Regulation of plant growth and metabolism by alpha-tocopherol under stress and non-stress conditions

Introduction

Tocopherols are lipophilic antioxidant compounds that belong to vitamin E family. They are naturally synthesized in green photosynthetic organisms (Munné-Bosch & Alegre, 2002) [74] . Tocopherols play an important role in protecting plant cells from oxidative damage. Ronald M. Evans et al., (1974) first described α-and β-tocopherols and isolated them from the seed oil of wheat. (Triticum aestivum L.). Subsequently, γ-and δ-tocopherols were extracted from other oilproducing plants (Stern et al., 1947) [102] . All tocopherols contain a hydrophilic chromanol head that is oriented toward the outer membrane surface and a hydrophobic phytol tail that extends into the inner membrane. Structural differences among tocopherols arise due to variations in the methyl groups on the chromanol ring (Munné-Bosch, 2007) [73] . Among these forms, α-and γ-tocopherols are commonly present in plant membranes. α-Tocopherol is mainly found in the photosynthetic membranes of plant leaves, whereas γ-tocopherol is predominantly present in fruits, seeds and nuts (Grilo et al., 2014) [37] . Tocopherol biosynthesis mainly occurs in the chloroplast, particularly in the envelope, thylakoid membranes, and plastoglobuli, although it may also occur in the protoplast and vacuole of plant cells (Rautenkranz et al., 1994) [84] . In plants, singlet oxygen is produced under photo-oxidative stress due to the formation of reactive oxygen species (ROS) (Asada, 2006) [7] . Tocopherols, which are abundant in cellular membranes, help scavenge these free radicals and prevent lipid peroxidation in membranes (Munné-Bosch, 2007) [73] . The tocopheroxyl radical can be reduced back to tocopherol by antioxidants such as vitamin C, ubiquinol, glutathione, or lipoic acid, there by maintaining the antioxidant defence system in plant cells. The tocopheroxyl radical can be reduced back to tocopherol by antioxidants such as vitamin C, ubiquinol, glutathione, or lipoic acid, which are further regenerated by NADH or NADPH (Yang et al., 2011) [110] . Therefore, tocopherols play an important role in both plants and animals. Photosynthetic organisms are the primary source of tocopherol production for other living organisms. Deficiency of tocopherols may lead to severe physiological disorders and may even be lethal in both plants and animals (Azzi & Stocker, 2000; Maeda & DellaPenna, 2007) [13,14] . In addition to protecting cellular membranes from oxidative damage, tocopherols also regulate the expression of certain genes (Munné-Bosch, 2007; Li et al., 2008; Suo et al., 2017) [73] . In recent decades, considerable attention has been given to improving the oxidative defence system of plants exposed to various environmental stresses. This can be achieved either through genetic engineering to enhance antioxidant production or through the exogenous application of antioxidant compounds such as tocopherols. The exogenous application approach has gained more attention because it is relatively simple, efficient, and economical. However, it is still unclear whether tocopherol alone or in combination with other antioxidants can significantly improve plant growth under normal or stress conditions. Therefore, recent studies have focused on understanding the role of tocopherols, particularly αtocopherol, in enhancing plant growth and stress tolerance under both stress and non-stress conditions. Studies on Arabidopsis thaliana revealed that mutation in the vte1 gene results in the deficiency of all four tocopherol isoforms due to the lack of tocopherol cyclase. This enzyme is located in the plastoglobuli of chloroplasts and is essential for the synthesis of the chromanol ring of tocopherol. Similarly, limited expression of the vte1 gene in Oryza sativa (rice) plants made them more sensitive to stress, whereas its overexpression increased stress tolerance in rice and Nicotiana tabacum (tobacco). Furthermore, overexpression of the γ-tocopherol methyltransferase gene in genetically engineered Brassica juncea plants converted γtocopherol into α-tocopherol and enhanced stress tolerance (Yusuf et al., 2010) [112] . These findings suggest that, transgenic approaches involving genes responsible for the synthesis and accumulation of α-tocopherol can improve stress tolerance, particularly in oilseed crops. Successful manipulation of such genes may also help in improving the oil quality of oilseed plants like soybean.

