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Review

Selenium’s Role in Plant Secondary Metabolism: Regulation and Mechanistic Insights

by
Yan Zhou
1,2,
Kaiqin Nie
1,2,
Lulu Geng
1,2,
Yixin Wang
1,2,
Linling Li
1,* and
Hua Cheng
2
1
School of Modern Industry for Selenium Science and Engineering, Wuhan Polytechnic University, Wuhan 430048, China
2
National R&D Center for Se-Rich Agricultural Products Processing, Wuhan Polytechnic University, Wuhan 430023, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(1), 54; https://doi.org/10.3390/agronomy15010054
Submission received: 19 November 2024 / Revised: 21 December 2024 / Accepted: 27 December 2024 / Published: 28 December 2024
(This article belongs to the Section Plant-Crop Biology and Biochemistry)
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Abstract

:
Selenium (Se) is an indispensable trace element for humans and other animals. Various studies have demonstrated the beneficial effects of Se on plants, including the promotion of growth, accumulation of secondary metabolites, and enhancement of antioxidant capacity, thereby improving plant stress resistance. Consequently, Se biofortification has emerged as an effective strategy to elevate Se content and nutritional quality in plants, attracting widespread attention. The mechanism of selenium (Se) at the plant secondary metabolic level has not yet been fully elucidated, and it remains an unanswered question as to how selenium affects plant secondary metabolic pathways and how these metabolic pathways respond to selenium biofortification. Although it has been shown that selenium can affect the antioxidant system and defense mechanisms in plants, detailed mechanisms of selenium’s action on plant secondary metabolic pathways, including its effects on specific metabolic enzymes and regulatory genes, still need to be revealed by further in-depth studies. The present study aims to elucidate the mechanisms of Se absorption, transport, and metabolism in plants under Se-rich conditions and to investigate the impact of various Se biofortification methods on the content of plant secondary metabolites. By integrating existing research progress, this paper will delve into the potential molecular regulatory mechanisms of Se on plant secondary metabolism, aiming to unravel the interplay between Se and plant secondary metabolism. This study provides a novel perspective and direction for future research on plant secondary metabolism and the biological utilization of Se.

Graphical Abstract

1. Introduction

Selenium (Se) is a rare element belonging to Group VIA in the fourth period of the periodic table of elements and serves as an essential trace element for the human body [1]. The recommended daily intake of selenium for adults is 55~70 μg, of which 400 μg is a toxic concentration. Selenium deficiency has been observed in some parts of the world, including China (about 72% of the total area), New Zealand, etc. Selenium deficiency can cause a variety of health problems, most notably Keshan disease, a cardiomyopathy that primarily affects the function of the heart, and Kashin–Beck disease, a deforming bone and joint disease that affects the normal development of bones and joints. At the same time, there are a few areas in the world with high soil selenium levels, such as Enshi Province in China, where the soil selenium level can reach up to 11.4 mg Se/kg. Long-term intake of food produced in these high-selenium areas may lead to selenotoxicity symptoms and growth abnormalities [2].
In nature, Se primarily exists in both inorganic and organic forms. Inorganic Se, such as selenite, selenate, and elemental Se, can be absorbed and utilized by plants from soil. Conversely, within plants, Se primarily occurs as organic Se species, including selenocysteine (SeCys) and selenomethionine (SeMet), as well as Se-containing compounds like Se polysaccharides [3]. An optimal amount of Se is beneficial to most plants, enhancing their yield and quality and improving their stress resistance. Se has been extensively studied for its application in agricultural practices and has shown remarkable potential in coping with biotic and abiotic stresses. For example, it has been demonstrated that selenium not only improves plant resistance to a wide range of diseases but also mitigates oxidative damage induced by salt stress by enhancing its antioxidant capacity [4,5,6,7,8,9] (Table 1). In addition, in the field of phytoremediation, the process of the uptake, accumulation, and transformation of Se by plants is used to remove excess Se from the environment so as to reduce environmental pollution [10]. However, when selenium concentration exceeds a safe threshold, selenium-induced accumulation of superoxide radicals damages cell membranes or produces non-specific selenoproteins, which can be toxic to the plant, leading to symptoms such as yellowing, necrosis, or growth restriction [11]. In particular, when selenium concentrations are too high, the antioxidant effect is transformed into a pro-oxidant, leading to the accumulation of reactive oxygen species (ROS), which inhibits the growth rate of plant tissues and causes lipid oxidation associated with malondialdehyde formation [12].
In recent years, with the intensifying research on Se enrichment in plants, Se biofortification has emerged as an effective strategy to elevate Se content in plants. Studies have demonstrated [13] that through foliar spraying, soil application, or hydroponics, plants can absorb and convert various exogenous Se forms into organic Se. This not only augments Se content in plants and fosters their growth and development but also increases the concentration of plant secondary metabolites, thereby enhancing resistance to abiotic stress.
Plant secondary metabolites, produced as a result of their long-term adaptation to the ecological environment, are classified into phenols, terpenes, and alkaloids based on their structural diversity. These compounds exhibit a broad spectrum of biological activities and pharmacological functions, holding significant potential for applications in anti-inflammatory, antioxidant, and anti-cancer treatments [14,15]. Selenium not only promotes the accumulation of certain metabolites (e.g., phenols, terpenes) but also influences the synthesis of secondary metabolites through the regulation of primary and secondary metabolism, phytohormones, and oxidation–reduction balance, enhancing the antioxidant capacity of the whole plant and consequently improving its resistance to changes. However, current research on the relationship between Se and plant secondary metabolism remains insufficient. Specifically, different plant species may exhibit varied responses to Se, and the mechanisms underlying Se’s effects on secondary metabolites are not fully elucidated, necessitating further investigation. Elucidating the molecular regulatory mechanisms of Se on plant secondary metabolism is crucial for understanding the alterations in the secondary metabolism network in Se-rich soil or Se bioaugmented crops. Therefore, this paper summarizes the mechanisms of Se absorption, transport, and metabolism in plants, the impacts of Se on plant secondary metabolite content, and the potential molecular mechanisms regulating related metabolic pathways. This provides a theoretical foundation for the future cultivation of Se-rich crops.

