Role of root exudates in metal acquisition and tolerance

Rare earth elements in seagrass beds: Contamination, bioaccumulation, and biomonitoring

Coastal environments are increasingly vulnerable to contamination from rare earth elements (REE) due to expanding anthropogenic activities, yet the fate and ecological risks of REE in ecologically critical seagrass ecosystems remain poorly understood. This study deciphered the behavior, fractionation, and compartmentalization of REE in both seagrass sediments and tissues. Total REE concentrations in sediments ranged from 70.5 to 258.8 mg kg-1, with Ce emerging as the most enriched REE in both matrices. Pollution Load Index varied from 0.7 to 3.0, indicating slight to moderate REE pollution, with localized enrichment of some REE (e.g., Tb, Lu) pointing to anthropogenic influences such as industrial effluents and marine traffic. Principal component and enrichment factor analyses attribute approximately 66 % of REE patterns to geogenic weathering, while 22.6 % reflect anthropogenic contributions. Geochemical partitioning revealed that Fe-Mn oxides serve as major REE sinks, while organic matter plays a dual role-enhancing total REE retention through complexation yet reducing mobility by stabilizing labile fractions. Correlations between REE concentrations in seagrass tissues and sediments suggest species-specific uptake and limited translocation. These findings underscore the capacity of seagrasses to serve as sensitive bioindicators for REE pollution and highlight the importance of organic matter and rhizosphere processes in modulating REE bioavailability.

Metabolomics and microbiome analysis elucidate the detoxification mechanisms of Hemarthria compressa, a low cadmium accumulating plant, in response to cadmium stress

Cadmium (Cd) is recognized as one of the most toxic heavy metal in the environment that causes pronounced phytotoxicity. This study investigated the physiological and biochemical responses and detoxification mechanisms of Hemarthria compressa under various concentrations of Cd stress (0, 30, 60, 90, and 270 mg·kg-1). Our research findings indicate that the growth and photosynthetic capacity of H. compressa reach their peak at a Cd concentration of 60 mg·kg-1. At this concentration, the Cd concentration in the shoots of H. compressa is 0.67 mg·kg-1, the total Cd accumulation is 0.25 μg, and the MDA content is 6.25 nmol·g-1, which represents the lowest values among all treatments.Metabolomics analysis reveals that sugar is related to Cd stress resistance, and the levels of organic acids involved in metabolic processes show only minor changes. H. compressa alters the composition of its root exudates by secreting substantial quantities of organic acids (such as citric acid, fumaric acid, and malic acid), sugars (such as trehalose, maltose, and glucose), and fatty acids (such as citraconic acid). These organic acids modulate the pH of the rhizosphere soil and recruit beneficial microorganisms, including Gp6, Sphingoaurantiacus, Devosia, and Neobacillus species, thereby enhancing plant growth and mitigating Cd accumulation.

Physiological, biochemical and elemental responses of grafted grapevines under drought stress: insights into tolerance mechanisms

The selection of appropriate grapevine grafts and optimizing irrigation practices for enhancing water use efficiency are critical for viticulture production in the arid regions of UAE, apart from mitigating the effects of changing environmental conditions. Extremely high arid temperatures leading to depleted soil moisture status limit grape production in the country. In order to streamline the production, it is imperative to focus on specific objectives of screening drought-tolerant grafts utilizing several laboratory analytical tools and irrigation management. Five grapevine cultivar-rootstock combinations were evaluated in an open field experiment under induced drought conditions by regulating irrigation at 100%, 75% and 50% field capacity (FC) in an arid region. The net photosynthetic rate increased in Flame Seedless [Formula: see text] Ramsey (V1), Thompson Seedless [Formula: see text] Ramsey (V2), and Crimson Seedless [Formula: see text] R110 (V3) at 50% FC. Stomatal conductance was reduced in V1, V3, Crimson Seedless [Formula: see text] Ramsey (V4) and Thompson Seedless x P1103 (V5) at 50% FC. Intercellular CO2 and transpiration rates were significantly reduced at 50% FC. Water use efficiency, calculated as Pn/gs ratio to relate photosynthesis to stomatal closure, was elevated in all the grafts at 75% FC and 50% FC compared to the control (100% FC). The relative water content (RWC) showed a declining trend in all the grafts with reduced water supply. Nevertheless, the V1 and V4 grafts exhibited the highest RWC at an FC of 50%. The V2 graft produced the highest total dry mass and fresh biomass compared to other grafts. The Chl a content decreased, but the Chl b content increased at 50% FC in V2. Lutein significantly decreased for V1, while V3 showed an increase at 50% FC. The N, P and K contents in all the grafts, except V3, showed an increasing trend at 50% FC. The scanning electron microscopy observations point to the strong responses of stomatal behaviour upon changes in irrigation, thus facilitating the drought tolerance of the grafts. The findings emphasize the importance of selecting drought-tolerant grapevine grafts, and our study results could serve as guideposts for developing sustainable viticulture in arid regions, providing valuable insights for future research and practical applications in grape production.

Abilities of herbaceous plant species to phytoextract Cd, Pb, and Zn from arable soils after poly-metallic mining and smelting

Potentially toxic element (PTE) contamination deteriorates agricultural land. This study explored the accumulation of excess PTEs (Cd, Pb, and Zn) in soils by shoots of herbaceous plants growing on alluvial sediments of an abandoned mining/smelting site near the Litavka River, Czech Republic, as a means of soil remediation. Determination of total Cd, Pb, and Zn, contents in soil and plant samples decomposed with HNO3 + HCl + HF, HNO3, and H2O2, respectively, were carried out by inductively coupled optical emission spectrometry. The soil Cd, Pb, and Zn contents in the studied site ranged from 40 to 65, 3183 to 3897, and 5108 to 6553 mg kg-1, respectively, indicating serious soil contamination compared to the limits allowed by the FAO/WHO and the Czech Republic. Slightly acidic soil reactions and negative correlations between the pH, C, and N supported the assumption of relative solubility, mobility, and accumulation of studied PTEs by herbaceous species. Shoot accumulation of Cd, Pb, and Zn varied in 22 of 23 species recording a Cd content above the permissible limit. The Zn content in all plants was above the WHO limit. Except for Arabidopsis halleri, with a bioaccumulation factor (BAFshoot) > 1 for Cd and Zn, Equisetum arvense recorded a comparatively higher Cd content (10.3-28 mg kg-1) than all other species. Silene vulgaris (Moench), Leucanthemum vulgare, E. arvense, Achillea millefolium, Carex sp., Dianthus deltoides, Campanula patula, Plantago lanceolata, and Rumex acetosa accumulated more Zn than many plants (> 300 mg kg-1). Although E. arvense had a BAF < 1, it accumulated > 1000 mg Zn kg-1 and supported the phytoextraction of Zn. Only 10 species accumulated Pb above the limit permissible in plants, with L. vulgare recording the highest concentration (40 mg kg-1) among all species. Therefore, the shoots of several plant species showed promising PTE accumulation abilities and deserve more detailed studies concerning their potential use for phytoremediation of Cd-, Pb-, or Zn-contaminated soils.

Barriers and carriers for transition metal homeostasis in plants

Transition metals are types of metals with high chemical activity. They play critical roles in plant growth, development, reproduction, and environmental adaptation, as well as in human health. However, the acquisition, transport, and storage of these metals pose specific challenges due to their high reactivity and poor solubility. In addition, distinct yet interconnected apoplastic and symplastic diffusion barriers impede their movement throughout plants. To overcome these obstacles, plants have evolved sophisticated carrier systems to facilitate metal transport, relying on the tight coordination of vesicles, enzymes, metallochaperones, low-molecular-weight metal ligands, and membrane transporters for metals, ligands, and metal-ligand complexes. This review highlights recent advances in the homeostasis of transition metals in plants, focusing on the barriers to transition metal transport and the carriers that facilitate their passage through these barriers.

Harnessing the rhizosphere sponge to smooth pH fluctuations and stabilize contaminant retention in biofiltration system

Fluctuating pH conditions can affect heavy metal mobility, thereby limiting the efficiency of biofiltration systems (BS). To address this, we developed an innovative rhizosphere sponge, biochar-based bioreactor (RBB), designed to stabilize Cd2+ removal across a pH range of 5 to 9. RBB consistently outperformed the control, achieving a notable 91.3 % Cd2+ removal at pH 5. By creating optimized oxygen and redox zoning, the rhizosphere sponge enhanced both biochar surface reactions and microbial activity. Under acidic conditions, biochar facilitated Fe2+/Mn2+ precipitation into stable (oxy)hydroxides, a process further driven by microbial oxidation. Consequently, RBB accumulated 1.54 times more Fe-Mn oxide-bound Cd than the control, effectively reducing Cd2+ mobility. Additionally, loosely bound extracellular polymeric substances claimed preferential Cd2+ sequestration after acidification. The stabilized microecology and increased ecological niches, allowing RBB to better buffer against pH fluctuations, presenting it as a robust solution for sustainable heavy metal remediation in variable environments.

