 mg/kg) toxicity in maize were investigated. Our results showed, under Cd stress, E. pisciphila inoculation enhanced the biomass of maize and reduced both inorganic and soluble forms of Cd (high toxicity) by 52.6% in maize leaves, which may be potentially contributing to Cd toxicity mitigation. Besides, E. pisciphila inoculation significantly affected the expression of genes involved in the signal transduction and polar transport of phytohormone, and then affected abscisic acid (ABA) and indole-3-acetic acid (IAA) contents in maize roots, which was the main reason for promoting maize growth. In addition, E. pisciphila also made a 27% increase in lignin content by regulating the expression of genes involved in the synthesis of it, which was beneficial to hinder the transport of Cd. In addition, E. pisciphila inoculation also activated glutathione metabolism by the up-regulation of genes related to glutathione S-transferase. This study helps to elucidate the functions of E. pisciphila under Cd stress, sheds light on the mechanism of detoxifying Cd and provides new insights into the protection of crops from heavy metals.

Utilization of Indole Acetic Acid with Leucadendron rubrum and Rhododendron pulchrum for the Phytoremediation of Heavy Metals in the Artificial Soil Made of Municipal Sewage Sludge

The development of phytoremediation by garden plants is an effective way to deal with the dilemma of municipal sewage sludge disposal. In this study, two ornamental plants were used as phytoremediation plants to rehabilitate heavy-metal-contaminated municipal sewage sludge in field experiments, and the role of exogenous phytohormone IAA was also tested. Ornamental plants Loropetalum chinense var. rubrum (L. rubrum) and Rhododendron pulchrum (R. pulchrum) adapted well to the artificial soil made of municipal sewage sludge, and the concentrations of Cu, Zn, Pb, and Ni were decreased by 7.29, 261, 20.2, and 11.9 mg kg−1, respectively, in the soil planted with L. rubrum, and 7.60, 308, 50.1, and 17.7 mg kg−1, respectively, in the soil planted with R. pulchrum, accounted for 11−37% of the total amounts and reached significant levels (p < 0.05), except Cd. The concentration of Pb in all parts of the two ornamental plants was increased, as well as most heavy metals in L. rubrum root. As a result, three months after transplant, the phyto-extraction amounts in L. rubrum were 397, 10.9, and 1330 μg for Ni, Cd, and Pb, respectively, increased by 233% to 279%. The phyto-extraction amount in R. pulchrum were 1510, 250, and 237 μg for Zn, Pb, and Cu, respectively, increased by 143% to 193%. These results indicated a potential to remediate heavy metals of the two ornamental plants, especially L. rubrum. The results of correlation analysis implied that the interaction of heavy metals in the plant itself played an important role in the uptake of heavy metals. This seemed to explain why applying IAA in the experiment had little effect on plant growth and phytoremediation of heavy metals. This study provided a green and feasible idea for the proper disposal of municipal sewage sludge.

The role of auxins and auxin-producing bacteria in the tolerance and accumulation of cadmium by plants

Cadmium (Cd) is one of the most toxic heavy metals for plant physiology and development. This review discusses Cd effects on auxin biosynthesis and homeostasis, and the strategies for restoring plant growth based on exogenous auxin application. First, the two well-characterized auxin biosynthesis pathways in plants are described, as well as the effect of exogenous auxin application on plant growth. Then, review describes the impacts of Cd on the content, biosynthesis, conjugation, and oxidation of endogenous auxins, which are related to a decrease in root development, photosynthesis, and biomass production. Finally, compelling evidence of the beneficial effects of auxin-producing rhizobacteria in plants exposed to Cd is showed, focusing on photosynthesis, oxidative stress, and production of antioxidant compounds and osmolytes that counteract Cd toxicity, favoring plant growth and improve phytoremediation efficiency. Expanding our understanding of the positive effects of exogenous auxins application and the interactions between bacteria and plants growing in Cd-polluted environments will allow us to propose phytoremediation strategies for restoring environments contaminated with this metal.

Inhibition of shoot-expressed NRT1.1 improves reutilization of apoplastic iron under iron-deficient conditions

Iron deficiency is a major constraint for plant growth in calcareous soils. The interplay between NO₃ ^- and Fe nutrition affects plant performance under Fe-deficient conditions. However, how NO₃ ^- negatively regulates Fe nutrition at the molecular level in plants remains elusive. Here, we showed that the key nitrate transporter NRT1.1 in Arabidopsis plants, especially in the shoots, was markedly downregulated at post-translational levels by Fe deficiency. However, loss of NRT1.1 function alleviated Fe deficiency chlorosis, suggesting that downregulation of NRT1.1 by Fe deficiency favors plant tolerance to Fe deficiency. Further analysis showed that although disruption of NRT1.1 did not alter Fe levels in both the shoots and roots, it improved the reutilization of apoplastic Fe in shoots but not in roots. In addition, disruption of NRT1.1 prevented Fe deficiency-induced apoplastic alkalization in shoots by inhibiting apoplastic H⁺ depletion via NO₃ ^- uptake. In vitro analysis showed that reduced pH facilitates release of cell wall-bound Fe. Thus, foliar spray with an acidic buffer promoted the reutilization of Fe in the leaf apoplast to enhance plant tolerance to Fe deficiency, while the opposite was true for the foliar spray with a neutral buffer. Thus, downregulation of the shoot-part function of NRT1.1 prevents apoplastic alkalization to ensure the reutilization of apoplastic Fe under Fe-deficient conditions. Our findings may provide a basis for elucidating the link between N and Fe nutrition in plants and insight to scrutinize the relevance of shoot-expressed NRT1.1 to the plant response to stress.

