Cadmium Tolerance in Tea Plants (Camellia sinensis): Physiological, Biochemical, and Molecular Insights

Abstract

Cadmium (Cd), a toxic and mobile heavy metal, poses significant risks to agricultural systems due to industrial pollution. Tea plants (Camellia sinensis L.) efficiently absorb and accumulate Cd from soil, leading to contamination in leaves. Chronic consumption of Cd-laden tea can cause severe health issues, including neurological, reproductive, and immunological disorders, as well as increased cancer risk. Despite growing concerns, the molecular mechanisms of Cd stress response in tea plants remain poorly understood. Current research highlights key physiological adaptations, including activation of antioxidant defenses and modulation of secondary metabolite pathways, which influence tea quality. Cd disrupts photosynthesis, induces oxidative stress, and alters the biosynthesis of flavor-related compounds. Several critical genes involved in Cd transport (e.g., CsNRAMP5, CsHMA3, CsZIP1), sequestration (CsPCS1), and stress regulation (CsMYB73, CsWRKY53, CsbHLH001) have been identified, offering insights into molecular responses. This review systematically examines Cd dynamics in the soil-tea plant system, its effects on growth, photosynthesis, and quality, and the physiological and biochemical mechanisms underlying Cd tolerance. By consolidating recent findings on Cd-responsive genes and regulatory pathways, this study provides a theoretical foundation for breeding Cd-resistant tea varieties and ensuring production safety. Furthermore, it identifies future research directions, emphasizing the need for deeper mechanistic insights and practical mitigation strategies. These advancements will contribute to safer tea consumption and sustainable cultivation practices in Cd-contaminated regions.

1. Introduction

Tea, the world’s second-largest beverage after water, plays a pivotal role in global agriculture and trade. In 2023, tea production reached 6.5 million tons, cultivated across 4.3 million hectares [1]. China is the world’s largest producer, contributing 2.9 million tons (45% of the global total) from its 2.4 million hectares of tea plantations [2]. India follows as the second producer (1.3 million tons, 20%), followed by Kenya (0.5 million tons), Sri Lanka (0.3 million tons), and Vietnam (0.18 million tons) [3]. Tea production is globally diversified, with distinct regional specializations. Green tea accounts for 20% of global production, with China contributing 70% of global output. Black tea accounts for 60%, mainly produced in India, Kenya, and Sri Lanka. Oolong tea is mainly produced in China’s Fujian and Taiwan. White and yellow teas are almost exclusively produced in China. Pu’erh tea is exclusive to Yunnan, China. Global tea consumption exceeds 6 million tons annually, with China (2.1 million tons) and India (1.1 million tons) as the largest consumer markets, and the European Union, the United States, and the Middle East being the central import regions [4]. Kenya ranks as the world’s largest tea exporter with an annual export of 0.5 million tons, contributing to a global tea market valued at over 50 billion U.S. dollars [5] (

Table 1. Global tea production and types.

Category	Aspect	Details	Ref.
Global Tea Production & Consumption	Global Production (2023)	6.5 million tons cultivated across 4.3 million hectares	[1]
	Largest Producer	China—2.9 million tons (45%) from 2.4 million ha	[2]
	Second Producer	India—1.3 million tons (20%)	[3]
	Other Producers	Kenya (0.5 Mt), Sri Lanka (0.3 Mt), Vietnam (0.18 Mt)	[3]
	Tea Types & Distribution	Green tea = 20% (China supplies 70% of this); Black tea = 60% (India, Kenya, Sri Lanka); Oolong tea = Fujian & Taiwan; White & Yellow = China; Pu’erh = Yunnan, China
	Consumption	China—2.1 Mt, India—1.1 Mt, the EU, USA & Middle East are the leading importers	[4]
	Largest Exporter	Kenya—0.5 Mt annually	[5]
	Market Value	>50 billion USD	[5]
	Industry Shift	Growing demand for organic & specialty teas; adoption of sustainable cultivation, high-yield cultivars, automation	[6]

).

The global tea industry is experiencing a paradigm shift driven by growing consumer demand for organic and specialty teas [6]. In response, major producing nations, including China, India, and Kenya, are implementing sustainable cultivation practices while adopting innovative technologies such as high-yield cultivars and automated processing systems to enhance productivity while reducing environmental impacts. Industrialization increased heavy metal pollution globally, and the presence of toxic metals in food and plants has become a serious issue worldwide [7]. Cd contamination in agricultural soils mainly caused by intensive farming practices, industrial operations, and mining activities, etc., has emerged as a critical public health concern [6]. Other sources of Cd contamination include pesticides, manure, and fertilizers, which introduce Cd into soils through over-application [2]. For higher organisms in the food chain, Cd-contaminated crops serve as the primary exposure route. Even trace amounts are hazardous for the human body, as evidenced by the European Food Safety Authority’s (EFSA) establishment of the Tolerable Weekly Intake (TWI) of 2.5 μg per kilogram of body weight [8]. While natural weathering processes contribute to environmental Cd fluxes, anthropogenic sources dominate current loading. So the use of Cd in plastics and jewelry, etc, was banned in European countries in 2011 (http://europa.eu/rapid/press-release_IP-11-620_en.htm, accessed on 10 August 2025).

Cd in soil has high mobility in the soil system and plant system, and therefore plant roots easily absorb it from the soil [9]. The lower amount of Cd with regular consumption can affect the immune, nervous, genital system, urinary, and specific types of cancers [10]. The most important cereal crops, like rice and wheat, were reported [9] with high Cd concentrations in their grains. The increase in soil Cd concentration is a significant threat to agricultural production as well as to human health concerns [11]; the predicted world’s population in 2050 will be closer to ten billion people. Cd exposure has been reported to be a significant health risk to humans. Kidneys accumulate Cd, leading to renal dysfunction (carcinogen) [12]. The regular and continuous accumulation may cause bone decalcification (Osteoporosis) [13]. Other serious health issues have also been reported, like effects on the respiratory system, hypertension, and heart disease [14]. Primarily, Cd comes into the soil from various natural sources, or it can be anthropogenic. Zinc, copper, and lead release Cd during the mining and refining of these metals, which contaminate the soil. As an impurity, the various phosphate fertilizers contain Cd, which also accumulates in agricultural soils. Industrial emissions often emit Cd-contaminated dust and waste, which lead to soil contamination [10].

Various studies reported the different methods for plant Cd uptake and transport reduction with soil pH regulation [15], water management, and low Cd accumulation accessions/varieties [16]. The breakdown of chemical compounds into ions forms due to acidic soil pH and penetrates plant cells easily. In acidification and oxidizing conditions, metals relatively show higher mobility, while in primary and reducing conditions, the metals’ mobility reduces [17]. It has been reported that the Cd bioavailability changes with pH concentration; at pH 2, the Cd concentration was much higher than at pH 7, where the concentration was very low [18]. It is also investigated that the contaminated soil with Cd treated with other soil alteration can influence the soil pH, and the Cd bioavailability was changed significantly [11]. These studies show that the liming process reduces the bioavailability and toxicity of Cd-contaminated soil. In combination with other treatments, bioavailability can be altered with different concentrations, and the pH can also be changed with different treatments. Moreover, for the reduction in soil Cd concentration, the soil amendments were an easy and inexpensive way to apply for Cd control in soil. The other way to reduce the Cd level in plants is the use of nanoparticles like nano-selenium (SeNPs), Silica (SiO₂NP), Zinc oxide, and manganese dioxide, etc. [10].

Cadmium (Cd) tolerance in Camellia sinensis exhibits several physiological and molecular distinctions from those observed in model plants such as Arabidopsis thaliana, rice (Oryza sativa), and tomato (Solanum lycopersicum) [9]. In tea, Cd accumulation is primarily restricted to root tissues, with limited translocation to aerial parts, a pattern contrasting with the higher Cd mobility seen in Arabidopsis and rice. This is achieved through enhanced apoplastic binding of Cd to cell wall components, such as pectins and hemicelluloses, and elevated expression of metal transporters that favor vacuolar sequestration over long-distance translocation. For instance, CsHMA3, a homolog of AtHMA3 in Arabidopsis, is strongly upregulated under Cd stress and facilitates Cd compartmentalization into root vacuoles, thereby preventing its movement to the shoots. Similarly, CsMTP1 and CsABCC2 contribute to vacuolar metal sequestration and detoxification, reinforcing the roots buffering capacity against Cd toxicity [1,2]. The model species, such as rice and Arabidopsis, exhibit more systemic Cd movement. OsHMA2 and AtHMA4 are key transporters mediating Cd efflux from root cells into the xylem, promoting translocation to shoots. Rice also employs OsNRAMP5 for Cd uptake and OsHMA3 for sequestration, but the efficiency of these processes varies with cultivar and genetic background. Tomato plants, although less extensively studied for Cd tolerance, display Cd translocation patterns intermediate between those of tea and rice, with moderate accumulation in leaves and fruits [5]. The unique expression patterns in tea suggest that selective suppression of xylem loading transporters and upregulation of vacuolar sequestration genes are adaptive traits that minimize Cd accumulation in young leaves and buds, the economically important parts of the plant.