Enhancing Miscanthus floridulus remediation of soil cadmium using Beauveria bassiana FE14: Plant growth promotion and microbial interactions

Soil heavy metal pollution presents substantial risks to food security and human health. This study focused on the efficiency of plant growth-promoting fungus-Beauveria bassiana FE14 and Miscanthus floridulus on the synergistic remediation of soil Cd contamination. Results revealed that B. bassiana FE14 significantly enhanced the growth of M. floridulus, substantially decreased Cd content in soil by 79.39 %, and modified enzyme activities (superoxide dismutase, peroxidase, and catalase) to alleviate Cd-induced oxidative stress in plants, determined by the physical and chemical indicators and enzyme activities of soil and plant. Based on microbiome analysis, this study also found significant changes in the composition, structure, and molecular ecological network of endophytic bacterial communities in roots, but this study had little effect on the bacterial and fungal communities in rhizosphere soil. In addition, the key genera (including Sphingomonas, unclassified_Comamonadaceae, Massilia, Bradyrhizobium, and Paraglomus) and key genes/enzymes (including cadC, zinc transporter, zinc and cadmium transporter, exoZ/Y/Z, catalase-peroxidase, superoxide dismutase, nitrite reductase, acid phosphatase, etc.) were involved in promoting plant growth and alleviating Cd stress. These findings revealed the potential of B. bassiana FE14 and M. floridulus working in synergy to enhance the phytoremediation efficiency of Cd-contaminated soils, thus presenting a promising approach for integrated plant-microbe remediation strategies.

Mycorrhizal fungi mitigate cadmium leaching losses by decreasing the inorganic cadmium proportion in soil solutions

Arbuscular mycorrhizal fungi (AMF) are common in cadmium (Cd)-contaminated soil. However, the effects of AMF on Cd migration in contaminated soil are still poorly understood. A pot experiment involving a control without AMF inoculation (CK), inoculation with AMF (AMF), and bacterial filtrate of the AMF inoculant (LY) was conducted in the present study. AMF caused an increase in total glomalin-related soil protein (T-GRSP) of 12.2 % and in the exudation of low-molecular-weight organic acids (LMWOAs), such as citric acid, malic acid, oxalic acid, and free amino acids, by maize roots of 30.4 %-200.0 % but caused a decrease in the Cd contents of the maize shoots and roots by 53.2 % and 47.2 %, respectively compared those of the CK. Moreover, AMF decreased the Cd concentration in the leachate by 36.0 %-76.3 % by reducing the proportion of Cd2 + and inorganic Cd complexes in the soil solution and leachate by 26.2 %-85.7 %, and increasing the proportion of organic Cd complexes. Structural equation analysis revealed that the GRSP and LMWOAs were the primary factors driving the potential of AMF to reduce Cd leaching loss from polluted soil. The adsorption of Cd ions by quartz sand increased with the application of GRSP and LMWOAs, which resulted in a 27.6 %-69.5 % reduction in Cd leaching loss in the sand column. In the soil with AMF, the proportion of organic-bound Cd increased and the proportion of inorganic-bound Cd decreased by promoting the secretion of LMWOAs via mycelium, thus reducing Cd leaching loss.

Mechanisms of N-doped microporous biochar decreased Cd transition in rhizosphere soils and its impact on soil bacterial community composition

Soil cadmium (Cd) contamination has garnered considerable attention. This study employed batch sorption experiments and rhizobox experiments to examine the impact of nitrogen-doped microporous biochar (NBB) on the temporal and spatial distribution of Cd in the rhizosphere of rice plants, with the aim of elucidating the underlying mechanisms. The results indicated that the adsorption of Cd(II) onto NBB was predominantly governed by chemical reactions. When applied to soil, the NBB significantly hindered the migration of Cd from the bulk soil to the rhizosphere. Additionally, the application of NBB decreased the redox potential (Eh) in the rhizosphere soil and increased the relative abundance of Anaeromyxobacteraceae, Geobacteraceae, Desulfurisporaceae, and Syntrophomonadaceae, which could facilitate the reduction of soil Cd availability. Furthermore, the NBB2 treatment encouraged the formation of iron plaque on the root surface, thereby limiting the uptake of Cd from the soil into the root system. Moreover, the N-doped microporous biochar treatment resulted in lower Cd levels in the stele of root, an effect that was associated with increased sulfur (S) content in the stele and epidermis, suggesting a potential role for S in Cd sequestration. Ultimately, the application of N-doped microporous biochar resulted in diminished Cd accumulation in the rice tissues.

Transcriptomic response of wetland microbes to root influence

Wetlands are hotspots for carbon and nutrient cycling. The important role of plant-microbe interactions in driving wetland biogeochemistry is widely acknowledged, prompting research into their molecular biological basis for a deeper understanding of these processes. We analyzed transcriptomic responses of soil microbes to root exudates in coastal wetland soils using 13CO2 pulse labeling. Metatranscriptomics revealed 388 upregulated and 11 downregulated genes in response to root exudates. The Wood-Ljungdahl pathway and dissimilatory sulfate reduction/oxidation were the most active microbial pathways independent of root influence, whereas pathways with the strongest upregulation in response to root influence were related to infection, stress response, and motility. We demonstrate shifts within the active community toward higher relative abundances of Betaproteobacteria, Campylobacterota, Kiritimatiellota, Lentisphaerota, and Verrucomicrobiota in response to exudates. Overall, this study improves our mechanistic understanding of wetland plant-soil microbe interactions by revealing the phylogenetic and transcriptional response of soil microorganisms to root influence and exudate input.

Multifaceted impacts of nanoparticles on plant nutrient absorption and soil microbial communities

With the growth of the global population and the increasing scarcity of resources, the sustainability and efficiency improvement of agricultural production have become urgent needs. The rapid development of nanotechnology provides new solutions to this challenge, especially the application of nanoparticles in agriculture, which is gradually demonstrating its unique advantages and broad prospects. Nonetheless, various nanoparticles can influence plant growth in diverse manners, often through distinct mechanisms of action. Beyond their direct effects on the plant itself, they frequently alter the physicochemical properties of the soil and modulate the structure of microbial communities in the rhizosphere. This review focuses intently on the diverse methods through which nanoparticles can modulate plant growth, delving deeply into the interactions between nanoparticles and plants, as well as nanoparticles with soil and microbial communities. The aim is to offer a comprehensive reference for the utilization of functionalized nanoparticles in the agricultural sector.

Water-extractable metals as indicators of wheat metal accumulation: Insights from Cd, Pb, Mn, Cu, and Zn

There is a long-standing debate over the effectiveness of chemical extraction methods in assessing soil metal phytoavailability. This study addresses the limitations of widely-used chemical extraction methods and presents the water-extractable pool as a more reliable indicator based on wheat pot experiments using homogenized agricultural soil amended with lime materials, phosphate, and biochar. Over 120 days' pot experiments, Cd accumulation in whole wheat plants and tissues exhibited positive relationships with water-extractable Cd concentrations at heading and maturity stage (Spearman's rho: 0.521-0.851; P < 0.05), revealing that the water-extractable pool instead of other pools better indicates wheat metal accumulation. Water-extractable metal concentrations are effective in assessing phytoavailability of metals primarily in ionic forms in soil solution (e.g, Zn, Cd), but less reliable for metals strongly complexed with dissolved organic matter (DOM) or sensitive to redox conditions. It demonstrated that water-extractable metal concentrations and chemical forms are key factors, fundamentally determined by metal properties and impacted by environmental factors. This study clarifies a more direct link between chemical extraction and plant metal uptake mechanisms. Given the extensive application of chemical extraction methods over several decades, this study will help advance soil metal risk assessment and remediation practices.

Miscanthus sp. root exudate alters rhizosphere microbial community to drive soil aggregation for heavy metal immobilization

The heavy metals (HMs) spatial distribution in soil is intricately shaped by aggregation processes involving chemical reactions and biological activities, which modulate HMs toxicity, migration, and accumulation. Pioneer plants play a central role in preventing HMs at source, yet the precise mechanisms underlying their involvement in soil aggregation remain unclear. This study investigates HMs distribution within rhizosphere and bulk soil aggregates of Miscanthus sp. grown in tailings to elucidate the impact of root exudates (REs) and rhizosphere microbes. The results indicate that Miscanthus sp. enhance soil stability, increasing the proportion of macroaggregates by 4.06 %-9.78 %. HMs tend to concentrate in coarse-aggregates, particularly within rhizosphere environments, while diminishing in fine-aggregates. Under HMs stress, lipids and lipid-like molecules are the most abundant REs produced by Miscanthus sp., accounting for under up to 26.74 %. These REs form complex with HMs, promoting microaggregates formation. Charged components such as sugars and amino acids further contribute to soil aggregation. REs also regulates rhizosphere bacteria and fungi, with Acidobacteriota, Chloroflexi were the dominant bacterial phyla, while Ascomycota and Basidiomycota dominate the fungal community. The synergistic effect of REs and microorganisms impact soil organic matter and nutrient content, facilitating HMs nanoparticle heteroaggregation and macroaggregates formation. Consequently, soil structure and REs shape the distribution of HMs in soil aggregation. Pioneer plants mediate REs interaction with rhizosphere microbes, promoting the distribution of HMs into macroaggregates, leading to immobilization. This study sheds light on the role of pioneer plants in regulating soil HMs, offering valuable insights for soil remediation strategies.

