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Review
. 2017 Dec 25;7(1):2.
doi: 10.3390/antiox7010002.

Vitamin E Biosynthesis and Its Regulation in Plants

Laurent Mène-Saffrané  1
Affiliations
Review

Vitamin E Biosynthesis and Its Regulation in Plants

Laurent Mène-Saffrané. Antioxidants (Basel). .

Abstract

Vitamin E is one of the 13 vitamins that are essential to animals that do not produce them. To date, six natural organic compounds belonging to the chemical family of tocochromanols-four tocopherols and two tocotrienols-have been demonstrated as exhibiting vitamin E activity in animals. Edible plant-derived products, notably seed oils, are the main sources of vitamin E in the human diet. Although this vitamin is readily available, independent nutritional surveys have shown that human populations do not consume enough vitamin E, and suffer from mild to severe deficiency. Tocochromanols are mostly produced by plants, algae, and some cyanobacteria. Tocochromanol metabolism has been mainly studied in higher plants that produce tocopherols, tocotrienols, plastochromanol-8, and tocomonoenols. In contrast to the tocochromanol biosynthetic pathways that are well characterized, our understanding of the physiological and molecular mechanisms regulating tocochromanol biosynthesis is in its infancy. Although it is known that tocochromanol biosynthesis is strongly conditioned by the availability in homogentisate and polyprenyl pyrophosphate, its polar and lipophilic biosynthetic precursors, respectively, the mechanisms regulating their biosyntheses are barely known. This review summarizes our current knowledge of tocochromanol biosynthesis in plants, and highlights future challenges regarding the understanding of its regulation.

Keywords: homogentisate; nutrigenomics; plastochromanol-8; polyprenyl pyrophosphate; tocochromanol; tocomonoenol; tocopherol; tocotrienol; vitamin E.

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Conflict of interest statement

The author declares no conflict of interest.

Figures

Figure 1
Figure 1
Tocochromanol biosynthetic pathways in plants. Tocochromanol and prenyl benzoquinol chemical structures and biosynthetic enzymes (highlighted in orange). Tocochromanol and prenyl benzoquinol names are color-coded to distinguish each tocochromanol pathway: red for the tocopherol pathway, blue for the tocotrienol pathway, green for the tocomonoenol pathway, and orange for the PC-8 and methyl PC-8 pathway. The α-, β-, γ-, and δ-forms of tocopherols, tocotrienols, and tocomonoenols have been identified in plants. For solanesyl-derived tocochromanols, only PC-8 has been identified in wild-type plants, and only methyl PC-8 has been identified in transgenic Arabidopsis overexpressing the γ-TMT/VTE4 gene. Abbreviations: HGA, homogentisate; HGGT, homogentisate geranylgeranyltransferase; HPT, homogentisate phytyltransferase; HST, homogentisate solanesyltransferase; GGPP, geranylgeranyl pyrophosphate; γ-TMT, γ-tocopherol methyltransferase; MT, methyltransferase; PC-8, plastochromanol-8; PPi, pyrophosphate; PPP, phytyl pyrophosphate; PQ-9, plastoquinol-9; SAM, S-adenosyl-l-methionine; SAH, S-adenosyl-l-homocysteine; SPP, solanesyl pyrophosphate; TC, tocopherol cyclase; THGGPP, tetrahydrogeranylgeranyl pyrophosphate. Prenyl benzoquinol acronyms are detailed in the main text.
Figure 2
Figure 2
Biosynthesis and transport of homogentisate in plants. Red, orange, and blue dotted lines correspond to transgenic plants overexpressing the coding sequence of yeast PDH, bacterial CM/PDH, and plant HPPD, respectively, all fused to a sequence encoding a chloroplast transit peptide. Biosynthetic enzymes demonstrated to be involved in tocochromanol synthesis are highlighted in blue or gray. In species such as Arabidopsis, HPPD is localized in the cytoplasm. In contrast, in maize, tomato, and cotton, HPPD genes exhibit a typical chloroplast transit signal, suggesting that this enzyme is localized in the chloroplasts of these species (HPPD in grey). In soybean, HPPD have been localized in both compartments. Plant species in which HGA biosynthesis is localized in the cytosol must have chloroplast membrane transporters (blue boxes) exporting Tyr and HPP into the cytosol, and importing HGA back into chloroplasts. Abbreviations: CM/PDH, bacterial bi-functional chorismate mutase/prephenate dehydrogenase; E4P, erythrose 4-phosphate; GGPP, geranylgeranyl pyrophosphate; Glu, glutamate; HGA, homogentisate; HGO, homogentisate dioxygenase; HPP, 4-hydroxyphenylpyruvate; HPPD, 4-hydroxyphenylpyruvate dioxygenase; 4-MA, 4-maleylacetoacetate; 2-OG, 2-oxoglutarate; PDH, prephenate dehydrogenase; PEP, phosphoenolpyruvate; PPP, phytyl pyrophosphate; TAT, tyrosine aminotransferase; Tyr, l-tyrosine.
Figure 3
Figure 3
HPPD expression pattern during Arabidopsis development. The expression of the Arabidopsis HPPD gene (At1g06570) was assessed with ePlant (http://bar.utoronto.ca/eplant). HPPD expression level is equal to 697.5 in green cotyledons (SD = 36.3; n = 3), and is the third highest HPPD expression after sepals (level = 971.7; SD = 84.2; n = 3) and senescent leaves (level = 797.7; SD = 6.9; n = 3).
Figure 4
Figure 4
Plastidic intersection of the shikimate pathway, the methyl erythritol phosphate pathway, and lipogenesis. The shikimate pathway, methyl erythritol phosphate (MEP) pathway, and fatty acid synthesis are localized in plastids, and share phosphoenolpyruvate and pyruvate as common biosynthetic precursors. The biosynthesis of triacylglycerols occurs in the endoplasmic reticulum. Arabidopsis Genome Initiative numbers in red indicate that the corresponding gene is downregulated in Arabidopsis wri1 developing embryos, or upregulated in transgenic plants constitutively expressing the WRI1 transcription factor. AGI numbers in black indicate that the expression of the corresponding gene is WRI1-independent. AGI numbers in blue indicate that no transcriptomic data was available. Abbreviations: DGAT1, acylCoA:diacylglycerol acyltransferase 1; E4P, erythrose 4-phosphate; G3P, glyceraldehyde 3-phosphate; phytyl-PP, phytyl pyrophosphate.
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