Tocopherol and Its Types

The vitamin E family consists of two major groups:tocopherols and tocotrienols. Both groups contain a 6chromanol ring as the head and an isoprenoid side chain. Each group includes four forms-alpha (α), beta (β), gamma (γ) and delta (δ)-which differ in the position of methyl groups on the chromanol ring (Shahidi & de Camargo, 2016) [97,98] . Thus, the vitamin E family includes eight compounds in total: four tocopherols and four tocotrienols. Among these, only the RRRform occurs naturally in biological systems. Among the vitamin E compounds, α-tocopherol is the most abundant form found in plant and animal tissues and more than 95% of research studies have focused on this form (Sen et al., 2007) [94] . The main structural difference between tocopherols and tocotrienols is that tocotrienols have unsaturated side chains, whereas tocopherols possess saturated side chains (Fu et al., 2017) [33] . Tocopherols are synthesized mainly in photosynthetic organisms such as algae and higher plants (Hussain et al., 2013) [48] . They act as antioxidants by donating phenolic hydrogen to neutralize reactive free radicals. Among the different types of tocopherols, α-tocopherol has the highest antioxidant activity. In comparison, β-and γ-tocopherols show moderate antioxidant activity, while δ-tocopherol exhibits relatively lower antioxidant potential (Kapoor et al., 2015) [53] .

Physio-Biochemical Role of Alpha-Tocopherol in Plants

All plants contain lipophilic antioxidants such as fatty acids, carotenoids and α-tocopherol as important components of photosynthetic membranes (Semida et al., 2016) [93] . Plastid membranes are rich in polyunsaturated fatty acids and αtocopherol plays a key role in the antioxidant defence system of mesophyll cells in plant. α-Tocopherol helps to prevent lipid peroxidation by scavenging reactive oxygen species and their by-products, thereby maintaining the stability and integrity of biological membranes (Wang & Quinn, 2000; Sharma et al., 2012; Spicher et al., 2016) [99,109] . For instance, a single molecule of α-tocopherol can neutralize nearly 120 molecules of singlet oxygen under stress conditions before being degraded (Semida et al., 2014) [92] . The α-tocopherol molecule is involved in the direct scavenging of singlet oxygen (1O2) as well as in the conversion of lipid peroxyl radical (LOO-) to lipid hydro-peroxide (LOOH). Regeneration of α-tocopherol from α-tocopheroxyl radical is mediated through ascorbate-glutathione redox couple. Tocopherol acts as an antioxidant by donating an electron from its chromanol ring to reactive lipid radicals, such as lipid peroxy radicals (LOO•), converting them into more stable products (LOOH). Through this mechanism, α-tocopherol protects cellular membranes from oxidative damage and helps maintain normal physiological and biochemical processes in plants. During the antioxidant reaction, α-tocopherol is converted into a tocopheroxyl radical after donating an electron to neutralize free radicals. This radical can be regenerated back to α-tocopherol through interactions with other antioxidants such as ascorbate and glutathione. The protective role of α-tocopherol in preventing the oxidation of polyunsaturated fatty acids in membranes involves several mechanisms (Sandorf & Hollander-Czytko, 2002) [90] . Reactive oxygen species (ROS) are produced as by-products of photosynthesis and other metabolic processes in plants (Bhattacharya, 2015). Excess accumulation of ROS can cause lipid degradation and membrane damage (Munné-Bosch & Alegre, 2003) [75] . However, α-tocopherol acts as an effective antioxidant that scavenges these free radicals and protects cellular membranes from lipid peroxidation. For example, Clement et al., (2002) [23] reported that α-tocopherol effectively reduced free radical activity and prevented membrane lipid peroxidation in soybean.

