Research Progress on the Physiological Mechanism by Which Selenium Alleviates Heavy Metal Stress in Plants: A Review

Abstract

Human activities, such as mining, industrialization, industrial waste emissions, and agricultural practices, have caused heavy metals to become widespread and excessively accumulated in soil. The high concentrations of heavy metals in soil can be toxic to plants, severely affecting crop yield and quality. Moreover, these heavy metals can also enter the food chain, affecting animals and humans and leading to various serious illnesses. Selenium (Se) is not only an essential element for animals and humans but is also beneficial for plants, as it promotes their ability to respond actively to biotic and abiotic stresses. The global issue of Se deficiency in diets has made plants the primary source for human Se supplementation. This paper comprehensively reviews the effects of heavy metal stress on plant growth and development, physiological responses of plants to such stress, and the intracellular transport processes of heavy metals within plants. It particularly focuses on the mechanisms by which Se alleviates heavy metal stress in plants. Additionally, the study delves into how Se significantly enhances plant tolerance mechanisms against typical heavy metals, such as cadmium (Cd), lead (Pb), and mercury (Hg). This integrative research not only expands the boundaries of research in the field of plant heavy metal stress and Se application but also provides new perspectives and solutions for understanding and addressing complex environmental heavy metal pollution issues. By integrating these aspects, this paper not only fills existing gaps in the literature but also offers comprehensive scientific basis and strategic recommendations for environmental protection and sustainable agriculture development.

1. Introduction

Heavy metal pollution refers to the presence of excessive amounts of high-density and toxic metallic elements in the environment, such as Pb, Hg, Cd, chromium (Cr), and arsenic (As), exceeding the safe levels tolerated by both humans and ecosystems. This disrupts the ecological balance, leads to a reduction in biodiversity, and poses a serious threat to ecosystems and human health [1]. Heavy metals primarily originate from human activities, including industrial, agricultural, and urban development sectors [2]. Plants are an indispensable part of nature and are the first to be affected when the soil environment is contaminated with heavy metals. When plants are excessively exposed to multiple metal elements, such as Cd, Cr, Hg, Pb, As, copper (Cu), nickel (Ni), iron (Fe), and antimony (Sb), plant roots absorb heavy metal elements from the soil and transport them to various plant parts, such as stems, leaves, and fruits. This causes issues with nutrient absorption, metabolism, and the growth and development of plants [3]. Cd primarily affects plant protein synthesis and root water absorption. Plants grown in soils with high Cd contents exhibit chlorosis, growth inhibition, and browning of root tips, leading to severe damage and even death [4,5]. Furthermore, Cr stress can affect plant CO₂ fixation, electron transfer, photophosphorylation, and various enzyme activities, thereby influencing plant respiration and photosynthesis [6]. At the cellular level, Pb reacts with the thiol groups of enzymes, inhibiting their activity and causing water imbalance. It also alters membrane permeability and interferes with mineral nutrient absorption and transport. High concentrations of Pb further induce oxidative stress by increasing the production of reactive oxygen species (ROS) in plants [7]. The adverse effects of toxic metal pollution on plants can also pose a threat to human health through the food chain. Therefore, the issue of toxic metal pollution has gained widespread attention in society, necessitating urgent measures to reduce its harm to both ecosystems and humans.

Se is a crucial trace element in human and animal life. It plays a vital role as the active center of glutathione peroxidase (GPX), which safeguards cell membranes against damage caused by hydrogen peroxide and lipid peroxides. Furthermore, Se has extensive applications in medical care, agriculture, and poultry farming [8]. Research has demonstrated that Se deficiency in the body may increase susceptibility to certain tumor diseases [9]. To address this issue, Se biofortification has emerged as a promising biochemical method to increase the organic Se content in plants, thereby meeting daily human requirements [10]. Numerous studies have also indicated that Se significantly influences the absorption and accumulation of heavy metals in plants through various pathways. It enhances membrane stability, maintains mineral nutrition homeostasis, improves antioxidant responses, and boosts photosynthesis, among other cellular functions [11]. Additionally, Se restricts the absorption of heavy metals by plant roots and their subsequent transport to stems [12]. Adequate Se can increase the content of pectin and hemicellulose, thereby increasing the thickness of the cell wall to improve the binding of toxic metals and increase plant tolerance to heavy metals [13]. Exogenous Se application has been reported to significantly reduce the accumulation of heavy metals in crops, such as rice (Oryza sativa), lettuce (Lactuca sativa), cucumber (Cucumis sativus), and rapeseed (Brassica napus L.), while also promoting their growth [14]. This paper provides a comprehensive review of the effects of heavy metal stress on plants and examines the various mechanisms through which Se regulates the absorption of heavy metals by plants, modulates reactive oxygen metabolism in plants, and enhances plant photosynthesis. These mechanisms are closely intertwined with plant growth and development, yield, and the enhancement of agricultural product quality. Hence, these findings provide an important theoretical basis for further exploration of Se-induced plant heavy metal tolerance.

2. Mechanisms of Plant Resistance to Heavy Metal Stress

In response to heavy metal pollution, plants employ a range of mechanisms to withstand the stress caused by heavy metals (Figure 1). Initially, heavy metal ions bind to particles in the soil and subsequently move toward plant roots via water percolation in the soil. Root cell surfaces possess ion channels and internal transport proteins that selectively absorb or exclude specific heavy metal ions, facilitating their transport to other plant tissues and organs [15,16]. Moreover, heavy metal ions in the soil can bind with gelatinous substances on the surface of plant roots, resulting in deposits that impede or prevent the further movement of heavy metal ions into the plant (Figure 1). This barrier process aids in reducing the uptake of highly concentrated heavy metals from the soil, thereby minimizing their toxic effects on plants [17].