Speciation, phytoavailability, and accumulation of toxic elements and sulfur by humic acid-fertilized lemongrass and common sage in a sandy soil treated with heavy oil fly ash: A trial for management of power stations wastes

Globally, power stations generate huge amounts of the hazardous waste heavy oil fly ash (HOFA), which is rich in Ni, V, Fe, S, and dumped into landfills. Thus, exploring new approaches for a safe recycling and sustainable management of HOFA is needed and of great environmental interest. The potential application of HOFA as an amendment to sandy soils has not been studied yet. This is the first research investigating the potentiality of using HOFA as a soil conditioner. To this end, we conducted a greenhouse experiment in order to investigate the impacts of HOFA addition (1.2, 2.4, 3.6 t ha-1) to sandy soil on the total and available content of nutrients (e.g., S, Fe, Mn, Cu, Zn) and toxic elements (TEs; e.g., Cd, Co, Cr, Ni, Pb, V) in the soil and their phytoextraction and translocation by lemongrass (Cymbopogon citratus) and common sage (Salvia officinalis). We also assessed the impact of humic acid (HA) foliar application (50 and 100 l ha-1) on the growth and elements accumulation by the two plants. The studied HOFA was acidic and highly enriched in S (43,268.0), V (3,527.0), Ni (1774.0), and Fe (15,159.0) (units in mg kg-1). The X-ray absorption near edge structure (XANES) data showed that V in HOFA was composed primarily of V(IV) sorbed onto goethite, V(V) sorbed onto humic substances, in the forms of V2O3, and VCl4. Addition of the lower doses of HOFA (1.2 and 2.4 t ha-1) did not change significantly soil pH, salinity, and the total and available elements content compared to the unamended soil. Although the elements content in the 3.6 t ha-1 HOFA-treated soil was significantly higher than the untreated, the total content of all elements (except for Ni) was lower than the maximum allowable concentrations in soils. HOFA addition, particularly in the highest dose (3.6 t ha-1), decreased significantly the growth and biomass of both plants. Common sage accumulated more elements than lemongrass; however, the elements content in the plants was lower than the critical concentrations for sensitive plants. The foliar application of humic acid enhanced significantly the plant growth and increased their tolerance to the HOFA-induced stress. We conclude that the addition of HOFA up to 2.4 t ha-1 in a single application as amendment to sandy soils is not likely to create any TE toxicity problems to plants, particularly if combined with a foliar application of humic acid; however, repeated additions of HOFA may induce toxicity. These findings should be verified under field conditions.

The Role of Low-Molecular-Weight Organic Acids in Metal Homeostasis in Plants

Low-molecular-weight organic acids (LMWOAs) are essential O-containing metal-binding ligands involved in maintaining metal homeostasis, various metabolic processes, and plant responses to biotic and abiotic stress. Malate, citrate, and oxalate play a crucial role in metal detoxification and transport throughout the plant. This review provides a comparative analysis of the accumulation of LMWOAs in excluders, which store metals mainly in roots, and hyperaccumulators, which accumulate metals mainly in shoots. Modern concepts of the mechanisms of LMWOA secretion by the roots of excluders and hyperaccumulators are summarized, and the formation of various metal complexes with LMWOAs in the vacuole and conducting tissues, playing an important role in the mechanisms of metal detoxification and transport, is discussed. Molecular mechanisms of transport of LMWOAs and their complexes with metals across cell membranes are reviewed. It is discussed whether different endogenous levels of LMWOAs in plants determine their metal tolerance. While playing an important role in maintaining metal homeostasis, LMWOAs apparently make a minor contribution to the mechanisms of metal hyperaccumulation, which is associated mainly with root exudates increasing metal bioavailability and enhanced xylem loading of LMWOAs. The studies of metal-binding compounds may also contribute to the development of approaches used in biofortification, phytoremediation, and phytomining.

A combined bibliometric and sustainable approach of phytostabilization towards eco-restoration of coal mine overburden dumps

Extraction of coal through opencast mining leads to the buildup of heaps of overburden (OB) material, which poses a significant risk to production safety and environmental stability. A systematic bibliometric analysis to identify research trends and gaps, and evaluate the impact of studies and authors in the field related to coal OB phytostabilization was conducted. Key issues associated with coal extraction include land degradation, surface and groundwater contamination, slope instability, erosion and biodiversity loss. Handling coal OB material intensifies such issues, initiating additional environmental and physical challenges. The conventional approach such as topsoiling for OB restoration fails to restore essential soil properties crucial for sustainable vegetation cover. Phytostabilization approach involves establishing a self-sustaining plant cover over OB dump surfaces emerges as a viable strategy for OB restoration. This method enhanced by the supplement of organic amendments boosts the restoration of OB dumps by improving rhizosphere properties conducive to plant growth and contaminant uptake. Criteria essential for plant selection in phytostabilization are critically evaluated. Native plant species adapted to local climatic and ecological conditions are identified as key agents in stabilizing contaminants, reducing soil erosion, and enhancing ecosystem functions. Applicable case studies of successful phytostabilization of coal mines using native plants, offering practical recommendations for species selection in coal mine reclamation projects are provided. This review contributes to sustainable approaches for mitigating the environmental consequences of coal mining and facilitates the ecological recovery of degraded landscapes.

Plants' molecular behavior to heavy metals: from criticality to toxicity

The contamination of soil and water with high levels of heavy metals (HMs) has emerged as a significant obstacle to agricultural productivity and overall crop quality. Certain HMs, although serving as essential micronutrients, are required in smaller quantities for plant growth. However, when present in higher concentrations, they become very toxic. Several studies have shown that to balance out the harmful effects of HMs, complex systems are needed at the molecular, physiological, biochemical, cellular, tissue, and whole plant levels. This could lead to more crops being grown. Our review focused on HMs' resources, occurrences, and agricultural implications. This review will also look at how plants react to HMs and how they affect seed performance as well as the benefits that HMs provide for plants. Furthermore, the review examines HMs' transport genes in plants and their molecular, biochemical, and metabolic responses to HMs. We have also examined the obstacles and potential for HMs in plants and their management strategies.

Diverse factors influence the amounts of carbon input to soils via rhizodeposition in plants: A review

Rhizodeposition encompasses the intricate processes through which plants generate organic compounds via photosynthesis, store these compounds within aboveground biomass and roots through top-down transport, and subsequently release this organic matter into the soil. Rhizodeposition represents one of the carbon (C) cycle in soils that can achieve long-term organic C sequestration. This function holds significant implications for mitigating the climate change that partly stems from the greenhouse effect associated with increased atmospheric carbon dioxide levels. Therefore, it is essential to further understand how the process of rhizodeposition allocates the photosynthetic C that plants create via photosynthesis. While many studies have explored the basic principles of rhizodeposition, along with the associated impact on soil C storage, there is a palpable absence of comprehensive reviews that summarize the various factors influencing this process. This paper compiles and analyzes the literature on plant rhizodeposition to describe how rhizodeposition influences soil C storage. Moreover, the review summarizes the impacts of soil physicochemical, microbial, and environmental characteristics on plant rhizodeposition and priming effects, and concludes with recommendations for future research.

Compost-diatomite-based phytostabilization course under extreme environmental conditions in terms of high pollutant contents and low temperatures

The effects of changes in environmental temperatures on the immobilization or removal of cationic potentially toxic elements (PTE) in heavily polluted soils are often poorly understood, although both are widely studied in the context of phytostabilization. To address this issue, a novel compost-diatomite hybrid (CDH) amendment was developed and applied for assisted phytostabilization at two external temperature regimes. (Cd/Ni/Cu/Zn)-extremely polluted soils (unenriched and CDH-enriched) were cultivated with perennial ryegrass and native soil microbiome under greenhouse conditions and then transferred to freeze-thaw conditions (FTC). The decrease in metal potential toxicity in soils subjected to phytostabilization following both temperature treatments was characterized by a combination of sequential extraction and atomic absorption measurements. The soil microbiome was characterized by high-throughput sequencing. In a relative comparison, the greatest decrease in the content of all PTEs in CDH-enriched soil (compared to unenriched soil) appeared in FTC. Furthermore, under the influence of FTC, in the relative comparison between two CDH-enriched soils (exposed-, and not-exposed- to FTC) and two unenriched soils (exposed-, and not-exposed- to FTC), the content of all PTEs decreased more sharply in the CDH-enriched series than in the unenriched series. The largest redistribution into four sequentially extracted fractions in CDH-enriched soil was found for Zn. Based on the distribution pattern, Zn immobilization was greater in CDH-enriched soil in FTC. CDH increased species richness in the soil, while FTC stimulated the growth of Bacteroidia, Alphaproteobacteria, Theromomicrobia, and Gammaproteobacteria. The analysis of the functionalities of the microbiome indicated enhanced metal transportation and defense systems in samples exposed to FTC. The current research is crucial for understanding how extreme environmental conditions in both cases high pollutant levels and low temperatures affect the movement and transformation of PTEs in polluted soils during phytostabilization.

The Phosphorus-Iron Nexus: Decoding the Nutrients Interaction in Soil and Plant

Phosphorus (P) and iron (Fe) are two essential mineral nutrients in plant growth. It is widely observed that interactions of P and Fe could influence their availability in soils and affect their homeostasis in plants, which has received significant attention in recent years. This review presents a summary of latest advances in the activation of insoluble Fe-P complexes by soil properties, microorganisms, and plants. Furthermore, we elucidate the physiological and molecular mechanisms underlying how plants adapt to Fe-P interactions. This review also discusses the current limitations and presents potential avenues for promoting sustainable agriculture through the optimization of P and Fe utilization efficiency in crops.