Molecular Aspects of Tocopherol-Induced Stress Tolerance

Tocopherols are lipid-soluble compounds belonging to the vitamin E family. It includes four isoforms: α, β, γ, and δ. These compounds play an important role in protecting plants from oxidative stress by scavenging and neutralizing reactive oxygen species (ROS) (Ouchi et al., 2014) [80] . Among these isoforms, α-tocopherol showed the highest antioxidant activity due to the presence of methyl groups in its phenolic ring structure (Hussain et al., 2013) [48] . The structure of tocopherol, which includes a benzoquinone ring and a phytol side chain, enables it to participate in redox reactions and neutralize singlet oxygen. It has been reported that, one molecule of α-tocopherol can neutralize nearly 120 singlet oxygen molecules in the photosynthetic system before it is degraded (Gill & Tuteja, 2010) [36] . The oxidized form of tocopherol can be regenerated by other antioxidants such as coenzyme Q, glutathione (GSH) and ascorbic acid (AsA) (Karuppanapandian et al., 2011) [54] . Under the stress conditions, the synthesis of tocopherol may increase to compensate for its degradation in plant cells (Szarka et al., 2012) [104] . In plants, the hydroxyl group of α-tocopherol can form α-tocopheryl acetate, which retains vitamin E activity (Gregory, 2007) [38] . α-Tocopherol also plays a role in the photosynthetic process. It participates in electron transport around Photosystem II by interacting with cytochrome b559 (Kruk & Strzalka, 2001) [58] . In addition, α-tocopherol influences the formation of the light-driven trans-membrane proton gradient (ΔpH) across the thylakoid membrane, which is important for ion transport and ATP synthesis. The proton gradient also regulates the xanthophyll cycle, where violaxanthin is converted into zeaxanthin. This process helps dissipate excess light energy as heat in chloroplasts and protects the photosynthetic apparatus from damage under stress conditions (Demmig-Adams et al., 2008) [26] .

Physiological Basis of Tocopherol-Induced Stress Tolerance

Alpha-tocopherol is an important component of the seeds of oilseed crops such as safflower, sunflower and wheat germ (Reboul et al., 2006) [85] . It plays a significant role in the germination of oil seeds by protecting the tissues from oxidative stress during the germination process (Sattler et al., 2004) [91] . Generally, oilseeds contain higher levels of tocopherol compared to photosynthetic tissues. A considerable amount of αtocopherol is present in plastoglobuli vesicles of chloroplasts. It helps to protect photosynthetic tissues from the harmful effects of abiotic stresses. The distribution of α-tocopherol between thylakoid membranes and plastoglobuli is an important physiological feature and varies with the growth stages of plants such as Nicotiana tabacum and Arabidopsis thaliana (Matringe et al., 2008) [68] . Under low temperature conditions, α-tocopherol helps to prevent the destabilization of cellular membranes. In phospholipid bilayer membranes, α-tocopherol maintains membrane stability by influencing the movement of fatty acid chains in the liquid crystal phase (Hincha, 2008) [45] . Through these mechanisms, αtocopherol contributes to improved physiological tolerance of plants under different environmental stresses.

Photo-Protective Function of α-Tocopherol in Plants

Photosynthesis converts solar energy into chemical energy in the form of carbohydrates. During photosynthesis process, excess light energy is safely dissipated as heat to maintain a balance between light absorption and energy utilization. Under normal conditions, most of the captured energy is used for plant growth, development, and only a small photoprotective mechanism is required. However, under unfavourable environmental conditions, photosynthesis may be inhibited, leading to the accumulation of excess light energy. In such situations, plants activate protective mechanisms to dissipate this extra energy and prevent damage to the photosynthetic system. Plants may modify their leaf structure and physiological processes to cope with excess energy, including mesophyll adaptation, changes in pigment composition, accumulation of anthocyanins to reduce light intensity and adjustments in light absorption efficiency (Terashima et al., 2001) [107] . Excess energy can lead to the formation of reactive oxygen species (ROS) in chloroplasts, which may cause oxidative damage to cellular components (Asada, 2006) [7] . α-Tocopherol plays an important role in protecting the photosynthetic apparatus by scavenging ROS and preventing lipid peroxidation. It also contributes to the dissipation of excess energy as heat within chloroplasts, thereby protecting plants from photooxidative damage.