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

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

Improvement of the Cd and Zn phytoremediation efficiency of rice (Oryza sativa) through the inoculation of a metal-resistant PGPR strain

Cadmium (Cd) and zinc (Zn) in contaminated soil inhibit rice yield and produce toxic effects on human body through rice accumulation. Plant growth promoting rhizobacteria (PGPR) assisted phytoremediation is an effective ecological measure to improve the remediation efficiency of heavy metal contaminated soil. The purpose of this study was to investigate the efficiency of the combination of rice and Cd/Zn-tolerant PGPR strain Bacillus sp. ZC3-2-1 for the remediation of Cd-Zn contaminated soil. Moreover, the effects of inoculations on rhizosphere bacterial communities and ion homeostasis of rice under Cd-Zn exposure will also be explored. The results showed that compared with the treatment without inoculation, ZC3-2-1 decreased the bioavailable Cd and Zn concentrations in soil by 39.3% and 32.0%, respectively, and increase the phytoextraction of Cd²⁺ and Zn²⁺ by rice to 48.2% and 8.0%, respectively. This inoculation process significantly increased the rice biomass, resulting that the contents of Cd²⁺ and Zn²⁺ per biomass unit of rice didn't change significantly. This fact meant that ZC3-2-1 could improve the phytoremediation efficiency of Cd-Zn contaminated soil by promoting the phytoextraction and immobilization of the metal, while might not affect the crop food safety. Besides, through regulation of the Na⁺ and Mg²⁺ concentration in rice, ZC3-2-1 played a positive role in maintaining ion homeostasis which was disrupted by Zn or Cd. Moreover, ZC3-2-1 could modulate the beneficial bacterial communities in rice rhizosphere soil, and then enhanced Cd-Zn immobilization and enzyme activities in soil, leading to the enhancement of rice growth and phytoremediation efficiency. Above all, this study provided novel insights into developing an efficient phytoremediation system and safe production of rice in Cd-Zn contaminated soil with the application of Bacillus sp. ZC3-2-1, as well as advance our understanding of the principles of rhizosphere bacterial community assemble and maintaining ion homeostasis in rice during this phytoremediation process.

Long term application of plant growth-promoting bacterium improved grain weight and reduced arsenic accumulation in rice grain: A comparison of 10 bacteria

Rice (Oryza sativa L.) is one of the main food crops, it plays an important role in the human diet. Arsenic (As) contamination in paddy soil inhibits rice growth and reduces rice yield seriously. In addition, As accumulated in rice grains was harmful to human health through the food chain. Using the exogenous method to alleviate As stress and reduce As accumulation in rice grain is one of the potential ways to achieve food safety in polluted farmland. In the present study, 10 bacteria was applied to evaluate the effects of plant growth-promoting bacteria (PGPBs) on rice growth and As accumulation in rice grain. The results showed higher levels of As inhibited PGPB growth, the most tolerant and sensitive bacteria were Bj05 and Ls09, with the growth reduction of 16.9% and 96.7% under 50 mM As, respectively. Most of 10 PGPBs enhanced rice growth and improved rice grain weight under As exposure, among them, Ts06 showed the most effective one. Six of 10 PGPBs reduced rice grain As levels significantly, the highest reduction of grain As was observed in Ts06 inoculated rice, with grain As deceasing to 46.3% of the control. Bj05 was the only one which caused the increase in grain As of Yangdao 6. The Pearson correlation analysis showed grain As concentration negatively correlated with leave As concentration, while did not correlated with total As accumulated in shoot, and soil available As and P. The present results indicated that some PGPBs inhibited As translocation from leave to grain, thus reduced As accumulation in rice grain. Ts06 was suggested to be a candidate as microbial amendments for As-contaminated paddy fields.

Cadmium removal potential of hyperaccumulator Solanum nigrum L. under two planting modes in three years continuous phytoremediation

Solanum nigrum L. is a Cd hyperaccumulator, but the potential for continuous remediation, or different planting methods have not been fully characterized. The potential for continuous phytoremediation of Cd-contaminated farmland soil (2.08 mg kg^-1 Cd) by 2 planting methods (flowering harvest twice a year and maturity harvest once a year) was studied in a 3-year pot experiment. The total Cd accumulation (ug plant^-1) of the 3-year flowering stage treatments was 26.3% higher than that of the maturity stage treatments, which was mainly due to that flowering harvest twice a year caused 65.5% increase of shoot biomass. Similarly, the Cd decreased concentration in soil and Cd removal rate in the flowering stage treatments were 29.2% and 27.9% higher than that in the maturity stage treatments, respectively. After 3 years of phytoremediation, the extractable Cd concentration in soil was reduced by 36.4% in the flowering stage treatments and by 27.6% in the maturity stage treatments, which also led to the same decreasing trend of Cd accumulation of S. nigrum. In conclusion, the study results have demonstrated that the planting mode of two harvests a year at the flowering stage seems to be a viable option to apply for continuous phytoremediation of Cd-contaminated farmland soil.

Plant growth-promoting bacteria in metal-contaminated soil: Current perspectives on remediation mechanisms

Heavy metal contamination in soils endangers humans and the biosphere by reducing agricultural yield and negatively impacting ecosystem health. In recent decades, this issue has been addressed and partially remedied through the use of “green technology,” which employs metal-tolerant plants to clean up polluted soils. Furthermore, the global climate change enhances the negative effects of climatic stressors (particularly drought, salinity, and extreme temperatures), thus reducing the growth and metal accumulation capacity of remediating plants. Plant growth-promoting bacteria (PGPB) have been widely introduced into plants to improve agricultural productivity or the efficiency of phytoremediation of metal-contaminated soils via various mechanisms, including nitrogen fixation, phosphate solubilization, phytohormone production, and biological control. The use of metal-tolerant plants, as well as PGPB inoculants, should hasten the process of moving this technology from the laboratory to the field. Hence, it is critical to understand how PGPB ameliorate environmental stress and metal toxicity while also inducing plant tolerance, as well as the mechanisms involved in such actions. This review attempts to compile the scientific evidence on this topic, with a special emphasis on the mechanism of PGPB involved in the metal bioremediation process [plant growth promotion and metal detoxification/(im)mobilization/bioaccumulation/transformation/translocation] and deciphering combined stress (metal and climatic stresses) tolerance.