Phytochelatins Bind Zn(II) with Micro- to Picomolar Affinities without the Formation of Binuclear Complexes, Exhibiting Zinc Buffering and Muffling Rather than Storing Functions

Phytochelatins (PCs) are poly-Cys peptides containing a repeating γ-Glu-Cys motif synthesized in plants, algae, certain fungi, and worms by PC synthase from reduced glutathione. It has been shown that an excess of toxic metal ions induces their biosynthesis and that they are responsible for the detoxification process. Little is known about their participation in essential metal binding under nontoxic, basal conditions under which PC synthase is active. This study presents spectroscopic and thermodynamic interactions with the PC2-PC5 series, mainly focusing on the relations between Zn(II) complex stability and cellular Zn(II) availability. The investigations employed mass spectrometry, UV-vis spectroscopy, potentiometry, competition assays with zinc probes, and isothermal titration calorimetry (ITC). All peptides form ZnL complexes, while ZnL2 was found only for PC2, containing two to four sulfur donors in the coordination sphere. Binuclear species typical of Cd(II)-PC complexes are not formed in the case of Zn(II). Results demonstrate that the affinity for Zn(II) increases linearly from PC2 to PC4, ranging from micro- to low-picomolar. Further elongation does not significantly increase the stability. Stability elevation is driven mainly by entropic factors related to the chelate effect and conformational restriction rather than enthalpic factors related to the increasing number of sulfur donors. The affinity of the investigated PCs falls within the range of exchangeable Zn(II) concentrations (hundreds of pM) observed in plants, supporting for the first time a role of PCs both in buffering and in muffling cytosolic Zn(II) concentrations under normal conditions, not exposed to zinc excess, where short PCs have been identified in numerous studies. Furthermore, we found that Cd(II)-PC complexes demonstrate significantly higher metal capacities due to the formation of polynuclear species, which are lacking for Zn(II), supporting the role of PCs in Cd(II) storage (detoxification) and Zn(II) buffering and muffling. Our results on phytochelatins' coordination chemistry and thermodynamics are important for zinc biology and understanding the molecular basis of cadmium toxicity, leaving room for future studies.

Regulation of Root Exudation in Wheat Plants in Response to Alkali Stress

Soil alkalization is an important environmental factor limiting crop production. Despite the importance of root secretion in the response of plants to alkali stress, the regulatory mechanism is unclear. In this study, we applied a widely targeted metabolomics approach using a local MS/MS data library constructed with authentic standards to identify and quantify root exudates of wheat under salt and alkali stresses. The regulatory mechanism of root secretion in alkali-stressed wheat plants was analyzed by determining transcriptional and metabolic responses. Our primary focus was alkali stress-induced secreted metabolites (AISMs) that showed a higher secretion rate in alkali-stressed plants than in control and salt-stressed plants. This secretion was mainly induced by high-pH stress. We discovered 55 AISMs containing -COOH groups, including 23 fatty acids, 4 amino acids, 1 amino acid derivative, 7 dipeptides, 5 organic acids, 9 phenolic acids, and 6 others. In the roots, we also discovered 29 metabolites with higher levels under alkali stress than under control and salt stress conditions, including 2 fatty acids, 3 amino acid derivatives, 1 dipeptide, 2 organic acids, and 11 phenolic acids. These alkali stress-induced accumulated carboxylic acids may support continuous root secretion during the response of wheat plants to alkali stress. In the roots, RNAseq analysis indicated that 5 6-phosphofructokinase (glycolysis rate-limiting enzyme) genes, 16 key fatty acid synthesis genes, and 122 phenolic acid synthesis genes have higher expression levels under alkali stress than under control and salt stress conditions. We propose that the secretion of multiple types of metabolites with a -COOH group is an important pH regulation strategy for alkali-stressed wheat plants. Enhanced glycolysis, fatty acid synthesis, and phenolic acid synthesis will provide more energy and substrates for root secretion during the response of wheat to alkali stress.

Two microbes assisting Miscanthus floridulus in remediating multi-metal(loid)s-contaminated soil

Miscanthus has good tolerance to multi-metal(loid)s and has received increasing attention in remediated studies of metal(loid)s-contaminated soil. In this study, we conducted phytoextraction techniques to investigate the synergic effects of remediation of multi-metal(loid)s-contaminated soil by Miscanthus floridulus (Lab.) and two plant growth-promoting bacteria (PGPB), TS8 and MR2, affiliated to Enterobacteriaceae. The results exhibited a decrease of arsenic (15.27-21.50%), cadmium (8.64-15.52%), plumbum (5.92-12.76%), and zinc (12.84-24.20%) except for copper contents in the soil in bacterial inoculation groups, indicating that MR2 and TS8 could enhance the remediation of metal(loid)s. Moreover, increased fresh/dry weight and height indicated that inoculated bacteria could promote Miscanthus growth. Although the activities of antioxidant enzymes and the content of chlorophyll in the overground tissues showed no significant increase or even decrease, the activities of antioxidant enzymes in the underground tissues and soil were elevated by 48.95-354.17%, available P by 19.07-23.02%, and available K by 15.34-17.79% (p < 0.05). Bacterial inoculants could also decrease the soil pH. High-throughput sequencing analysis showed that the bacterial inoculant affected the rhizosphere bacterial community and reduced community diversity, but the relative abundance of some PGPB was found to increase. Phylogenetic molecular ecological networks indicated that bacterial inoculants reduced interactions between rhizosphere bacteria and thereby led to a simpler network structure but increased the proportion of positive-correlation links and enhanced the metabiosis and symbiosis of those bacteria. Spearman's test showed that OTUs affiliated with Enterobacteriaceae and soil nutrients were critical for metal(loid) remediation and Miscanthus growth. The results of this study provide a basis for the synergic remediation of multi-metal(loid)s-contaminated soils by Miscanthus and PGPB and provide a reference for the subsequent regulation of Miscanthus remediation efficiency by the other PGPB or critical bacteria.

The activation of iron deficiency responses of grapevine rootstocks is dependent to the availability of the nitrogen forms

BACKGROUND

In viticulture, iron (Fe) chlorosis is a common abiotic stress that impairs plant development and leads to yield and quality losses. Under low availability of the metal, the applied N form (nitrate and ammonium) can play a role in promoting or mitigating Fe deficiency stresses. However, the processes involved are not clear in grapevine. Therefore, the aim of this study was to investigate the response of two grapevine rootstocks to the interaction between N forms and Fe uptake. This process was evaluated in a hydroponic experiment using two ungrafted grapevine rootstocks Fercal (Vitis berlandieri x V. vinifera) tolerant to deficiency induced Fe chlorosis and Couderc 3309 (V. riparia x V. rupestris) susceptible to deficiency induced Fe chlorosis.

RESULTS

The results could differentiate Fe deficiency effects, N-forms effects, and rootstock effects. Interveinal chlorosis of young leaves appeared earlier on 3309 C from the second week of treatment with NO3-/NH4+ (1:0)/-Fe, while Fercal leaves showed less severe symptoms after four weeks of treatment, corresponding to decreased chlorophyll concentrations lowered by 75% in 3309 C and 57% in Fercal. Ferric chelate reductase (FCR) activity was by trend enhanced under Fe deficiency in Fercal with both N combinations, whereas 3309 C showed an increase in FCR activity under Fe deficiency only with NO3-/NH4+ (1:1) treatment. With the transcriptome analysis, Gene Ontology (GO) revealed multiple biological processes and molecular functions that were significantly regulated in grapevine rootstocks under Fe-deficient conditions, with more genes regulated in Fercal responses, especially when both forms of N were supplied. Furthermore, the expression of genes involved in the auxin and abscisic acid metabolic pathways was markedly increased by the equal supply of both forms of N under Fe deficiency conditions. In addition, changes in the expression of genes related to Fe uptake, regulation, and transport reflected the different responses of the two grapevine rootstocks to different N forms.

CONCLUSIONS

Results show a clear contribution of N forms to the response of the two grapevine rootstocks under Fe deficiency, highlighting the importance of providing both N forms (nitrate and ammonium) in an appropriate ratio in order to ease the rootstock responses to Fe deficiency.

Mechanisms of low cadmium accumulation in crops: A comprehensive overview from rhizosphere soil to edible parts

Cadmium (Cd) is a toxic heavy metal often found in soil and agricultural products. Due to its high mobility, Cd poses a significant health risk when absorbed by crops, a crucial component of the human diet. This absorption primarily occurs through roots and leaves, leading to Cd accumulation in edible parts of the plant. Our research aimed to understand the mechanisms behind the reduced Cd accumulation in certain crop cultivars through an extensive review of the literature. Crops employ various strategies to limit Cd influx from the soil, including rhizosphere microbial fixation and altering root cell metabolism. Additional mechanisms include membrane efflux, specific transport, chelation, and detoxification, facilitated by metalloproteins such as the natural resistance-associated macrophage protein (Nramp) family, heavy metal P-type ATPases (HMA), zinc-iron permease (ZIP), and ATP-binding cassette (ABC) transporters. This paper synthesizes differences in Cd accumulation among plant varieties, presents methods for identifying cultivars with low Cd accumulation, and explores the unique molecular biology of Cd accumulation. Overall, this review provides a comprehensive resource for managing agricultural lands with lower contamination levels and supports the development of crops engineered to accumulate minimal amounts of Cd.

