New insight into the mechanisms of preferential encapsulation of metal(loid)s by wheat phytoliths under silicon nanoparticle amendment

A novel layered culture device reveals spatial dynamics of root element uptake and optimal silicon application site for mitigating chromium uptake by rice

Understanding root uptake mechanisms for various elements is crucial for optimizing heavy metal remediation strategies and enhancing plant-nutrient interactions. However, simple and effective methods to differentiate the contributions of specific root segments in element uptake are lacking. Here, we developed a layered culture device consisting of a culture box and a plant suspension mechanism, which isolates different root segments through solid media and waterproof coating. Then, we used the device to investigate the roles of distinct root segments (0-1 cm and 1-2 cm from the tip) in heavy metal chromium (Cr) and beneficial element silicon (Si) uptake in rice. The results indicated that the 0-1 cm root segment contributed approximately 58% of leaf Cr(VI), with higher efflux compared to the 1-2 cm segment. Conversely, the 1-2 cm root segment served as the primary source of leaf Si and Cr(III), accounting for 62% and 54%, respectively. The translocation factors for Cr(VI) were similar for both segments (0.039 and 0.032), while the Cr(III) translocation factor for the 0-1 cm root segment (0.061) was 2.8 times that of the 1-2 cm segment. Notably, Si application to the 0-1 cm segment most effectively alleviated Cr (III) and Cr (VI) stress, boosting shoot length, fresh weight, and chlorophyll concentration and reducing Cr concentrations in roots and leaves by 24.7%-65.7%. In contrast, Si application to the 1-2 cm segment had minimal impact on rice growth or Cr uptake. These results suggest a deep Si application strategy for remediating Cr-contaminated soil. The innovative device provides a scientific foundation for distinguishing element uptake contributions of different root segments and enhancing the utilization efficiency of remediation materials and nutrient management in agriculture.

Rare earth elements sequestration in phytoliths: Partitioning patterns and influencing mechanism

Rare earth elements (REEs) are integral to numerous high-tech industries, yet their biogeochemical cycling within ecosystems remains inadequately characterized. Recently, phytoliths have been identified as potentially significant sinks for REEs; however, their role in the cycling of these elements has been underestimated. In this work, we investigate the accumulation of REEs in phytoliths (PhytREEs) within the Greater Khingan Mountains region, employing an optimized wet oxidation method combined with heavy liquid flotation to quantify PhytREEs contents in surface soils. The results revealed an elevation-dependent pattern of PhytREEs concentration, with heightened levels at higher altitudes and diminishing concentrations towards the eastern plains. The enrichment coefficient of PhytREEs (ECPhytREEs) was found to be approximately 2.7 %, indicative of a moderately selective sequestration process. The multivariate analysis indicated that terrain complexity, climatic patterns, soil texture, and organic matter significantly influence the uptake and storage of REEs in plants, subsequently affecting their partitioning in phytoliths. Among these factors, the complexation of REEs with organic matter emerged as a pivotal mechanism facilitating their immobilization within phytoliths. Soil characteristics also play a non-negligible role in modulating REEs dynamics. Our findings highlight the predominant influence of climate on PhytREE storage, suggesting that climatic variables are the primary drivers modulating the bioavailability and ultimate sequestration of REEs within phytoliths. This study enhances our understanding of the biotic-abiotic interplay in the sequestration of REEs and underscores the need to incorporate phytoliths into models of terrestrial REE cycling.