2. Absorption, Transport, and Metabolism of Se in Plants

2.1. Se Absorption Mechanism and Transport Pathway

The mechanism of Se uptake and transport in plants is influenced by various factors, including plant species, Se forms and concentrations, soil pH, microbial activity, and transporter activity. The plants vary in their capacity to accumulate Se. Se absorbed by plant roots primarily consists of selenite (SeO32−) and selenate (SeO42−). Selenite predominantly occurs in weak acid to neutral soil, whereas selenate is found in alkaline soil. Elemental se is usually difficult to be absorbed directly by plants and needs to undergo a complex chemical transformation process before it can be converted into a plant-available form. In plants, the absorption and transport of Se are regulated by a series of transporters that facilitate Se movement from roots to stems and leaves, ensuring normal plant growth and development. Numerous studies have confirmed that the absorption of selenate by plants through active energy consumption [16] is intimately linked to the sulfate transporter (SULTR) located in chloroplasts and vacuoles [17].
After being absorbed by roots, selenate can easily enter the upper part of the plant through the roots and is reduced to selenite in the leaves of the plant, which is further converted into organic Se compounds. The plastid serves as the central site for the conversion of selenate into organic selenium compounds. SULTR3;1, located in the chloroplast membrane, catalyzes the transport of selenate into the plastid [18,19]. Plant roots can also transport SeO42− through sulfate transport channels such as Sultr1;1, Sultr1;2, and Sultr1;3 and transport them to the aboveground parts to complete Se metabolism [20,21]. However, compared to selenate, there are fewer studies on the absorption mechanism of selenite in plants. Early research suggests that the absorption process of selenite may be completed through passive diffusion [22]. As selenite is a polybasic weak acid, it generates various forms of H2SeO3, SeO32−, and HSeO3− under different pH conditions [23,24]. Research on tobacco and rice [25,26] found that the uptake of HSeO3− and some SeO32− by plants is mainly completed through phosphate transporters in the roots. The overexpression of specific phosphate transporters such as OsPT8 can significantly improve the absorption efficiency of plants for selenite. Selenite is easily absorbed by plant roots and converted into other forms, including SeMet and its oxides, etc. These substances are mainly accumulated in the roots, with only a small portion being transported to the upper part of the plant [27]. In addition, selenite is transported by the silicon transporter LSI1 and the water channel protein OsNIP2 in the neutral form of H2SeO3 from the plant roots’ transport and absorption [28].

2.2. Metabolic Pathways of Se in Plants

Se and sulfur share similar metabolic pathways in plants. Se is absorbed by plant roots and transported to chloroplasts via the xylem. The oxidized form of Se (Se6+) is activated by ATP sulfurylase to generate adenosine 5′-phosphosulfate (APSe). Under the catalysis of glutathione (GSH) and APS reductase, APSe is reduced to the reduced form of Se (Se4+), which is then further reduced to Se2+ by sulfite reductase in chloroplasts [29]. Subsequently, SeCys is produced through the reaction of cysteine synthase with O-acetylserine (OAS). The metabolism of SeCys in plants can be broadly classified into three pathways depending on plant species and environmental conditions; SeCys can either be directly converted into elemental Se under the action of its lyase, methylated by SeCys methyltransferase (SMT) to produce methylselenocysteine (MeSeCys), or synthesized into SeMet through the combined action of cystathionine γ-synthase (CγS) and cystathionine β-lyase (CβL) [30]. MeSeCys and SeMet can further be catalyzed into selenoproteins and volatile compounds such as dimethylselenide (DMSe) and dimethyldiselenide (DMDSe). SULTRs, NIP, PHT, etc., are the major genes mediating the entry of Se into plant roots, while APS, APR, SIR, CS, SL, SMT, and CBL are involved in the metabolic pathway of Se in chloroplasts [3] (Figure 1).
Selenium (Se) uptake and metabolism play key roles in plant physiological processes, and its effects are not only limited to the promotion of plant growth and enhancement of antioxidant capacity but also significantly regulate the biosynthesis and accumulation of secondary metabolites. Particularly, the role of selenium is important in the synthesis of sulfur-containing secondary metabolites. The metabolic pathway of selenium as a trace element in plants is closely related to sulfur metabolism because selenium and sulfur are chemically similar. This similarity may affect the activity of key enzymes in the sulfur metabolic pathway, which in turn affects the synthesis of sulfur-containing secondary metabolites.

2.3. Effect of Se on Primary Metabolites in Plants

Se can influence primary metabolic pathways within plants through various mechanisms, indirectly affecting growth rates and biomass accumulation and further regulating secondary metabolic pathways. Treatment with low concentrations of Se enhances photosynthetic efficiency and primary metabolism related to growth, thereby promoting plant development. In hydroponic experiments with Astragalus adsurgens, low Se concentrations upregulated the expression of amino acid metabolism-related pathways, such as proline and arginine pathways, whereas high Se concentrations reduced Se translocation to aerial parts, directing more primary metabolites towards secondary metabolic pathways, thereby enhancing Se tolerance in A. adsurgens [31]. Appropriate Se application to Medicago sativa increased photosynthetic pigments and soluble sugar and protein contents, facilitating biomass accumulation [32]. Similarly, a study on selenite application to Brassica rapa [33] demonstrated that low Se concentrations (0.1–0.4 mM) promoted growth and enhanced nutritional quality by increasing soluble protein, soluble sugar, and free amino acid contents.
However, the effects of Se are not always positive, as high Se concentrations elicit various metabolic changes in plants to counteract Se toxicity. High selenite concentrations not only alter primary metabolite contents in Brassica napus roots, such as increased amino acid levels and decreased tricarboxylic acid (TCA) cycle intermediates, but also elevate glucose content to maintain higher respiratory rates and ATP levels in plants. These high Se levels can also lead to increased mitochondrial superoxide concentrations, subsequently inhibiting aconitase activity in the TCA cycle and enhancing alternative oxidase 1 (AOX1) and cyanide-resistant respiration, thereby activating the AOX pathway to reduce superoxide accumulation and ensure metabolic homeostasis under stress conditions [34]. High Se concentrations (1.5 mM) exert negative effects on the growth of different rice genotypes under hydroponic conditions. Se treatment significantly decreases soluble protein, chlorophyll, and carotenoid contents in plants, leading to leaf chlorosis and even necrosis; however, primary metabolic compounds such as sucrose, total sugars, nitrate, and amino acids increase correspondingly to mitigate Se-induced toxicity [35].
Moreover, Se can enhance plant tolerance to stress conditions. A study on the effects of Se on the growth of maize under cadmium stress found that low Se concentrations (2 mg/L) improved germination rates, while high Se concentrations (4 mg/L) increased biomass in plants under cadmium stress. Intriguingly, all Se concentrations effectively alleviated the toxic effects of cadmium on photosynthetic pigments, while higher Se concentrations significantly mitigated cadmium-induced oxidative stress and increased flavonoid content in plants [36]. In Brassica juncea under chromium stress, selenate application was found to act as an osmoregulator, contributing to increased total sugar, reducing sugar, and non-reducing sugar contents [37]. Research on broccoli sprouts treated with selenate indicated that selenate treatment affected the metabolism of β-alanine and glutathione, as well as the biosynthesis of plant metabolites related to glucosinolate precursors [38]. More importantly, the presence of Se promoted an increase in sulfur transporter genes such as Sultr1, Sultr2, and ATP sulfurylase at the transcriptome level, leading to enhanced sulfur absorption and the synthesis of primary and secondary metabolites containing these elements [39]. In a study on rice grains, it was found that the use of selenite and selenate increased fatty acids (oleic acid, linoleic acid, and palmitic acid) by threefold; moreover, compared to Na2SeO4, Na2SeO3 biofortification resulted in higher Se accumulation in grains, although the specific metabolic pathways affected remain unclear [40]. The effects of exogenous Se on plants are not limited to changes in primary metabolite contents but are more pronounced in its influence on the accumulation of secondary metabolites.