The role of plants in ironstone evolution: iron and aluminium cycling in the rhizosphere

The Carajás plateaus in Brazil host endemic epilithic vegetation ("campo rupestre") on top of ironstone duricrusts, known as canga. This capping rock is primarily composed of iron(III) oxide minerals and forms a physically resistant horizon. Field observations reveal an intimate interaction between canga's surface and two native sedges (Rhynchospora barbata and Bulbostylis cangae). These observations suggest that certain plants contribute to the biogeochemical cycling of iron. Iron dissolution features at the root-rock interface were characterised using synchrotron-based techniques, Raman spectroscopy and scanning electron microscopy. These microscale characterisations indicate that iron is preferentially leached in the rhizosphere, enriching the comparatively insoluble aluminium around root channels. Oxalic acid and other exudates were detected in active root channels, signifying ligand-controlled iron oxide dissolution, likely driven by the plants' requirements for goethite-associated nutrients such as phosphorus. The excess iron not uptaken by the plant can reprecipitate in and around roots, line root channels and cement detrital fragments in the soil crust at the base of the plants. The reprecipitation of iron is significant as it provides a continuously forming cement, which makes canga horizons a 'self-healing' cover and contributes to them being the world's most stable continuously exposed land surfaces. Aluminium hydroxide precipitates ("gibbsite cutans") were also detected, coating some of the root cavities, often in alternating layers with goethite. This alternating pattern may correspond with oscillating oxygen concentrations in the rhizosphere. Microbial lineages known to contain iron-reducing bacteria were identified in the sedge rhizospheric microbiome and likely contribute to the reductive dissolution of iron(III) oxides within canga. Drying or percolation of oxygenated water to these anaerobic niches have led to iron mineralisation of biofilms, detected in many root channels. This study sheds light on plants' direct and indirect involvement in canga evolution, with possible implications for revegetation and surface restoration of iron mine sites.

Amendment-driven soil health restoration through soil pH and microbial robustness in a Cd/Cu-combined acidic soil: A ten-year in-situ field experiment

Soil health arguably depends on biodiversity and has received wide attention in heavy-metal (HM) contaminated farmland remediation in recent years. However, long-term effects and mechanisms of soil amendment remain poorly understood with respect to soil microbal community. In this in-situ field study, four soil amendments (attapulgite-At, apatite-Ap, montmorillonite-M, lime-L) at three rates were applied once only for ten years in a cadmium (Cd)-copper (Cu) contaminated paddy soil deprecated for over five years. Results showed that after ten years and in compared with CK (no amendment), total Cd concentration and its risk in plot soils were not altered by amendments (p > 0.05), but total Cu concentration and its risk were significantly increased by both Ap and L, especially the former, rather than At and M (p < 0.05), through increased soil pH and enhanced bacterial alpha diversity as well as plant community. Soil microbial communities were more affected by amendment type (30%) than dosage (11%), microbial network characteristics were dominated by rare taxa, and soil multifunctionality was improved in Ap- and L-amended soils. A structural equation model (SEM) indicated that 57.3% of soil multifunctionality variances were accounted for by soil pH (+0.696) and microbial network robustness (-0.301). Moreover, microbial robustness could be potentially used as an indicator of soil multifunctionality, and Ap could be optimized to improve soil health in combined with biomass removal. These findings would advance the understanding of soil microbial roles, especially its network robustness, on soil multifunctionality for the remediation of metal contaminated soils and metal control management strategies in acidic soils. ENVIRONMENTAL IMPLICATION: Farmland soil contamination by heavy metals (HMs) has been becoming a serious global environmental challenge. However, most studies have been conducted over the short term, leading to a gap in the long-term remediation efficiency and ecological benefits of soil amendments. For the successful deployment of immobilization technologies, it is critical to understand the long-term stability of the immobilized HMs and soil health. Our study, to the best of our knowlege, is the first to state the long-term effects and mechanisms of soil amendments on soil health and optimize an effective and eco-friendly amendment for long-term Cd/Cu immobilization.

Urban green space area mitigates the accumulation of heavy metals in urban soils

Despite that the heavy metals in urban soils pose a threat to public health, the critical factors that influence their concentrations in urban soils are not well understood. In this study, we conducted a survey of surface soil samples from urban green spaces in Shanghai, to analyze the concentrations of the key heavy metals. The results showed that Zn was the most abundant metal with an average concentration of 122.99 mg kg-1, followed by Pb (32.72 mg kg-1) and Cd (0.23 mg kg-1). All concentrations were found to be below the risk screening values defined by the National Environmental Quality Standards for soils of development land in China (GB36600-2018), indicating no current risk in Shanghai. However, there was a clear accumulation of heavy metals, as the mean concentrations were significantly higher than the background values. Furthermore, we explored the relationships between key heavy metals with population density, GDP and green space area. Both Spearman correlation and Random Forest analysis indicated that per capita green space area (pGSA) and population density were the most crucial factors influencing the status of heavy metals in urban soils, unlike edaphic factors e.g. SOM content in farmland soils. Specifically, there was a significantly positive linear correlation between heavy metal concentrations and population density, with correlation coefficients ranging from 0.3 to 0.4. However, the correlation with pGSA was found to be non-linear. The nonlinear regression analysis revealed threshold values between heavy metals concentrations and pGSA (e.g Zn 22.22 m2, Pb 24.92 m2, and Cd 25.92 m2), with a sharp reduction in heavy metal concentrations below the threshold and a slow reduction above the threshold. It suggests that an increase in per capita green space area can mitigate the accumulation of heavy metals caused by growing population density, but the effect is limited after the threshold. Our findings not only provide insights into the distribution patterns of heavy metals in the urban soils at the local scale, but also contribute to the urban greening at the global scale and offer guidance for city planning in the face of increasing population densities over the coming decades.

Recent advances in nano-fertilizers: synthesis, crop yield impact, and economic analysis

The escalating global demand for food production has predominantly relied on the extensive application of conventional fertilizers (CFs). However, the increased use of CFs has raised concerns regarding environmental risks, including soil and water contamination, especially within cereal-based cropping systems. In response, the agricultural sector has witnessed the emergence of healthier alternatives by utilizing nanotechnology and nano-fertilizers (NFs). These innovative NFs harness the remarkable properties of nanoparticles, ranging in size from 1 to 100 nm, such as nanoclays and zeolites, to enhance nutrient utilization efficiency. Unlike their conventional counterparts, NFs offer many advantages, including variable solubility, consistent and effective performance, controlled release mechanisms, enhanced targeted activity, reduced eco-toxicity, and straightforward and safe delivery and disposal methods. By facilitating rapid and complete plant absorption, NFs effectively conserve nutrients that would otherwise go to waste, mitigating potential environmental harm. Moreover, their superior formulations enable more efficient promotion of sustainable crop growth and production than conventional fertilizers. This review comprehensively examines the global utilization of NFs, emphasizing their immense potential in maintaining environmentally friendly crop output while ensuring agricultural sustainability.

Arabidopsis response to copper is mediated by density and root exudates: Evidence that plant density and toxic soils can shape plant communities

PREMISE

Plants grown at high densities show increased tolerance to heavy metals for reasons that are not clear. A potential explanation is the release of citrate by plant roots, which binds metals and prevents uptake. Thus, pooled exudates at high plant densities might increase tolerance. We tested this exclusion facilitation hypothesis using mutants of Arabidopsis thaliana defective in citrate exudation.

METHODS

Wild type Arabidopsis and two allelic mutants for the Ferric Reductase Defective 3 (FRD3) gene were grown at four densities and watered with copper sulfate at four concentrations. Plants were harvested before bolting and dried. Shoot biomass was measured, and shoot material and soil were digested in nitric acid. Copper contents were determined by atomic absorption.

RESULTS

In the highest-copper treatment, density-dependent reduction in toxicity was observed in the wild type but not in FRD3 mutants. For both mutants, copper concentrations per gram biomass were up to seven times higher than for wild type plants, depending on density and copper treatment. In all genotypes, total copper accumulation was greater at higher plant densities. Plant size variation increased with density and copper treatment because of heterogeneous distribution of copper throughout the soil.

CONCLUSIONS

These results support the hypothesis that citrate exudation is responsible for density-dependent reductions in toxicity of metals. Density-dependent copper uptake and growth in contaminated soils underscores the importance of density in ecotoxicological testing. In soils with a heterogeneous distribution of contaminants, competition for nontoxic soil regions may drive size hierarchies and determine competitive outcomes.