Role of Alpha-Tocopherol in Plant Resistance against Abiotic Stresses

Tocopherols play an important role in plant adaptation to various abiotic stresses such as drought, salinity, extreme temperatures, radiation and toxic metals. Different plant species show variation in tocopherol-mediated stress tolerance depending on their sensitivity and metabolic responses to stress conditions (Kapoor et al., 2015) [53] . Studies have shown that, stresstolerant plant species generally contain higher levels of αtocopherol compared to stress-sensitive species (Munné-Bosch & Alegre, 2002) [74] . The level of α-tocopherol in plants exposed to stress is usually regulated in two phases. In the first phase, the synthesis of α-tocopherol increases to scavenge reactive oxygen species (ROS) produced during stress. In the second phase, under severe stress conditions, degradation of tocopherol may occur. The first phase is more prominent in stress-tolerant species, while the degradation phase is mainly observed in stress-sensitive plants ( Munné-Bosch, 2005). Environmental stresses such as drought, salinity, extreme temperatures, heavy metals and radiation negatively affect plant growth and lead to the accumulation of reactive oxygen species in plant cells and tissues. A gradual increase in ROS can disrupt important metabolic processes and cause oxidative damage to cellular components. Under severe stress conditions, the accumulation of reactive oxygen species (ROS) can damage cellular components and may ultimately lead to cell death. To protect themselves from such oxidative damage, plants activate defence mechanisms by producing both low and high molecular weight antioxidants (Lushchak, 2011) [65] . The accumulation of tocopherols in plants depends on the intensity and severity of the stress conditions, as well as the species-specific responses of plants. Higher levels of tocopherols help plants reduce oxidative damage and improve their tolerance to adverse environmental conditions.

Drought Stress

Under drought stress, plants often close their stomata to reduce water loss. This leads to increased photo-oxidation and the production of reactive oxygen species (ROS), which can disrupt photosynthesis through lipid peroxidation (Cela et al., 2009b) [20,21] . To counteract this effect, many plant species increase the production of α-tocopherol under water-deficit conditions. Increased α-tocopherol levels have been reported in plants such as Arabidopsis thaliana and Nicotiana tabacum (Liu et al., 2008) [64] , Rosmarinus officinalis (Munné-Bosch et al., 1999) [76] , Salvia officinalis and Melissa officinalis (Munné-Bosch & Alegre, 2003) [75] [74] and Quercus ilex (Munné-Bosch et al., 2004) [77] . Studies have also shown that, transgenic drought-tolerant plants often produce higher levels of tocopherols. For example, transgenic brassica juncea plants showed that, greater drought tolerance due to increased α-tocopherol content compared to wild types (Kumar et al., 2013) [59] . Similarly, Cela et al., (2009a) reported higher α-tocopherol levels in a drought-tolerant transgenic line of Arabidopsis thaliana. The VTE2 gene as an important gene involved in α-tocopherol synthesis under water-stress conditions, helping to protect the photosynthetic system from photo-oxidative damage (Espinoza et al., 2013) [29] . In Cistus clusii, early drought stress activated the expression of the homogentisate phytyl transferase gene, which increased α-tocopherol levels by about 62% (Munné-Bosch et al., 2009) [78] . Additionally, Liu et al., (2008) [64] observed that, higher total tocopherol content in transgenic Nicotiana tabacum plants under drought stress due to overexpression of the VTE1 gene from Arabidopsis thaliana. Abdallah et al., (2013) [2] studied the combined effect of mycorrhiza and α-tocopherol on Helianthus annuus plants grown under 60% field capacity conditions. Their results showed that, significant improvement in growth parameters, mineral ion uptake and yield components compared to untreated plants. Similarly, Sadiq et al., (2016) [86] reported that, foliar application of α-tocopherol enhanced plant growth and increased antioxidant enzyme activity in Vigna radiata under moisturedeficit conditions. These findings suggest that, α-tocopherol can improve plant tolerance to drought stress by enhancing antioxidant defence mechanisms.