Combined Plant Growth-Promoting Bacteria Inoculants Were More Beneficial than Single Agents for Plant Growth and Cd Phytoextraction of Brassica juncea L. during Field Application

Single or combined plant growth-promoting bacteria (PGPB) strains were widely applied as microbial agents in cadmium (Cd) phytoextraction since they could promote plant growth and facilitate Cd uptake. However, the distinct functional effects between single and combined inoculants have not yet been elucidated. In this study, a field experiment was conducted with single, double and triple inoculants to clarify their divergent impacts on plant growth, Cd uptake and accumulation at different growth stages of Brassica juncea L. by three different PGPB strains (Cupriavidus SaCR1, Burkholdria SaMR10 and Sphingomonas SaMR12). The results show that SaCR1 + SaMR10 + SaMR12 combined inoculants were more effective for growth promotion at the bud stage, flowering stage, and mature stage. Single/combined PGPB agents of SaMR12 and SaMR10 were more efficient for Cd uptake promotion. In addition, SaMR10 + SaMR12 combined the inoculants greatly facilitated Cd uptake and accumulation in shoots, and enhanced the straw Cd extraction rates by 156%. Therefore, it is concluded that the application of PGPB inoculants elevated Cd phytoextraction efficiency, and the combined inoculants were more conductive than single inoculants. These results enriched the existing understanding of PGPB agents and provided technical support for the further exploration of PGPB interacting mechanisms strains on plant growth and Cd phytoextraction, which helped establish an efficient plant–microbe combined phytoremediation system and augment the phytoextraction efficiency in Cd-contaminated farmlands.

Novel Pseudomonas sp. SCA7 Promotes Plant Growth in Two Plant Families and Induces Systemic Resistance in Arabidopsis thaliana

Pseudomonas sp. SCA7, characterized in this study, was isolated from roots of the bread wheat Triticum aestivum. Sequencing and annotation of the complete SCA7 genome revealed that it represents a potential new Pseudomonas sp. with a remarkable repertoire of plant beneficial functions. In vitro and in planta experiments with the reference dicot plant A. thaliana and the original monocot host T. aestivum were conducted to identify the functional properties of SCA7. The isolate was able to colonize roots, modify root architecture, and promote growth in A. thaliana. Moreover, the isolate increased plant fresh weight in T. aestivum under unchallenged conditions. Gene expression analysis of SCA7-inoculated A. thaliana indicated a role of SCA7 in nutrient uptake and priming of plants. Moreover, confrontational assays of SCA7 with fungal and bacterial plant pathogens revealed growth restriction of the pathogens by SCA7 in direct as well as indirect contact. The latter indicated involvement of microbial volatile organic compounds (mVOCs) in this interaction. Gas chromatography-mass spectrometry (GC-MS) analyses revealed 1-undecene as the major mVOC, and octanal and 1,4-undecadiene as minor abundant compounds in the emission pattern of SCA7. Additionally, SCA7 enhanced resistance of A. thaliana against infection with the plant pathogen Pseudomonas syringae pv. tomato DC3000. In line with these results, SA- and JA/ET-related gene expression in A. thaliana during infection with Pst DC3000 was upregulated upon treatment with SCA7, indicating the ability of SCA7 to induce systemic resistance. The thorough characterization of the novel Pseudomonas sp. SCA7 showed a remarkable genomic and functional potential of plant beneficial traits, rendering it a promising candidate for application as a biocontrol or a biostimulation agent.

Cadmium phytoextraction through Brassica juncea L. under different consortia of plant growth-promoting bacteria from different ecological niches

Combined bioaugmentation inoculants composed of two or more plant growth-promoting bacteria (PGPB) were more effective than single inoculants for plant growth and cadmium (Cd) removal in contaminated soils. However, the principles of consortia construction still need to be discovered. Here, a pot experiment with Cd natural polluted soil was conducted and PGPB consortia with different ecological niches from hyperaccumulator Sedum alfredii Hance were used to compare their effects and mechanisms on plant growth condition, Cd phytoextraction efficiency, soil enzymatic activities, and rhizospheric bacterial community of Brassica juncea L. The results showed that both rhizospheric and endophytic PGPB consortia inoculants promoted plant growth (6.9%-22.1%), facilitated Cd uptake (230.0%-350.0%) of oilseed rape, increased Cd phytoextraction efficiency (343.0%-441.0%), and enhanced soil Cd removal rates (92.0%-144.0%). PGPB consortia inoculants also enhanced soil microbial carbon by 22.2%-50.5%, activated the activities of soil urease and sucrase by 74.7%-158.4% and 8.4%-61.3%, respectively. Simultaneously, PGPB consortia inoculants increased the relative abundance of Flavobacterium, Rhodanobacter, Kosakonia, Pseudomonas and Paraburkholderia at the genus level, which may be beneficial to plant growth promotion and bacterial phytopathogen biocontrol. Although the four PGPB consortia inoculants promoted oilseed growth, amplified Cd phytoextraction, and changed bacterial community structure in rhizosphere soil, their original ecological niches were not a decisive factor for the efficiency of PGPB consortia. therefore, the results enriched the present knowledge regarding the significant roles of PGPB consortia as bioaugmentation agents and preliminarily explored construction principles of effective bioaugmentation inoculants, which will provide insights into the microbial responses to combined inoculation in the Cd-contaminated soils.