Genome-wide profiling of genetic variations reveals the molecular basis of aluminum stress adaptation in Tibetan wild barley

Aluminum (Al) toxicity in acidic soil is a major factor affecting crop productivity. The extensive genetic diversity found in Tibetan wild barley germplasms offers a valuable reservoir of alleles associated with aluminum tolerance. Here, resequencing of two Al-tolerant barley genotypes (Tibetan wild barley accession XZ16 and cultivated barley Dayton) identified a total of 19,826,182 and 16,287,277 single nucleotide polymorphisms (SNPs), 1628,052 and 1386,973 insertions/deletions (InDels), 61,532 and 57,937 structural variations (SVs), 248,768 and 240,723 copy number variations (CNVs) in XZ16 and Dayton, respectively, and uncovered approximately 600 genes highly related to Al tolerance in barley. Comparative genomic analyses unveiled 71 key genes that contain unique genetic variants in XZ16 and are predominantly associated with organic acid exudation, Al sequestration, auxin response, and transcriptional regulation. Manipulation of these key genes at the genetic and transcriptional level is a promising strategy for developing optimal haplotype combinations and new barley cultivars with improved Al tolerance. This study represents the first comprehensive examination of genetic variation in Al-tolerant Tibetan wild barley through genome-wide profiling. The obtained results make the deep insight into the mechanisms underlying barley adaptation to Al toxicity, and identified the candidate genes useful for improvement of Al tolerance in barley.

Citrus sinensis manganese tolerance: Insight from manganese-stimulated secretion of root exudates and rhizosphere alkalization

We used manganese (Mn)-tolerant 'Xuegan' (Citrus sinensis) seedlings as materials and examined the characterization of Mn uptake and Mn-activated-release of root exudates under hydroponic conditions. We observed that root and shoot Mn bioaccumulation factor (BCF) reduced with the increase of Mn supply, and that Mn transfer factor (Tf) reduced greatly as Mn supply increased from 0 to 500 μM, beyond which Tf slightly increased with increasing Mn supply, suggesting that Mn supply reduced the ability to absorb and accumulate Mn in roots and shoots, as well as root-to-shoot Mn translocation. Without Mn, roots alkalized the solution pH from 5.0 to above 6.2, while Mn supply reduced root-induced alkalization. As Mn supply increased from 0 to 2000 μM, the secretion of root total phenolics (TPs) increased, while the solution pH decreased. Mn supply did not alter the secretion of root total free amino acids, total soluble sugars, malate, and citrate. Mn-activated-release of TPs was inhibited by low temperature and anion channel inhibitors, but not by protein biosynthesis inhibitor. Using widely targeted metabolome, we detected 48 upregulated [35 upregulated phenolic compounds + 13 other secondary metabolites (SMs)] and three downregulated SMs, and 39 upregulated and eight downregulated primary metabolites (PMs). These findings suggested that reduced ability to absorb and accumulate Mn in roots and shoots and less root-to-shoot Mn translocation in Mn-toxic seedlings, rhizosphere alkalization, and Mn-activated-release of root exudates (especially phenolic compounds) contributed to the high Mn tolerance of C. sinensis seedlings.

The Mechanisms of Molybdate Distribution and Homeostasis with Special Focus on the Model Plant Arabidopsis thaliana

This review article deals with the pathways of cellular and global molybdate distribution in plants, especially with a full overview for the model plant Arabidopsis thaliana. In its oxidized state as bioavailable molybdate, molybdenum can be absorbed from the environment. Especially in higher plants, molybdenum is indispensable as part of the molybdenum cofactor (Moco), which is responsible for functionality as a prosthetic group in a variety of essential enzymes like nitrate reductase and sulfite oxidase. Therefore, plants need mechanisms for molybdate import and transport within the organism, which are accomplished via high-affinity molybdate transporter (MOT) localized in different cells and membranes. Two different MOT families were identified. Legumes like Glycine max or Medicago truncatula have an especially increased number of MOT1 family members for supplying their symbionts with molybdate for nitrogenase activity. In Arabidopsis thaliana especially, the complete pathway followed by molybdate through the plant is traceable. Not only the uptake from soil by MOT1.1 and its distribution to leaves, flowers, and seeds by MOT2-family members was identified, but also that inside the cell. the transport trough the cytoplasm and the vacuolar storage mechanisms depending on glutathione were described. Finally, supplying the Moco biosynthesis complex by MOT1.2 and MOT2.1 was demonstrated.

Phytoaccumulation of trace elements (As, Cd, Co, Cu, Pb, Zn) by Nicotiana glauca and Euphorbia segetalis growing in a Technosol developed on legacy mine wastes (Domingo Rubio wetland, SW Spain)

Sulfidic mine wastes have the potential to generate acid mine drainage (AMD) and release acid leachates containing high levels of iron, sulfate and potentially toxic elements (PTEs). Soils receiving AMD discharges are generally devoid of vegetation. Only a few metal-tolerant plant species can survive under such adverse soil conditions. This work investigates two plant species, Nicotiana glauca and Euphorbia segetalis, that have successfully colonized an AMD-impacted wetland area in south-western Spain. The uptake of PTEs from the soil by roots and their transfer and accumulation in the above-ground biomass were quantified. Results showed that these pioneer plants grew in patches of neutral soil within the wasteland despite the high concentrations of PTEs in the rhizosphere soil (up to: 613 mg kg-1 As, 18.7 mg kg-1 Cd, 6370 mg kg-1 Cu, 2210 mg kg-1 Pb and 5250 mg kg-1 Zn). The target organs of As, Cu and Pb accumulation were: root > leaf > stem in N. glauca, and root > stem > leaf in E. segetalis. Zinc and Cd showed a significant decrease in roots relative to aerial parts of N. glauca, and Co was preferentially partitioned in stems of N. glauca and leaves of E. segetalis. The soil-plant transfer coefficient values of PTEs in all parts of both plants were well below unity with the only exception of Cd in leaves of N. glauca (1.254), suggesting that roots acted as a barrier limiting the uptake of PTEs by plants. Interestingly, under the same soil conditions, N. glauca absorbed Cd in considerable proportions from soil and accumulated it in its leaves, while E. segetalis was not effective in transferring PTEs from roots shoots except for Co. In conclusion, soil pH and plant-related factors greatly influence the stabilization of PTE in the rhizospheric soil and produce inconsistencies in PTE phytoavailability. The findings of this study provide criteria to assist in natural remediation in other legacy contaminated sites worldwide.

Effects of different remediation methods on phosphorus transformation and availability

The effects of different heavy metal pollution remediation methods on soil nutrient transformation and soil health remain unclear. In this study, the effects of phytoextraction (PE) and passivation remediation (PR) on Cd-polluted soil phosphorus transformation and availability were compared by pot experiment. The results showed that PE significantly reduced the concentrations of total and available Cd (both H2O-Cd and DTPA-Cd) in soil, PR also decreased available Cd content but had no significant effect on total Cd content. PE slightly increased soil pH and NH4+-N content, while PR significantly increased soil pH, NO3--N and AK content. PE promoted the conversion of stable P (including HCl-Pi and residual-Pt), and increased the content of labile P (including H2O-Pi, NaHCO3-Pi and NaHCO3-Po) and the proportion of moderately labile P (including NaOH-Pi and NaOH-Po), while PR showed the opposite trend. PE showed a higher soil phoC gene abundance and acid phosphatase (ACP) activity, while PR showed a higher phoD gene copies and alkaline phosphatase (ALP) activity. Soil bacteria and phoD-harboring bacteria community was significantly affected by remediation methods and soil types. Compared with PR, PE reduced phoD-harboring bacterial diversity but significantly increased the abundance of genera associated with P dissolution (Streptomyces) and P conversion (Bradyrhizobium and Frankia), both of which were significantly positively correlated with labile P or moderately labile P. In general, compared with PR, PE can effectively remove soil Cd pollution, while maintaining a higher content of labile P and a higher proportion of moderately labile P, which can be considered as a green and sustainable remediation strategy conducive to soil quality.

Rhizosphere environmental factors regulated the cadmium adsorption by vermicompost: Influence of pH and low-molecular-weight organic acids

Vermicompost is a promising amendment for immobilization of cadmium (Cd) in soils; however, its effectiveness can be influenced by rhizosphere environment conditions, such as pH and the presence of low-molecular-weight organic acids (LMWOAs). In this study, a batch experiment was conducted to examine the characteristics of Cd adsorption by vermicompost at different pH (pH = 3, 5, and 7) and after the addition of different LMWOAs (oxalic acid; citric acid; malic acid). Furthermore, a series of morphology and structural analyses were conducted to elucidate the mechanisms of observed effects. The results showed that the adsorption capacity of vermicompost for Cd increased as pH increased, and chemisorption dominated the adsorption process. Changes in pH altered adsorption performance by affecting the -OH groups of alcohol/phenol and the -CH2 groups of aliphatics. Further, the addition of oxalic acid promoted Cd adsorption, and the effect was concentration dependent. Modifying the verimicompost surface with more adsorption sites might be the main reason. Conversely, citric acid and malic acid showed the ability to inhibit Cd adsorption by vermicompost. Citric acid caused a blocking effect by covering flocculent substances on the vermicompost surface while reducing surface adsorption sites by dissolving mineral components such as iron oxides. However, the action of malic acid did not appear to be related to changes in morphology or the structure of vermicompost. Overall, the results of this study partially explain the limited effectiveness of Cd immobilization within the rhizosphere by vermicompost, and provide theoretical support for regulating rhizosphere environments to improve the effectiveness of vermicompost immobilization of Cd.