At the cellular level, heavy metals primarily interact with the cell wall of plant cells through physical adsorption or chemical bonding. Components such as pectin and cellulose in the cell wall possess a high affinity for heavy metal ions, leading to their binding and formation of complexes or precipitates. This immobilizes heavy metal ions on the surface of the cell wall (Figure 1), limiting their entry into the cell and thus mitigating their harmful effects on plant cells [18]. A fraction of the heavy metal ions will permeate the membranes of root hair cells, entering the cytoplasm through Fe carrier proteins, ion channels, and ion pumps [19]. Subsequently, within the cytoplasm, transport proteins facilitate the further transport of heavy metal ions to different organelles, including chloroplasts, mitochondria, and the endoplasmic reticulum (Figure 1). The accumulation of heavy metal ions in these organelles significantly contributes to their toxicity. For instance, metallothionein in chloroplasts can bind heavy metal ions, forming stable chelates, thereby alleviating the detrimental effects of heavy metal ions on chloroplasts and minimizing their impact on photosynthesis [20]. Additionally, intracellular chelators, such as glutathione (GSH), are utilized by plant cells to form complexes with heavy metal ions, reducing their toxicity. This chelation process aids in stabilizing heavy metal ions, which are subsequently transported to the cell wall or roots for excretion through transport proteins [21].

Excessive uptake of heavy metals by plants results in heavy metal accumulation in plant tissues, causing subsequent changes in gene expression. Genes linked to antioxidant defense, ion balance, and detoxification pathways are activated to enhance resistance to heavy metal ions [22]. Additionally, plants adjust their metabolic pathways to cope with heavy metal ion stress. These enzymes activate antioxidant enzyme systems to reduce oxidative damage, regulate ion channels and pumps to maintain ion balance, and modulate secondary metabolite synthesis to minimize damage caused by heavy metal ions. These response mechanisms enable plants to mitigate the toxic effects of heavy metal ions, enhance tolerance, and maintain cellular environment stability and normal functions [23].

2.1. Effects of Heavy Metal Stress on Plant Growth and Development

Despite the various mechanisms by which plants respond to heavy metal stress, heavy metals still have numerous adverse effects on plants. Macroscopically, heavy metal stress inhibits plant growth and causes leaf yellowing, leaf curling, and root necrosis (Figure 2) [24]. Studies on leguminous plants have shown that excessive nickel and cobalt hinder the growth of leguminous crops, leading to reduced yields [25]. Phenolic compounds are important secondary metabolites in plants, playing critical roles in lignification processes within plant cell walls. Lignification involves the polymerization of phenolic monomers to form lignin, a key substance underlying cell wall reinforcement and hardening. Polymerization of phenolics typically requires the catalytic action of peroxidases and involves hydrogen peroxide (H₂O₂) as an oxidizing agent. This process not only enhances the mechanical strength of plant tissues but also improves plant resistance to environmental stressors. However, in the presence of heavy metals, hydroxyl (OH) groups of phenolic compounds can be activated, transforming into pro-oxidants. Heavy metals, such as Pb, Cd, and Hg, can complex with hydroxyl groups of phenolic compounds, promoting H₂O₂ generation and yielding highly reactive hydroxyl radicals (-OH). These radicals can attack cell membrane lipids, proteins, and nucleic acids, leading to structural and functional damage, thereby exacerbating oxidative stress and cellular injury (Figure 2) [26]. Heavy metal stress also results in leaf chlorosis, necrosis, and inhibited root growth in leguminous plants. At the microscopic level, heavy metals initially affect plant cell walls by interacting with components such as pectin and cellulose. This disrupts cell wall synthesis and structure, reducing strength and stability. Heavy metal ions can also compromise plant cell membranes, primarily by impacting membrane integrity. Interactions with lipid molecules in the cell membrane damage the lipid bilayer, increasing membrane permeability, disrupting intracellular and extracellular environments, and affecting normal cell functions [27]. Furthermore, heavy metal ions can alter the fluidity of cell membranes, disrupting membrane protein function and, consequently, affecting substance transport and intracellular environment stability. Consequently, heavy metal stress affects the structural integrity and functionality of plant cell walls and membranes, ultimately influencing plant growth and development (Figure 2). In addition to their effects on cell walls and membranes, heavy metals also affect cellular organelles. Heavy metal ions can cause damage to the structure and function of chloroplasts, disrupting pigment synthesis and photosynthesis. This results in a decrease in energy conversion efficiency, a reduction in pigment content, and the generation of ROS and free radicals. These effects lead to uncontrolled oxidation and free radical chain reactions, ultimately resulting in damage to cellular biomolecules, such as nucleic acids, lipids, and proteins. Consequently, chloroplasts experience oxidative damage [28]. Moreover, heavy metal stress can disrupt enzyme activity within chloroplasts, disturbing normal photosynthesis and related metabolic pathways. As a result, plant growth, development, and physiological functions are ultimately affected [29].

Furthermore, heavy metal ions can influence mitochondrial respiration by inhibiting enzyme activity within the mitochondria. This disruption affects oxidative phosphorylation, leading to reduced energy production and cellular metabolism efficiency. Moreover, heavy metals have the potential to impact the function of the ER. This disruption interferes with proper protein folding and translation processes, causing endoplasmic reticulum stress and disruption of protein synthesis. Consequently, cellular protein synthesis and quality control mechanisms are affected. These various impacts disrupt the normal structure and function of vital organelles within the cell, leading to adverse effects on plant growth, development, and physiological activities. Research has shown that high concentrations of Cd inhibit the growth of almond (Prunus dulcis) plants, significantly decrease chlorophyll content, and increase oxidative stress, particularly in leaves and roots. Furthermore, these high concentrations also result in a decrease in the concentrations of various nutrients, such as Ca, Mg, K, and Fe [30]. Similarly, high concentrations of heavy metals, such as mustard (Brassica juncea) greens, inhibit the growth and development of vegetables. Treating mustard (B. juncea) with high concentrations of Cd and Pb inhibited mustard (B. juncea) growth, with Cd exhibiting a more severe inhibitory effect than Pb. Additionally, the total carotenoid content of mustard (B. juncea) greens significantly decreased [31].