Plants-Microorganisms-Based Bioremediation for Heavy Metal Cleanup: Recent Developments, Phytoremediation Techniques, Regulation Mechanisms, and Molecular Responses

Rapid industrialization, mine tailings runoff, and agricultural activities are often detrimental to soil health and can distribute hazardous metal(loid)s into the soil environment, with harmful effects on human and ecosystem health. Plants and their associated microbes can be deployed to clean up and prevent environmental pollution. This green technology has emerged as one of the most attractive and acceptable practices for using natural processes to break down organic contaminants or accumulate and stabilize metal pollutants by acting as filters or traps. This review explores the interactions between plants, their associated microbiomes, and the environment, and discusses how they shape the assembly of plant-associated microbial communities and modulate metal(loid)s remediation. Here, we also overview microbe-heavy-metal(loid)s interactions and discuss microbial bioremediation and plants with advanced phytoremediation properties approaches that have been successfully used, as well as their associated biological processes. We conclude by providing insights into the underlying remediation strategies' mechanisms, key challenges, and future directions for the remediation of metal(loid)s-polluted agricultural soils with environmentally friendly techniques.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

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

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

Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation

Heavy metal pollution in soil is a global problem with serious impacts on human health and ecological security. Phytoextraction in phytoremediation, in which plants uptake and transport heavy metals (HMs) to the tissues of aerial parts, is the most environmentally friendly method to reduce the total amount of HMs in soil and has wide application prospects. However, the molecular mechanism of phytoextraction is still under investigation. The uptake, translocation, and retention of HMs in plants are mainly mediated by a variety of transporter proteins. A better understanding of the accumulation strategy of HMs via transporters in plants is a prerequisite for the improvement of phytoextraction. In this review, the biochemical structure and functions of HM transporter families in plants are systematically summarized, with emphasis on their roles in phytoremediation. The accumulation mechanism and regulatory pathways related to hormones, regulators, and reactive oxygen species (ROS) of HMs concerning these transporters are described in detail. Scientific efforts and practices for phytoremediation carried out in recent years suggest that creation of hyperaccumulators by transgenic or gene editing techniques targeted to these transporters and their regulators is the ultimate powerful path for the phytoremediation of HM contaminated soils.

Rhodococcus qingshengii facilitates the phytoextraction of Zn, Cd, Ni, and Pb from soils by Sedum alfredii Hance

The enhanced heavy metal (HM) phytoextraction efficiency of hyperaccumulating plants via plant-growth-promoting microbes has been proposed as an effective strategy to remove HMs from contaminated soil. Nevertheless, it remains unclear whether catabolizing the abscisic acid (ABA) in hyperaccumulating plants via rhizobacteria can facilitate HM phytoextraction. In the present study, a hyperaccumulator, Sedum alfredii Hance, inoculated with an ABA-catabolizing bacterium Rhodococcus qingshengii, showed higher concentrations of Zn, Cd, Ni, and Pb in the contaminated paddy-grown plant shoots by 35%, 63%, 49%, and 49%, and in plants grown in mine soils by 112%, 105%, 46%, and 49%, respectively, than in the control_{bacteria-free} plants. However, no significant changes were observed in Cu content between these plants. Furthermore, parameters indicating phytoremediation potential, including the translocation factor (TF) and bioconcentration factor (BCF), revealed that bacterial inoculation could markedly increase the efficacy of Zn, Cd, Ni, and Pb phytoextraction from the soil. Notably, the bioavailabilities of HMs in soils were not influenced by R. qingshengii; however, the expression of transporters related to the uptake of these HMs, including SaIRT1, SaZIP1, SaZIP2, SaZIP3, SaNramp1, SaNramp3, SaNramp6, SaHMA2, and SaHMA3, was upregulated. These findings indicate that R. qingshengii inoculation could increase the HM-uptake ability of plants by catabolizing ABA and may provide a promising strategy for enhancing the phytoremediation efficacy in HM-contaminated soils.

Enhanced effects of walnut green husk solution on the phytoextraction of soil Cd and Zn and corresponding microbial responses

Walnut green husk (WGH) is a common agricultural waste, but it may be conducive to phytoremediation of heavy metals owing to its abundant phenolic hydroxyl, carboxyl, and other functional groups. In this study, WGH solution was used as an enhancer in the phytoextraction process via Sedum plumbizincicola, a hyperaccumulator of Zn and Cd. Microbial responses in the soil and plants were seamlessly analyzed to determine the underlying mechanisms of heavy metal extraction in this process. The results showed that the addition of 0.05 g/mL WGH solution increased the accumulation of Cd (by 153%) and Zn (by 220%) in plants as well as the availabilities of Cd (by 29.46%) and Zn (by 9.71%) in the soil, which can be attributed to an increase in the relative abundance of plant growth-promoting rhizobacteria that benefit phytoextraction. Furthermore, co-occurrence network analysis indicated the keystone taxa in the microbial community. Particularly, one of the keystone taxa, Pseudomonas, was also identified as distinct taxon in soil, and it was dominant among the endophytic bacteria. These results indicated that Pseudomonas, a taxon responded to WGH solution, may play a key role in enhanced phytoextraction in both soil and plant root. These results help to a better understanding of the microbial mechanisms for heavy metal accumulation in hyperaccumulator.