Phytotoxicity of Prussian blue nanoparticles to rice and the related defence mechanisms: In vivo observations and physiological and biochemical analysis

While the nanotoxic effects on plants have been extensively studied, the underlying mechanisms of plant defense responses and resistance to nanostress remain insufficiently understood. Particularly, Prussian blue nanoparticles (PB NPs) have been extensively used in pigments, pharmaceuticals, electrocatalysis, biosensors and energy storage. However, the impact of PB NPs on plants' health and growth are largely unknown. Herein, the phytotoxicity of PB NPs to rice and trace the uptake, accumulation and biotransformation of PB NPs was explored, along with the underlying defence mechanisms. The results showed that PB NPs (≥50 mg L-1) significantly inhibited the growth of rice seedling up to 16.16%, 27.80%, and 29.37% in plant height, shoot biomass and root biomass, respectively. The X-ray spectroscopic studies and in vivo elemental and particle-imaging demonstrated that PB NPs were transported through the cortex via xylem from root to shoot. However, most of the PB NPs and their transformation products were retained in the root, where they were blocked owing to root cell wall (RCW) remodeling, and 81.4%-83.4% of Fe accumulated in the RCW compared to 66.6% in the control. Specifically, PB NPs stimulated pectin methylesterase activity by promoting hydrogen peroxide production to participate in RCW remodeling. More interestingly, Si was specifically regulated to covalently bind to hemicellulose to form the Si-hemicellulose complex that strongly bound with PB NPs during RCW remodeling, resulting in the strong defense against PB NPs. These findings provide new insights into the phytotoxicity of artificial NPs and the defense mechanisms of plants.

Silicon reduces toxicity and accumulation of arsenic and cadmium in cereal crops: A meta-analysis, mechanism, and perspective study

Arsenic (As) and cadmium (Cd) are two toxic metal(loid)s that pose significant risks to food security and human health. Silicon (Si) has attracted substantial attention because of its positive effects on alleviating the toxicity and accumulation of As and Cd in crops. However, our current knowledge of the comprehensive effects and detailed mechanisms of Si amendment is limited. In this study, a global meta-analysis of 248 original articles with over 7000 paired observations was conducted to evaluate Si-mediated effects on growth and As and Cd accumulation in rice (Oryza sativa L.), wheat (Triticum aestivum L.), and maize (Zea mays L.). Si application increases the biomass of these crops under As and/or Cd contamination. Si amendment also decreased shoot As and Cd accumulation by 24.1 % (20.6 to 27.5 %) and 31.9 % (29.0 to 31.9 %), respectively. Furthermore, the Si amendment reduced the human health risks posed by As (2.6 %) and Cd (12.9 %) in crop grains. Si-induced inhibition of Cd accumulation is associated with decreased Cd bioavailability and the downregulation of gene expression. The regulation of gene expression by Si addition was the driving factor limiting shoot As accumulation. Overall, our analysis demonstrated that Si amendment has great potential to reduce the toxicity and accumulation of As and/or Cd in crops, providing a scientific basis for promoting food safety globally.

Silicon a key player to mitigate chromium toxicity in plants: Mechanisms and future prospective

Chromium is a serious heavy metal (HM) and its concentration in plant-soil interface is soaring due to anthropogenic activities, unregulated disposals, and lack of efficient treatments. High concentration of Cr is toxic to ecosystems and human health. Cr stress also diminishes the plant performance by changing the plant's vegetative and reproductive development that ultimately affects sustainable crop production. Silicon (Si) is the second-most prevalent element in the crust of the planet, and has demonstrated a remarkable potential to minimize the HM toxicity. Amending soils with Si mitigates adverse effects of Cr by improving plant physiological, biochemical, and molecular functioning and ensuring better Cr immobilization, compartmentation, and co-precipitation. However, there is no comprehensive review on the role of Si to mitigate Cr toxicity in plants. Thus, in this present review; the discussion has been carried on; 1) the source of Cr, 2) underlying mechanisms of Cr uptake by plants, 3) how Si affects the plant functioning to reduce Cr toxicity, 4) how Si can cause immobilization, compartmentation, and co-precipitation 5) strategies to improve Si accumulation in plants to counter Cr toxicity. We also discussed the knowledge gaps and future research needs. The present review reports up-to-date knowledge about the role of Si to mitigate Cr toxicity and it will help to get better crop productivity in Cr-contaminated soils. The findings of the current review will educate the readers on Si functions in reducing Cr toxicity and will offer new ideas to develop Cr tolerance in plants through the use of Si.

