Silicon mediated heavy metal stress amelioration in fruit crops

Fruit crops are essential for human nutrition and health, yet high level of heavy metal levels in soils can degrade fruit quality. These metals accumulate in plant roots and tissues due to factors like excessive fertilizer and pesticide use, poor waste management, and unscientific agricultural practices. Such accumulation can adversely affect plant growth, physiology, and yield. Consuming fruits contaminated with toxic metals poses significant health risks, including nervous system disorders and cancer. Various strategies, such as organic manuring, biomaterials, and modified cultivation practices have been widely researched to reduce heavy metal accumulation. Recently, silicon (Si) application has emerged as a promising and cost-effective solution for addressing biological and environmental challenges in food crops. Si, which can be applied to the soil, through foliar application or a combination of both, helps reduce toxic metal concentrations in soil and plants. Despite its potential, there is currently no comprehensive review that details Si's role in mitigating heavy metal stress in fruit crops. This review aims to explore the potential of Si in reducing heavy metal-induced damage in fruit crops while enhancing growth by alleviating heavy metal toxicity.

Silicon efficacy for the remediation of metal contaminated soil

In the course of past two decade anthropogenic activities have reinforced, begetting soil and water defilement. A plethora of heavy metals alters and limits plant growth and yield, with opposing effect on agricultural productivity. Silicon often perceived as plant alimentary 'nonentity'. A suite of determinants associated with silicon have been lately discerned, concerning plant physiology, chemistry, gene regulation/expression and interaction with different organisms. Exogenous supplementation of silicon renders resistance against heavy-metal stress. Predominantly, plants having significant amount of silicon in root and shoot thus are barely prone to pest onset and manifest greater endurance against abiotic stresses including heavy-metal toxicity. Silicon-mediated stress management involves abatement of metal ions within soil, co-precipitation of metal ions, gene modulation associated with metal transport, chelation, activation of antioxidants (enzymatic and non-enzymatic), metal ion compartmentation and structural metamorphosis in plants. Silicon supplementation also stimulates expression of stress-resistant genes under heavy-metal toxicity to provide plant tolerance under stress conditions. Ergo, to boost metal tolerance within crops, immanent genetic potential for silicon assimilation should be enhanced. Current study, addresses the potential role and mechanistic interpretation of silicon induced mitigation of heavy-metal stress in plants.

Silicon nanoparticles vs trace elements toxicity: Modus operandi and its omics bases

Phytotoxicity of trace elements (commonly misunderstood as 'heavy metals') includes impairment of functional groups of enzymes, photo-assembly, redox homeostasis, and nutrient status in higher plants. Silicon nanoparticles (SiNPs) can ameliorate trace element toxicity. We discuss SiNPs response against several essential (such as Cu, Ni, Mn, Mo, and Zn) and non-essential (including Cd, Pb, Hg, Al, Cr, Sb, Se, and As) trace elements. SiNPs hinder root uptake and transport of trace elements as the first line of defence. SiNPs charge plant antioxidant defence against trace elements-induced oxidative stress. The enrolment of SiNPs in gene expressions was also noticed on many occasions. These genes are associated with several anatomical and physiological phenomena, such as cell wall composition, photosynthesis, and metal uptake and transport. On this note, we dedicate the later sections of this review to support an enhanced understanding of SiNPs influence on the metabolomic, proteomic, and genomic profile of plants under trace elements toxicity.

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.

Interactive effect of silicon and nitric oxide effectively contracts copper toxicity in Salvia officinalis L

Excess copper (Cu) causes the toxic effects in plants and health hazards to humans. Therefore, in this study, the effect of sodium silicate (1 mM Si) and sodium nitroprusside (200 µM SNP as a releasing NO), was assessed on Cu tolerance in Salvia officinalis L. plants exposed to 400 µM CuSO4. Results revealed that the combined supplementation with Si and SNP rather than the single application of these chemicals lowered Cu concentrations and translocation factor and increased Mg, Zn, and Fe concentrations in roots and shoots. Furthermore, combined treatment more efficiently decreased electrolyte leakage enhanced the activities of POD and APX in the leaves and roots, and improved relative water content and the content of Chl. a and Chl. b in leaves and consequently further increased tolerance index. Silicon supply enhanced NO content and applying Si + SNP more than the treatment of Si alone increased Si concentrations in the roots and shoots under Cu stress. Therefore, the reciprocal interaction of Si and NO might enhance Cu tolerance in plants, and the combined application of Si and SNP might be a promising strategy to decrease heavy metal accumulation in medicinal plants grown in polluted lands.

