New insights into the function of tocopherols in plants
- First Online:
- Cite this article as:
- Munné-Bosch, S. & Falk, J. Planta (2004) 218: 323. doi:10.1007/s00425-003-1126-0
- 329 Views
Vitamin E represents a group of lipophilic antioxidants, which are important in human health. Biotechnological approaches to manipulate vitamin E content in plants, with the aim of improving human nutrition, has been a major focus of research in recent years (Grusak and DellaPenna 1999; Cahoon et al. 2003; Collakova and DellaPenna 2003; Qi et al. 2003). In biochemical terms, vitamin E includes closely related tocopherol and tocotrienol derivatives. Both groups of compounds consist of a chromanol head group with one, two or three methyl groups (δ-, β- or γ-, α-form) and an isoprenoid (prenyl) side chain. Tocopherols have a saturated phytyl tail while tocotrienols have a 3-fold unsaturated side chain. Though tocopherols and tocotrienols are exclusively synthesized in photosynthetic organisms, their role in plants has been far less understood than in humans. Usually, α-tocopherol is the major vitamin E form present in green plant tissue, while γ-tocopherol and tocotrienols accumulate to higher levels in seeds. To date, several lines of evidence indicate that tocopherols perform several functions in plants. Apart from their role as antioxidants and in maintaining membrane stability, tocopherols have been suggested to participate in intracellular signalling and in cyclic electron transport around photosystem II (reviewed by Munné-Bosch and Alegre 2002). Genomic-based approaches are now permitting new insights into the study of the function of tocopherols in plants (Porfirova et al. 2002; Sandorf and Hollander-Czytko 2002; Cahoon et al. 2003; Falk et al. 2003; Qi et al. 2003; Sattler et al. 2003; Welsch et al. 2003).
To date, all genes necessary to produce tocopherols have been identified in Synechocystis and Arabidopsis thaliana. In plants, the synthesis of tocopherols, tocotrienols and plastoquinones is closely linked. These three groups of compounds share the same aromatic precursor, homogentisic acid, which is synthesized by the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPD). A lack of HPD activity by a mutation in the hpd gene or by HPD-specific inhibitors impairs tocopherol and plastoquinone biosynthesis in higher plants leading to an albino phenotype (Norris et al. 1995). However, only tocopherol biosynthesis is impaired by a defect in the hpd gene in Synechocystis, thus suggesting an alternative plastoquinone biosynthetic pathway in this cyanobacterium (Dähnhardt et al. 2002). Moreover, it has been shown that different polyprenyltransferases are required for the synthesis of tocopherols and plastoquinones both in cyanobacteria and higher plants (Collakova and DellaPenna 2001; Schledz et al. 2001; Savidge et al. 2002). After the prenylation of homogentisic acid, three further enzymatic modifications occur, including two methylation reactions and a cyclization reaction forming the chromanol head group (Porfirova et al. 2002). Depending on the order and number of these reactions δ-, β-, γ- and α-tocopherol are obtained. The gene coding for the 2-methyl-6-phytylbenzoquinone methyltransferase, the first of two methyltransferases of the tocopherol biosynthetic pathway, has been biochemically recognized as a major inner-envelope protein, but could only recently be assigned to the tocopherol and plastoquinone biosynthetic pathway in the A. thaliana mutant apg1 (Motohashi et al. 2003). With all necessary DNA sequence data of the tocopherol biosynthetic pathway in A. thaliana at hand, it is now possible to address the regulation of the genes of the tocopherol biosynthetic pathway. Welsch et al. (2003) found an ATCTA element present in the promoter of photosynthesis-related genes and genes coding for the enzymes of the tocopherol and carotenoid biosynthetic pathway in A. thaliana, thus suggesting common regulatory mechanisms of photosynthesis and photoprotection.
Given the general assumption that tocopherols have antioxidant and photoprotective functions in plants, impairment of tocopherol synthesis is expected to increase sensitivity to oxidative stress. However, Synechocystis mutants with impaired tocopherol biosynthesis survive under both optimal and high light conditions in the absence of tocopherols (Collakova and DellaPenna 2001; Dähnhardt et al. 2002). In another study, an A. thaliana mutant totally devoid of tocopherols, vte1, showed a growth reduction of only 10–15% and small reductions in chlorophyll content and photosynthetic quantum yield when exposed to a light intensity of 850 μmol photons m−2 s−1 (Porfirova et al. 2002). These data suggest that tocopherols themselves may be dispensable in the protection of the photosynthetic apparatus under certain conditions, and it is speculated that alternative antioxidants or protective mechanisms may compensate for the tocopherol deficiency. In agreement with this, Havaux et al. (2003) showed that tocopherol deficiency is compensated in young and developing leaves of chlP transgenic tobacco plants, which have reduced geranygeranyl reductase activity, by a selective accumulation of xanthophyll cycle pigments, thus allowing photoacclimation in the long term and avoiding photooxidative damage. In addition, Trebst et al. (2002) have shown that tocopherols play a major role in protection of the D1 protein and the maintenance of photosystem II structure and function under high light in Chlamydomonas reinhardtii, and that various artificial chemical scavengers of singlet oxygen may be effective in compensating for a tocopherol deficiency. Taken together, these studies suggest that, while tocopherols contribute in maintaining the integrity of the photosynthetic apparatus, other alternative mechanisms of photo- and antioxidative protection may be up-regulated to afford adequate protection of the photosynthetic apparatus under tocopherol deficiency. On the other hand, in the npq1 mutant of A. thaliana, which is deficient in violaxanthin de-epoxidase, leaves accumulated higher amounts of α-tocopherol during photoacclimation (Havaux et al. 2000). Compensatory mechanisms between α-tocopherol, ascorbate and glutathione have also been shown in two sunflower cell lines differing in α-tocopherol biosynthetic capacity (Caretto et al. 2002). Moreover, interdependence between α-tocopherol, ascorbate and carnosic acid has been shown. In some carnosic acid-free plants of the Labiatae, enhanced α-tocopherol and ascorbate accumulation in chloroplasts helps to avoid damage to the photosynthetic apparatus, thus compensating for the lack of carnosic acid in leaves (Munné-Bosch and Alegre 2003). All these reports point to a cooperative action between tocopherols and other photoprotective and antioxidative mechanisms, thus highlighting the complexity and multitude of protective mechanisms that have evolved in plants to keep them alive. Only when all mechanisms of photo- and/or antioxidative protection processes are affected, or when stress is very severe, is control of cellular redox state lost; compensatory mechanisms are then unable to avoid oxidative damage and cell death occurs.
In conclusion, genomic-based approaches presently bring together invaluable tools to study the function of tocopherols and tocotrienols in plants. While studies on the function of tocopherols in plants have increased in the past years, much less attention has unfortunately been paid to tocotrienols. Though it is assumed that tocotrienols may prevent oxidation of lipids in seeds, the function of tocotrienols in plants remains poorly understood. One possible explanation of why tocotrienols have often been overlooked may be the erroneous view that dicots do not produce tocotrienols, though it has been reported that amaranths and tobacco seeds contain tocotrienols (Lehmann et al. 1994; Falk et al. 2003). In addition, though current data suggest an interaction between plant regulators and antioxidants in the control of the cellular redox state, further research is needed to better understand the role of tocopherols and tocotrienols in intracellular signalling. A combination of molecular, cellular, biochemical and physiological approaches will undoubtedly bring a deeper understanding of the function of tocopherols and tocotrienols in plants in the near future.