In conclusion, high concentrations of heavy metals have a negative impact on the growth and development of plants. As the foundation of the ecological pyramid, plants play a crucial role as primary producers. Heavy metals entering plant bodies are then transferred to the food chain. Considering their bio-accumulative nature and resistance to degradation and metabolism within plants, this problem has become increasingly severe [32,33].

2.2. Impact of Heavy Metal Stress on Plant Cell Walls

Plant cell walls play a crucial role in maintaining cell morphology and functionality. They provide mechanical support, ensuring cell shape stability, and enable adhesion between neighboring cells, reducing intercellular compression [34]. However, heavy metal stress can have several detrimental effects on plant cell walls. First, heavy metal stress triggers the lignification process of the secondary cell wall, characterized by an increase in lignin and phenolic compounds. Lignin formation relies on the phenylpropanoid pathway, which becomes activated under heavy metal stress. Second, heavy metal stress indirectly affects plant cell walls by promoting the generation of ROS. These ROS can lead to the degradation and remodeling of cell walls, impacting plant growth and development processes [27]. Furthermore, heavy metal stress disrupts cross-linking within cell walls, causing structural changes. Excessive concentrations of heavy metal ions can compromise the cross-linking structure, resulting in a loss of stability. Another consequence of heavy metal accumulation is the alteration of pectin content within the cell wall. This affects the negative charge density of the wall, subsequently impacting its binding capacity with heavy metals. Moreover, heavy metal stress can induce changes in the levels of lignin and cellulose in the cell wall, further influencing its cross-linking properties [35].

2.3. Impact of Heavy Metal Stress on Plant Cell Membrane Permeability

The plant cell membrane serves as a natural barrier, preventing the free entry of external substances and providing a stable environment for physiological and biochemical reactions in plants. With selective permeability, the plant cell membrane regulates the transport and exchange of substances inside and outside the cell. Under heavy metal stress, the selective function of the cell membrane is impaired, leading to increased permeability and cellular ion leakage, disrupting ion homeostasis. The content of malondialdehyde (MDA) in plant tissues reflects the degree of cell membrane damage caused by heavy metals, with higher MDA levels indicating greater damage [36]. Studies have shown that as the Cd concentration increases, the MDA content in barley tissues also increases [37]. Additionally, studies using high concentrations of zinc (Zn) and Cr to treat wheat (Triticum durum cv Creso) found that the MDA content in wheat (T. durum) leaf tissues was significantly positively correlated with heavy metal concentrations [38].

The toxic effects of the terbium ion Tb³⁺ on horseradish cell membranes revealed that high concentrations of Tb³⁺ damaged both the horseradish cell membrane and chloroplast membrane, causing the leakage of radiating thylakoids and stroma from chloroplasts [39]. Moreover, high concentrations of Cu and Cd, by altering H⁺-ATPase pumps and lipid composition, cause varying degrees of cell membrane damage, leading to increased potassium leakage from roots [40]. High concentrations of heavy metals also induce oxidative stress in plants. Under heavy metal stress, damage to the ethylene glycol enzyme system reduces the GSH content, leading to increased accumulation of the cytotoxic compound methylglyoxal and an overproduction of free radicals. These free radicals can disrupt the structure and function of the cell membrane, resulting in changes in cell membrane permeability [41]. Collectively, these studies indicate that high concentrations of heavy metals cause varying degrees of damage to plant cell membranes, compromising their protective barrier function and leading to the leakage of ions and organic substances and the entry of harmful substances into the cell, thereby affecting intracellular enzyme and metabolic reactions.

2.4. Effects of Heavy Metal Stress on Free Proline in Plants

Proline (Pro) is the most abundant water-soluble amino acid in plants and is characterized by a structure containing hydrophobic and hydrophilic ends. The hydrophobic end has the ability to bind to proteins, while the hydrophilic end tends to associate with water molecules. Through its strong hydration ability, Pro can bind to water molecules, thereby preventing dehydration in plants under osmotic stress conditions. Consequently, Pro plays a crucial role in maintaining protein structure and function, as well as regulating osmotic pressure in plants [42]. In instances where the water content in plant cells is low, free Pro can supply additional water, aiding in the maintenance of normal physiological activities. Notably, the content of free Pro significantly increased under adverse stress conditions, particularly in the roots [43,44,45]. This increase was instrumental in regulating osmotic pressure and thereby maintaining the stability of cell osmotic pressure. Thus, the content of free Pro can, to some extent, indicate the environmental conditions in which plants are situated [46]. By producing a certain amount of Pro, plants mitigate water loss caused by heavy metal stress in plant tissues, thereby reducing cell damage [47]. The determination of the free Pro content in legume plant needles under various heavy metal pollution environments revealed that the highest content occurred in urban legume plant needles, followed by those in suburban and mountainous areas [48]. Therefore, under heavy metal stress, plants respond by increasing the accumulation of free Pro to support normal physiological functions [49].