A field study reveals links between hyperaccumulating Sedum plants-associated bacterial communities and Cd/Zn uptake and translocation

Hyperaccumulating ecotypes of Sedum plants are promising Cd/Zn phytoextractors, with potential for leveraging its rhizospheric or endophytic microbiomes to improve phytoremediation efficiency. However, research of bacteria associated with Sedum at field scale is still lacking. Here, we presented a detailed investigation of the bacterial microbiome of hyperaccumulating Sedum ecotypes (S. alfredii and S. plumbizincicola) and a non-hyperaccumulating S. alfredii ecotype, which grow at different soil environments. Moreover, we evaluated the heavy metal uptake and translocation potential of Sedum plants at different locations. The results showed that both HE ecotypes, contrary to the NHE, were efficient for phytoremediation in mine areas and farmlands. For NHE plants, rhizosphere co-occurrence networks were more complex than the networks of other compartments, while networks of HE plants were more complex in bulk soil and roots. The proportion of positive correlations within co-occurrence networks was higher for the HE plants, suggesting a greater potential for mutualistic interactions. Plant compartment and location predominantly shaped the microbiome assembly, and Proteobacteria, Actinobacteria and Acidobacteria dominated the bacterial communities of Sedum plants. Keystone taxa related to Zn hyperaccumulation are similar to those related to Cd hyperaccumulation, and nine bacterial genera had significantly positive correlation with Cd/Zn hyperaccumulation. Taxa, linked to phytoremediation in both mine and farmland (i.e. Actinospica and Streptomyces from Actinobacteria), could be targets for further investigation of their ability to promote metal phytoremediation of Sedum species.

Roles of exogenous plant growth regulators on phytoextraction of Cd/Pb/Zn by Sedum alfredii Hance in contaminated soils

Plant growth regulators (PGRs) assisted phytoextraction was investigated as a viable phytoremediation technology to increase the phytoextraction efficiency in contaminated soils. This study aimed to evaluate the cadimum (Cd)/lead (Pb)/zinc (Zn) phytoextraction efficiency by a hyperaccumulator Sedum alfredii Hance (S. alfredii) treated with 9 PGRs, including indole-3-acetic acid (IAA), gibberellin (GA₃), cytokinin (CKs), abscisic acid (ABA), ethylene (ETH), brassinosteroid (BR), salicylic acid (SA), strigolactones (SL) and jasmonic acid (JA), in slightly or heavily contaminated (SC and HC, respectively) soil. Results demonstrated that PGRs were able to improve S. alfredii biomass, the most significant increases were observed in GA₃ and SL for HC soil, while for SC soil, IAA and BR exhibited positive effects. The levels of Cd, Pb and Zn in the shoots of S. alfredii treated with ABA and SL were noticeably greater than in the CK treatment in HC soil, while the uptake of metals were increased by IAA and CKs in SC soil. Combined with the results of biomass and metal contents in shoots, we found that ABA showed the highest Cd removal efficiency and the maximum Pb and Zn removal efficiency was observed with GA₃, which was 62.99%, 269.23%, and 41.18%, respectively higher than the control in HC soil. Meanwhile, compared to control, the maximum removal efficiency of Cd by IAA treatment (52.80%), Pb by JA treatment (165.1%), and Zn by BR treatment (44.97%) in the SC soil. Overall, our results suggested that these PGRs, especially, ABA, SL, IAA, BR and GA₃ had great potential in improving phytoremediation efficiency of S. alfredii grown in contaminated soils.

Endophytic bacteria promote biomass production and mercury-bioaccumulation of Bermuda grass and Indian goosegrass

Plant growth-promoting endophytic bacteria can potentially improve the biomass production of Hg-accumulating grasses, resulting in improved Hg extraction from contaminated soils. This study aimed to analyze the effect of inoculation of Hg-resistant endophytic bacteria (i) Jeotgalicoccus huakuii (B1) and (ii) Bacillus amyloliquefaciens (B2), as single and consortium inoculant, on biomass production and Hg bioaccumulation of Bermuda grass (R1) and Indian goosegrass (R2) planted as monoculture and mixed cropping. The grass seeds were surface-sterilized before the inoculation. Both inoculated seeds with B1 and B2 (treatments) and uninoculated seeds were sown separately in sterilized sand. Grass seedlings of both treatments and control were replanted in the washed and sterilized sand medium, spiked with HgCl₂ (100 mg kg^-1). A subset of grass was harvested at 0, 4, 6, and 10 weeks after planting to measure biomass production and Hg bioaccumulation. The results showed that bacterial inoculation enhanced the grass biomass by 52.68% and Hg bioaccumulation by 47.76%. Mercury residue of Hg-spiked sand treated with the bacterial consortium was reduced by 80%. This suggests that endophytic bacteria can improve grass biomass production and enhance Hg bioaccumulation in grass biomass.

Plant Growth-Promoting Bacteria (PGPB) integrated phytotechnology: A sustainable approach for remediation of marginal lands

Land that has little to no utility for agriculture or industry is considered marginal land. This kind of terrain is frequently found on the edge of deserts or other arid regions. The amount of land that can be used for agriculture continues to be constrained by increasing desertification, which is being caused by climate change and the deterioration of agriculturally marginal areas. Plants and associated microorganisms are used to remediate and enhance the soil quality of marginal land. They represent a low-cost and usually long-term solution for restoring soil fertility. Among various phytoremediation processes (viz., phytodegradation, phytoextraction, phytostabilization, phytovolatilization, phytofiltration, phytostimulation, and phytodesalination), the employment of a specific mechanism is determined by the state of the soil, the presence and concentration of contaminants, and the plant species involved. This review focuses on the key economically important plants used for phytoremediation, as well as the challenges to plant growth and phytoremediation capability with emphasis on the advantages and limits of plant growth in marginal land soil. Plant growth-promoting bacteria (PGPB) boost plant development and promote soil bioremediation by secreting a variety of metabolites and hormones, through nitrogen fixation, and by increasing other nutrients' bioavailability through mineral solubilization. This review also emphasizes the role of PGPB under different abiotic stresses, including heavy-metal-contaminated land, high salinity environments, and organic contaminants. In our opinion, the improved soil fertility of marginal lands using PGPB with economically significant plants (e.g., Miscanthus) in dual precession technology will result in the reclamation of general agriculture as well as the restoration of native vegetation.