Comparative genomic and transcriptomic analyses further reveal lineage-specific expansion of several metal transporter gene families in tea, particularly the ZIP (ZRT/IRT-like Protein), HMA (Heavy Metal ATPase), and ABC (ATP-binding cassette) transporters [8]. The CsZIP gene family has undergone significant duplication events, resulting in multiple paralogs with divergent expression profiles under Cd stress. Some CsZIP members are induced predominantly in roots, suggesting specialization for low-affinity Cd uptake or Zn/Cd discrimination [15]. Similarly, the expansion of CsHMA genes, particularly CsHMA2, CsHMA3, and CsHMA5, points toward functional diversification in Cd sequestration and intercellular transport. ABC transporters such as CsABCC1, CsABCC2, and CsABCC3 also exhibit lineage-specific diversification and are implicated in vacuolar sequestration of Cd-phytochelatin complexes, enhancing tolerance. Notably, these expansions are more pronounced in tea than in Arabidopsis or rice, reflecting possible adaptive evolution to Cd-rich or acidic soils typical of tea-growing regions [12]. Recent evidence indicates that Camellia sinensis exhibits several species-specific responses to cadmium (Cd) stress that differ from many other crop plants, underscoring the need for a tea-focused synthesis. Tea’s distinctive metabolic profile, characterized by high levels of catechins, theanine, caffeine, and diverse polyphenols, leads to unique transcriptomic, proteomic, and metabolomic adjustments when exposed to Cd. Studies have revealed tea-specific regulatory networks involving metal transporters, antioxidative enzymes, and secondary-metabolic pathways that modulate Cd uptake, compartmentalization, and detoxification [17]. Moreover, Cd contamination poses a significant concern for tea-producing regions due to soil acidity, long-term fertilization patterns, and high leaf-harvesting frequency, which can influence Cd accumulation in marketable leaves. Despite these factors, no existing review has systematically integrated the multi-omics findings exclusive to C. sinensis. Therefore, a dedicated review is essential to consolidate current knowledge and highlight the distinct physiological and molecular mechanisms underlying Cd stress responses in tea.

2. Cadmium Sources and Uptake in Tea Ecosystems

Cadmium (Cd) is a Group IIB (Group 12) transitional metal with an ionic radius similar to that of Ca²⁺ and Mg²⁺ ions [19]. Accordingly, it can be absorbed via Ca²⁺ and Mg²⁺ transporters and channels [20]. These include the CorA-MRS2-ALR family of magnesium (Mg²⁺) and AtCNGC (Cyclic Nucleotide-Gated Channel) transport systems on the plasma membrane, where over 85 metal transporters are localized [21]. Cd is indeed absorbed by plants relatively easily because of its high biochemical activity. Indeed, most of these transporters do facilitate Cd uptake in plants. In recent decades, more attention has been paid to Cd because of the rising Cd pollution in agricultural lands. Most harmful are crop plants that accumulate high concentrations of heavy metals in their edible parts [22]. Cd is actively taken up by plants and transported in the plant body via various transport processes, which typically involve the plant’s expenditure of energy [23]. However, a small amount of the metal could also enter the plant body in the absence of energy expenditure. Cd ions at low concentrations present in soil solutions and irrigation water can enter the interior of a plant through this pathway. This free entry of metal for the plant is called diffusion-based or non-specific uptake. A more significant part of the Cd inside the plant is removed by transpiration and growth. This rate of removal depends on the soil solution and plant physiological properties [24].

Active uptake of ions involves the presence of transport proteins or ion channels in the root cell plasma membrane. It is believed that the Cd²⁺ ions that are absorbed by plant roots share the same transport pathways used for the uptake of essential divalent cations such as Fe²⁺, Zn²⁺, or Mn²⁺ [25]. When the concentrations of these crucial ions are high, they compete with Cd for active uptake. This indirect effect decreases the amount of this metal that is absorbed by plant roots. The main pathways for metallic ions to diffuse through the plant from roots to leaves are the vascular tissues of the xylem and phloem. When toxic elements, like Cd, are taken up and loaded into the xylem, there usually is a physical barrier across the endodermis, the Casparian strip, discouraging the free movement of pollutants between the root cortex and vascular bundles. The driving force presents the physical nature of how transpiration can bring Cd into the xylem sap and initiate the long-distance conferment of Cd, since this process largely depends on the environmental conditions that plants are in or the regulation of their physiology to a certain extent. Due to the significant variations in water flow patterns in the xylem, it remains difficult but also necessary to exactly predict Cd distribution in edible parts based on a mechanistic understanding of xylem and phloem transport [26].

Cadmium Accumulation in Tea Leaves

The accumulation of Cd in tea plants is a quick process [27]. The uptake of Cd in tea plants has two different pathways, namely, the symplastic route, which is passive, and the apoplastic route, which is an active transport [17]. Metal stress can disrupt membrane integrity, causing an inverse accumulation of Cd. Tea plants can adapt to the stress of Cd concentrations below a certain level by active transport to reduce cell accumulation, with the ability of antioxidant enzymes to respond significantly [28]. The accumulation of Cd in tea plants includes Cd in the root, branch, and leaf, with the content of Cd in the young leaf being the highest. The accumulation of Cd in roots is mainly in the form of citric acid and inorganic acid combined with acidic substances, while the Cd absorbed by tea plants is primarily in a free state. In the experiment of adding Cd to the nutrient solution, some elements in tea leaves show a clear trend of change with Cd concentration [29]. In contrast, young tea leaves have a more significant response to the addition of Cd. It can be seen that there are substantial differences in the Cd content in tea leaves of different varieties and cultivars. The Cd content in tea depends on the soil environment, variety, and processing technology, and the intake of tea also leads to different results [30].

The taste of tea changes significantly with the increase in Cd when the Cd concentration exceeds food safety levels. It will have direct sensory changes, subtly change the aroma and taste of tea, and affect the quality of the tea. The presence of a large number of non-biological nutrients, such as heavy metals, will interfere with the biochemical metabolic pathway in tea plants [10]. Cd can promote the absorption of nitrogen, phosphorus, and potassium, inhibit the absorption of calcium, magnesium, sulfur, and other essential elements, and affect the quality of the tea picked. The emergence of Cd has a more significant impact on tea plants, involving the key enzyme activity of flavonoid biosynthesis and photosynthetic capacity in tea leaves, reducing the accumulation of flavonoids, anthocyanins, and other substances, and reducing the aroma and quality of tea leaves [21].

3. Physiological and Biochemical Responses

Due to its accumulation in soils and harmful effects on soil microorganisms and plants, Cd pollution is a worldwide environmental problem, especially in agricultural ecosystems [31]. Once absorbed, Cd is phloem-mobile in most plant species, can be accumulated in edible portions, and is difficult to metabolize, making it toxic [32]. Tolerant plants do not change their access to pollutant accumulation. Still, they do reduce the adverse effects of heavy metals on plant growth through some physiological and other strategies in their systems. Cd shows phytotoxicity even at low concentrations (Figure 1). When plants absorb Cd, they exercise chelation, complexation, and precipitation by cell wall and vacuolar-bound substances to restrict the free metal pool, prevent Cd translocation to the edible part, reduce its toxicity in plants, and enhance food safety levels. The major Cd detoxification pathways include sequestration, chelation, and transformation. The significant proteins and enzymes involved in these toxicant detoxification pathways are transporters, proteins, members of the family, and major facilitator superfamily transporters [33].