3. Effect of Se on Secondary Metabolites in Plants

3.1. Phenolic Compounds

Phenolic compounds, which are widely present in plants and contain benzene rings and hydroxyl groups, primarily include flavonoids, phenolic acids, lignans, and stilbenes. They are produced through the shikimate or acetate–malonate biosynthetic pathways and exhibit pharmacological properties such as antioxidant and antibacterial activities. Notably, selenium concentration is one of the factors influencing the biosynthesis of phenolic compounds, which may play a role by regulating the accumulation of plant secondary metabolites or affecting the balance of metabolic pathways (Table 2).
Huang et al. [41] treated soybeans with varying concentrations of Na2SeO3, resulting in an 87.3-fold increase in total Se content compared to the control group. The converted organic Se forms were primarily SeCys and SeMet. Low Se concentrations significantly enhanced the total phenol, isoflavone, and amino acid contents in soybeans, whereas high concentrations exhibited inhibitory effects. Similarly, soaking chickpeas with a low concentration of Na2SeO3 (24 mg/L) increased their isoflavone content compared to the control [42]. Se biofortification of basil through foliar spray application by Skrypnik et al. demonstrated an increase in the total contents of hydroxycinnamic acids, anthocyanins, and phenolic compounds in basil leaves [43]. In a hydroponic study of red-leaf lettuce treated with Na2SeO3, low Se concentrations (4 and 8 μM) significantly elevated anthocyanin content, whereas high Se concentration (16 μM) led to a notable decrease in anthocyanin content [44]. Se enrichment using nano-Se not only effectively accumulated Se in red-fleshed pitaya but also promoted the enhancement of phenolic acids (16.9–94.2%), total phenols (15.7%), total flavonoids (29.5%), and betacyanins (34.1%) in the pulp and increased the activity of antioxidant enzymes. Further analysis revealed a positive correlation between the accumulation of secondary metabolites and the differential expression of phenylalanine (Phe) and tyrosine (Tyr), precursors in the shikimate pathway [45]. Schiavon observed an increased accumulation of phenolic acids, flavonoids such as kaempferol, and several amino acids excluding proline in Se-enriched tomato fruits and radish roots [46,47]. When different wheat varieties were treated with Se through soil and foliar application, it was found that compared to common wheat (Shannong 129), foliar Se application not only significantly increased the grain yield and organic Se content of purple-grained wheat (202w17) but also substantially elevated its anthocyanin content [48]. Similar phenomena were observed in studies on chili peppers, where soil Se application not only reduced the bioavailability and content of the heavy metal chromium but also increased the levels of flavonoid metabolites (rutin, luteolin, etc.) in plant leaves [49].
Low concentrations of Se treatment significantly enhance the content of secondary metabolites in most plants, whereas high concentrations inhibit plant growth and the accumulation of secondary metabolites. Additionally, Se exhibits differential effects on the content changes in secondary metabolites in various plant parts.
Table 2. Effect of Se on the accumulation of phenolic compounds in plants.
Table 2. Effect of Se on the accumulation of phenolic compounds in plants.
PlantSe ApplicationSecondary Metabolites
(Content)		Overall Changes
(Compared to Control)		Reference
Soybeanhydroponics
Na2SeO3		total phenolic compound↑[41]
total flavonoidIt shows a tendency of increasing at the beginning and decreasing later.
Ocimum basilicum L.hydroponics/foliar application
Na2SeO4		total hydroxycinnamic acid↑[43]
total phenolic compound↑
total flavonoidNS
total anthocyanin↑
Purple lettucehydroponics
Na2SeO3		anthocyanin↑[44]
Red pitaya fruitsoil/foliar application
NPs-Se (50–78 nm)		chlorogenic acid↑[45]
caffeic acid↑
ferulic acid↑
ellagic acid↓
rutin↓
betacyanins↑
total phenolic compound↑
total flavonoid↑
Peppersoil application
NPs-Se		chlorogenic acid↑ (roots)[49]
caffeic acid↑ (roots)
vanillic acid↑ (leaves)
p-hydroxybenzonic acid↑ (leaves)
syringic acid↑ (leaves)
ferulic acid↑ (fruits)
apigenin↑ (fruits)
rutin↑
luteolin↑
Tomatohydroponics/foliar application
Na2SeO4		chlorogenic acid↑[46]
4-O-caffeoylquinic acid↑
caffeic acid hexose 1↑
quinic acid derivatives↑
kaempferol↑
rutin↑
Radishhydroponics/foliar application
Na2SeO4		total phenolic compound↑ (leaves) ↓ (roots)[47]
caffeic acid↑ (leaves)
coumaric acid↑ (leaves) ↓ (roots)
sinapic acid↑ (leaves)
ferulic acid↑ (leaves) ↓ (roots)
Chickpeahydroponics
Na2SeO3		total phenolic compoundIt shows a tendency of increasing at the beginning and decreasing later.[42]
isoflavones
Purple-grained wheat
(202w17 and Shannong 129)		soil and foliar applications
as the form of Se4+ (Se-enriched solid fertilizer and Se-enriched nutrient solution) 		total anthocyanin↑ (foliar application > soil application, 202w17)
NS (Shannong 129)		[48]
Lycium chinense L.nutrient solution
Na2SeO3		chlorogenic acid↑[50]
rutin↓
The arrows ↑ and ↓ show increases and decreases in the content of secondary metabolites, respectively, while NS indicates no significant changes.