Combination of intercropping maize and soybean with root exudate additions reduces metal mobility in soil-plant system under wastewater irrigation

The effects of root exudates and irrigation with treated wastewater on heavy metal mobility and soil bacterial composition under intercropping remain poorly understood. We conducted a pot experiment with maize and soybean grown in monocultures or intercultures, irrigated with either groundwater or treated wastewater. In addition, the pre-collected root exudates from hydroponic culture with mono- or inter-cropped maize and soybean were applied to the soil at four levels (0 %, 16 %, 32 % and 64 %). The results showed that application of root exudates increased plant growth and soil nutrient content. The analysis of "Technique for Order of Preference by Similarity to Ideal Solution" for higher plant biomass and lower soil Cd and Pb concentrations indicated that the best performance of soybean under treated wastewater irrigation was recorded under intercropping applied with 64 % of exudates, with a performance score of 0.926 and 0.953 for Cd and Pb, respectively. The second-best performance of maize under treated wastewater irrigation was also observed under intercropping applied with 64 % of exudates. Root exudate application reduced heavy metals migration in the soil-plant system, with a greater impact in intercropping than in monocropping. In addition, certain soil microorganisms were also increased with root exudate application, regardless of irrigation water. This study suggests that appropriate application of root exudates could potentially improve plant growth and soil health, and reduce toxic heavy metal concentrations in soils and plants irrigated with treated wastewater.

Electrokinetic remediation of estuarine sediments using a large reactor: spatial variation of physicochemical, mineral, and chemical properties

The treatment and beneficial use of polluted or contaminated environmental matrices have become major issues, especially as the world strives toward a zero-waste policy. In this regard, dredged sediments need to be treated before they can be used in an environmentally safe and sustainable manner. Therefore, this work aims to treat estuarine sediments and, more importantly, use physicochemical, mineral, organic, and chemical information to understand the reactions that occur upon treatment. Dredged estuarine sediments were collected from Tancarville (Seine River estuary, France) and subjected to electrokinetic (EK) remediation using a 128-L laboratory-scale reactor. The sediments were treated 8 h per day for 21 days. The electric (voltage and current) and physicochemical (pH and electric conductivity) parameters were monitored during treatment. Sediments were collected from various sections in the reactor at the end of the experiment (lengthwise, widthwise, and depthwise). The spatial variation was investigated in terms of organic, mineral, and metal contents. Statistical analyses proved that the variation occurred only in the lengthwise direction. Furthermore, three main phases described the treatment, which were mainly linked to carbonate dissolution and pH variation. The results also showed that the trace elements Ni and Zn were reduced by 21% and 19%, respectively, without a direct link to pH, while Ca and Mg were only redistributed. The buffering capacity of the anodic sediment was reduced due to carbonate dissolution. The treated sediments showed reduced contents in trace metals without affecting major elements that can be useful in agriculture (i.e., Ca and Mg).

Unraveling plant adaptation to single and combined nutrient deficiencies in a dicotyledonous and a monocotyledonous plant species

Nutrient deficiencies considerably limit agricultural production worldwide. However, while single deficiencies are widely studied, combined deficiencies are poorly addressed. Hence, the aim of this paper was to study single and combined deficiencies of iron (Fe) and phosphorus (P) in barley (Hordeum vulgare) and tomato (Solanum lycopersicum). Plants were grown in hydroponics and root exudation was measured over the growing period. At harvest, root morphology and root and shoot ionome was assessed. Shoot-to-root-ratio decreased in both species and in all nutrient deficiencies, besides in -Fe tomato. Barley root growth was enhanced in plants subjected to double deficiency behaving similarly to -P, while tomato reduced root morphology parameters in all treatments. To cope with the nutrient deficiency barley exuded mostly chelants, while tomato relied on organic acids. Moreover, tomato exhibited a slight exudation increase over time not detected in barley. Overall, in none of the species the double deficiency caused a substantial increase in root exudation. Multivariate statistics emphasized that all the treatments were significantly different from each other in tomato, while in barley only -Fe was statistically different from the other treatments. Our findings highlight that the response of the studied plants in double deficiencies is not additive but plant specific.

CsBPC2 is a key regulator of root growth and development

BASIC PENTACYSTEINE (BPCs) transcription factors are important regulators of plant growth and development. However, the regulatory mechanism of BPC2 in roots remains unclear. In our previous study, we created Csbpc2 cucumber mutants by the CRISPR/Cas9 system, and our studies on the phenotype of Csbpc2 mutants showed that the root growth was inhibited compared with wide-type (WT). Moreover, the surface area, volume and number of roots decreased significantly, with root system architecture changing from dichotomous branching to herringbone branching. Compared with WT, the leaf growth of the Csbpc2 mutants was not affected. However, the palisade and spongy tissue were significantly thinner, which was not beneficial for photosynthesis. The metabolome of root exudates showed that compared with WT, amino acids and their derivatives were significantly decreased, and the enriched pathways were mainly regulated by amino acids and their derivatives, indicating that knockout of CsBPC2 mainly affected the amino acid content in root exudates. Importantly, transcriptome analysis showed that knockout of CsBPC2 mainly affected root gene expression. Knockout of CsBPC2 significantly reduced the gene expression of gibberellins synthesis. However, the expression of genes related to amino acid synthesis, nitrogen fixation and PSII-related photosynthesis increased significantly, which may be due to the effect of knocking out CsBPC2 on gibberellins synthesis, resulting in the inhibition of seedling growth, thus forming negative feedback regulation. Generally, we showed for the first time that BPC2 is a key regulator gene of root growth and development, laying the foundation for future mechanisms of BPC2 regulation in roots.

Role of mycorrhizas and root exudates in plant uptake of soil nutrients (calcium, iron, magnesium, and potassium): has the puzzle been completely solved?

Anthropogenic global change is driving an increase in the frequency and intensity of drought and flood events, along with associated imbalances and limitation of several soil nutrients. In the context of an increasing human population, these impacts represent a global-scale challenge for biodiversity conservation and sustainable crop production to ensure food security. Plants have evolved strategies to enhance uptake of soil nutrients under environmental stress conditions; for example, symbioses with fungi (mycorrhization) in the rhizosphere and the release of exudates from roots. Although crop cultivation is managed for the effects of limited availability of nitrogen (N) and phosphorus (P), there is increasing evidence for limitation of plant growth and fitness because of the low availability of other soil nutrients such as the metals potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe), which may become increasingly limiting for plant productivity under global change. The roles of mycorrhizas and plant exudates on N and P uptake have been studied intensively; however, our understanding of the effects on metal nutrients is less clear and still inconsistent. Here, we review the literature on the role of mycorrhizas and root exudates in plant uptake of key nutrients (N, P, K, Ca, Mg, and Fe) in the context of potential nutrient deficiencies in crop and non-crop terrestrial ecosystems, and identify knowledge gaps for future research to improve nutrient-uptake capacity in food crop plants.

Intercropping efficiency of Pteris vittata with two legume plants: Impacts of soil arsenic concentrations

Intercropping of hyperaccumulators with crops has emerged as a promising method for remediating arsenic (As)-contaminated soil in agroecosystems. However, the response of intercropping hyperaccumulators with different types of legume plants to diverse gradients of As-contaminated soil remains poorly understood. In this study, we assessed the response of plant growth and accumulation of an As hyperaccumulator (Pteris vittata L.) intercropped with two legume plants to three gradients of As-contaminated soil. Results indicated that soil As concentration had a substantial effect on the As uptake by plants. P. vittata growing in slightly As-contaminated soil (80 mg kg^-1) exhibited higher As accumulation (1.52-5.49 folds) than those in higher As-contaminated soil (117 and 148 mg kg^-1), owing to the lower soil pH in high As-contaminated soil. Intercropping with Sesbania cannabina L. increased As accumulation in P. vittata by 19.3%- 53.9% but decreased in intercropping with Cassia tora L. This finding was attributed to S. cannabina providing more NO₃^--N to P. vittata to support its growth, and higher resistance to As. The decreased rhizosphere pH in the intercropping treatment also resulted in the increased As accumulation in P. vittata. Meanwhile, the As concentrations in the seeds of the two legume plants met the national food standards(<0.5 mg kg^-1). Therefore, the intercropping P. vittata with S. cannabina is a highly effective intercropping system in slightly As-contaminated soil and provides a potent method for As phytoremediation.

Speciation and distribution of chromium (III) in rice root tip and mature zone: The significant impact of root exudation and iron plaque on chromium bioavailability

Evidence on the contribution of root regions with varied maturity levels in iron plaque (IP) formation and root exudation of metabolites and their consequences for uptake and bioavailability of chromium (Cr) remains unknown. Therefore, we applied combined nanoscale secondary ion mass spectrometry (NanoSIMS) and synchrotron-based techniques, micro-X-ray fluorescence (µ-XRF) and micro-X-ray absorption near-edge structure (µ-XANES) to examine the speciation and localisation of Cr and the distribution of (micro-) nutrients in rice root tip and mature region. µ-XRF mapping revealed that the distribution of Cr and (micro-) nutrients varied between root regions. Cr K-edge XANES analysis at Cr hotspots attributed the dominant speciation of Cr in outer (epidermal and sub-epidermal) cell layers of the root tips and mature root to Cr(III)-FA (fulvic acid-like anions) (58-64%) and Cr(III)-Fh (amorphous ferrihydrite) (83-87%) complexes, respectively. The co-occurrence of a high proportion of Cr(III)-FA species and strong co-location signals of ⁵²Cr¹⁶O and ¹³C¹⁴N in the mature root epidermis relative to the sub-epidermis indicated an association of Cr with active root surfaces, where the dissolution of IP and release of their associated Cr are likely subject to the mediation of organic anions. The results of NanoSIMS (poor ⁵²Cr¹⁶O and ¹³C¹⁴N signals), dissolution (no IP dissolution) and µ-XANES (64% in sub-epidermis >58% in the epidermis for Cr(III)-FA species) analyses of root tips may be indicative of the possible re-uptake of Cr by this region. The results of this research work highlight the significance of IP and organic anions in rice root systems on the bioavailability and dynamics of heavy metals (e.g. Cr).