Silicon in action: Between iron scarcity and excess copper

Essential micronutrients belonging to the transition metals, such as Fe and Cu, are indispensable for plant growth and stress tolerance; however, when present in excess, they can become potentially dangerous producers of reactive oxygen species. Therefore, their homeostases must be strictly regulated. Both microelement deficiencies and elevated concentrations of heavy metals in the soil are global problems that reduce the nutritional value of crops and seriously affect human health. Silicon, a beneficial element known for its protective properties, has been reported to alleviate the symptoms of Cu toxicity and Fe deficiency stress in plants; however, we are still far from a comprehensive understanding of the underlying molecular mechanisms. Although Si-mediated mitigation of these stresses has been clearly demonstrated for some species, the effects of Si vary depending on plant species, growing conditions and experimental design. In this review, the proposed mechanistic models explaining the effect of Si are summarized and discussed. Iron and copper compete for the common metal transporters and share the same transport routes, hence, inadequate concentration of one element leads to disturbances of another. Silicon is reported to beneficially influence not only the distribution of the element supplied below or above the optimal concentration, but also the distribution of other microelements, as well as their molar ratios. The influence of Si on Cu immobilization and retention in the root, as well as Si-induced Fe remobilization from the source to the sink organs are of vital importance. The changes in cellular Cu and Fe localization are considered to play a crucial role in restoring homeostasis of these microelements. Silicon has been shown to stimulate the accumulation of metal chelators involved in both the mobilization of deficient elements and scavenging excess heavy metals. Research into the mechanisms of the ameliorative effects of Si is valuable for reducing mineral stress in plants and improving the nutritional value of crops. This review aims to provide a thorough and critical overview of the current state of knowledge in this field and to discuss discrepancies in the observed effects of Si and different views on its mode of action.

Multifaceted roles of silicon in mitigating environmental stresses in plants

Food security relies on plant productivity and plant's resilience to climate change driven environmental stresses. Plants employ diverse adaptive mechanisms of stress-signalling pathways, antioxidant defense, osmotic adjustment, nutrient homeostasis and phytohormones. Over the last few decades, silicon has emerged as a beneficial element for enhancing plant growth productivity. Silicon ameliorates biotic and abiotic stress conditions by regulating the physiological, biochemical and molecular responses. Si-uptake and transport are facilitated by specialized Si-transporters (Lsi1, Lsi2, Lsi3, and Lsi6) and, the differential root anatomy has been shown to reflect in the varying Si-uptake in monocot and dicot plants. Silicon mediates a number of plant processes including osmotic, ionic stress responses, metabolic processes, stomatal physiology, phytohormones, nutrients and source-sink relationship. Further studies on the transcriptional and post-transcriptional regulation of the Si transporter genes are required for better uptake and transport in spatial mode and under different stress conditions. In this article, we present an account of the availability, uptake, Si transporters and, the role of Silicon to alleviate environmental stress and improve plant productivity.

Interactions of Silicon With Essential and Beneficial Elements in Plants

Silicon (Si) is not classified as an essential element for plants, but numerous studies have demonstrated its beneficial effects in a variety of species and environmental conditions, including low nutrient availability. Application of Si shows the potential to increase nutrient availability in the rhizosphere and root uptake through complex mechanisms, which still remain unclear. Silicon-mediated transcriptional regulation of element transporters for both root acquisition and tissue homeostasis has recently been suggested as an important strategy, varying in detail depending on plant species and nutritional status. Here, we summarize evidence of Si-mediated acquisition, uptake and translocation of nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), chlorine (Cl), and nickel (Ni) under both deficiency and excess conditions. In addition, we discuss interactions of Si-with beneficial elements: aluminum (Al), sodium (Na), and selenium (Se). This review also highlights further research needed to improve understanding of Si-mediated acquisition and utilization of nutrients and vice versa nutrient status-mediated Si acquisition and transport, both processes which are of high importance for agronomic practice (e.g., reduced use of fertilizers and pesticides).