Phytochelatins (PCs) are small, multifunctional peptides of the glutathione (GSH) metabolic pathway capable of metal-binding and antioxidant activities [15]. They are synthesized enzymatically from GSH within the cytosol in a reaction catalyzed by phytochelatin synthase, and they can also be catabolized. PCs are thought to be involved in vacuolar sequestration and detoxification of cationic heavy metals that gain cellular entry, in particular, in cysteine- and glutathione-rich plants such as Brassicaceae, Fabaceae, Poaceae, and many more. Synthesis of phytochelatins from Glu-Cys (PC2) is initiated by γGluCys dipeptidyl transpeptidation of GSH, followed by one to three condensation steps involving Cys and GSH [34]. PCs generally terminate in a γGluCys-Cys dipeptide, which has a thiol moiety capable of further reduction of metals. Chemical crosslinking for bioconjugation and chelation, the antioxidant activity of both a-dithiols present in the cysteine ligands and the amino acids, stimulated or slightly impaired Fenton activity, and lipid radical trapping were shown for PCs. Phytochelatins move metals from the site of the chelating reaction to the cellular vacuole, where detoxification in some tolerant opportunists is thought to occur. PC synthesis is regulated via the phytohormone ABA (Abscisic acid), via elevation of oxidative stress-responsive cell wall proteins, and phytoene synthase (PSY) progress is self-inhibited by the synthesized PCs. Genetic manipulation, co-overexpression, and comparison of GSH/GSH1, 2-PCs, extremely sensitive, and metal excluder suggest different GS-conjugate transport chains. The binding capacity of polyphenols and metal ions gives the tea plant a unique character of ‘preventing external accumulation’. Polyphenols can chelate and bind free metal ions inside the cells, and the existing heavy metal ions can already be precipitated and bound, thus reducing cytotoxicity [35].

The differences in gene expression between the Gushu and Longjing cultivars under Cd treatment showed that Cd treatment triggered a series of detoxification mechanisms in Gushu tea plants [36]. Longjing plants did not respond as much as Gushu plants under these conditions. Among the essential genes, iron and transcription factor bHLH9 may mediate the response of Gushu plants to Cd stress. The GLUT gene plays a vital role in Cd detoxification of tea plants, and the effect of this gene on Longjing plants is worth exploring further. The previous studies show that the Gushu and Longjing tea cultivars have a difference in leaf Cd detoxification. The Gushu cultivar accumulates more Cd in leaves than the Longjing cultivar, but it detoxifies Cd more effectively [37]. These results are consistent with the findings that suggested that the expression of OsHMA3, which prevents Cd from being transported into the grain, was lower in the low Cd rice cultivar. As a result, the reduced expression of OsHMA3 in the low Cd rice cultivar leads to an increase in Cd transportation and distribution in the shoot [38]. The concentrations of Cd in roots and in shoots between the two tea cultivars were not significantly different, suggesting that the accumulation capability was similar between the two cultivars and was not the main reason for the differences in Cd accumulation found in mature leaves [39].

Figure 1. Mechanisms of cadmium (Cd) uptake, transport, and detoxification in plants. Note: Cd enters roots via ZIP, NRAMP, and HMA transporters, then moves through the phloem. Detoxification involves cell wall binding, vacuolar sequestration, and chelation by phytochelatins (PCs) and glutathione (GSH). The Gushu tea cultivar shows higher Cd tolerance than Longjing due to upregulated genes (e.g., bHLH9, GLUT). Polyphenols further reduce Cd toxicity [31,37,39].

Figure 1. Mechanisms of cadmium (Cd) uptake, transport, and detoxification in plants. Note: Cd enters roots via ZIP, NRAMP, and HMA transporters, then moves through the phloem. Detoxification involves cell wall binding, vacuolar sequestration, and chelation by phytochelatins (PCs) and glutathione (GSH). The Gushu tea cultivar shows higher Cd tolerance than Longjing due to upregulated genes (e.g., bHLH9, GLUT). Polyphenols further reduce Cd toxicity [31,37,39].

ROS-Mediated Cd Toxicity

In particular, Cd-stressed tea plants produce excessive reactive oxygen species, including hydroperoxide, hydrogen peroxide, singlet oxygen, superoxide anion, and hydroxyl radical, which readily oxidize lipids, proteins, and nucleic acids [40]. Under stressed conditions, reactive oxides destroy the electron transport chain, thereby reducing the light-capture energy to inhibit plant growth and development (Figure 2). A moderate increase in hydrogen peroxide can efficiently destroy, synthesize, or transport the entire range of reactive oxygen species. Numerous genes related to reactive oxygen species can be overexpressed to increase plant non-enzymatic organic metabolic content. Furthermore, reactive oxygen species also act as signal molecules triggering the scavenging system, and numerous ROS-responding genes in plants encode peroxidase, ascorbate peroxidase, catalase, and superoxide dismutase to scavenge ROS [32]. In chloroplasts, excess accumulation of ROS can produce chlorophyll, destroy thylakoid membranes, and degrade the oxygen-evolving complex in photosystem II, possibly resulting in photo-oxidation in photosystem II. Chloroplasts are the primary sites of ROS production in photosynthetic eukaryotic cells. Mitochondria are the primary producers of ROS in cells because of the small amount of energy ATP produced by aerobic respiration. Peroxisomes can play a central role in organellar ROS metabolism. As essential regulators and sensors of cellular signals in plants, ROS regulate plant growth, development, and responses to the external environment. Increased peroxidation of the cellular membrane, a common stress injury in organisms, has been used as a marker of oxidative damage. Defense enzymes are responsible for the effective disposal of excess ROS. These enzymes include superoxide dismutase, ascorbate peroxidase, peroxidase, and catalase, and they detoxify ROS and ultimately produce water and oxygen [41].

Plants can generate unpaired electrons and unstable reactive oxygen species (ROS) as by-products of aerobic metabolism. The ROS consists of some forms, including superoxide anion radical, hydroxyl radical, hydrogen peroxide, and singlet oxygen [42]. Among them, hydrogen peroxide can penetrate cell membranes readily and generally exists in a larger quantity than the other forms, which is commonly identified to evaluate oxidative stress. Therefore, the plant’s ability to resist oxidative stress is closely related to the ROS detoxification process, including antioxidative enzyme systems and non-enzymatic scavengers. These ROS scavenging and signaling activities contribute to the maintenance of intracellular homeostasis and the stress acclimatization of tea plants during the Cd tolerance course. Unlike other metals, Cd can stimulate the increase in ROS contents in plant cells dramatically, which causes a poisonous attack in the plant [43]. It has been reported that higher hydrogen peroxide levels accumulated in the leaves of Cd-treated tea plants, and many genes and proteins involved in the ROS scavenging process were significantly upregulated, including a nucleus-encoded Cu/Zn superoxide dismutase-like in the SOD gene family, a nucleus-encoded ascorbate peroxidase-like in the APX gene family, a nucleus-encoded peroxidase-like in the POD gene family, a nucleus-encoded catalase-like in the CAT gene family, and a putative gene beta, beta-carotene 9′,10′-cyclases-like. In addition, an enzyme involved in photorespiration, ascorbate peroxidase, and monodehydroascorbate reductase, which affect the scavenging of hydrogen peroxide in chloroplasts, were upregulated [44].

Tea is rich in catechins, and epigallocatechin gallate accounts for 80%. Tea also contains aspartic acid, α-methylchlorogenate, acyl chlorophyll, amino acids, protein, carbohydrates, and polysaccharides, and it is also rich in caffeine. Polyphenols, amino acids, and other nutrients in tea have significant antioxidant effects, which consumers widely recognize. The enhanced expression of several antioxidative genes by Cd stress has been analyzed in the tea plant [35]. These genes encode proteins, including superoxide dismutase, peroxidase, catalase, glutathione S-transferase, and ascorbate peroxidase. The accumulation of transcripts for the superoxide dismutase gene is induced by Cd stress. Peroxidase activity is also known to be caused by Cd stress in the tea plant. Still, the currently available reports focus only on its enhanced expression and not on the accumulation of the corresponding transcripts. The accumulated levels of transcripts for the catalase gene and their encoded protein are enhanced by the increased exposure to Cd stress. Tea plants overexpressing the glutathione S-transferase and Cu/Zn-superoxide dismutase genes show increased tolerance to Cd stress, as well as lower levels of H₂O₂ and lipid peroxidation. In addition, some transcription factors involved in regulating the expression of these antioxidative genes have been described [45]. These transcription factors include the heat shock transcription factor, NAC transcription factor, and MeHIRD11, encoding a gene induced by high light, oxidative stress, and abscisic acid signaling pathways. The heat shock transcription factor conferred enhanced tolerance to Cd stress on tobacco plants via the reduced accumulation of reactive oxygen species. The H₂O₂ and Cd stress strengthen the accumulation of transcripts for the MeHIRD11 gene in wild tea plants [10].