2.5. Impact of Heavy Metal Stress on the Plant Antioxidant Enzyme System

The response to oxidative stress is a vital mechanism by which plants counteract external stressors. Under heavy metal stress conditions, the plant antioxidant enzyme system is activated to mitigate the abnormal increase in ROS, such as superoxide radicals (O²⁻), hydrogen peroxide (H₂O₂), and oxygen (O₂), thereby reducing oxidative damage to cells. Within this antioxidant enzyme system, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) all play crucial roles [50]. SOD primarily targets superoxide O²⁻, catalyzing its reduction to more stable H₂O₂ and O₂, thus preventing superoxide radicals from initiating reactions that produce more toxic oxidizing agents. H₂O₂, a potent oxidizer, can be harmful to cells at high concentrations. CAT facilitates the breakdown of H₂O₂ into H₂O and O₂, effectively lowering its concentration and alleviating cellular oxidative damage. APX catalyzes the reaction between H₂O₂ and ascorbate, resulting in the production of water and dehydroascorbic acid, thereby converting H₂O₂ into harmless substances. POD, on the other hand, catalyzes reactions between H₂O₂ and other reducing agents, such as phenols and phenolic alcohols. This process transforms H₂O₂ into water and the corresponding oxidized products, effectively breaking down and removing peroxides to minimize cellular damage [51].

Numerous studies have demonstrated that exposure to low concentrations of heavy metals can enhance the activity of protective enzymes through the induction of ROS. However, as the intensity of stress increases, the activities of key enzymes, such as SOD, POD, and CAT, may decrease, ultimately resulting in cellular damage [50]. Research has indicated that Cu, Fe, and Cd all contribute to plant toxicity by promoting the accumulation of free radicals, with Cu having the most pronounced effect and Cd being the least potent.

The primary mechanism of Cu-induced toxicity is its direct involvement in plant physiological reactions, leading to an elevated level of free radicals in plant tissues. On the other hand, Cd induces the accumulation of H₂O₂ by inhibiting the enzyme activity responsible for scavenging H₂O₂, thus disrupting the plant’s protective enzyme system and compromising its resistance to heavy metal stress [52]. Notably, the stimulating or inhibitory effects of heavy metal stress on the plant protective enzyme system depend on the specific type and concentration of heavy metals, as well as the duration of the stress. Furthermore, different plant species and environmental conditions may exhibit varied responses to heavy metal stress.

3. Mitigation Effect of Se on Heavy Metal Stress in Plants

The mitigating effect of Se on heavy metal stress in plants can be attributed to multiple mechanisms. First, Se facilitates the production of substances, such as cellulose and pectin, in the cell wall, thereby reducing the absorption of heavy metals. Second, Se enhances the antioxidant capacity of plants, thereby mitigating oxidative damage. Additionally, Se promotes photosynthesis by supporting chlorophyll synthesis and protecting the structure of chloroplast membranes. Finally, Se increases the content of glutathione and essential mineral nutrients in plants, further alleviating heavy metal stress (Figure 3).

3.1. Mechanisms by Which Se Alleviates Cd Stress in Plants

Cd, a common soil pollutant, has been found to accumulate in soil over time, causing detrimental effects on plant growth and development. Studies have shown that the ionization and coordination of Cd can interfere with crucial biochemical reactions within plant cells, disrupting processes such as photosynthesis, respiration, nutrient and water absorption, cell division, nitrogen metabolism, and protein expression. This ultimately leads to the disruption of plant metabolic functions [53]. On the other hand, Se, a beneficial element for plant growth, has gained significant attention for its role in promoting plant growth through increased accumulation of carbohydrates and plant hormones, as well as its ability to protect plants against biotic and abiotic stresses [54]. One notable benefit of Se is its ability to mitigate Cd stress in plants. This is achieved by regulating the uptake, translocation, and sequestration of Cd. Furthermore, Se forms complexes with Cd, reducing the absorption of heavy metals by plants [55]. Research has also shown that Se can enhance plant tolerance to Cd stress by modulating gene expression. In a study conducted on rice (O. sativa) suspension cells exposed to Cd and treated with different concentrations of Se, the expression levels of certain genes (such as OsLCT1, OsNramp5, OsNramp1, OsIRT1, and OsIRT2) were lower in Se-treated rice cells than in untreated cells. Additionally, the uptake of Cd decreased [56].

3.1.1. Se Mitigates Plant Cd Uptake and Accumulation

Further studies have demonstrated that Se enhances the Cd retention capacity of cell walls. This effectively reduces the Cd content within plant cells, subsequently diminishing the absorption and accumulation of Cd by plants [54]. The cell wall serves as the first barrier against Cd ingress into plants. It is composed of three layers: the intercellular layer, primary wall, and secondary wall. The middle lamella primarily consists of pectin, while the primary wall mainly consists of cellulose, hemicellulose, and pectin. The secondary wall is primarily composed of cellulose and lignin [57]. These components contain functional groups, such as hydroxyl, carboxyl, thiol, and other negatively charged groups that are capable of attracting and binding Cd ions to form stable compounds [58]. This facilitates the deposition of Cd ions within the cell wall. Research has indicated that more than 80% of Cd in maize (Zea mays) and rice (O. sativa) is adsorbed onto the cell wall [59].

Research has demonstrated that the application of Se leads to an increase in the biosynthesis of various components, such as pectin, cellulose, hemicellulose, and lignin, in root cell walls. This enhancement facilitates greater sequestration of Cd within the cell wall and subsequently alleviates Cd-induced stress. Moreover, Se treatment increased the content of pectin and activated pectin methylesterase in rapeseed (B. napus) plants under Cd stress [60]. This increase in pectin content promoted the binding of carboxyl groups to Cd, resulting in a reduction in the proportion of soluble Cd within the plant cells [61]. Furthermore, Se supplementation led to an increase in the pectin and hemicellulose contents in rapeseed (B. napus) plants under Cd stress, resulting in a significant reduction in the proportion of soluble Cd within the cells [54]. These findings were further confirmed in a study on winter wheat (T. durum), highlighting the beneficial role of Se in both plant growth and the reduction in Cd uptake [62]. Hence, it can be concluded that Se plays a crucial role in enhancing the tolerance of plants to Cd stress.