Potential of Grasses in Phytolith Production in Soils Contaminated with Cadmium

Cadmium (Cd) is a very toxic heavy metal occurring in places with anthropogenic activities, making it one of the most important environmental pollutants. Phytoremediation plants are used for recovery of metal-contaminated soils by their ability to absorb and tolerate high concentrations of heavy metals. This paper aims to evaluate the potential of grasses in phytolith production in soils contaminated with Cd. The experiments, separated by soil types (Typic Quartzipsamment, Xanthic Hapludox and Rhodic Hapludox), were conducted in a completely randomized design with a distribution of treatments in a 3 × 4 factorial scheme with three replications. The factors were three grasses (Urochloa decumbens, Urochloa brizantha andMegathyrsus maximus) and four concentrations of Cd applied in soils (0, 2, 4 and 12 mg kg⁻¹). Grass growth decreased and increased Cd concentration in shoots of grasses with the increased Cd rates in soils. The toxic effect of Cd resulted in production and Cd occlusion in phytoliths produced in shoots of the grasses. Grasses showed potential for phytolith production, independent of soil type, providing phytoextraction of Cd in phytoliths. Megathyrsus maximus was the grass with the highest tolerance to Cd, evidenced by higher production and Cd capture in phytoliths for the evaluated soils. Phytolith production by grasses in Cd-contaminated soils is related to genetic and physiological differences of the evaluated grasses and Cd availability in soils.

Silicon and Iron Differently Alleviate Copper Toxicity in Cucumber Leaves

Copper (Cu) toxicity in plants may lead to iron (Fe), zinc (Zn) and manganese (Mn) deficiencies. Here, we investigated the effect of Si and Fe supply on the concentrations of micronutrients and metal-chelating amino acids nicotianamine (NA) and histidine (His) in leaves of cucumber plants exposed to Cu in excess. Cucumber (Cucumis sativus L.) was treated with 10 µM Cu, and additional 100 µM Fe or/and 1.5 mM Si for five days. High Cu and decreased Zn, Fe and Mn concentrations were found in Cu treatment. Additional Fe supply had a more pronounced effect in decreasing Cu accumulation and improving the molar ratio between micronutrients as compared to the Si supply. However, the simultaneous supply of Fe and Si was the most effective treatment in alleviation of Cu-induced deficiency of Fe, Zn and Mn. Additional Fe supply increased the His but not NA concentration, while Si supply significantly increased both NA and His whereby the NA:Cu and His:Cu molar ratios exceeded the control values indicating that Si recruits Cu-chelation to achieve Cu tolerance. In conclusion, Si-mediated alleviation of Cu toxicity was directed toward Cu tolerance while Fe-alleviative effect was due to a dramatic decrease in Cu accumulation.

Silicon (Si): Review and future prospects on the action mechanisms in alleviating biotic and abiotic stresses in plants

In the era present, due to increasing incidences of a large number of different biotic and abiotic stresses all over the world, the growth of plants (principal crops) may be restrained by these stresses. In addition to beneficial microorganisms, use of silicon (Si)-fertilizer is known as an ecologically compatible and environmentally friendly technique to stimulate plant growth, alleviate various biotic and abiotic stresses in plants, and enhance the plant resistance to multiple stresses, because Si is not harmful, corrosive, and polluting to plants when presents in excess. Here, we reviewed the action mechanisms by which Si alleviates abiotic and biotic stresses in plants. The use of Si (mostly as industrial slags and rice straw) is predicted to become a sustainable strategy and an emerging trend in agriculture to enhance crop growth and alleviate abiotic and biotic stresses in the not too distant future. In this review article, the future research needs on the use of Si under the conditions of abiotic and biotic stresses are also highlighted.