Salinity Stress

Salinity is more harmful than drought because it causes both ionic stress and osmotic stress in plants. It negatively affects important physiological processes such as water and nutrient uptake, respiration and photosynthesis. Salinity also increases the production of reactive oxygen species (ROS) and leads to membrane lipid peroxidation, which ultimately reduces plant growth and yield (Mahajan & Tuteja, 2005) [66] . Skłodowska et al., (2009) [100] studied salt-stressed tomato plants (Solanum lycopersicum) treated with 50 and 150 mM NaCl. They observed increased lipid peroxidation, reduced chlorophyll content and changes in antioxidant enzyme activities along with an increase in α-tocopherol levels. The antioxidative role of α-Tocopherol was associated with higher activities of glutathione S-transferase (GST) and glutathione peroxidase (GPX) during the early growth stage. However, at later stages, α-Tocopherol was involved in signalling pathways related to plant senescence. Similarly, treatment with 100-200 mM NaCl increased total tocopherol content in Brassica juncea plants. However, at 400 mM NaCl, total tocopherol levels decreased due to a reduction in α-tocopherol and an increase in γ-tocopherol. Overexpression of the γ-tocopherol gene did not change the total tocopherol content but increased the conversion of γ-tocopherol to αtocopherol (Yusuf et al., 2010) [112] . Hussein et al., (2007) [49] reported that, exogenous application of 200 mg L⁻¹ α-tocopherol improved the growth of cowpea plants. Similarly, Mohamed and Aly (2008) [70] found that, α-tocopherol application reduced the harmful effects of salt stress in onion plants and increased the levels of total phenolics and flavonoids. Farouk (2011) [32] reported that, foliar application of αtocopherol significantly increased antioxidant enzyme activity and the accumulation of ascorbic acid (AsA), phenolics, accessory pigments, calcium, potassium and magnesium in wheat plants compared with untreated plants. In addition, soaking sunflower seeds in different concentrations of αtocopherol helped reduce the negative effects of salinity stress (Rady et al., 2011) [82] . These studies indicate that, foliar application of α-tocopherol under salinity stress enhances plant tolerance by stabilizing important biochemical processes. As a result, it helps maintain plant growth and improves biomass and seed yield production.

Extreme Temperatures

Increasing global warming creates stress conditions for crops, particularly in tropical regions (Kumar et al., 2013) [59] . High temperature stress adversely affects plant growth and development at different growth stages (Hatfield and Prueger, 2015) [43] . Elevated temperatures can denature cellular and subcellular proteins, leading to disruption of membrane integrity (Hatfield and Prueger, 2015). In addition, high temperature causes enzyme inactivation in chloroplasts and mitochondria, which blocks many metabolic reactions (Howarth, 2005) [47] . Generally, high temperature stress reduces photosynthesis, causes nutrient deficiency and impairs transpiration (Oukarroum et al., 2012) [81] . It also leads to pigment degradation and slower plant growth. However, several studies have reported a positive relationship between high temperature stress and the increased synthesis or accumulation of tocopherols (Falk and Munné-Bosch, 2010) [31] . For example, exposure of Helianthus annuus plants to 35 °C increased tocopherol levels during the reproductive stage and also enhanced seed oil content (Dong et al., 2007) [27] . In lettuce, high temperature activated tocopherol cyclase, which increased the biosynthesis of vitamin E (Tang et al., 2011) [106] . Generally, tocopherol accumulation in plants is considered a lightdependent process (Lichtenthaler, 2007) [63] , as reported in Arabidopsis (Szymańska and Kruk, 2010) [105] . However, Chennupati et al., (2011) [22] reported that, high temperature conditions of 33/25 °C (day/night) significantly increased αtocopherol levels while decreasing γ-tocopherol and δtocopherol concentrations at different developmental stages of soybean plants. Similarly, Carrera and Seguin (2016) [19] observed that, increased temperature combined with soil moisture deficiency enhanced the accumulation of α-tocopherol but reduced δ-tocopherol levels in soybean.