3.1.2. Enhancement of Cd Stress Tolerance in Plants by Se

Se not only reduces the absorption of Cd by plants but also enhances their antioxidant capacity to withstand heavy metal Cd stress. Se achieves this primarily by scavenging intracellular free radicals, increasing the activity of antioxidant enzymes, and elevating the levels of nonenzymatic antioxidants, thereby bolstering the plant’s antioxidant defense system [63,64,65]. Studies have indicated that treatment with either Se acid salts or selenite enhances the antioxidant defense capacity of cabbage (Brassica oleracea), reducing the accumulation of hydrogen peroxide and malondialdehyde within the cells and alleviating the toxic effects of Cd stress on cabbage (B. oleracea) [66]. Similarly, research has shown that treating rapeseed (B. napus) under Cd stress with 15 mg/L of selenite reduced the levels of superoxide anions, hydrogen peroxide, and malondialdehyde, thereby inhibiting Cd-induced oxidative damage to rapeseed (B. napus) [67]. Studies on plants such as wheat (T. durum), ginseng (Panax ginseng), and mustard (B. juncea) [68,69,70] have demonstrated that Se can directly regulate and significantly enhance the activity of antioxidant enzymes, such as SOD, APX, CAT, POD, and GR, under Cd stress. As a result, the quantity of ROS within the cells is reduced, the plant’s antioxidant defense capabilities are strengthened, and the accumulation of Cd in plants is decreased. One of the possible reasons for Se’s promotion of antioxidant enzyme activity is its ability to regulate the absorption and distribution of the coenzyme factors of these enzymes, such as Fe, Mn, Cu, and Zn, thereby increasing the effective utilization of these elements within the plant. This enhances the activity of relevant antioxidant enzymes, facilitates the efficient removal of ROS by these enzymes, and enhances plant tolerance to Cd stress [71]. Therefore, the regulation of the absorption and distribution of certain essential elements by Se is considered a key mechanism for regulating antioxidant enzyme activity, reducing ROS levels, and enhancing plant tolerance to environmental stress. In addition to promoting the antioxidant defense capabilities of plants through the ROS scavenging system, Se can also alleviate oxidative stress induced by Cd by regulating the activity of enzymes related to plant nitrogen metabolism. Research has revealed that Se can enhance the activity of nitrogen metabolism-related enzymes, such as nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase, in potato (Solanum tuberosum L.) plants under Cd stress, improving nitrate assimilation pathways and enhancing their antioxidant capacity [72]. In plants, Se can be transformed into various selenoproteins, among which selenocysteine is most significant. Selenocysteine, a Se-containing amino acid, is present in both animals and plants and constitutes many important selenoproteins. These selenoproteins not only participate in antioxidant activity but also play critical roles in various metabolic reactions. Serving as potent antioxidants, selenoproteins effectively protect cells from damage caused by excessive reactive oxygen species, such as superoxide and hydrogen peroxide. This antioxidant function is beneficial not only for the growth and development of plants themselves but also for enhancing their stress resistance and environmental adaptability. Particularly under Cd stress, Se alleviates oxidative stress responses in plants, thereby mitigating the negative impacts of Cd. Cd typically induces the production of reactive oxygen species, leading to increased oxidative stress within cells, which damages cell membranes, proteins, and nucleic acids, affecting normal physiological processes in plants. Se effectively neutralizes these harmful oxidants, preserving the structural integrity and function of cells, and maintaining normal growth and development in plants [73].

It should be noted that the effects of Se on antioxidant enzymes vary, and there are significant differences due to differences in plant species, tissues, treatment methods, and durations. This variability may be attributed to variations in the sensitivity to Cd or Se among different genotypes of plants or tissues and organs.

3.1.3. Se Ameliorates the Impact of Cd Stress on Plant Photosynthesis

Leaf photosynthesis is highly sensitive to Cd stress [74,75]. Studies have demonstrated that Cd stress has multiple effects on plant photosynthesis, including reduced photosynthetic efficiency and the inhibition of key photochemical processes. These effects include damage to the chloroplasts and thylakoid structures in plant cells, as well as a decrease in chlorophyll content [76,77,78]. However, Se has been found to improve the adverse effects of Cd stress on plant photosynthesis. Research has shown that exogenous Se treatment can promote tobacco (Nicotiana tabacum) growth, increase chlorophyll content, enhance photosynthetic performance, and increase nitrogen content [79]. Additionally, Se is involved in the recovery and reconstruction of the photosynthetic system, cellular membranes, and chloroplasts under Cd stress [65,80]. Studies have revealed that Cd stress leads to the degradation of the chloroplast inner membrane in cabbage (B. oleracea), whereas Se application results in the reconstitution of the chloroplast ultrastructure and reorganization of thylakoid and stroma lamellae structures. This ultimately leads to an increase in chloroplast size, fatty acid unsaturation, and cellular membrane fluidity [66]. Se is taken up by plants through the root system in the form of selenate or selenite from the soil and is subsequently converted into organic Se (Se methionine and Se cysteine) within chloroplasts. Therefore, the Se metabolic process is closely linked to the chloroplast [81].

Se not only helps plants rebuild damaged chloroplast structures but also regulates and promotes chlorophyll synthesis metabolism while inhibiting the degradation of chlorophyll under Cd stress. Research has shown that Se treatment significantly increases the chlorophyll content in Triticum aestivum leaves, with concentrations of 10, 20, and 30 mg/L of Se increasing the chlorophyll content during the wheat (T. durum) flowering stage [82]. Similarly, in rice (O. sativa), corn (Zea mays), and spinach (Spinacia oleracea), low concentrations of Se effectively increase chlorophyll a and b levels and the ratio of chlorophyll a to chlorophyll b [83,84,85]. The increase in photosynthetic pigment content may be attributed to the close relationship between Se and Fe, a key element in the chlorophyll biosynthesis process. Se can help plants maintain a high absorption capacity for Fe [86]. This regulatory mechanism may involve the Se-mediated control of Fe-sulfur (S) proteins in chloroplasts.