Silicon occurrence, uptake, transport and mechanisms of heavy metals, minerals and salinity enhanced tolerance in plants with future prospects: A review

Recently, heavy metals pollution due to industrialization and urbanization, use of untreated wastewater and unreasonable use of pesticides and fertilizers is increasing rapidly, resulting in major threat to the environment and contaminate soils. Silicon (Si) is the second most abundant element in the earth crust after oxygen. Although it's higher accumulation in plants, yet Si has not been listed as essential nutrient however, considered as beneficial element for growth of plants particularly in stressed environment. Research to date has demonstrated that silicon helps the plants to alleviate the various biotic and abiotic stresses. This review article presents a comprehensive update about Si and heavy metals, minerals and salinity stresses, and contained the progress about Si so far done worldwide in the light of previous studies to evaluate the ecological importance of Si. Moreover, this review will also be helpful to understand the Si uptake ability and its benefits on plants grown under stressed environment. Further research needs for Si-mediated mitigation of heavy metals and mineral nutrients stresses are also discussed.

Mechanisms of silicon-mediated alleviation of heavy metal toxicity in plants: A review

In present era, heavy metal pollution is rapidly increasing which present many environmental problems. These heavy metals are mainly accumulated in soil and are transferred to food chain through plants grown on these soils. Silicon (Si) is the second most abundant element in the soil. It has been widely reported that Si can stimulate plant growth and alleviate various biotic and abiotic stresses, including heavy metal stress. Research to date has explored a number of mechanisms through which Si can alleviate heavy metal toxicity in plants at both plant and soil levels. Here we reviewed the mechanisms through which Si can alleviate heavy metal toxicity in plants. The key mechanisms evoked include reducing active heavy metal ions in growth media, reduced metal uptake and root-to-shoot translocation, chelation and stimulation of antioxidant systems in plants, complexation and co-precipitation of toxic metals with Si in different plant parts, compartmentation and structural alterations in plants and regulation of the expression of metal transport genes. However, these mechanisms might be associated with plant species, genotypes, metal elements, growth conditions, duration of the stress imposed and so on. Further research orientation is also discussed.

Assessment of the role of silicon in the Cu-tolerance of the C4 grass Spartina densiflora

An experiment was designed to investigate the effect of silicon supply (0 and 500 μM) on Spartina densiflora plants grown at two copper (Cu) concentrations: 0 and 15 mM. Growth parameters together with total concentrations of calcium, Cu, potassium, magnesium, manganese, sodium and nitrogen were determined in roots and leaves. Photosynthetic traits were followed by measurement of leaf gas exchange, efficiency of PSII biochemistry, total content of photosynthetic pigments and concentration and carbamylation of Rubisco sites concentration ([Rubisco]). Respiration and oxygen isotope fractionation were measured in roots to study the in vivo activities of cytochrome oxidase (COX) and alternative oxidase (AOX) pathways, as well as AOX capacity. The results confirm that Si supply improves growth of S. densiflora under Cu stress. Improved growth was associated with higher net photosynthetic rate. Beneficial effect of Si on S. densiflora photosynthetic apparatus was associated with a reduction of the Cu impact on active Rubisco sites, as well as on the photochemical apparatus and chlorophyll concentration. Moreover, ameliorative effects of Si were associated with the avoidance of Cu translocation from roots to leaves. Finally in vivo activities of COX and AOX pathways were strongly inhibited in Cu-treated plants, and this reduction was not mitigated by Si-treatment. Therefore, Si appears to play an important role in Cu-tolerance of S. densiflora, not by avoiding its uptake by roots, but via some mechanism to avoid Cu translocation from roots to leaves, resulting in a general reduction of Cu-induced deleterious effects on the leaf photosynthetic apparatus.

Effect of silicon on wheat seedlings (Triticum turgidum L.) grown in hydroponics and exposed to 0 to 30 µM Cu