Elemental composition of grass phytoliths: Environmental control and effect on dissolution

Plant phytoliths, which represent the main pool of silica (Si) in the form of hydrous Si oxide, are capable of providing valuable information on different aspect of environmental issues including paleo-environmental reconstruction and agricultural sustainability. Phytoliths may have different chemical composition, which, in turn, affects their preservation in soils ad impacts terrestrial cycle of the occluded elements including micro-nutrients and environmental toxicants. Yet, in contrast to sizable work devoted to phytoliths formation, dissolution and physico-chemical properties, the mechanisms that control total (major and trace) elemental composition and the impact that various elements exert on phytolith reactivity and preservation in soils remains poorly known. In order to fil this gap in knowledge, here we combined two different approaches - analytical trace element geochemistry and experimental physical chemistry. First, we assessed full elemental composition of phytoliths from different plants via measuring major and trace elements in 9 samples of grasses collected in northern Eurasia during different seasons, 18 grasses from Siberian regions, and 4 typical Si-concentrating plants (horsetail, larch, elm and tree fern). We further assessed the dissolution rates of phytoliths exhibiting drastically different concentrations of trace metals. In the European grasses, the variations of phytolith chemical composition among species were highly superior to the variations across vegetative season. Compared to European samples, Siberian grass phytoliths were impoverished in Ca and Sr, exhibited similar concentrations of Li, B, Na, Mg, K, V, Zn, Ni, Mo, As, Ba, and U, and were strongly enriched (x 100-1000) in lithogenic elements (trivalent and tetravalent hydrolysates), P, Mn, Fe and divalent metals. Overall, the variations in elemental composition between different species of the same region were lower compared to variations of the same species from distant regions. The main factors controlling phytoliths elemental composition are the far-range atmospheric (dust) transfer, climatic conditions (humidity), and, in a lesser degree, local lithology and anthropogenic pollution. Despite significant, up to 3 orders of magnitude, difference in TE composition of grass and other plant phytoliths, the dissolution rates of grass phytoliths measured in this study were similar, within the experimental uncertainty, to those of other plants studied in former works. Therefore, elemental composition of phytoliths has relatively minor impact on their preservation in soils.

Silicon dioxide nanoparticles enhance plant growth, photosynthetic performance, and antioxidants defence machinery through suppressing chromium uptake in Brassica napus L

Chromium (Cr) is a highly toxic heavy metal that is extensively released into the soil and drastically reduces plant yield. Silicon nanoparticles (Si NPs) were chosen to mitigate Cr toxicity due to their ability to interact with heavy metals and reduce their uptake. This manuscript explores the mechanisms of Cr-induced toxicity and the potential of Si NPs to mitigate Cr toxicity by regulating photosynthesis, oxidative stress, and antioxidant defence, along with the role of transcription factors and heavy metal transporter genes in rapeseed (Brassica napus L.). Rapeseed plants were grown hydroponically and subjected to hexavalent Cr stress (50 and 100 μM) in the form of K2Cr2O7 solution. Si NPs were foliar sprayed at concentrations of 50, 100 and 150 μM. The findings showed that 100 μM Si NPs under 100 μM Cr stress significantly increased the leaf Si content by 169% while reducing Cr uptake by 92% and 76% in roots and leaves, respectively. The presence of Si NPs inside the plant leaf cells was confirmed by using energy-dispersive spectroscopy, inductively coupled plasma‒mass spectrometry, and confocal laser scanning microscopy. The study's findings showed that Cr had adverse effects on plant growth, photosynthetic gas exchange attributes, leaf mesophyll ultrastructure, PSII performance and the activity of enzymatic and nonenzymatic antioxidants. However, Si NPs minimized Cr-induced toxicity by reducing total Cr accumulation and decreasing oxidative damage, as evidenced by reduced ROS production (such as H2O2 and MDA) and increased enzymatic and nonenzymatic antioxidant activities in plants. Interestingly, Si NPs under Cr stress effectively increased the NPQ, ETR and QY of PSII, indicating a robust protective response of PSII against stress. Furthermore, the enhancement of Cr tolerance facilitated by Si NPs was linked to the upregulation of genes associated with antioxidant enzymes and transcription factors, alongside the concurrent reduction in metal transporter activity.