The oxidative stress caused by excess reactive oxygen species (ROS) is thought to be the primary mode of action of Cd’s toxic effects. Plants have developed a series of signaling pathways to adjust ROS levels when exposed to Cd finely. The involved signaling molecules are calcium ions and various protein kinases. The well-studied Ca²⁺-ATPase and CDPK respond to Cd by involving the influx of extracellular Ca²⁺. The ROS levels can be adjusted through the ascorbate-glutathione cycle enzymes under the regulation of two major transcription factors. Meanwhile, a series of antioxidants, including GSH, catalase, superoxide dismutase, and others, are effective for ROS reduction [46]. The chloroplast is a significant organelle mediating photosynthesis in the light, while it produces complex metabolic pathways to eliminate ROS in the dark. The main regulatory pathways here are shown to be the Ca²⁺ signaling pathway and the methylerythritol phosphate pathway, enhanced by salicylic acid. The Ca²⁺ signaling pathway adjusts the redox status of photosynthesis and antioxidant systems to cope with variations in ROS [32]. The GSH system and glutathione S-transferase efficiently eliminate excess ROS via the regulation of MeSA function. Under Cd stress, GSH and amino acid biosynthesis and metabolism are mainly enriched in pathways to alleviate oxidative stress, cell cycle regulation, and fatty acid elongation and degradation since Cd changes the concentration distribution of amino acid metabolites, disturbs fatty acid synthesis, and results in the decline of GSH content in tea plants [47].

Higher plants have a variety of antioxidant and non-antioxidant enzymes that can maintain the redox homeostasis of cells by scavenging reactive oxygen species. Transcription factors are also involved in these antioxidant systems to regulate the expression of antioxidant genes. Members of DREB/CBF transcription factors, ERF transcription factors, MYC transcription factors, NAC transcription factors, WRKY transcription factors, bZIP transcription factors, and SDK1 are supposed to be involved in lead stress response in Oryza sativa, which may also have possible functions in Cd stress response [46]. HsfA2, a member of heat shock factors, enhanced Pb tolerance in transgenic Pb-exposed plants through regulation of ROS homeostasis and expression of other transcription factors and antioxidant genes. Exogenous glutathione significantly increases Cd tolerance of O. sativa seedlings with significant modulation of the expression of HsfA3 and HsfA4a and three DNA-binding proteins, which are substantial for Pb tolerance response [48].

Cadmium (Cd) stress significantly affects tea plant physiology and metabolism through the reactive oxygen species (ROS)-mediated oxidative damage pathway [49]. At the molecular level, Cd²⁺ causes excessive accumulation of reactive oxygen species (-O₂-, H₂O₂, -OH, etc.) by interfering with the mitochondrial and chloroplast electron transport chain, while inhibiting the activities of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT), which trigger a series of toxic effects such as membrane lipid peroxidation, protein denaturation and DNA [50]. This causes a series of toxic effects such as membrane lipid peroxidation, protein denaturation, and DNA damage. The tea plant responds to Cd stress through a multilevel defense system: on the one hand, it activates antioxidant enzymes (SOD/CAT/APX) and non-enzymatic antioxidants (glutathione GSH, ascorbic acid AsA, and polyphenols such as catechins) to scavenge for ROS synergistically; on the other hand, it sequesters Cd²⁺ through phytochelatant peptides (PCs) and metallothionein proteins (MTs) [51]. Relies on vesicular membrane transport proteins such as HMA3 for compartmentalized storage of toxic substances. Significant differences were observed among varieties: tolerant varieties (e.g., Gushu tea) showed stronger GSH synthesis capacity, PCs production efficiency, and up-regulated expression of key genes such as bHLH9 and GLUT, whereas sensitive varieties (e.g., Longjing) were more susceptible to oxidative damage due to insufficient ROS scavenging capacity [23].

4. Molecular and Transcriptional Regulation

Cadmium toxicity directly interferes with mineral nutrient uptake by inhibiting the activity of various transporters and non-enzymatic proteins [52]. At higher concentrations, Cd directly induces the generation of reactive oxygen species, which can damage cells and whole plants due to oxidative stress [53]. Furthermore, the accumulated Cd leads to the synthesis of highly oxidizing reactive oxygen species. These free radicals can react with organic molecules, resulting in changes in protein structure, oxidative stress, and loss of cell integrity. The four pathways and their related components or signaling are comprehensively listed in various plants. The MAPK signaling pathway, ABA signaling pathway, ethylene signaling pathway, and JA (Jasmonic acid) signaling pathway in plant response to Cd stress [48].

The MAPK pathway is, therefore, a cumulative representation of a series of enzymatically controlled signals that are linked to and converted into unique cellular responses for the induction of resistance isoform enzymes into proteins as a form of transcriptional and post-transcriptional regulator-triggered events, etc. Once activated by Cd, MPK can be quickly translocated to functions in the nucleus, where it might serve as a kinase-dependent effector molecule to lead to the expression of a specific set of downstream resistance-related genes [54]. Abscisic acid (ABA) is considered to be the primary stress hormone in plants that perceives stress stimuli in the environment, especially in the case of water scarcity or high salinity [55]. In response to stress, ABA initiates plant physiological changes that help in stress adaptation and provide plant survival under stress conditions: the ABA signaling pathway and its connection with other hormone signaling pathways. In plants, ABA can be directly perceived by a soluble protein called ABA receptor PYR/PYL/RCAR. These proteins sequester group A type 2C protein phosphatases (PP2C) when ABA is perceived. In the absence of ABA, PP2C dephosphorylates and thereby activates SNF1-related protein kinase 2 (SnRK2) kinase [56]. Activation of the SnRK2 protein kinases results in the phosphorylation of SNF1S-related proteins (bZIP) transcription factors. These factors enhance the expression of ABA-responsive genes and signal transduction, occurring in three significant cascades, including ion channels controlling developmental programs and water balance. Plant stress adaptation to ABA signaling includes genes such as ABA biosynthetic genes, PP2C, PYR/PYL/RCAR, and regulons ABA-responsive element binding gene and DREB, late embryogenesis abundant proteins, and aquaporin genes [49].

Ethylene, known as a gaseous hormone, regulates many processes involved in plant growth, development, and stress responses [35]. Meanwhile, ethylene is also involved in the reaction of plants to Cd stress by limiting root growth to prevent Cd from entering the plant body. With the development of tea plant physiology research, the manipulation of the ethylene pathway will become an important strategy used for planting and processing in the tea industry. Jasmonic acid (JA) is a long-alkyl group oxy-substituted derivative of linoleic acid, a general C6 volatilizing substance [57]. JA is biosynthesized from α-linolenic acid through the lipoxygenase pathway, which is an integral part of the octadecanoic carbonyl-rice keto-7-esters isomerase pathway that is independent of the JA conjugate synthase pathway. Oxylipin 12-oxo-phytodienoic acid (OPDA) is then transported to the peroxisome, where α-oxidation and acyl acid oxidases degrade the OPDA into JA-Ile, which undergoes regulation by the F-box protein complex Ubiquitin system 26S proteasome, where core suppressors of (Jasmonate-Zim Domain) JAZs are combined and degraded. JAZ is a repressor transcription factor of JA response genes, and the degradation of JAZ enables the release and activation of JA response genes [58] (

Table 2. Cadmium Toxicity Effects and Plant Defense Systems in Camilla sinensis.

Category	Key Components/Pathways	Mechanisms	Ref.
Cd Uptake & Primary Effects	Inhibition of mineral transporters (Fe2+, Zn2+)	Cd competitively blocks nutrient uptake, disrupting ion homeostasis	[52]
	Reactive oxygen species (ROS: H2O2, O2−, OH·)	Cd induces oxidative stress, damaging proteins, lipids, and DNA	[53]
Signaling Pathways	MAPK Pathway	MPK3/6 kinases activate stress-responsive genes via nuclear translocation	[54]
	ABA Pathway	PYR/PYL receptors → PP2C inhibition → SnRK2/bZIP activation → stress gene expression	[49]
	Ethylene Pathway	Limits root growth to reduce Cd uptake; regulates stress acclimation	[10]
	Jasmonic Acid (JA) Pathway	JAZ repressor degradation → MYC TF activation → antioxidant synthesis	[58]
Antioxidant Systems	Enzymatic (SOD, CAT, APX, GST)	Detoxify ROS (e.g., SOD converts O2− to H2O2; CAT breaks down H2O2)	[40]
	Non-enzymatic (GSH, polyphenols)	Scavenge ROS directly; tea-specific EGCG and catechins enhance resilience	[21]
Genetic Regulation	Upregulated Genes (CsSOD, CsGST)	Enhance ROS scavenging and Cd chelation	[44]
	Transcription Factors (NAC, WRKY, bZIP)	Bind stress-responsive elements to activate defense genes	[59]
Therapeutic Targets	Exogenous GSH application	Boosts cellular glutathione pools to mitigate oxidative stress	[47]
	TF engineering (e.g., HsfA2 overexpression)	Improves ROS homeostasis and Cd tolerance	[47]

).