Furthermore, Se enhances photosynthetic processes, such as light absorption, photosynthetic electron transport, and carbon assimilation. It has been observed that foliar application of Se improves photosystem II and the electron transport rate under Cd stress in perilla leaves, while root application of Se increases the photosynthetic rate, stomatal conductance, transpiration rate, and stomatal limitation value under Cd stress [87]. Similarly, the findings of this study on rice (O. sativa) are consistent with these observations [88]. Taken together, these studies suggest that Se can mitigate the effects of Cd stress on plant photosynthesis through various mechanisms.

3.2. Mechanism by Which Se Alleviates Pb Stress in Plants

Pb, a common heavy metal element, significantly impacts the growth and development of plants. Adequate Se can ameliorate the Pb stress experienced by plants [89]. Research has shown that appropriate Se concentrations can inhibit the accumulation of Pb in plants. Additionally, exogenous biosynthesized nano-Se has been found to significantly reduce Pb accumulation in mustard (B. juncea) greens. Furthermore, when applied as a foliar spray, biosynthesized nano-Se effectively enhances the activities of SOD and GPX in mustard (B. juncea) greens. This leads to increased levels of ascorbic acid and GSH, thereby alleviating the toxicity and oxidative damage caused by heavy metals to plants [90].

Studies have also revealed that foliar spraying of nano-Se significantly increases the Se content in roots and alleviates Pb stress. Moreover, different concentrations (5, 10, and 20 mg·L⁻¹) of nano-Se had a positive influence on root activity. This is likely due to the protective effect of biosynthesized nano-Se on oilseed rape (B. napus) root tips, which mitigates the inhibition of cell division and root elongation induced by Pb [91]. Similarly, Se has been shown to have a strong ability to bind Pb, forming nontoxic complexes and thereby reducing Pb-induced stress in plants [92,93]. The addition of Se to lettuce (L. sativa), rape (B. napus), and spinach (S. oleracea) has been found to significantly reduce Pb accumulation while promoting the absorption of some trace elements, including Se [67,94].

3.2.1. Se Enhances Plant Photosynthesis to Counteract Pb Stress

Treatment with Pb reduces the efficiency of photosynthesis in plants, while proper treatment with Se can enhance photosynthetic efficiency [95]. Previous studies have indicated that lower levels of Se can improve the photochemical efficiency of plants [96]. Se supplementation can partially alleviate Pb stress in plants, as evidenced by the greater photosynthetic rate in the Pb + Se treatment group than in the Pb treatment group. Research suggests that Pb treatment decreases the levels of chlorophyll a and chlorophyll b, whereas Se supplementation increases their levels [97]. Decreased levels of photosynthetic pigments have been observed in coontails (Ceratophyllum demersum L.) due to the addition of Pb [98], as well as in wheat (T. durum) seedlings [99]. The reduction in chlorophyll content can be attributed to decreased biosynthesis of chlorophyll, chloroplast damage, impaired absorption of essential elements, such as Fe and Mn, or chlorophyll degradation [100]. Furthermore, other studies have shown that chlorophyll b is more affected than chlorophyll a, and additional Se treatment can prevent the degradation of chlorophyll b [101]. Research has shown that plants treated with Se exhibit increased chlorophyll content, which contributes to improved photosynthetic efficiency [102]. Additionally, plants treated with Se also exhibited greater Fv/Fm (maximum photosynthetic efficiency) and PI (photosynthesis performance index) values, indicating that Se alleviates the inhibition of photosynthesis caused by Pb stress by enhancing photosynthesis.

3.2.2. Se Alleviates Pb Stress in Plants by Enhancing the Content of GSH

Studies have shown that Pb can lead to the inactivation of GSH. Pb has the ability to share electrons, which makes it prone to forming covalent bonds. Pb deactivates GSH by binding to the thiol groups present in GSH, resulting in a decrease in GSH content. This decrease in GSH content ultimately affects normal plant growth and development [103]. On the other hand, treatment with low concentrations of Se can increase GSH levels in plants. For example, compared to the control treatment, the application of 0.1 mg·L⁻¹ of Se(IV) to Berula erecta resulted in significant increases of 22% and 13% in the GSH levels in the roots and shoot parts, respectively [96]. Similarly, research has shown a significant increase in the GSH content in plants after treatment with low Se concentrations (0.01 mg·L⁻¹ of Se(IV)) and has shown that the low Se absorbed by plants may be utilized for synthesizing important substances, such as GSH and GPX, which can rebalance excessive ROS under stress conditions [104]. In Brassica chinensis, the application of exogenous nanoscale Se at 5 mg·L⁻¹ resulted in a maximum increase in the GSH content, with a 184% increase compared to that in the control group [100]. Additionally, the increase in GSH induced by Se application can promote the ascorbate (AsA)–GSH cycle to some extent, and GSH can quench Pb-induced ROS, indicating the important role of GSH in detoxifying Pb. Therefore, Se application can alleviate Pb stress in plants to some extent [105].