3.2. Terpenes

Terpenes are a class of compounds composed of isoprene units and their derivatives, with secondary metabolites playing a pivotal role in the plant defense system as their main component. These secondary metabolites assist plants in resisting pathogenic microorganisms and pests or participate in plant growth and development processes.
Se biofortification generally enhances the terpene content in most plants (Table 3). In a study on Se-enriched nutrition in maize, the soil application of Na2SeO3 significantly increased the carotenoid content (zeaxanthin and lutein) in maize grains and significantly enhanced the antioxidant activity of the plants [50]. Low doses of NPs-Se (30 and 50 mg/L) significantly increased peroxidase and superoxide dismutase activity, chlorophyll content, and carotenoid content in alfalfa, while significantly reducing malondialdehyde content [51]. Compared to control plants, treating the same rice seedlings with selenite (20 mg/L) resulted in an increase in carotenoid content [52]. In a study exploring different Se enrichment methods on the physiological growth of Ginkgo biloba, it was found that compared to Se application at the roots, foliar Se spray significantly increased the total terpene lactone content in G. biloba [53]. Research on Se biofortification in Lycium barbarum leaves showed that adding Se in the form of Na2SeO3 to the nutrient medium (from 0.01 g/kg to 0.05 g/kg) increased the carotenoid content of the plants by 200–400% [54]. Different concentrations of Se treatment affected the volatile oil content of Salvia officinalis, with the highest content of components such as α- and β-thujone and camphor observed at a Se concentration of 8 mg/L compared to the control and other treatments [55].
In summary, appropriate Se treatment not only significantly promotes the accumulation of terpenes in plants but also enhances their overall antioxidant capacity. Additionally, different Se application methods have varying effects on the changes in terpene content in plants, which requires selection and optimization based on specific conditions in practical applications.

3.3. Alkaloids

Alkaloids are a class of small nitrogen-containing organic compounds with basic and biological activities naturally synthesized by plants. Based on their molecular structural characteristics, alkaloids are categorized into over ten classes, including isoquinolines, indolines, tropanes, piperidines, and pyrroles. The biosynthetic pathways of alkaloid compounds primarily include the citric acid cycle and shikimate pathway.
The accumulation of alkaloids in Iranian borage cultivated under different Se concentrations exhibits variability. The form, concentration, and treatment period of Se all affect its alkaloid content (Table 4). Specifically, the foliar application of 4 mg/L Na2SeO4 significantly enhances plant growth characteristics and total alkaloid content [56]. Se treatment not only enhances the activity of antioxidant enzymes in onion plants under salt stress but also increases the content of proline, glycine, betaine, and total soluble sugars in plant leaves and bulbs [57]. During the tissue culture of Sophora tonkinensis, an appropriate amount of Se not only promotes the growth of seedling roots but also enhances the accumulation of matrine and oxymatrine [58]. In hydroponic experiments with octoploid strawberry fruits, a Se concentration of 100 µM was found to increase the content of gramine [59]. In contrast, foliar Se application reduces the total alkaloid content in the seeds of Lupinus albus [60].
There are significant differences in the ability of different plants or different parts of the same plant to absorb exogenous Se, which correspondingly leads to variations in their alkaloid content. However, there is currently limited literature on this aspect, and further exploration is needed.

3.4. Other Secondary Metabolites

Glucosinolates (GSLs) are significant secondary metabolites in Brassicaceae plants, specifically those belonging to the genus Brassica. They are classified into aromatic, aliphatic, and indole GSLs based on their side chain structures. The precursors for their biosynthetic pathways primarily include eight amino acids, and they play crucial roles in plant growth, development, and defense mechanisms. When plant tissues are damaged or invaded by pests and pathogens, GSLs readily degrade into isothiocyanate compounds under the action of myrosinase, exhibiting pharmacological effects such as anti-cancer and antioxidant activities [61].
Se, with chemical properties similar to sulfur, can participate in sulfur metabolism pathways in plants, thereby influencing the synthesis and accumulation of sulfur-containing metabolites (Table 4). The application of 8 mg/kg yeast Se significantly increased Se content, growth quality, and antioxidant capacity in Brassica oleracea, including free amino acids, soluble sugars, GSLs, and SOD activity, which were increased by 81.6%, 46.5%, 44.8%, and 25.2%, respectively, compared to the control group [62]. Different forms of Se (100 μM selenite and selenate) had no overall effect on the total glucosinolate content in broccoli sprouts but increased myrosinase activity, leading to the accumulation of sulforaphane [63]. In another study, the biofortification of broccoli with yeast Se and Na2SeO3 at concentrations of 0.1–1.6 mM increased the total glucosinolate content in florets [64]. Research on the combined effects of Se and sulfur on glucosinolate content showed that the application of ZnSO4 solution alone caused growth stress in Chinese cabbage sprouts, while the combined application of ZnSO4 and Na2SeO3 solutions significantly increased Se content and total glucosinolate content in the sprouts [65]. Se foliar application resulted in a 2–3-fold increase in thiol-containing cysteine and glutathione content in the roots of radish plants and a 35% increase in total glucosinolate content; however, the addition of Se to the nutrient solution had no significant effect on GSLs in the roots but decreased their content in the leaves [47]. During the cultivation of two Brassica juncea varieties, the addition of different concentrations of Na2SeO4 to the nutrient solution resulted in a decrease in glucosinolate content [66].
Based on the above studies, the effects of selenium on the accumulation of GSLs varied according to plant species and se concentration, and even for the same plant species, there were some differences in GSLs between cultivars, especially when the differences were specific to particular types of GSLs (Table 5).
The bioavailability of Se is closely related to the accumulation of secondary metabolites in plants, showing a dual effect. Appropriate Se concentrations can induce the accumulation of plant secondary metabolites such as phenols, terpenes, and alkaloids; however, excessively high Se concentrations can cause an imbalance in redox reactions in plants, leading to growth inhibition. Plants adopt corresponding defense mechanisms, such as increasing the synthesis of antioxidant enzymes to scavenge excessive free radicals in the body.