Low Temperature

Low temperature also adversely affects the plant growth and development. This problem is more common in countries that experience continuously low temperatures, although chilling stress can also occur during the winter season in tropical regions. Both high and low temperature extremes have been reported to promote tocopherol accumulation in several plant species (Kanayama et al., 2013) [52] . However, the concentration of αtocopherol varies among different plant species under chilling stress (Streb et al., 1999) [103] . Under low temperature conditions, tocopherols helps to protect the plastid membranes from oxidative damage (Matringe et al., 2008) [68] . For example, exposure of Medicago sativa plants to low temperature (10 °C) increased lipid peroxidation (MDA) and hydrogen peroxide (H₂O₂) levels. During the recovery period, αtocopherol helped to scavenge the free radicals produced due to oxidative stress. The increased levels of α-tocopherol were associated with higher activities of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD) and ascorbate peroxidase (APX), which helped to reduce the accumulation of reactive oxygen species (ROS) (Bafeel and Ibrahim, 2008) [16] . In response to cold stress, plants maintain their metabolic activities by accumulating various cryoprotective compounds, which enhance their tolerance to chilling conditions.

Metal Toxicity

Metal toxicity can negatively affect various physiological and biochemical processes in plants (Hasanuzzaman and Fujita, 2013) [39,40] . Similar to other environmental stresses, metal stress can also influence the levels of tocopherols in plants processes. For example, Artetxe et al., (2002) [6] reported that duckweed plants exposed to zinc (Zn) and cadmium (Cd) stress showed the increased levels of tocopherols in plant. A similar response was observed in lupine plants subjected to lead (Pb) stress, where tocopherol synthesis was enhanced. In wheat, treatment with 50 and 100 mM nickel (Ni) for nine days resulted in 38% and 60% higher tocopherol levels, respectively, compared with the control plants (Gajewska and Skłodowska, 2007) [34] . Similarly, in Arabidopsis, copper (Cu) and cadmium (Cd) stress induced approximately six-fold and five-fold increases in tocopherol content compared with control plants (Collin et al., 2008) [24] . In Brassica juncea, Yusuf et al., (2010) [112] reported that, 1.5-fold and 9.3-fold increases in αtocopherol and γ-tocopherol contents, respectively, in plants treated with CdCl₂ (20 mM).

Ultraviolet (UV) Radiation

Global warming and climate change are causing depletion of the stratospheric ozone layer, which increases the exposure of the earth to ultraviolet (UV) radiation. The reduction in ozone concentration leads to higher levels of UV-B radiation reaching the Earth's surface (Mpoloka, 2008) [72] . UV-B radiation can cause various harmful effects in plants, including reduced growth, biochemical changes and decreased yield. α-Tocopherol is considered an important protective compound against UV-B stress. Endogenous α-Toc present in plastids helps reduce reactive oxygen species (ROS), lipid peroxidation and hydrogen peroxide (H₂O₂) levels, thereby maintaining membrane integrity under UV-B stress in different plant species (Sharma et al., 2012) [99] . For example, exposure of Spinacia oleracea leaves to UV-B radiation increased malondialdehyde (MDA) formation, indicating enhanced lipid peroxidation (DeLong and Steffen, 1998) [25] . However, increased endogenous α-Toc levels helped reduce lipid peroxidation and neutralize oxidative stress. In addition, α-Tocopherol also protects the membranous antioxidant system (DeLong and Steffen, 1998). In contrast, some studies have reported that, no positive effect of UV-B on α-Tocopherol levels. For instance, UV-B stress significantly reduced α-Tocopherol content in maize (Carletti et al., 2003) [18] and cucumber seedlings (Jain et al., 2003) [50] , while no significant effect was observed in soybean leaves (Galatro et al., 2001) [35] .

Therefore, α-tocopherol may help reduce the harmful effects of UV radiation in some plant species under certain conditions.