Interspecific Neighbor Stimulates Peanut Growth Through Modulating Root Endophytic Microbial Community Construction

Plants have evolved the capability to respond to interspecific neighbors by changing morphological performance and reshaping belowground microbiota. However, whether neighboring plants influence the microbial colonization of the host's root and further affect host performance is less understood. In this study, using 16S rRNA high-throughput sequencing of peanut (Arachis hypogaea L.) roots from over 5 years of mono- and intercropping field systems, we found that neighbor maize can alter the peanut root microbial composition and re-shape microbial community assembly. Interspecific maize coexistence increased the colonization of genera Bradyrhizobium and Streptomyces in intercropped peanut roots. Through endophytic bacterial isolation and isolate back inoculation experiments, we demonstrated that the functional potentials of available nutrient accumulation and phytohormones production from Bradyrhizobium and Streptomyces endowed them with the ability to act as keystones in the microbial network to benefit peanut growth and production with neighbor competition. Our results support the idea that plants establish a plant-endophytic microbial holobiont through root selective filtration to enhance host competitive dominance, and provide a promising direction to develop modern diversified planting for harnessing crop microbiomes for the promotion of crop growth and productivity in sustainable agriculture.

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

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

A Mini-Review on the Co-growth and Interactions Among Microorganisms (Fungi and Bacteria) From Rhizosphere of Metal-Hyperaccumulators

The co-growth and synergistic interactions among fungi and bacteria from the rhizosphere of plants able to hyper accumulate potentially toxic metals (PTMs) are largely unexplored. Fungi and bacteria contribute in an essential way to soil biogeochemical cycles mediating the nutrition, growth development, and health of associated plants at the rhizosphere level. Microbial consortia improve the formation of soil aggregates and soil fertility, producing organic acids and siderophores that increase solubility, mobilization, and consequently the accumulation of nutrients and metals from the rhizosphere. These microorganism consortia can both mitigate the soil conditions promoting plant colonization and increase the performance of hyperaccumulator plants. Indeed, microfungi and bacteria from metalliferous soils or contaminated matrices are commonly metal-tolerant and can play a key role for plants in the phytoextraction or phytostabilization of metals. However, few works deepen the effects of the inoculation of microfungal and bacterial consortia in the rhizosphere of metallophytes and their synergistic activity. This mini-review aimed to collect and report the data regarding the role of microbial consortia and their potentialities known to date. Moreover, our new data had shown an active fungal-bacteria consortium in the rhizosphere of the hyperaccumulator plant Alyssoides utriculata.

Phytoremediation of Cd-contaminated farmland soil via various Sedum alfredii-oilseed rape cropping systems: Efficiency comparison and cost-benefit analysis

Phytoremediation is a green technology for heavy metal removal from contaminated soil, and its remediation efficiency and economic feasibility in field trial should be evaluated before large-scale application. However, there is still lacking relevant analysis, especially for phytoremediation with different cropping patterns. In the present study, we performed phytoremediation on slightly Cd-contaminated farmland soil via three cropping systems, i.e. Sedum alfredii monoculture, oilseed rape monoculture, and S. alfredii-oilseed rape intercropping. Dry weights of S. alfredii and oilseed rape were both enhanced under intercropping pattern, while the highest total Cd extraction amount (148 g ha^-1) were observed under S. alfredii monoculture. Furtherly, a cost-benefit analysis via Monta Carlo simulation in a ten-year lifetime was conducted. The benefits of S. alfredii monoculture and intercropping schemes would offset the total costs in 6 and 8 years, respectively. S. alfredii monoculture achieved a higher net present value of 1.88 × 10⁴ US$ as compared with intercropping (9.53 × 10³ US$). These results indicate that S. alfredii monoculture scheme could be a promising phytoremediation strategy for slightly Cd-contaminated soil owing to better remediation efficiency and economic feasibility. Moreover, the enhancement in mechanization level and the reduction of seedling cost could further improve its economic viability.

Plant Growth-Promoting Rhizobacteria as a Green Alternative for Sustainable Agriculture

Environmental stress is a major challenge for sustainable food production as it reduces yield by generating reactive oxygen species (ROS) which pose a threat to cell organelles and biomolecules such as proteins, DNA, enzymes, and others, leading to apoptosis. Plant growth-promoting rhizobacteria (PGPR) offers an eco-friendly and green alternative to synthetic agrochemicals and conventional agricultural practices in accomplishing sustainable agriculture by boosting growth and stress tolerance in plants. PGPR inhabit the rhizosphere of soil and exhibit positive interaction with plant roots. These organisms render multifaceted benefits to plants by several mechanisms such as the release of phytohormones, nitrogen fixation, solubilization of mineral phosphates, siderophore production for iron sequestration, protection against various pathogens, and stress. PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. Genetically engineered PGPR strains play significant roles to alleviate the abiotic stress to improve crop productivity. Thus, the present review will focus on the impact of PGPR on stress resistance, plant growth promotion, and induction of antioxidant systems in plants.