4. Potential Mechanism of Se Regulation on Secondary Metabolites in Plants

4.1. Effects of Se on Primary Carbon and Nitrogen Metabolic Pathways in Plants

Se biofortification can enhance the stress resistance of alfalfa by boosting the antioxidant system to scavenge reactive oxygen species (ROS) and upregulating the expression of genes related to carbohydrate metabolism, such as PPC4, RBCS-3A, RBCS-3C, and GAPA, which are involved in the carbon fixation pathway [32]. Low concentrations of Se can increase sugar content by upregulating the expression of enzymes related to starch and sucrose catabolism, accompanied by an enhancement in the cellular respiration rate to meet the energy requirements for mung bean growth [67]. Metabolomic analysis of maize after the foliar application of Se nanoparticles (SeNPs) revealed that SeNP treatment enhanced metabolic pathways such as glutathione and the TCA cycle, improved antioxidant capacity, ultimately improved various physiological indicators of maize, and promoted its growth [68]. Under arsenic stress conditions, the addition of Se to rice was observed to further increase the total soluble sugar and reducing sugar contents in the plant, enhance the activity of sucrose phosphate synthase in leaves to promote sucrose accumulation, and enhance tolerance to arsenic stress [69]. Furthermore, Se-enriched potatoes increased soluble sugars (fructose, glucose, and sucrose) by enhancing the activities of neutral invertase (NI), sucrose synthase (SS), and sucrose phosphate synthase (SSP) in the carbon metabolism pathway and reduced Cd and As contents in plant tissues to alleviate the toxicity of Cd and As [70]. Compared to the control group, there were significant differences in the expression of sugar metabolism-related genes in peas cultivated with Se nanoparticles, specifically upregulating genes such as hexokinase (HK) to promote the synthesis of soluble sugars in sprouts [71] (Figure 2).
The primary forms of nitrogen absorbed by plant roots are nitrate (NO3−) and ammonium (NH4+). During nitrogen reduction and assimilation, nitrate reductase (NR) is a key enzyme for plant nitrogen uptake, as it catalyzes the conversion of NO3− to nitrite. The GS/GOGAT cycle is the primary pathway for plant nitrogen assimilation, where glutamine synthetase (GS) is a crucial enzyme for nitrogen absorption. GS catalyzes the ATP-mediated combination of glutamate and ammonia to produce glutamine, which is then converted back to glutamate by NADH-dependent glutamate synthase (GOGAT) [72]. Se intake reduces the plant toxicity of arsenic (As) and significantly increases the activity of nitrogen metabolism enzymes (NR, GOGAT, and GS) in rice, thereby mitigating the negative effects of As on nitrogen metabolism. This alleviating effect of Se also enhances chlorophyll content and photosynthetic efficiency in rice [73]. Various exogenous Se treatments not only increase the activity of nitrate reductase in legumes such as peanut, soybean, and cowpea, promoting the efficiency of nitrogen assimilation to increase amino acid and protein content in plant leaves, but also promote the biosynthesis of daidzein and genistein in roots [74,75,76]. In hydroponic studies of lettuce, Se application in the form of selenate significantly increased NR and GOGAT activity, positively regulating nitrogen metabolism [77]. Similar observations were made in wheat grains, where Se promoted nitrogen metabolism and increased total nitrogen content, thereby enhancing plant biomass [78]. Under drought conditions, Se can improve drought tolerance in maize by enhancing nitrogen metabolism efficiency [79].

4.2. Effect of Se on Biosynthesis Pathway of Secondary Metabolism in Plants

4.2.1. Direct Effect of Se on Key Enzymes

Within plant cells, Se directly or indirectly participates in the regulation of secondary metabolite biosynthesis through key enzymes and transcription factors. Precursor substances within the plant undergo a series of complex secondary metabolic pathways to convert into biologically active secondary metabolites. The phenylpropanoid metabolic pathway serves as the initiating step for flavonoid synthesis. Studies have demonstrated that Se can promote the expression of the phenylalanine ammonia lyase (PAL) gene, which in turn enhances the accumulation of flavonoids [80]. Treatment of G. biloba with inorganic Se has an impact on the genes involved in flavonoid biosynthesis in Ginkgo leaves [81]. Compared to the control group, the transcription levels of CHS, FOMT, FLS, and PAL were significantly increased in the Se-treated group. In lettuce plants treated with low concentrations of selenite, an increase in the expression of the UFGT and F3H genes involved in anthocyanin metabolism was observed, leading to an enhanced biosynthesis of this pigment [44].
HMGR plays a crucial role in the biosynthesis and metabolism of terpenoids in plants, serving as the first rate-limiting enzyme in the mevalonate pathway [82]. TCS1 catalyzes the N-3 and N-1 methylation steps and is the primary enzyme involved in caffeine biosynthesis in tea plants [83]. Se treatment can upregulate the relative expression levels of genes (CsHMGR and CsTCS1) in tea leaves, thereby accelerating the biosynthesis of caffeine and terpenoids [84]. Appropriate concentrations of Se applied through foliar spraying significantly increase the content of ginkgolides in G. biloba leaves. Transcriptome data analysis reveals that the expression levels of genes such as DXS, DXR, and HMGR are upregulated after Se treatment, and these three genes encode key enzymes in the terpenoid biosynthetic pathway [85]. Under treatment with high Se concentrations of 24 μM, the photosynthesis and normal growth of apple plants are inhibited. Concurrently, the phenylpropanoid biosynthetic pathway is activated accordingly, leading to a significant upregulation of genes related to the enzymes CYP73A, HCT, and CYP98A, which results in the accumulation of more flavonoids and antioxidants, thereby reducing cellular membrane damage. However, secondary metabolism consumes a substantial amount of energy and carbon skeletons, inhibiting plant growth [86].
BCAT (branched-chain amino acid aminotransferase) and MAM (malonate semialdehyde dehydrogenase) play pivotal roles in the side-chain elongation process during the regulation and accumulation of GSLs in plants [87]. Studies have confirmed that Se treatment can significantly downregulate the expression of BCAT4 and MAM1 in the glucosinolate biosynthetic pathway of broccoli [88]. Following the completion of main-chain elongation, the synthesis of the glucosinolate core structure occurs, primarily involving key enzymes such as those in the cytochrome P450 family (CYP79 and CYP83) and UGT74B1 (UDP-glucuronosyltransferase). In a study of radish plants, it was found that the application of a Se concentration of 40 µM downregulated the expression of UGT74B1 in leaves while upregulating the expression of the gene Sultr2;1 [47]. Other studies have demonstrated that the reduction in carotenoid biosynthesis expression in Arabidopsis under Se treatment is achieved through the downregulation of phytoene synthase, an enzyme involved in the initial stage of the carotenoid biosynthetic pathway [89].