Tocopherols and Cellular Signalling

Antioxidants play an important role in redox reactions and help regulate the expression of genes involved in plant acclimatization to abiotic stress. Stress conditions and environmental factors trigger various signals that activate the plant's internal defence mechanisms. In cellular systems, αtocopherol plays an important role in initiating signalling processes in cell. (Azzi, 2007) [13] . The level of bioactive α-Tocopherol involved in signalling varies depending on the severity of stress conditions. This highlights the important role of α-Tocopherol in maintaining redox homeostasis in plant cells (Traber and Atkinson, 2007) [108] . In addition, α-Tocopherol can influence gene expression and modify source-sink relationships in plants, indicating its involvement in cellular signalling processes (Hofius et al., 2004) [46] . Studies on transgenic potato plants have shown that, tocopherol can regulate gene expression and increase the transcription of several defence-related proteins, such as proline biosynthesis enzyme (P5CS), jasmonic acid synthesis enzyme (AOC) and proteinase inhibitor II (PIN2) (Lichtenthaler, 2007) [63] . Furthermore, plants deficient in α-tocopherol were found to be more sensitive to stress, whereas mutant plants containing γtocopherol instead of α-tocopherol showed greater tolerance to osmotic stress (Abbasi, 2007) [1] .

Exogenous Application of Tocopherols

Exogenous application is considered a simple, effective method to help plants cope with various abiotic stresses (Ashraf et al., 2010) [9] . Under stress conditions, the internal levels of phytohormones and antioxidants in plants often decrease. These reduced levels can be improved through external application of such compounds. The exogenous application of non-enzymatic antioxidants has become a common practice to reduce the harmful effects of abiotic stresses in plants (Kaya et al., 2010) [55] . Vitamins act as antioxidants and also function as coenzymes in many metabolic processes. Although they are required in small amounts, they may act as limiting factors for plant growth (Hassanein et al., 2009) [42] . Several studies have reported that, beneficial effects of applying vitamins, phytohormones, and antioxidants to protect plants against abiotic stresses. These protective substances enhance plant stress tolerance by regulating metabolic processes, which ultimately improves plant growth and crop productivity (Hasanuzzaman et al., 2013) [39,40] . To maintain cellular homeostasis in response to intra and extracellular stimuli, chloroplasts regulate many intercellular signalling pathways. Plastids are often located close to plasmodesmata, which allows external molecules to easily reach these organelles (Bobik and Burch-Smith, 2015) [17] . The movement of molecules between cells is mediated by proteins located in plasmodesmata and by the activity of callose synthase (Lee et al., 2011) [62] . These findings indicate that, existence of cross-talk between low molecular weight compounds, including tocopherols and plasmodesmata-mediated signalling. Tocopherols, including αtocopherol, are lipophilic compounds, which enables them to easily enter cells through biological membranes (Sattler et al., 2004) [91] . The chloroplast is the primary site of α-tocopherol biosynthesis and most of the enzymes involved in this process are located on the inner membrane of the chloroplast envelope. In addition, plastoglobules associated with the thylakoid membrane are also believed to participate in tocopherol biosynthesis (Atkinson et al., 2008) [11] . Therefore, tocopherols or related chemicals can be applied externally to plants through different methods such as seed treatment, foliar spray, or application through the growing medium via roots (Tables 1 and2).

Pre-sowing Seed Treatment

Seed soaking is an effective technique to improve seed germination and seedling growth under both stress (Agholaghi and Sedghi, 2014) [4] and non-stress conditions (Saleem et al., 2014) [89] . It is a controlled hydration process in which metabolic activities within the seed are initiated, but radicle emergence is prevented (Ashraf and Foolad, 2005) [8] . Jie et al., (2008) [51] reported that, pre-treatment of Leymus chinensis seeds with 0.05-10.0 mmol L⁻¹ vitamin E increased antioxidant enzyme activity and proline content, while reducing malondialdehyde (MDA) levels in plants grown from treated seeds. Similarly, soaking sunflower seeds in an α-tocopherol (α-Tocopherol) solution reduced the harmful effects of salinity stress (Rady et al., 2011) [82] . vSakr and El-Metwally (2009) [88] also reported that, wheat seeds pre-treated with α-Tocopherol produced plants with higher levels of ascorbic acid (AsA) and phenolic compounds. These plants showed that, improved antioxidant activity, which helped protect them from the damaging effects of salinity stress. Although only a limited number of studies are available, seed soaking with α-tocopherol appears to be a practical and economical method for improving seed germination under stress conditions. This treatment can lead to better crop establishment and increased yield.