Aqueous Si limits Cu uptake by a Si-accumulating plant via physicochemical mechanisms occurring at the root level. Sufficient Si supply may alleviate Cu toxicity in Cu-contaminated soils. Little information is available on the role of silicon (Si) in copper (Cu) tolerance while Cu toxicity is widespread in crops grown on Cu-contaminated soils. A hydroponic study was set up to investigate the influence of Si on Cu tolerance in durum wheat (Triticum turgidum L.) grown in 0, 0.7, 7.0 and 30 µM Cu without and with 1.0 mM Si, and to identify the mechanisms involved in mitigation of Cu toxicity. Si supply alleviated Cu toxicity in durum wheat at 30 µM Cu, while Cu significantly increased Si concentration in roots. Root length, photosynthetic pigments concentrations, macroelements, and organic anions (malate, acetate and aconitate) in roots, were also increased. Desorption experiments, XPS analysis of the outer thin root surface (≤100 Å) and µXRF analyses showed that Si increased adsorption of Cu at the root surface as well as Cu accumulation in the epidermis while Cu was localised in the central cylinder when Si was not applied. Copper was not detected in phytoliths. This study provides evidences for Si-mediated alleviation of Cu toxicity in durum wheat. It also shows that Si supplementation to plants exposed to increasing levels of Cu in solution induces non-simultaneous changes in physiological parameters. We propose a three-step mechanism occurring mainly at the root level and limiting Cu uptake and translocation to shoots: (i) increased Cu adsorption onto the outer thin layer root surface and immobilisation in the vicinity of root epidermis, (ii) increased Cu complexation by both inorganic and organic anions such as aconitate and, (iii) limitation of translocation through an enhanced thickening of a Si-loaded endodermis.

Evidence of sulfur-bound reduced copper in bamboo exposed to high silicon and copper concentrations

We examined copper (Cu) absorption, distribution and toxicity and the role of a silicon (Si) supplementation in the bamboo Phyllostachys fastuosa. Bamboos were maintained in hydroponics for 4 months and submitted to two different Cu (1.5 and 100 μm Cu(2+)) and Si (0 and 1.1 mM) concentrations. Cu and Si partitioning and Cu speciation were investigated by chemical analysis, microscopic and spectroscopic techniques. Copper was present as Cu(I) and Cu(II) depending on plant parts. Bamboo mainly coped with high Cu exposure by: (i) high Cu sequestration in the root (ii) Cu(II) binding to amino and carboxyl ligands in roots, and (iii) Cu(I) complexation with both organic and inorganic sulfur ligands in stems and leaves. Silicon supplementation decreased the visible damage induced by high Cu exposure and modified Cu speciation in the leaves where a higher proportion of Cu was present as inorganic Cu(I)S compounds, which may be less toxic.

Effects of silicon and copper on bamboo grown hydroponically

Due to its high growth rate and biomass production, bamboo has recently been proven to be useful in wastewater treatment. Bamboo accumulates high silicon (Si) levels in its tissues, which may improve its development and tolerance to metal toxicity. This study investigates the effect of Si supplementation on bamboo growth and copper (Cu) sensitivity. An 8-month hydroponic culture of bamboo Gigantocloa sp. "Malay Dwarf " was performed. The bamboo plants were first submitted to a range of Si supplementation (0-1.5 mM). After 6 months, a potentially toxic Cu concentration of 1.5 μM Cu(2+) was added. Contrary to many studies on other plants, bamboo growth did not depend on Si levels even though it absorbed Si up to 218 mg g(-1) in leaves. The absorption of Cu by bamboo plants was not altered by the Si supplementation; Cu accumulated mainly in roots (131 mg kg(-1)), but was also found in leaves (16.6 mg kg(-1)) and stems (9.8 mg kg(-1)). Copper addition did not induce any toxicity symptoms. The different Cu and Si absorption mechanisms may partially explain why Si did not influence Cu repartition and concentration in bamboo. Given the high biomass and its absorption capacity, bamboo could potentially tolerate and accumulate high Cu concentrations making this plant useful for wastewater treatment.

Response of drought and fertilization in Erica andevalensis seed banks: significance for conservation management

The present study attempts to investigate the size, composition and seedling dynamics of the seed bank of a metalliferous and vulnerable species, Erica andevalensis. Samples were taken during spring and autumn from two different sites. We also studied the effects of nutrient solution, irrigation from the river Tinto and irrigation deficit on seeding establishment and survival. Only E. andevalensis and Poaceae species emerged from the seed banks, although the former was dominant (98%). Germination and seedling establishment was totally inhibited by the waters of the river Tinto. Seed density was high in the soils of both seed banks irrigated with water and nutrient solution. We found no seasonal differences in the seed bank and number of germinated seeds and mortality rate and density were similar. From the standpoint of restoration management, the results indicate that the seed bank is a very important factor for successful species establishment.