5. Epigenetic, Proteomic, and Metabolomic Perspectives

Several studies have been conducted to gain insight into candidate genes that are activated under a toxic metal environment [60]. Various genes that play essential roles in the plant stress response have recently been investigated using the genomics approach in tea plants [36]. In addition, candidate genes were classified according to their functional attributes during the stress response or disease resistance, for example, after Camellia sinensis cv. ‘Baicha 1’ was treated with 20 µM Cd; a total of 85,811 unigenes were obtained from both the control and treated libraries, and 207 Cd-responsive genes were found [60,61]. Cd-responsive genes had putative functions in detoxification processes, such as antiporters and low molecular weight metal-binding proteins. To protect cells from metal toxicity, activation of genes involved in tolerance reactions is required. In plants, after perceiving an external signal, a signal transduction cascade is activated, transmitting the signal into the nucleus and inducing downstream transcriptional reprogramming [62].

Signal transduction pathways often involve transiently induced protein phosphorylation and dephosphorylation mediated by kinases and phosphatases [63]. It was revealed recently that the mechanisms of regulation on gene expression by stress are mainly signal transduction pathways, with protein kinases including CDPK, MAPK, and CIPK, as well as Zn-finger, bZIP, MYB, and WRKY, all essential transcription factors. There are 16 and 10 genes in the MAPK, ethylene, ABA, and JA pathways that have shown different changes under Cd stress in leaves and roots [64]. These genes mainly play roles in the biosynthesis of stress hormones, signal transduction, H₂O₂ and Ca²⁺ signaling, and CYP450 synthesis [65]. Members among those four signaling pathways are enriched in the spliceosome, plant hormone signal transduction, and protein processing in endoplasmic reticulum-related pathways in the tea plant’s response to Cd stress. MAPK, ABA, ethylene, and JA can relieve Cd pressure by enhancing SOD, GSH, and AsA, scavenging free radicals, and inhibiting the accumulation of TA and MDA, thus playing crucial roles in the response of the tea plant to Cd stress [65].

5.1. Cadmium-Responsive Transcription Factors

Transcription factors (TFs) are a group of regulatory proteins that can specifically bind to the cis-acting element of the DNA sequence, thereby modulating the activity of the target gene during plant growth, development, and reproduction processes [35]. With the increasing levels of environmental pollutants, such as heavy metals and salinity, these reports on TFs in response to environmental stimuli are becoming a hot issue again [24]. Cd is a toxic heavy metal for the physiological activities of plants and animals. Some of these Cd-inducible or Cd-enhanced TFs regulate the expression of stress response-related genes, which are mainly involved in signaling functions. Critical roles of various families of TFs in the regulation of plant responses to Cd stress can be indicated as WRKY, MYB, bZIP, NAC, and ERF families, which are involved in the transcriptional reprogramming of stress response-related genes [66]. These TFs bind to the W-boxes of stress-related genes, MBS sites of the target gene stocks to regulate their expression, suggesting part of stress signaling systems in cells following exposure to heavy metal Cd [67].

The WRKY is known as a large transcription factor family, with a typical characteristic of the WRKY domain, including 60 amino acids, which in turn includes one conserved WRKYGQK sequence and a zinc-finger motif at its C-terminus [44]. In terms of protein structure, the WRKY transcription factors were divided into three distinct groups based on the structural characteristics of the zinc-finger motifs, which include C₂H₂/C₂HC-type, C₂HC/C₂HC-type, and the lack of a zinc-finger motif [68]. The WRKY proteins are numerous in autumn crops. In plants, WRKY transcription factors have been demonstrated to regulate downstream gene expression by binding to the promoters of Cd-stress-responsive genes. The conserved WRKY transcription factors commonly function as activators of Cd-stress-responsive genes. Afterward, the conserved WRKY transcription factors modulate a broad spectrum of Cd-stress-responsive genes and exhibit cross-talk with other stress signals [33]. Furthermore, most of the conserved WRKY transcription factors are also regulated by different transcription factors with cross-talk pathways under Cd stress. The WRKY genes in many plants were found to be upregulated by Cd stress. The WRKY genes that have been demonstrated to play a crucial role under different abiotic stresses have been upregulated under Cd stress. The WRKY proteins could be found in diverse environments due to their ability to interact with multiple Cd-associated signaling pathways [11].

The MYB transcription factors (TFs) are one of the largest and most diverse groups of TFs in plants [69]. They can be classified into four categories based on the number of consecutive MYB repeats (1R, 2R, 3R, and four or more R). Reports on the identification of MYB-domain-containing proteins show that there are 153 and 127 MYB genes in Arabidopsis and Vitis vinifera, respectively [21]. Rapid responses to metal exposures such as Cd, copper, zinc, and mercury at the transcriptional, translational, and post-translational levels play a crucial role in protecting cells from these metal toxicities. Studies have shown that MYB proteins can modulate a variety of physiological processes that could contribute to plant adaptation to changes in environmental conditions [44]. In terms of Cd stress, the roles of MYB TFs in Cd-induced stress-responsive gene expression have been identified in many plant species. New biological functions of MYB TFs were identified in regulating the expression of the gene associated with antioxidant production and metal chelation mechanisms for Cd. For example, MYB29 transgenes exhibited enhanced tolerance by adjusting the reaction related to glutathione homeostasis without overaccumulating anthocyanins [70]. In Arabidopsis thaliana, overexpressing MYB15, microarray data have shown that MYB15 can rapidly regulate the gene expression of peroxidase and other oxidative stress proteins, encoding enzymes and functional proteins in cytokinin signaling, glutathione S-transferase, an ascorbate peroxidase gene, and other genes encoding an aquaporin [71]. More evidence for MYB20 in cucumber was also found through transcriptome analysis. Thus, the multiple signaling pathways of MYB genes appear to be involved in the abiotic stress response process [72].

Many landmark studies have arisen that directly or indirectly deal with bZIP family transcription factors, which are involved in different pathways of stress and hormone signaling [73]. It is primarily confirmed that, in the stress-regulated antioxidant encoding genes, the signaling pathway containing bZIP genes would be employed in the induction mechanism of ABREs [74]. Several studies have shown that bZIPs play crucial roles in plant responses to Cd. For example, bZIPs were involved in the regulation of the expression of transcription factor genes, which are the main components of detoxification mechanisms under the stress of Cd. The bZIP transcription factors are shown to regulate Cd-responsive gene expression in poplars [41]. In addition, the Cd treatment of S. hupehensis showed many differentially expressed bZIPs, indicating that the bZIPs were under the influence of the Cd response and could support plant development under severe Cd conditions. Other researchers have also screened the expression patterns of the bZIP family transcription factor members under Cd stress in different poplars and demonstrated their role in response to heavy metal stresses and their association with the biosynthesis of other phytohormones, such as abscisic acid, indoleacetic acid, and cytokinins, as well as stress-resilient molecules [75]. These transcription factors can form dimers with some non-bZIP proteins, including MYCs, EIL1, or EIL2, in the cytokinin signaling pathway to diminish the binding on the promoter regions of a set of target genes and mainly affect binding to the genes involved in cell division and senescence of cotyledons, leaf primordia, and root tips [3,41]. bZIP members play crucial roles in nitric oxide and H₂O₂-mediated signaling and the cross-activities of ABA-dependent and ABA-independent signaling transduction pathways in response to Cd stress. In P. bartlettii, expression pattern analysis of PbZFP85, PbZIP1, PbZIP11, and PbZIP73 members under Cd, copper, and zinc treatments indicated their importance in signaling transduction and their association in mediating metal cation tolerance. Evolutionary conservation of bZIP members for the acceptance of H₂O₂ and other reactive oxygen species induced by Cd stress was screened in tobacco, tomato, Chinese cabbage, and poplar [76].