Abiogenic silicon: Interaction with potentially toxic elements and its ecological significance in soil and plant systems

Abiogenic silicon (Si), though deemed a quasi-nutrient, remains largely inaccessible to plants due to its prevalence within mineral ores. Nevertheless, the influence of Si extends across a spectrum of pivotal plant processes. Si emerges as a versatile boon for plants, conferring a plethora of advantages. Notably, it engenders substantial enhancements in biomass, yield, and overall plant developmental attributes. Beyond these effects, Si augments the activities of vital antioxidant enzymes, encompassing glutathione (GSH), catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), among others. It achieves through the augmentation of reactive oxygen species (ROS) scavenging gene expression, thus curbing the injurious impact of free radicals. In addition to its effects on plants, Si profoundly ameliorates soil health indicators. Si tangibly enhances soil vitality by elevating soil pH and fostering microbial community proliferation. Furthermore, it exerts inhibitory control over ions that could inflict harm upon delicate plant cells. During interactions within the soil matrix, Si readily forms complexes with potentially toxic metals (PTEs), encapsulating them through Si-PTEs interactions, precipitative mechanisms, and integration within colloidal Si and mineral strata. The amalgamation of Si with other soil amendments, such as biochar, nanoparticles, zeolites, and composts, extends its capacity to thwart PTEs. This synergistic approach enhances soil organic matter content and bolsters overall soil quality parameters. The utilization of Si-based fertilizers and nanomaterials holds promise for further increasing food production and fortifying global food security. Besides, gaps in our scientific discourse persist concerning Si speciation and fractionation within soils, as well as its intricate interplay with PTEs. Nonetheless, future investigations must delve into the precise functions of abiogenic Si within the physiological and biochemical realms of both soil and plants, especially at the critical juncture of the soil-plant interface. This review seeks to comprehensively address the multifaceted roles of Si in plant and soil systems during interactions with PTEs.

Silicon in Plants: Alleviation of Metal(loid) Toxicity and Consequential Perspectives for Phytoremediation

For the majority of higher plants, silicon (Si) is considered a beneficial element because of the various favorable effects of Si accumulation in plants that have been revealed, including the alleviation of metal(loid) toxicity. The accumulation of non-degradable metal(loid)s in the environment strongly increased in the last decades by intensified industrial and agricultural production with negative consequences for the environment and human health. Phytoremediation, i.e., the use of plants to extract and remove elemental pollutants from contaminated soils, has been commonly used for the restoration of metal(loid)-contaminated sites. In our viewpoint article, we briefly summarize the current knowledge of Si-mediated alleviation of metal(loid) toxicity in plants and the potential role of Si in the phytoremediation of soils contaminated with metal(loid)s. In this context, a special focus is on metal(loid) accumulation in (soil) phytoliths, i.e., relatively stable silica structures formed in plants. The accumulation of metal(loid)s in phytoliths might offer a promising pathway for the long-term sequestration of metal(loid)s in soils. As specific phytoliths might also represent an important carbon sink in soils, phytoliths might be a silver bullet in the mitigation of global change. Thus, the time is now to combine Si/phytolith and phytoremediation research. This will help us to merge the positive effects of Si accumulation in plants with the advantages of phytoremediation, which represents an economically feasible and environmentally friendly way to restore metal(loid)-contaminated sites.