3.2.3. Se Alleviates Pb Stress in Plants by Alleviating Oxidative Stress

Pb and other heavy metal ions often reduce cell viability in plants by inducing oxidative stress or inhibiting enzyme reactions. Pb stress decreases the activity of GPX by 15% in Vicia faba L. roots, while the addition of an appropriate concentration of Se enhances GPX activity under Pb stress, suggesting that Se may have antioxidant effects [106]. Numerous studies indicate that supplementation with lower concentrations of Se contributes to increased GPX activity in plants, thereby exerting beneficial effects on the plant antioxidant system. However, relatively high Se concentrations can decrease plant GPX activity, and plants may exhibit pro-oxidative properties [107]. According to the literature, the antioxidant properties of Se are associated with its beneficial effects on GPX activity [108]. The application of Se or selenites can enhance GPX activity [109]. The effects of Se on root growth, vitality, reactive oxygen species (ROS) production, and antioxidant activity under Pb stress are not well understood, as the protective role of Se primarily depends on its concentration. Low concentrations enhance cell viability, while high concentrations exhibit pro-oxidative effects, promoting the accumulation of lipid peroxidation products and thereby increasing damage to the cell membrane.

3.3. Mechanism by Which Se Alleviates Hg Stress in Plants

According to the World Health Organization, Hg is highly toxic to biological communities and has emerged as a key concern for public health. A recent soil quality survey conducted by the Chinese Ministry of Environmental Protection revealed that approximately 1.6% of agricultural soils exceeded the safety standards for Hg [110]. Even at lower concentrations, Hg can harm plants by interfering with nutrient absorption, triggering oxidative stress, damaging cellular components, and inhibiting growth [111]. Thus, controlling Hg levels in soil and reducing Hg accumulation in crops are essential for ensuring crop production safety and food security [112,113].

3.3.1. Se Alleviates Hg Stress by Reducing Hg Uptake and Accumulation

Studies have demonstrated that Se can alleviate Hg stress in plants. First, nanoscale Se particles can bind with ionic Hg²⁺ to form relatively insoluble Hg selenide compounds, reducing the bioavailability of Hg. Second, nanoscale Se can suppress the expression of genes involved in the absorption and transport of heavy metals in plant roots, thereby reducing their uptake of Hg. This helps mitigate the detrimental effects of Hg on plant growth. Moreover, nanoscale Se can decrease the accumulation of Hg in plant leaves and roots, further alleviating its negative impact [114]. Under anaerobic soil conditions, exogenous Se undergoes transformations, ultimately forming Hg-Se precipitates with Hg²⁺. This process reduces the effective Hg²⁺ content and inhibits Hg accumulation in rice (O. sativa) [115]. Research has shown that Se application in soil inhibits Hg²⁺ absorption by rice (O. sativa), possibly due to the conversion of exogenous Se to Se₂O₃⁻ or Se₂O₄⁻ under anaerobic conditions. These compounds can then react with Hg²⁺ to form Hg-Se complexes [116]. The concentration of CH₃Hg⁺ in soil decreases continuously after Se treatment, likely due to the activity of sulfate-reducing bacteria (SRB) under anaerobic conditions. It has also been observed that Hg absorption by rice (O. sativa) tissues decreases gradually from roots to aboveground parts, possibly because SRB reduce free Se₂O₃⁻ or Se₂O₄⁻ to Se²⁻, thus reducing Hg accumulation [117,118]. Therefore, Se supplementation effectively reduces the uptake and accumulation of Hg by plants.

3.3.2. Se Modulates the Balance of Mineral Nutrition under Hg Stress

Hg toxicity disrupts the absorption and transport of essential plant mineral nutrients, leading to metabolic disturbances. Hg and plant nutrients compete for transport proteins, which can result in mineral nutrient deficiencies [119]. Se plays a crucial role in regulating the absorption and distribution of mineral nutrients in plants, maintaining the balance of mineral elements [120]. Research has shown that Hg inhibits the uptake of Se in plants [121]. Additionally, Hg treatment suppresses the uptake of trace elements, such as Ca, Mg, Mn, Cu, and Zn. However, the addition of appropriate levels of Se reversed the decrease in the concentrations of these elements, suggesting a regulatory mechanism. Se supplementation not only alleviates Hg toxicity but also helps balance plant mineral nutrition. Studies have also shown that Se enhances the accumulation of magnesium, zinc, and manganese in hydroponically grown maize (Z. mays) [122]. Moreover, Se enhances the activity of transmembrane ATPases, facilitating the transport of mineral elements and maintaining the intracellular ion balance. In tobacco (N. tabacum) plants, the application of Se at a concentration of 2.5 mg·kg⁻¹ has been found to promote the activity of H⁺-K⁺ ATPases, Na⁺-H⁺ ATPases, and Ca²⁺-Mg²⁺ ATPases, mitigating the disruption of cellular environments caused by Hg stress. Nanoscale Se has been shown to restore or increase the content of macro- and micro-nutrients in plants, supporting their growth under Hg stress [79]. These findings suggest that Se can be an effective additive for mitigating the impact of Hg stress on plants [90,115,123]. However, further research is needed to fully understand how Se regulates the absorption and distribution of mineral elements and to elucidate the mechanism by which Se promotes the balance of mineral ion metabolism in plants.

3.3.3. Se Alleviates Hg Protein Toxicity in Plants

Due to the strong affinity of Hg²⁺ for S ligands, the cytotoxicity of Hg²⁺ is believed to be associated with its binding to -SH groups in functional proteins [124]. Hg disrupts proteins involved in key cellular processes by altering cell membrane permeability and replacing essential metal ions with high affinity for -SH groups, which in turn may lead to protein precipitation [125]. Se plays a regulatory role in the stress response, sulfur metabolism, GSH metabolism, DNA replication, the cell cycle, and energy and carbohydrate metabolism, indicating its protective function against Hg toxicity [126]. In addition, Se effectively affects ATPase synthesis by preserving membrane lipid integrity, regulating pH and Ca²⁺ homeostasis, and competing with heavy metals for ion channels to enter root cells. Consequently, the addition of Se₃O₂⁻ increases the activity of root H⁺-ATPase and Ca²⁺-ATPase, significantly alleviating heavy metal toxicity in rice (O. sativa) tissues [127].