Phenotype Switching in Metal-Tolerant Bacteria Isolated from a Hyperaccumulator Plant

As an adaptation to unfavorable conditions, microorganisms may represent different phenotypes. Azolla filiculoides L. is a hyperaccumulator of pollutants, but the functions of its microbiome have not been well recognized to date. We aimed to reveal the potential of the microbiome for degradation of organic compounds, as well as its potential to promote plant growth in the presence of heavy metals. We applied the Biolog^TM Phenotypic Microarrays platform to study the potential of the microbiome for the degradation of 96 carbon compounds and stress factors and assayed the hydrolytic potential and auxin production by the microorganisms in the presence of Pb, Cd, Cr (VI), Ni, Ag, and Au. We found various phenotype changes depending on the stress factor, suggesting a possible dual function of the studied microorganisms, i.e., in bioremediation and as a biofertilizer for plant growth promotion. Delftia sp., Staphylococcus sp. and Microbacterium sp. exhibited high efficacy in metabolizing organic compounds. Delftia sp., Achromobacter sp. and Agrobacterium sp. were efficient in enzymatic responses and were characterized by metal tolerant. Since each strain exhibited individual phenotype changes due to the studied stresses, they may all be beneficial as both biofertilizers and bioremediation agents, especially when combined in one biopreparation.

Arnebia euchroma, a Plant Species of Cold Desert in the Himalayas, Harbors Beneficial Cultivable Endophytes in Roots and Leaves

The endophytic mutualism of plants with microorganisms often leads to several benefits to its host including plant health and survival under extreme environments. Arnebia euchroma is an endangered medicinal plant that grows naturally in extreme cold and arid environments in the Himalayas. The present study was conducted to decipher the cultivable endophytic diversity associated with the leaf and root tissues of A. euchroma. A total of 60 bacteria and 33 fungi including nine yeasts were isolated and characterized at the molecular level. Among these, Proteobacteria was the most abundant bacterial phylum with the abundance of Gammaproteobacteria (76.67%) and genus Pseudomonas. Ascomycota was the most abundant phylum (72.73%) dominated by class Eurotiales (42.42%) and genus Penicillium among isolated fungal endophytes. Leaf tissues showed a higher richness (S_chao1) of both bacterial and fungal communities as compared to root tissues. The abilities of endophytes to display plant growth promotion (PGP) through phosphorus (P) and potassium (K) solubilization and production of ACC deaminase (ACCD), indole acetic acid (IAA), and siderophores were also investigated under in vitro conditions. Of all the endophytes, 21.51% produced ACCD, 89.25% solubilized P, 43.01% solubilized K, 68.82% produced IAA, and 76.34% produced siderophores. Six bacteria and one fungal endophyte displayed all the five PGP traits. The study demonstrated that A. euchroma is a promising source of beneficial endophytes with multiple growth-promoting traits. These endophytes can be used for improving stress tolerance in plants under nutrient-deficient and cold/arid conditions.

Rhizosphere Microbial Communities and Heavy Metals

The rhizosphere is a microhabitat where there is an intense chemical dialogue between plants and microorganisms. The two coexist and develop synergistic actions, which can promote plants’ functions and productivity, but also their capacity to respond to stress conditions, including heavy metal (HM) contamination. If HMs are present in soils used for agriculture, there is a risk of metal uptake by edible plants with subsequent bioaccumulation in humans and animals and detrimental consequences for their health. Plant productivity can also be negatively affected. Many bacteria have defensive mechanisms for resisting heavy metals and, through various complex processes, can improve plant response to HM stress. Bacteria-plant synergic interactions in the rhizosphere, as a homeostatic ecosystem response to HM disturbance, are common in soil. However, this is hard to achieve in agroecosystems managed with traditional practices, because concentrating on maximizing crop yield does not make it possible to establish rhizosphere interactions. Improving knowledge of the complex interactions mediated by plant exudates and secondary metabolites can lead to nature-based solutions for plant health in HM contaminated soils. This paper reports the main ecotoxicological effects of HMs and the various compounds (including several secondary metabolites) produced by plant-microorganism holobionts for removing, immobilizing and containing toxic elements.

Exogenous hemin improves Cd2+ tolerance and remediation potential in Vigna radiata by intensifying the HO-1 mediated antioxidant defence system

The present study evaluated the effects of exogenous hemin on cadmium toxicity in terms of metal accretion and stress resilience in Vigna radiata L. (Wilczek). One-week-old seedlings were treated with CdCl₂ (50 μM) alone and in combination with hemin (0.5 mM) in half-strength Hoagland medium for 96 h. The optimum concentrations of Cd and hemin were determined on the basis of haem oxygenase-1 activity. The results demonstrated that under Cd stress, plants accumulated a considerable amount of metal in their tissues, and the accumulation was higher in roots than in leaves, which significantly reduced the plant biomass and chlorophyll content by increasing the oxidative stress (MDA and H₂O₂ content). However, hemin supplementation under Cd,-stress improved plant growth by enhancing the harvestable biomass and photosynthetic pigments, increasing antioxidant activities (SOD, APX, POD, HO-1 and proline), lowering oxidative damage and increasing Cd tolerance in plants. Furthermore, the application of hemin enhances the removal efficiency of Cd in V. radiata by increasing the uptake of Cd via roots and its translocation from roots to foliar tissues. Thus, the study suggests that hemin has the potential to improve the stress tolerance and phytoremediation ability of heavy metal-tolerant plants so that they can be used instead of hyperaccumulators for remediation of Cd-contaminated environments.