Cd-responsive NAC TFs are a subfamily of NAC family proteins. NAC072 is strongly upregulated under heavy metal stress, including Cd ions [77]. This TF directly binds to the promoter regions of HMA3, MTP1, and ZAT6 to upregulate their expression to detoxify excessive heavy metal ions [78]. A GmNAC085–GFP fusion protein was observed in the nucleus. Under Cd stress, the transcription of GmNAC085 might be regulated in a transcriptional silence-mediated manner. A promoter fragment of GmNAC085, which can upregulate gene expression, binds strongly to a GmNAP heterodimer [70]. The roles and functions of these novel NAC TFs in biotic and abiotic stress have not been sufficiently interrogated. Many researchers reported that ERF genes are involved in plant resistance under Cd stress. For example, the expression of 95 ERFs was found to be distinctly modulated in Solanum lycopersicum in response to Cd stress. Among them, 94% were reported to be upregulated, while 6% were considered downregulated. A total of 39 BnERF transcription factors were detected in Brassica napus by comprehensive genome-wide analysis, and 35 BnERF transcription factors were significantly induced under Cd stress compared to the control [52]. Simultaneously, the double mutant 35S: NtERF1a/35S: NtERF1b was challenged in Nicotiana tabacum and displayed high Cd tolerance by activating the enzymatic system, which resulted in a significant decrease in ROS content. Moreover, some ERF families might interact with other phytohormones to enhance plant tolerance to environmental stress. Overall, ERF transcription factor family proteins can regulate the downstream gene resistance to Cd [79].

5.2. Methylation-Mediated Adaptation

In-depth research has shown that DNA cytosine methylation is involved in maintaining the basal level of gene expression patterns in most of the plant genome, including those of stress-related genes [80]. Cytosine methylation displays strong antithetical effects on gene expression, acting as a repressive signal by recruiting proteins containing methyl-CpG-binding domains and Su(Var)3-9, enhancer-of-zest, trithorax, and ring finger-associated domains that remodel chromatin and prevent the initiation of gene expression [81]. In addition to genetic variations, epigenetic variations, including DNA methylation, histone tail modification, chromatin remodeling, and small RNA-mediated silencing, may also contribute to rapid gene expression reprogramming in response to different environmental stresses. DNA methylation, involving the addition of a methyl group to cytosine residues, is considered a central part of the epigenetic landscape at the DNA level. Generally, heavy metals, especially Cd, can lead to genome-wide DNA demethylation in plants and animals. In addition, changes in DNA methylation may occur in some genes under heavy metal stress [82].

In plants, methylation in CG, CHG, and CHH (where H is A, T, or C) context occurs mainly in the gene body, transposons, repeats, and intergenic regions [83]. The interaction between differential methylation in the gene body and gene expression is expected. CHG and CHH context hypermethylation are typically related to a decrease in gene expression. In contrast, CHH context hypomethylation in the promoter can increase gene expression [65]. In addition, it has been found that DNA methylation mediates the expression of ZmBADH in maize under Cd stress. DNA methylation in the promoter and gene body indicates a decrease in the transcription level of ZmBADH. SLC19 has also been verified as a folate transporter to increase folate accumulation under abiotic stress. Stressful environments can reprogram genes by changing the DNA methylation of related genes to help the plant adapt to stress [84]. DNA methyltransferase can gain de novo DNA methylation by adding a new methyl group to an unmethylated cytosine or hemimethylated DNA. DNA demethylases participate in removing the methyl group from methylated cytosine to antagonize DNA methylation. DEMETER-like DNA demethylase can remove the methyl group from methylated cytosine, 5-methylcytosine, to create an unmethylated cytosine as the genome demethylase. In addition, DME can remove the methyl group from 5-mC to create 5-hydroxymethylcytosine. Changes in the structure of histone proteins, the primary components of nucleosomes, are another vital process of epigenetic regulation in response to Cd stress [85].

Although stress-induced changes in histone modifications have not been widely reported in experimental plants, a series of studies show changes in the histones H3 and H4 of experimental animals. The addition of acetyl groups to lysine residues of histone N-terminal tails is closely associated with gene expression and can promote transcriptional activation. For example, in response to stress, the increased H4K5ac level in the open reading frame of the activated P5CS1 and ELIPaE genes with increased transcripts enhances salt tolerance in Arabidopsis and wheat [86]. The association between H3K9ac and the promoters of the genes that are upregulated by abscisic acid in Arabidopsis under salt, cold, osmotic, and drought conditions also enhances stress responses. Conversely, the reduced H4K8ac, H4K12ac, and H4K16ac levels with decreased nucleosome occupancy in the promoter and transcribed regions repress active gene expression levels by acting as a barrier for other acetyltransferases. Histone acetylations always occur on positively correlated genes with the expression levels of their mRNAs, such as in the ABA-related transcription factor genes and transporter genes that enhance stress responses to signals of organization. Improved Cd uptake and transport activities of HAT enzyme hyperacetylation substrates also promoted apoptosis in mice, resulting in acute-to-chronic effects. Such acetylated histones and related HAT enzymatic activities are downregulated or upregulated by OsSel activities and CdSCs9 repression [71].

DNA methylation serves as a critical epigenetic regulator in tea plants (Camellia sinensis), fine-tuning gene expression to mediate adaptive responses to environmental stresses and developmental transitions [54]. Under drought conditions, methylation changes in promoters of DREB and NCED3 genes modulate ABA-dependent stress responses, while cold exposure triggers hypomethylation of CBF/DREB1 transcription factors to activate cold-responsive genes like COR15A. Pathogen attacks induce dynamic methylation shifts in defense-related genes, including hypomethylation of NLR immune receptors and PR genes to enhance disease resistance. Methylation also directly influences tea quality by regulating secondary metabolism—low methylation in young leaves promotes TS1 expression for theanine synthesis, while shading reduces methylation of F3′H to boost EGCG production [87]. During development, demethylation of CsDAM1 releases bud dormancy, and progressive methylation silencing of EXPANSIN genes accompanies leaf maturation [88]. Remarkably, processing methods like fermentation alter methylation patterns in PPO and POD genes, affecting enzymatic browning [89]. These epigenetic modifications create a molecular memory, with heritable methylation changes in HSP70 and LEA genes enabling transgenerational stress adaptation [90] (Table 2).

Recent omics-driven studies have revealed significant progress in understanding cadmium responses in Camellia sinensis, providing insights far beyond general plant Cd stress mechanisms. Emerging transcriptomic, proteomic, and metabolomic datasets have identified key pathways involved in Cd uptake, transport, and detoxification, while functional analyses of genes such as CsHMA3, CsPCS1, and CsWRKY53 have demonstrated their crucial roles in regulating Cd sequestration, phytochelatin synthesis, and stress-responsive signaling. These advances underscore the need for a tea-specific synthesis that integrates molecular, biochemical, and physiological evidence to characterize better how tea plants uniquely respond to Cd exposure.

5.3. Proteomic and Metabolomic Responses

Utilizing the proteomic approach, several essential proteins involved in the Cd response have been identified in tea plants [91]. The main proteins, generally considered to be proteomic biomarkers of heavy metal response, are related to detoxification and antioxidants. The metal-thiolate cluster, metallothionein, is activated by the synthesis of the most potent intracellular Cd chelate, phytochelatins, while the H₂O₂-scavenging enzymes contribute to plant antioxidation. Several metallothioneins have been identified in the roots and leaves of Camellia sinensis [92]. It has been suggested that MeMT2 may play an essential role in the high-level response to Cd stress in Cd-resistant cultivars of C. sinensis. Several studies involving proteomic exploration of the Cd response have shown that many of the proteins identified have multiple functions. Many proteins can regulate the expression of catalase and peroxidase. The metabolism between the two groups showed that many pathways, such as the citrate cycle, amino acid metabolism, phenylalanine metabolism, and galactose and sucrose metabolism, were involved in the response of tea plants to Cd stress [16,93].