4. Potential Application of Se in Mitigating Heavy Metal Stress in Plants

In China, more than 70% of the population consumes less than the daily recommended intake of Se [65], with the average daily intake among adults ranging from only 26 to 32 μg, and in certain areas, it falls below 10 μg·d⁻¹ [128]. Considering the global concerns regarding Se deficiency and its effectiveness in reducing heavy metal accumulation in crops, the potential for utilizing Se to mitigate heavy metal pollution is significant. Moreover, the application of Se to reduce the uptake of heavy metals by crops is increasingly acknowledged [129]. This not only contributes to the reduction in heavy metal content in food but also meets the Se requirements of the human body [130].

Extensive research has been conducted on the interactions and toxicity relationships between Se and elements such as As, Cd, and Cr (III and VI). These studies have highlighted the potential role of Se in preventing and treating certain diseases [131]. Additionally, the detoxification mechanisms of Se against Pb, As, and Hg involve complex formation with heavy metals, enhancement of the plant’s antioxidative defense system, induction of metallothionein formation, alteration of metal distribution within the organism, restoration of antioxidant enzyme activity, and protection of cells from heavy-metal-induced damage (Figure 3) [132]. Therefore, Se plays a multifaceted role in mitigating heavy metal toxicity and has the potential to positively impact human health. Although Se supplementation has been shown to benefit plants in coping with various abiotic stresses, excessive Se concentrations can lead to Se toxicity. Se concentrations below 1 mg·kg⁻¹ are beneficial for most plants, whereas higher concentrations can be toxic to most crops. At lower Se concentrations, Se acts as a growth regulator, antioxidant, anti-aging agent, abiotic stress modulator, and defense molecule against pathogens. At higher concentrations, plants exhibit various toxic symptoms, including stunted growth, chlorosis, wilting, leaf desiccation, premature reduction in protein synthesis, and even plant death [133,134].

Choosing appropriate Se application methods is crucial. Foliar application of Se has been shown to be superior and more effective for biofortification compared to soil application. Se enrichment rates in the edible parts of plants are lower with soil-applied Se fertilizers, which may pose long-term toxic effects on nearby ecosystems; therefore, caution should be exercised in Se fertilizer use to avoid adverse effects [135]. Selenite and selenate have been demonstrated to be more biologically available to plants when directly applied to leaf surfaces rather than soil. Importantly, direct foliar exposure ensures efficient Se assimilation in plants, as it does not depend on root-to-shoot translocation. The bioconversion rates of inorganic Se absorbed by plants into different organic forms are species-specific and depend on specific biochemical pathways [136].

Plants primarily uptake Se in the forms of selenate or selenite, which are metabolized via the S assimilation pathway. Se and S share similar electronic structures, as they are positioned in the same period of the periodic table between oxygen and tellurium. Considering the biochemical similarities between Se and S, Se may competitively interact with S in many key enzymatic steps of sulfur assimilation pathways [137]. S is typically the primary S source required for cysteine synthesis in terrestrial plants. However, in regions facing sulfate scarcity, due to their similar uptake and metabolic pathways, Se supplementation can substitute for insufficient S [138]. This substitution is not limited to plants—Se can also partially substitute for S functions in animal organisms. Therefore, understanding the chemical resemblance between Se and S is crucial for agricultural practices. Considering Se’s ability to supplement S deficiencies in soils may aid in developing balanced Se and S fertilizers. This approach can potentially enhance nutrient uptake efficiency in plants and improve crop yield and quality.

In conclusion, the application of Se to plants not only reduces the uptake of heavy metals but also indirectly supplements the human body’s requirement for this essential element. This dual benefit offers protection against heavy metals and ensures an adequate supply of Se.

5. Conclusions and Outlook

The toxicity of heavy metals to plants and the mechanisms by which plants detoxify heavy metals are currently important areas of research. Heavy metals disrupt normal cellular metabolism and cause damage to plant structures, thus affecting plant growth and development. Considering the variations in plant metabolism and the toxicity of heavy metals, the adsorption, adaptation, and detoxification mechanisms of plants to heavy metals are complex and require further investigation. This review presented an overview of the physiological mechanisms through which heavy metals impact plant growth. Additionally, this study highlighted the significant benefits of Se in enhancing plants’ resistance to heavy metal stress. Se assists in improving plant antioxidant systems, repairing damaged cell structures and functions, and promoting the formation of Se–heavy metal complexes and metallothionein. Moreover, Se influences the absorption and translocation of heavy metal ions in plants, effectively mitigating the damage caused by heavy metal stress. Nevertheless, there are still research gaps to be addressed regarding the role of Se in plant resistance to heavy metal stress. This study investigated the mechanisms of Se’s interaction with various heavy metals, focusing on how Se forms complexes with specific heavy metal ions and enhances plant tolerance by regulating gene expression. Molecular genetic approaches were employed to uncover the genetic regulatory mechanisms of Se in plant resistance to heavy metal stress, elucidating the relationships among Se, genes related to heavy metal resistance, and their regulatory networks. Furthermore, the agricultural application potential of Se was explored to assess its impact on crop yield and quality, as well as its practical effectiveness in remediating heavy-metal-contaminated soils. These in-depth investigations not only advance our understanding of mechanisms of Se in alleviating heavy metal stress in plants but also provide scientific foundations for developing novel Se remediation agents and strategies to enhance crops’ heavy metal tolerance. These efforts aim to offer innovative solutions for addressing heavy metal pollution in modern agriculture and environmental conservation, thereby promoting sustainable agricultural development and improving environmental quality.