Cadmium level and soil type played a selective role in the endophytic bacterial community of hyperaccumulator Sedum alfredii Hance

Phytoremediation technology has been applied for heavy metal elimination for many years, however little research about the difference of remediation efficiency of hyperaccumulator in different soils was reported. Here, a pot experiment was conducted with a cadmium (Cd)/zinc hyperaccumulator Sedum alfredii Hance grown on different types of soils and the differences of its endophytic bacterial community were elucidated. The results showed that the biomass of S. alfredii grown on black soil under both low and high Cd treatment was much heavier than that grown on other soils, and Cd uptake and Cd accumulation of S. alfredii in paddy soil was the highest, suggesting that black soil was more suitable for S. alfredii growth while paddy soil was more efficient for Cd phytoextraction. Moreover, Cd treated level and soil type both affected the structure of plant endophytic bacterial community. The two shared genera in the four representative soils were Caulobacter and Acinetobacter under low Cd level, and Caulobacter and Lactobacillus under high Cd level. Cd treatment shifted the structure and abundance of plant endophytes in different types of soils, while black soil and paddy soil were more similar in the distribution and abundance of S. alfredii endophytic community. This study highlighted the understanding of response to Cd within S. alfredii endophytic community in different types of soils, which could be beneficial for enhanced phytoremediation efficiency and better S. alfredii cultivation.

Pseudomonas fluorescens accelerates a reverse and long-distance transport of cadmium and sucrose in the hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) can promote root uptake and shoot accumulation of cadmium (Cd) in hyperaccumulator plants, but the mechanisms by which PGPB accelerate root-to-shoot transport of Cd is still unknown. A better understanding of these mechanisms is necessary to develop the strategies that can promote the practical phytoextraction of Cd-polluted soils. In this study, we found that Pseudomonas fluorescens accelerates a reversed and a long-distance transport of Cd and sucrose in Sedum alfredii, by examining the xylem and phloem sap and by quantifying the concentrations of Cd and sucrose in shoot and root. The transcriptome sequencing has revealed the up-regulated expressions of starch metabolism and sucrose biosynthesis related genes in the shoots of Cd hyperaccumulator plant S. alfredii that was inoculated with PGPB P. fluorescens. In addition, the genes of sugar, cation and anion transporters were also up-regulated by bacterial treatment, showing a complicated co-expression network with sucrose biosynthesis related genes. The expression levels of Cd transporter genes, such as ZIP1, ZIP2, HMA2, HMA3 and CAX2, were elevated after PGPB inoculation. As a result, the PGPB successfully colonized the root, and promoted the sucrose shoot-to-root transport and Cd root-to-shoot transport in S. alfredii. Since non-photosynthetic root-associated bacteria usually obtain sugars from photosynthetic plants, our results highlight the importance of PGPB-induced changes in hyperaccumlator plants for both the host and the PGPB.

Fava bean intercropping with Sedum alfredii inoculated with endophytes enhances phytoremediation of cadmium and lead co-contaminated field

Phytoremediation coupled with agro-production is considered a sustainable strategy for remediation of trace element contaminated fields without interrupting crop production. In this study hyperaccumulator Sedum alfredii was intercropped with a leguminous plant fava bean (Vicia fava) in cadmium (Cd) and lead (Pb) co-contaminated field to evaluate the effects of intercropping on growth performance and accumulations of trace elements in plants with plant growth promoting endophyte (PGPE) consortium application. The results showed, compared with monoculture, intercropping coupled with inoculation application promoted biomass as well as Cd and Pb concentrations in individual parts of both plants, thus increasing the removal efficiencies of trace elements (4.49-folds for Cd and 5.41-folds for Pb). Meanwhile, this superposition biofortification measure maintained normal yield and nutrient content, and limited the concentration of Cd and Pb within the permissible limit (<0.2 mg kg^-1 FW) in fava bean during the grain production. These results demonstrated a feasible technical system for phytoremediation coupled with agro-production in slightly or moderately Cd and Pb co-contaminated field, and also provided useful information for further investigation of interaction mechanisms between intercropping and PGPEs inoculation.

Improving hybrid Pennisetum growth and cadmium phytoremediation potential by using Bacillus megaterium BM18-2 spores as biofertilizer

Environmental pollution with heavy metals becomes an issue of serious concern worldwide. Cadmium is considered one which adversely affects living organisms. Recently, the usage of endophytic bacteria to enhance the plant growth and phytoremediation of heavy metals contaminated sites is gaining great attention. The current study focused on utilizing the spores of Bacillus megaterium BM18-2 as biofertilizer for enhancing the growth of Cd hyperaccumulator Hybrid Pennisetum and Cd tolerance of the plant. Therefore, the production of the highest proportion of BM18-2 spores in short incubation time was investigated using different culture media. The results revealed that the maximum proportion of BM18-2 spores (90%) was obtained following incubation for 48 h in Tryptone- yeast extract media (TY). Furthermore, several growth parameters of H. Pennisetum were shown to be significantly improved by inclusion of BM18-2 spores into Cd contaminated soil in contrast to non- inoculated plant. The chlorophyll concentration of the leaves rose by 5%, 13%, and 22.89% with increasing Cd concentration of soil (20, 40 and 60 mg/Kg, respectively). The percentage of total nitrogen content of the root, stem and leaf was increased due to the bacterial spores inoculation and the highest percentage was recorded in the leaf in all treatments. Moreover, Cd phytoremediation capacity of H. Pennisetum greatly enhanced with the application of BM18-2 spores into the soil. An obvious correlation was also observed between Cd accumulation and bacterial colonization where the Cd accumulation enhanced by 21.9%, 16.5%, and 94.6% and the maximum count of BM18-2 (27 × 10⁵, 194 × 10⁴,and 145 × 10⁴ CFU/g) were recorded in the root system in 20, 40, and 60 mg/Kg Cd spiked soil, respectively. Consequently, the spores of BM18-2 was proven to succeed as biofertilizer to improve growth of H. pennisetum during Cd stress which subsequently improved the phytoremediation of Cd contaminated soil.