Some of the identified metabolites have multiple functions, such as proline, which enhances the antioxidant defense system and reduces the malondialdehyde content for regulating osmotic adjustment and stabilizing the structure and function of proteins and membrane lipids. Amino acids were also the central metabolites that increased on day 7 and then declined [94]. They play roles in protein synthesis and in the protective enzymes of energy metabolism, which reduce Cd toxicity. However, other amino acids have different roles in metabolism that could counteract the influence on amino acid metabolism. Moreover, reducing the contents of phenolic compounds can aid in the reduction of oxygen-free radicals, which enhances the antioxidant defense system of tea plants. Carbohydrates are also mobilized to provide alternative energy resources in tea plants in response to Cd stress. Critical enzyme metabolism plays a vital role in the synthesis of some flavonoids and phenolic compounds, which can affect the color, fragrance, and efficacy of tea. In addition, they are also showing some processes to counteract the opposing roles of Cd [95,96].

Tea plants (Camellia sinensis) exhibit dynamic proteomic and metabolomic responses to environmental stresses, developmental changes, and agricultural practices [35]. These molecular adaptations play a crucial role in tea quality, flavor, and stress resilience. Under biotic and abiotic stresses such as drought, pathogen attacks, or temperature fluctuations, tea plants modulate their protein expression and metabolite profiles [97]. Proteomic studies reveal shifts in stress-related proteins, including heat shock proteins (HSPs), antioxidant enzymes (e.g., peroxidases), and pathogenesis-related (PR) proteins [98]. Currently, metabolomic analyses highlight changes in key secondary metabolites, such as catechins, theanine, and flavonoids, which influence tea’s health benefits and sensory properties. During growth and processing, variations in light, soil nutrients, and plucking intervals further alter proteomic and metabolomic patterns [99]. For instance, shading enhances theanine synthesis, while fermentation triggers enzymatic oxidation of polyphenols, crucial for black tea production. Integrating proteomics and metabolomics provides deeper insights into tea plant physiology, aiding in the development of stress-resistant cultivars and optimized cultivation techniques to enhance yield and quality (

Table 3. List of the genes playing a role in cadmium stress in Camilla sinensis.

S. No.	Genes	Name	Function	Expression	Ref.
1	CsNRAMP1, CsNRAMP2, CsNRAMP3.	Natural Resistance-Associated Macrophage Protein	Gene family encodes metal ion transporters, growth, development, and stress responses.	Roots	[100]
2	CsABCC1, CsABCC2.	ATP-binding Cassette Transporters	Convert the energy gained from ATP hydrolysis into trans-bilayer movement	Root & leaves	[101]
3	CsIRT1, CsZIP1.	Zinc-Iron Permease	Transition metal ions, zinc and iron, are transported into the cytoplasm	Leaves	[102]
4	CsYSL2, CsYSL5.	Yellow Stripe-Like	Transportation of metal-phytosiderophores	Shoots	[103]
5	CsMT1, CsMT2, CsMT3.	Metallothioneins	Cis-elements related to stress and hormone responses	Shoots	[104]
6	CsPCS1, CsPCS2.	Phytochelatin Synthase	Important in stress response and adaptation.	Roots & leaves	[105]
7	CsGSTU1, CsGSTF2.	Glutathione S-Transferases	Stress response, detoxification, and biosynthesis of secondary metabolites like anthocyanins.	Leaves	[106]
8	CsSOD1, CsSOD2.	Superoxide Dismutase	Response to various abiotic stresses	Roots	[107]
9	CsCAT1, CsCAT2.	Catalase	Key enzymes involved in antioxidant defense systems	Roots	[27]
10	CsAPX1, CsAPX2.	Ascorbate Peroxidase	Development and defense against abiotic stresses.	Shoots	[106]
11	CsHSP70, CsHSP90.	HSP Families	Tolerance to heat and cold stresses.	Shoots	[46]
12	CsWRKY28, CsWRKY53.	WRKY	Response to biotic and abiotic stress.	Roots	[68]
13	CsMYB108, CsMYB30.	Myeloblastosis	Responses to biotic and abiotic stresses, development, and primary and secondary metabolism	Roots & leaves	[85]
14	CsbZIP60	Basic Leucine Zipper	Positive regulators of drought and salt stress responses	Shoots	[87]
15	CsNAC1, CsNAC5.	NAC	Essential for plant growth and development	Roots	[11]
16	CsMAPK3, CsMAPK6.	Mitogen-Activated Protein Kinase	Defense response against citrus canker	Leaves	[15]
17	CsCML19, CsCML37.	Calmodulin-like Proteins	Regulating plant growth and development, and response to abiotic stress	Roots & leaves	[88]
18	CsCDPK.	Calcium-Dependent Protein Kinase	Plant responses to abiotic and biotic stresses	Roots	[10]
19	CsPIP1, CsPIP2.	play roles in water and solute transport under stress	Mediating water transport	Leaves	[101]
20	CsFER1, CsFER2.	An iron storage protein that also helps in metal detoxification	Response to iron deficiency, growth, and development	Leaves	[103]
21	CsPHT1, CsPHT2.	Phytochelatin Transporter	Phosphate transport, essential for phosphate uptake, translocation, and homeostasis	Leaves & shoots	[106]
22	CsCAX1, CsCAX3	regulates vacuolar sequestration of metal ions	Contributing to ion homeostasis and stress responses	Roots	[103]
23	CsMTP1, CsMTP3, CsMTP8.	vacuolar transporters that sequester excess metals	Maintain metal homeostasis and confer tolerance to heavy metal stress	Leaves and shoots	[27]
24	CsGRX1, CsGRX2	Glutaredoxin	Regulation and defense against oxidative stress	Roots & leaves	[73]
25	CsPOD1, CsPOD2	Peroxidase	Antioxidant defense and enhancing stress tolerance	Leaves	[20]
26	CsMDHAR1, CsMDHAR2.	Monodehydroascorbate Reductase	Maintain redox balance and enhance tolerance to oxidative stress	Roots	[91]
27	CsDHAR1, CsDHAR2.	Dehydroascorbate Reductase	Sustaining antioxidant capacity and enhancing stress resilience	Leaves	[10]
28	CsHSP17, CsHSP22, CsHSP90.1, CsHSP90.2.	Heat shock proteins	Tolerance to heat and other stresses	Leaves	[58]
29	CsZAT6, CsZAT12.	zinc transporter	Drought, salt, and heavy metals enhance plant stress tolerance	Leaves & shoots	[104]
30	CsERF1, CsERF109.	Ethylene Response Factor	Role in plant responses to abiotic stresses such as drought and salinity	Leaves	[48]
31	CsbHLH38, CsbHLH39.	Basic Helix-Loop-Helix	Growth, development, and responses to environmental stresses	Roots	[38]
32	CsAP2/ERF1, CsAP2/ERF4.	AP2/ERF Family	Antioxidant defense and stress tolerance mechanisms	Leaves	[66]
33	CsNHX1, CsNHX2.	(Na+)/(H+) antiporter	Regulates ion homeostasis and pH balance	Roots and leaves	[86]
34	CsSULTR1, CsSULTR3.	Sulfate Transporter	Sulfate uptake and distribution, supporting sulfur assimilation and promoting stress resilience and growth	Roots	[27]
35	CsCAX1, CsACA8.	anti-CRISPR-associated (aca) genes	Calcium-transporting ATPase that helps regulate intracellular calcium levels and stress response.	Leaves & shoots	[103]
36	CsARF7, CsARF19.	Auxin response factors	Plant growth, development, and adaptation to environmental stresses	Leaves	[11]
37	CsJAZ1, CsJAZ2.	Jasmonate-ZIM domain	Modulating plant defense responses, growth, and stress adaptation		[104]
38	CsAREB1, CsABF4.	ABA-responsive element binding protein	Key role in stress tolerance, particularly under drought and salinity conditions	Roots & leaves	[27]
39	CsTRX1, CsTRX2.	Thioredoxin	Redox regulation aids in cellular homeostasis, stress response, and protection against oxidative damage	Roots & leaves	[91]
40	CsNIA1, CsNIA2.	Nitrate reductase	Nitrogen assimilation and supporting plant growth and development.	Leaves & shoots	[20]
41	CsLAC1, CsLAC4.	The lactose operon	Contributing to cell wall formation, stress tolerance, and defense responses	Leaves	[104]
42	CsHMT1, CsHMT2.	Histone methyltransferases	Role in plant development and stress responses	Leaves & shoots	[50]
43	CsVIT1, CsVIT2.	The vacuolar iron transporter	Contributing to iron homeostasis and overall plant health	Leaves	[15]
44	CsNIP1.	Necrosis-inducing protein	Playing a crucial role in root development and stress adaptation	Roots	[79]
45	CsP1B1, CsP1B2.	P1B-type ATPase	Contributing to plant responses to stress and developmental processes	Roots & leaves	[36]
46	CsTRR1, CsTRR2.	tRNA-Arg (anticodon ACG) 1-2 provided	Plays a key role in redox regulation, helping to maintain cellular homeostasis and protect against oxidative stress	Roots	[17]
47	CsGPX1, CsGPX2.	Glutathione peroxidase	Vital role in antioxidant defense and stress response mechanisms	Leaves & shoots	[25]
48	CsALDH7, CsALDH12.	Aldehyde Dehydrogenase	Contributing to stress tolerance and overall plant health	Roots & leaves	[91]
49	CsZFP1, CsZFP7.	Zinc Finger Protein	Response to environmental stresses and developmental cues	Leaves & shoots	[99]
50	CsHD-Zip1, CsHD-Zip2.	Homeodomain-leucine zipper	Regulates plant development and adaptation to abiotic stresses	Roots	[47]
51	CsGRAS1, CsGRAS2.	GIBBERELLIC ACID INSENSITIVE	Plant growth, development, and stress responses	Leaves	[25]
52	CsPP2C1, CsPP2C5.	Protein phosphatase 2C	Regulating plant growth, development, and stress responses	Roots & leaves	[27]
53	CsCaBP1, CsCaBP2.	calcium-binding protein 5provided	Role in stress response and adaptation, particularly under drought conditions	Leaves & shoots	[33]
54	CsGPCR1.	G protein-coupled receptors	Role in perceiving environmental signals and mediating stress responses	Leaves	[23]
55	CsCHI1, CsCHI3.	Chalcone Isomerase	Catalyzes the conversion of chalcones to flavanones, supporting plant defense	Roots & leaves	[99]
56	CsLTP1, CsLTP3.	Lipid Transfer Protein	Role in cuticle formation, pathogen defense, and stress adaptation	Leaves	[14]
57	CsPAO1, CsPAO2.	Polyamine oxidases	Cellular homeostasis, stress response, and defense against oxidative stress	Roots and leaves	[33]
58	CsGID1A, CsGID1B.	Gibberellin Insensitive Dwarf1	Regulating growth, development, and responses to environmental stimuli	Roots	[91]
59	CsSAMT1, CsSAMT2.	Salicylic Acid Methyltransferase	Role in the modulation of plant defense and stress responses	Roots	[23]
60	CsPIN1, CsPIN2.	PIN-FORMED	Regulating auxin distribution and polar transport to influence plant growth and development	Leaves & shoots	[14]
61	CsEXP1, CsEXP3.	Expansin gene family	Role in cell wall loosening, facilitating cell growth and expansion during plant development	Roots & leaves	[52]
62	CsCESA1, CsCESA3.	Cellulose Synthase	Contributing to cell wall structure, plant growth, and resistance to abiotic stresses	Roots & leaves	[45]
63	CsPME1, CsPME2.	Pectin Methylesterase	Influencing cell wall integrity, development, and stress responses	Roots & leaves	[105]
64	CsHSF1, CsHSF2.	Heat Shock Factor	Response to thermal stress and maintaining protein homeostasis	Roots	[36]
65	CsUBC1, CsUBC2.	Ubiquitin-Conjugating Enzyme	Regulates protein degradation, signaling, and responses to environmental stresses	Roots & leaves	[52]
66	CsCYP71, CsCYP85.	Cytochrome P450	Contributing to stress responses and development	Leaves & shoots	[49]
67	CsHMA1, CsHMA2.	Heavy Metal ATPases	Gene family encodes metal ion transporters, growth, development, and stress responses.	Roots	[108]