4.2.2. Transcriptional Regulation of Se on Secondary Metabolic Pathways

MYB transcription factors (TFs) participate in the regulation of plant flavonoid biosynthetic pathways, encompassing both transcriptional activators and repressors, which directly or indirectly modulate the expression of structural genes [90]. Elevated expression levels of MYB1 and MYB2 were observed in studies on Se foliar application in G. biloba leaves, resulting in increased expression levels of flavonoid biosynthetic genes such as PAL, CHS, FLS, and FOMT. MYB1 and MYB2 are implicated in the Se-induced accumulation of flavonoid compounds [91]. The bHLH TFs family represents the second largest transcription factor family in plants and is involved in various developmental and growth processes. Notably, it plays a pivotal role in the synthesis and metabolic pathways of plant secondary compounds [92]. For instance, when different concentrations of Na2SeO4 were applied to aloe vera leaves, Se was found to induce the accumulation of antioxidant-related metabolites, including phenolics, flavonoids, and terpene compounds. Further transcriptome analysis revealed that treatment with 400 mg·L-1 selenate significantly upregulated the expression levels of MYB, bHLH, GATA, and IBH1 in aloe vera plants compared to low-concentration Se treatments [93].
CYP79F1, CYP83A1, UGT74B1, and ST5b are key genes regulating the synthesis of GSLs, and their expression is further regulated by MYB28 [94,95]. Under high-temperature stress, Se–sulfur interaction treatments significantly increased the transcription levels of MYB28, UGT74B1, and ST5b in broccoli sprouts, leading to an increase in sulforaphane content [96]. However, other studies have reported less optimistic results, where exogenous Se application downregulated the expression of aliphatic GSL synthase-related genes (CYP79F1 and CYP83A1) but upregulated indolic GSL synthase-related genes (CYP79B2 and CYP83B1). Notably, the transcription level of MYB28 did not change significantly during this process. The upregulation of enzyme genes encoding indolic GSL biosynthesis and the downregulation of genes involved in aliphatic GSL biosynthesis may represent a mechanism to protect plant GSL replenishment while restricting the production of Se-GSLs [97]. Exogenous selenate treatment reduced the accumulation of total GSLs in broccoli, and further studies confirmed that Se not only decreased the levels of GSL precursors methionine and phenylalanine but also inhibited the expression of transcription factors MYB28 and MYB34 involved in the GSL synthesis pathway [88]. These studies indicate that Se exhibits the differential regulation of secondary metabolic pathways in different plants, potentially related to Se speciation and concentration. Se directly affects the synthesis of secondary metabolites by regulating the expression of key transcription factors and metabolic enzymes involved in secondary metabolism biosynthesis, thereby enabling plants to adapt to various growth environments. Notably, there are differences in the regulation of secondary metabolic genes among different species, meaning that certain genes may be upregulated in response to Se induction in some species but downregulated or non-responsive in others. This variability highlights the diversity and complexity of plant adaptation processes (Figure 3).

4.3. Se and Plant Hormones Regulate Secondary Metabolites

Plant hormones, also known as plant natural hormones or plant endogenous hormones, are trace organic compounds produced within plants that regulate (promote or inhibit) their physiological processes. There are six major classes of plant hormones that have been identified, namely auxins, gibberellins, cytokinins, abscisic acid, ethylene, and brassinosteroids. These hormones play crucial roles in the growth, development, and adaptation of plants to environmental conditions. Research has shown that Se exhibits a close regulatory relationship with plant hormones in terms of plant secondary metabolism. Se can influence the synthesis and accumulation of secondary metabolites by mediating the transduction and regulation of plant hormone signals.
A study conducted by Xu et al. found that treatment with gibberellic (GA) at a concentration of 300 mg/L could enhance the biomass and carotenoid content of Cyphomandra betacea under Se stress conditions, thereby promoting its growth. Correlation analysis indicated that under GA treatment, the absorption of Se by C. betacea was closely correlated with the contents of carotenoids, chlorophyll a, and chlorophyll b [98]. Treatment with methyl jasmonate (MeJA) can enhance the antioxidant defense system of Plantago asiatica and upregulate the expression levels of key enzymes in the phenylpropanoid pathway, PAL and CHI. This leads to increased contents of phenolic acids and luteoloside, thereby enhancing the plant’s tolerance to Se [99]. In another study, the application of MeJA alone was found to elevate the content of indolic GSLs in Brassica oleracea var. italica. However, under conditions of high Se exposure, the MeJA-mediated synthesis of GSLs was significantly inhibited [100]. In studies on the resistance of cucumbers to Botrytis cinerea, it was found that nano-Se can activate the MeJA biosynthesis pathway and promote the accumulation of phenolic acids and cucurbitacins, thereby enhancing the plant’s resistance to B. cinerea [101]. Zaji et al. used greenhouse pot cultivation experiments to study the effects of 24-epibrassinolide (EBL) at 0.5 μM and Se (10 μM) combined with foliar spraying on the volatile oil content of Dracocephalum moldavica. The application of EBL and Se promoted the accumulation of EOs in the plant and changed its chemical composition, with oxygen-containing monoterpenes accounting for the highest proportion [102]. Research has found that the key genes in the jasmonic acid signaling pathway of tea plants, the allene cyclase oxidase gene (CsAOC), and the lipoxygenase gene (CsLOX6) are induced by high concentrations of Se [103].

4.4. Relationship Between Se-Induced Oxidative Stress and Secondary Metabolites

Plants counteract the adverse effects of high Se levels by triggering antioxidant defense mechanisms, including the biosynthesis of secondary metabolites such as phenolics, to maintain redox homeostasis [104]. Various studies have demonstrated that moderate amounts of Se can significantly enhance the activity of multiple antioxidant enzymes in plants, thereby improving their tolerance to stress, with differential effects observed among different plant species, Se forms, and stress conditions. In tomatoes, Se treatment enhances the activity of catalase (CAT), ascorbate peroxidase (APX), and GSH under cadmium stress, with similar phenomena observed under drought conditions. Additionally, the activities of the non-enzymatic antioxidants ascorbic acid (ASA) and α-tocopherol increase significantly [105,106]. In another study, low concentrations of Se treatment significantly increased glutathione peroxidase (GSH-Px) activity in tea leaves. The high activity of GSH-Px contributes to maintaining cellular homeostasis, thereby influencing the accumulation of polyphenols and polysaccharides [107]. High concentrations of Se treatment result in a significant increase in glutathione content in Astragalus membranaceus cells while stimulating the production of flavonoids, phenolic acids, and saponins to collectively activate cellular defense mechanisms and alleviate oxidative stress [108]. Subsequent studies found that high Se concentrations increased alkaloid content in A. membranaceus cells, reducing plant lifespan and biomass [109]. Growth at the highest Se concentration significantly increased total phenolic content in coriander and tatsoi by 21% and 95%, respectively [110]. Studies on germinating rice and wheat revealed that high-dose Se treatment promoted phenylpropanoid biosynthesis [111,112]. Research has confirmed that selenite treatment increases PAL activity and enhances phenolic compound content, such as isoflavones, in chickpeas [42,104]. In response to oxidative stress caused by high levels of Se, plants often activate the phenylpropanoid pathway to produce phenolic compounds, greatly enhancing their ability to scavenge ROS [80].