Foliar Spray

Foliar application is an effective method for improving plant growth and crop yield (Shahi and Srivastava, 2016) [97] . It is commonly used to supply micronutrients directly to plant leaves in order to overcome nutrient deficiencies. The response of plants to foliar application depends on the crop species and the prevailing agro-climatic conditions (Aslam et al., 2016) [10] . Foliar application becomes particularly useful when the root system cannot absorb sufficient nutrients due to factors such as soil compaction, low soil moisture or nutrient loss through leaching. It can also compensate for the plant's inability to synthesize adequate amounts of certain nutrients (Fageria et al., 2009; Herrera et al., 2016) [30,44] . After foliar application, many water-soluble compounds can easily enter the plant through stomata, the cuticle or the entire leaf surface (Latha and Nadanassababady, 2003) [61] . Several studies have reported that, foliar nutrient application during the vegetative stage increases internal mineral levels in plants and ultimately improves crop yield (Kolota and Osinska, 2001) [56] . Vitamin E (α-tocopherol) is a non-enzymatic antioxidant which, when applied as a foliar spray, can enhance plant growth and yield, particularly under stress conditions (Soltani et al., 2012) [101] .

Sereflioglu et al., (2017) [95] reported that, α-tocopherol acts as a signalling molecule from soybean leaves to roots under salinity stress. The external application of α-Tocopherol improved root length and root weight, increased endogenous auxin (IAA) concentration and peroxidase enzyme activity and reduced the levels of malondialdehyde (MDA), proline and hydrogen peroxide (H₂O₂) in soybean roots. Ye et al., (2017) [111] treated Carex leucochlora plants with 0.8 mM α-Tocopherol under salinity stress and observed that it alleviated the harmful effects of salinity. The treatment increased carotenoid content, free proline and chlorophyll b, while significantly reducing the production of superoxide radicals and hydrogen peroxide.

Rahmawati and Damanik (2018) [83] reported that, foliar application of 500 mg L⁻¹ α-Tocopherol significantly increased root and shoot dry weight, number of branches, chlorophyll content and total soluble proteins in soybean under salt stress. Similarly, Sadiq et al., (2018) [87] studied the effect of exogenous α-Tocopherol on mungbean plants under field water deficit conditions. They found that, α-Tocopherol application increased plant biomass, photosynthetic pigments, proline, glycine betaine, sugars and antioxidant enzyme activities. It also reduced the accumulation of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA). Conclusions and Future Prospects Vitamin E i.e. α-Tocopherol is an important antioxidant that helps to improve the plant growth and enhances tolerance to various abiotic stresses. It plays a significant role in protecting plants from oxidative damage under both normal and stress environmental conditions. However, the exact mechanisms by which α-tocopherol regulates stress tolerance at physiological, biochemical and molecular levels are still not fully understood and require further investigation. Tocopherols may also be involved in complex signalling pathways associated with reactive oxygen species (ROS), phytohormones, antioxidants and secondary metabolites. Therefore, it is important to identify the factors that regulate tocopherol expression under both stress and non-stress conditions.

Several studies have shown that, exogenous application of αtocopherol increases its internal concentration in plants and improves growth and stress tolerance. However, more research is needed to understand how tocopherol-related enzymes regulate the redox signalling and how α-tocopherol interacts with other metabolic pathways to help the plants adapt to unfavourable environmental conditions. Further studies are also required to clarify the role of tocopherols in regulating metabolic processes in plant at both cellular and whole-plant levels. In addition, it is important to evaluate the practical use of tocopherols under field conditions, considering their cost-benefit ratio and environmental impact. So far, very few studies have confirmed the positive effects of exogenous tocopherol application on crops grown under natural field conditions. Therefore, more field-based experiments are required to determine whether the results obtained under laboratory or greenhouse conditions can be successfully applied under variable field environments.

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