).

6. Conclusions

As the world’s second most consumed beverage, tea faces critical threats to its safe production from Cd pollution. Studies have shown that Cd pollution through industrial emissions, agricultural fertilizer, and other ways into the soil, where it is readily absorbed by tea plants and accumulated in edible tissues. This not only affects the tea quality but also poses significant health risks to consumers’ health. Tea plants employ multi-level Cd defense mechanisms: physiological (cell wall fixation, vesicle compartmentalization), molecular (antioxidant activation, phytochelatin synthesis), and epigenetic (DNA methylation-mediated gene regulation). Current mitigation strategies include soil remediation, low-Cd cultivar breeding, and organic cultivation. In the future, in tea plants, it is necessary to uncover more Cd-related genes, elucidate molecular tolerance mechanisms, establish global Cd limits for tea, and implement blockchain-based traceability systems. This research on tea plants will not only guide safe production but also inform Cd control strategies for other crops.

Tea, the world’s second most consumed beverage, faces increasing challenges from cadmium (Cd) contamination, which threatens both product safety and consumer health. Tea plants use a combination of physiological defenses, molecular signaling networks, and epigenetic regulation to cope with Cd stress, but many of these pathways remain only partly understood. Key knowledge gaps persist, particularly in identifying the full range of genes, regulatory circuits, and metabolic processes involved in Cd uptake, transport, and detoxification.

Future research should prioritize multi-omics integration—including genomics, transcriptomics, proteomics, metabolomics, and epigenomics—to build a more complete picture of Cd-response networks in Camellia sinensis. Such integrative approaches will help reveal new candidate genes and biomarkers functional for breeding programs. Additionally, developing Cd-safe tea cultivars through genomic-assisted breeding and emerging gene-editing tools will be essential for long-term mitigation. Establishing harmonized global Cd limits for tea and improving traceability systems will further support safer production. Together, these directions provide a more precise roadmap for reducing Cd risks in tea and improving sustainability across the tea industry.

7. Future Perspective

To ensure the sustainable production of tea under cadmium (Cd) stress, a multi-faceted approach integrating molecular, agricultural, regulatory, and environmental strategies is essential. At the molecular level, comprehensive research should focus on elucidating the genetic and biochemical mechanisms underlying Cd tolerance in tea plants. This involves identifying and characterizing key functional genes such as metal transporters (NRAMP, ZIP families), sequestration proteins (HMA3), and detoxification enzymes (phytochelatin synthetases), coupled with multi-omics analyses to map the complex regulatory networks controlling the Cd response. Advanced gene-editing tools further validate these molecular targets, providing a foundation for genetic improvement programs.

For practical implementation, innovative breeding strategies must be developed to cultivate low-Cd tea varieties without compromising yield or quality. Marker-assisted selection can accelerate the identification and incorporation of favorable traits from existing tea germplasm, while gene pyramiding techniques allow stacking multiple beneficial alleles for enhanced Cd resistance. Modern approaches such as somaclonal variation and hybrid breeding offer additional avenues to develop improved cultivars. These breeding efforts should be complemented by strong regulatory frameworks establishing internationally balanced Cd limits for tea products, ensuring consumer safety across global markets.

Effective pollution control requires integrated soil management solutions. Environmentally friendly remediation techniques, including biochar amendment, microbial inoculation, and phytoremediation, can significantly reduce Cd bioavailability in tea plantations. Precision agriculture practices, particularly optimized fertilization and irrigation regimes, play a crucial role in minimizing Cd uptake while maintaining soil health.

The knowledge gained from studying Cd resistance mechanisms in tea plants has broader agricultural applications. Similar molecular approaches and breeding strategies could be adapted for other crops prone to Cd accumulation, such as rice and leafy vegetables. The remediation techniques developed for tea plantations may also be scaled for use in other heavy metal-contaminated agricultural systems. By combining these scientific advances with smart policy implementation and technological innovation, the tea industry can serve as a model for safe agricultural production in metal-polluted environments worldwide, ultimately contributing to global food security and public health protection.

Advances in plant genomics and genome editing offer promising avenues for accelerating the development of cadmium-resilient tea cultivars. Genomic-assisted breeding, supported by high-density SNP markers and whole-genome sequencing resources now available for Camellia sinensis, can enable the identification of quantitative trait loci (QTLs) and candidate genes controlling Cd uptake, transport, and sequestration. Integrating genomic selection with transcriptomic datasets may further improve predictive accuracy for low-Cd phenotypes. Additionally, CRISPR/Cas-based genome editing provides a precise strategy to modify key metal transporter genes (e.g., members of the ZIP, NRAMP, ABC, and HMA families) or enhance regulators involved in phytochelatin and glutathione biosynthesis pathways to reduce Cd accumulation in tea tissues. Although gene editing in tea is still emerging, recent progress in tissue culture regeneration and transformation efficiency suggests that CRISPR-driven manipulation of Cd detoxification pathways will be a feasible and robust approach for future breeding programs.