5. Conclusions and Prospects

Through extensive research on selenium biofortification in plants, we have determined that selenium exerts a bidirectional regulatory influence on plant secondary metabolites. The mechanisms involved are both complex and diverse; selenium enhances the plant’s antioxidant system, directly affects the activity of key enzymes involved in secondary metabolism, and indirectly influences the production and accumulation of secondary metabolites by modulating signaling pathways and gene expression in plants. This regulatory capacity contributes to improved stress resilience and enhances both the nutritional and medicinal value of plants. However, excessive selenium can inhibit plant growth and lead to oxidative stress-induced damage. Consequently, the precise calibration of selenium dosage in practical applications is crucial to maximize its positive effects while minimizing potential toxic impacts. Although progress has been made in elucidating the mechanisms by which selenium regulates common secondary metabolic pathways in plants, significant interspecies differences in the regulatory capacity of secondary metabolism in response to selenium stress have been observed. The specific mechanisms responsible for these differences require further investigation.
According to the existing literature, selenium biofortification at appropriate concentrations can enhance the biosynthesis and accumulation of secondary metabolites in most plants. However, in the context of medicinal plants used in clinical applications and product development, it remains unclear whether the accumulation of selenium exhibits synergistic effects with the bioactive components of traditional Chinese medicine. Moreover, the impact of this potential synergy on various medicinal components and disease control, whether positive or negative, lacks systematic and scientific evaluation. Therefore, there is a need to further strengthen research in this area.
Future research priorities in this domain encompass several key aspects. Firstly, it is essential to further investigate the mechanisms by which selenium precisely regulates plant secondary metabolites. This includes examining its specific effects on the activity of key transcription factors and enzymes involved in secondary metabolism, as well as its role in plant hormone regulation. Secondly, the interactions between selenium and other elements and their impact on the synthesis and accumulation of secondary metabolites also warrant thorough exploration. Lastly, exploring strategies to effectively utilize selenium biofortification for regulating the synthesis of secondary metabolites is crucial for enhancing crop quality and yield. In addressing the variability in selenium-enrichment mechanisms across different plants, it is possible to develop a range of targeted selenium–nutrient formulations that can effectively increase selenium content and bioavailability in plants or cultivate crop varieties with varying levels of selenium enrichment to align with diverse agricultural practices.