Biotic stress-induced changes in root exudation confer plant stress tolerance by altering rhizospheric microbial community

Every organism on the earth maintains some kind of interaction with its neighbours. As plants are sessile, they sense the varied above-ground and below-ground environmental stimuli and decipher these dialogues to the below-ground microbes and neighbouring plants via root exudates as chemical signals resulting in the modulation of the rhizospheric microbial community. The composition of root exudates depends upon the host genotype, environmental cues, and interaction of plants with other biotic factors. Crosstalk of plants with biotic agents such as herbivores, microbes, and neighbouring plants can change host plant root exudate composition, which may permit either positive or negative interactions to generate a battlefield in the rhizosphere. Compatible microbes utilize the plant carbon sources as their organic nutrients and show robust co-evolutionary changes in changing circumstances. In this review, we have mainly focused on the different biotic factors responsible for the synthesis of alternative root exudate composition leading to the modulation of rhizosphere microbiota. Understanding the stress-induced root exudate composition and resulting change in microbial community can help us to devise strategies in engineering plant microbiomes to enhance plant adaptive capabilities in a stressful environment.

Field evaluation of industrial non-food crops for phytomanaging a metal-contaminated dredged sediment

Phytomanagement is a concept fit for a bio-based circular economy that combines phytotechnologies and biomass production for non-food purposes. Here, ten annual and perennial industrial non-food crops (Sorghum Biomass 133, Sorghum Santa Fe red, Linum usitatissimum L., Eucalyptus sp., Salix Inger, Salix Tordis, Beta vulgaris L., Phacelia tanacetifolia Benth., Malva sylvestris L., and Chenopodium album L.) were studied under field conditions for phytomanaging a metal (Cd, Cu, Pb, and Zn)-contaminated dredged sediment in the North of France. The crops were selected according to their relevance to pedoclimatic and future climatic conditions, and one or more non-food end-products were proposed for each plant part collected, such as biogas, bioethanol, compost, natural dye, ecocatalyst, and fiber. Based on the soil-plant transfer of metals, eight out of the crops cultivated on field plots exhibited an excluder behavior (bioconcentration factor, BCF < 1), a trait suitable for phytostabilization. However, these crops did not change the metal mobilities in the dredged sediment. The BCF < 1 was not sufficient to characterize the excluder behavior of crops as this factor depended on the total dredged-sediment contaminant. Therefore, a BCF group ranking method was proposed accounting for metal phytotoxicity levels or yield decrease as a complemental way to discuss the crop behavior. The feasibility of the biomass-processing chains was discussed based on these results and according to a survey of available legislation in standard and scientific literature.

Type-dependent effects of microplastics on tomato (Lycopersicon esculentum L.): Focus on root exudates and metabolic reprogramming

Much attention has been paid to the prevalence of microplastics (MPs) in terrestrial systems. MPs have been shown to affect the physio-biochemical properties of plants. Different MPs may have distinctive behaviors and diverse effects on plant growth. In the present study, the effects of polystyrene (PS), polyethylene (PE), and polypropylene (PP) MPs on physio-biochemical properties, root exudates, and metabolomics of tomato (Lycopersicon esculentum L.) under hydroponic conditions were investigated. Our results show that MPs exposure has adverse effects on tomato growth. MPs exposure had a significant type-dependent effect (p < 0.001) on photosynthetic gas parameters, chlorophyll content, and antioxidant enzyme activities. After exposure to MPs, the content of low molecular weight organic acids in tomato root exudates was significantly increased, which was considered as a strategy to alleviate the toxicity of MPs. In addition, MPs treatment significantly changed the metabolites of tomato root and leaf. Metabolic pathway analysis showed that MPs treatment had a great effect on amino acid metabolism. We also found that plants exposed to PS and PP MPs produced more significant metabolic reprogramming than those exposed to PE MPs. This study provides important implications for the mechanism studies on the toxic effect of various MPs on crops and their future risk assessment.

Role of Root Exudates in Cadmium Accumulation of a Low-Cadmium-Accumulating Tobacco Line (Nicotiana tabacum L.)

Root exudates are tightly linked with cadmium (Cd) uptake by the root and thus affect plant Cd accumulation. A hydroponic experiment was carried out to explore the role of root exudates in Cd accumulation of a low-Cd-accumulating tobacco line (RG11) compared with a high-Cd- accumulating tobacco line (Yuyan5). Greater secretion of organic acids and amino acids by the roots was induced by an exogenous Cd addition in the two tobacco lines. The concentration of organic acid secreted by RG11 was only 51.1~61.0% of that secreted by Yuyan5. RG11 roots secreted more oxalic acid and acetic acid and less tartaric acid, formic acid, malic acid, lactic acid, and succinic acid than Yuyan5 under Cd stress. Oxalic acid accounted for 26.8~28.8% of the total organic acids, being the most common component among the detected organic acids, and was significantly negatively correlated with Cd accumulation in RG11. Propionic acid was only detected in the root exudates of RG11 under Cd stress. Lactic acid was positively linked with Cd accumulation in Yuyan5, being less accumulated in RG11. Similarly, RG11 secreted more amino acids than Yuyan5 under Cd stress. Aspartic acid, serine, and cysteine appeared in RG11 when it was exposed to Cd. Lysine was the most secreted amino acid in RG11 under Cd stress. RG11 roots secreted less lysine, histidine, and valine, but more phenylalanine and methionine than Yuyan5 under Cd stress. The results show that organic acids and amino acids in root exudates play a key role in Cd uptake by the root, and this contribution varied with cultivar/genotype. However, further research is still needed to explore the mechanisms underlying low Cd translocation to the leaf, which may be the key contribution of low Cd accumulation in RG11 to the security of tobacco leaf.

Cadmium tolerance and accumulation from the perspective of metal ion absorption and root exudates in broomcorn millet

Cadmium (Cd) is a persistent heavy metal that poses environmental and public health concerns. This study aimed to identify the potential biomarkers responsible for Cd tolerance and accumulation by investigating the response of the content of essential metal elements, transporter gene expression, and root exudates to Cd stress in broomcorn millet (Panicum miliaceum). A hydroponics experiment was conducted using two broomcorn millet cultivars with distinct Cd tolerance levels and accumulation phenotypes (Cd-tolerant and Cd-sensitive cultivars). Cd stress inhibited lateral root growth, especially in the Cd-sensitive cultivar. Furthermore, Cd accumulation was significantly greater in the Cd-tolerant cultivar than in the Cd-sensitive cultivar. Cd stress significantly inhibited the absorption of essential metal elements and significantly increased the calcium concentration. Differentially expressed genes involved in metal ion transport were identified via transcriptome analysis. Cd stress altered the composition of root exudates, thus increasing lipid species and decreasing alkaloid, lignan, sugar, and alcohol species. Moreover, Cd stress significantly reduced most alkaloid, organic acid, and phenolic acid exudates in the Cd-tolerant cultivar, while it increased most lipid and phenolic acid exudates in the Cd-sensitive cultivar. Some significantly changed root exudates (ferulic acid, O-coumaric acid, and spermine) are involved in the phenylalanine biosynthesis, and arginine and proline metabolic pathways, thus, may be potential biomarkers of Cd stress response. Overall, metal ion absorption and root exudates are critical for Cd tolerance and accumulation in broomcorn millet. These findings provide valuable insights into improving Cd phytoremediation by applying mineral elements or metabolites.

The Hormetic Response of Soil P Extraction Induced by Low-Molecular-Weight Organic Acids

The hormetic effect is a toxicological phenomenon in the soil ecosystem. The influence of low-molecular-weight organic acids (LMWOAs) on the release and activation of soil phosphorus (P) has become the focus of toxicological research. To what extent LMWOAs can regulate the hormetic effect of P release and then influence soil P nutrients is worth attention. This study aimed to investigate the effects of different types and concentrations of exogenous LMWOAs on P extraction, establish the relationship between the concentration of LMWOAs and P extraction efficiency, and calculate the hormetic parameters to understand the mechanism of types and concentrations of LMWOAs in P extraction efficiency. Four organic acids, i.e., citric, oxalic, tartaric, and malic acids, induced hormetic effects on P extraction that were concentration dependent. The relationship between LMWOAs and P extraction efficiency was explained by a quadratic polynomial equation. The critical threshold of citric acid concentration was similar to that of oxalic acid, whereas that of tartaric acid was similar to that of malic acid. The critical thresholds of the P concentration extracted by malic acid and citric acid were higher than those extracted by oxalic acid and tartaric acid due to the differences in the structure and properties of LMWOAs. The critical thresholds of P extraction efficiency of oxalic acid were lower than those of the other three organic acid types. These results provide evidence for the use of citric acid and malic acid to increase soil P.