Author Contributions

Investigation, writing—original draft preparation, Y.Z.; project administration, conceptualization, resources, L.L.; software, validation, L.G., Y.W., and K.N.; data curation, K.N.; supervision, writing—review and editing, funding acquisition, H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Discipline Construction Project of Wuhan Polytechnic University, grant number 315-01003009.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors extend their gratitude to Enshi Se-De Biotechnology Co., Ltd. for their financial support in the realm of this research endeavor.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Selenium cycling processes in plants. An enlarged view of the root structure shows that the major transport proteins for selenium uptake in plants (selenate transporter proteins Sultr1;2, selenite transporter proteins NIP2;1 and PHT2, and amino acid permeases AA Tr.) are located in the root peridermis. Selenite is the main form of selenium transported through the xylem to the leaves, where it is ultimately converted to selenoamino acids. Selenate is transported into the xylem via SULTR2;1 in the parenchyma cells of the xylem and pericycle, whereas the organic selenium compounds produced in the leaf cells, mainly selenoamino acids, are further transported by amino acid permease (AAP) through the xylem to the leaf and through the phloem and thus to the whole plant. Eventually, the volatile forms DMSe and DMDSe generated in the leaves are released to the atmosphere. Solid arrows represent confirmed catalytic steps, while dashed arrows represent omitted multi-step reactions or transport.
Figure 1. Selenium cycling processes in plants. An enlarged view of the root structure shows that the major transport proteins for selenium uptake in plants (selenate transporter proteins Sultr1;2, selenite transporter proteins NIP2;1 and PHT2, and amino acid permeases AA Tr.) are located in the root peridermis. Selenite is the main form of selenium transported through the xylem to the leaves, where it is ultimately converted to selenoamino acids. Selenate is transported into the xylem via SULTR2;1 in the parenchyma cells of the xylem and pericycle, whereas the organic selenium compounds produced in the leaf cells, mainly selenoamino acids, are further transported by amino acid permease (AAP) through the xylem to the leaf and through the phloem and thus to the whole plant. Eventually, the volatile forms DMSe and DMDSe generated in the leaves are released to the atmosphere. Solid arrows represent confirmed catalytic steps, while dashed arrows represent omitted multi-step reactions or transport.
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Figure 2. Possible factors influencing exogenous Se on secondary metabolic pathways in plants. By utilizing Se biofortification, it is possible to promote photosynthetic efficiency and carry out related primary metabolic pathways such as carbon metabolism, which in turn regulates secondary metabolic pathways in plants by influencing the transcriptional expression of key enzymes, transcription factors, or plant hormone signal transduction.
Figure 2. Possible factors influencing exogenous Se on secondary metabolic pathways in plants. By utilizing Se biofortification, it is possible to promote photosynthetic efficiency and carry out related primary metabolic pathways such as carbon metabolism, which in turn regulates secondary metabolic pathways in plants by influencing the transcriptional expression of key enzymes, transcription factors, or plant hormone signal transduction.
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Figure 3. Mechanisms of Se regulation on plant secondary metabolic pathways. The red font represents the enzymes or transcription factors associated with Se on plant secondary metabolic pathways at those sites of action mentioned in the above study. Circles represent enzymes associated with different secondary metabolic pathways in plants and squares represent transcription factors. The green dashed arrow indicated induction of gene expression, while the red dashed line plus a short horizontal line indicated inhibition of gene expression. The red solid circle represents hexavalent selenium, and the blue solid circle represents tetravalent selenium. Information was derived from previous reports. (A) Terpenoid biosynthesis; (B) phenolic biosynthesis; (C) glucosinolates biosynthesis.
Figure 3. Mechanisms of Se regulation on plant secondary metabolic pathways. The red font represents the enzymes or transcription factors associated with Se on plant secondary metabolic pathways at those sites of action mentioned in the above study. Circles represent enzymes associated with different secondary metabolic pathways in plants and squares represent transcription factors. The green dashed arrow indicated induction of gene expression, while the red dashed line plus a short horizontal line indicated inhibition of gene expression. The red solid circle represents hexavalent selenium, and the blue solid circle represents tetravalent selenium. Information was derived from previous reports. (A) Terpenoid biosynthesis; (B) phenolic biosynthesis; (C) glucosinolates biosynthesis.
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Table 1. Application of Se in biotic and abiotic stresses.
Table 1. Application of Se in biotic and abiotic stresses.
PlantSe ApplicationMajor EffectReference
Sugarcane
(Saccharum spp. hybrids)		Nano-SeSe increased the antioxidant and jasmonic acid content and reduced the accumulation of ROS induced by Xanthomonas albilineans infection, thus enhancing its quality.[4]
Common Bean
(Phaseolus vulgaris L.)		Nano-Se and nano-SiThe combined application of selenium and silicon nanoparticles showed remarkable results in suppressing plant pathogens.[5]
Rice
(Oryza sativa L.)		Se bio-nanocompositeSe reduced cadmium levels in rice and mitigated damage caused by cadmium-induced oxidative stress.[6]
Cotton
(Gossypium hirsutum L.)		Foliar Se applicationSe reduced oxidative damage from heat stress by reducing the accumulation of reactive oxygen species in cotton.[7]
Bitter MelonChitosan–selenium nanoparticleSe mitigated oxidative stress damage by enhancing antioxidant enzyme activity.[8]
Glycine max L.Na2SeO4 and H3BO3The application of Se and B in combination effectively enhanced the antioxidant defense system of plants and attenuated the oxidative damage induced by salt stress.[9]
Table 3. Effect of Se on the accumulation of terpenoids in plants.
Table 3. Effect of Se on the accumulation of terpenoids in plants.
PlantSe ApplicationSecondary Metabolites
(Content)		Overall Changes
(Compared to Control)		Reference
Zea mays L. grainssoil application
Na2SeO3		xanthophyll↑[51]
zeaxanthin↑
Medicago sativa L.foliar application
NPs-Se		carotenoids↑[52]
Oryza sativa L.hydroponics
Na2SeO3		carotenoids↑[53]
total phenolic compound↑
Ginkgo bilobasoil/foliar application
Na2SeO3		total terpene lactone↑ (foliar applications)[54]
↓ (soil applications)
Lycium chinense L.nutrient solution
Na2SeO3		carotenoids↑[50]
Salvia officinalis L.soil irrigationα-thujone↑[55]
β- thujone
camphor
Arabidopsis shoothydroponics
Na2SeO4		xanthophyll↓[56]
The arrows ↑ and ↓ show increases and decreases in the content of secondary metabolites, respectively.
Table 4. Effect of Se on the accumulation of alkaloids in plants.
Table 4. Effect of Se on the accumulation of alkaloids in plants.
PlantSe ApplicationSecondary Metabolites (Content)Overall Changes (Compared to Control)Reference
Iranian Boragefoliar application
Na2SeO4 and Na2SeO3		total alkaloids↑[57]
Onionfoliar application
Na2SeO4		choline↑[58]
betaine↑ (leaves); ↓ (bulbs)
Sophora tonkinensisnutrient solution
Se amino acids		matrine↑ (the whole plant)[59]
oxymatrine
Fragaria × ananassahydroponics
Na2SeO4		gramine↑ (100 µM)[60]
Lupinus albus L.foliar application
Na2SeO3		total alkaloids↓[61]
The arrows ↑ and ↓ show increases and decreases in the content of secondary metabolites, respectively.
Table 5. Effect of Se on the accumulation of other secondary metabolites in plants.
Table 5. Effect of Se on the accumulation of other secondary metabolites in plants.
PlantSe ApplicationSecondary Metabolites
(Content)		Overall Changes
(Compared to Control)		Reference
Brassica oleracea var. capitata L.soil application
Se yeast		total glucosinolate↑[63]
Broccoli sprouts
(FL60, WX90, SL120)		spraying
Na2SeO4, Na2SeO3		total glucosinolate↑ (Na2SeO4: WX90)[64]
sulforaphaneNS (FL60, SL120)
↑ (SL120: Na2SeO4)
↑ (FL60, WX90: Na2SeO3)
Brassica oleracea L. var. italicasoil application
Se yeast/Na2SeO3		total glucosinolate↑[65]
Chinese cabbage sproutsspraying
ZnSO4/Na2SeO3/mixture		total glucosinolate↑[66]
Radishfoliar application/hydroponics
Na2SeO4		total glucosinolate↑ (leaves, 5 mg/plant; roots, 20 mg/plant)[47]
↓ (leaves, 40 µM)
Eruca Sativa Mill. and Diplotaxis Tenuifoliahydroponics
Na2SeO4		total glucosinolate↓[67]
glucoraphanin↓
glucoerucin↓ (Eruca Sativa Mill.)
dimeric-4-mercaptobutyl glucosinolate (DMB-GLS)↓ (Eruca Sativa Mill.)
The arrows ↑ and ↓ show increases and decreases in the content of secondary metabolites, respectively, while NS indicates no significant changes.
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Zhou, Y.; Nie, K.; Geng, L.; Wang, Y.; Li, L.; Cheng, H. Selenium’s Role in Plant Secondary Metabolism: Regulation and Mechanistic Insights. Agronomy 2025, 15, 54. https://doi.org/10.3390/agronomy15010054

AMA Style

Zhou Y, Nie K, Geng L, Wang Y, Li L, Cheng H. Selenium’s Role in Plant Secondary Metabolism: Regulation and Mechanistic Insights. Agronomy. 2025; 15(1):54. https://doi.org/10.3390/agronomy15010054

Chicago/Turabian Style

Zhou, Yan, Kaiqin Nie, Lulu Geng, Yixin Wang, Linling Li, and Hua Cheng. 2025. "Selenium’s Role in Plant Secondary Metabolism: Regulation and Mechanistic Insights" Agronomy 15, no. 1: 54. https://doi.org/10.3390/agronomy15010054

APA Style

Zhou, Y., Nie, K., Geng, L., Wang, Y., Li, L., & Cheng, H. (2025). Selenium’s Role in Plant Secondary Metabolism: Regulation and Mechanistic Insights. Agronomy, 15(1), 54. https://doi.org/10.3390/agronomy15010054

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