Production of high-value compounds: carotenoids and vitamin E
Introduction
Plant breeding has been focusing over the years mainly on increasing crop productivity by pursuing higher yields and better adaptation to biotic and abiotic stresses. Improving nutrient composition of plant foods, however, is no less important. There are still vast populations, mainly in the developing world, whose diet lacks essential nutrients, such as vitamins and minerals. For example, it was estimated that over 124 million children worldwide are vitamin A deficient, and that improved vitamin A nutrition alone could prevent 1.3–2.5 million deaths among late infancy and preschool-age children that occur each year in the developing countries 1, 2. Modern breeding tools of gene transformation open up the possibility for metabolic engineering targeted at improving nutritional quality of plants. To manipulate complex metabolisms it is necessary to unravel the biochemical pathways and understand the regulation underlying metabolic fluxes. This paper examines the progress made in understanding the biosynthesis and regulation of carotenoids (vitamin A) and tocopherols (vitamin E) and describes attempts to manipulate these pathways.
Section snippets
Carotenoids (vitamin A)
Carotenoids are 40-carbon isoprenoids, which consist of eight isoprene units. Carotenes are hydrocarbons that are either linear or cyclized at one or both ends of the molecule. Oxygenated derivatives of carotenes are called xanthophylls. The polyene chain, consisting of conjugated double bonds, is responsible for their characteristic colors and their photochemical properties [3]. In plants, carotenoids are essential for photosynthesis where they serve as accessory light-harvesting pigments and
Tocopherols (vitamin E)
Tocopherols (collectively known as vitamin E) are a class of lipid-soluble antioxidants, which are essential ingredients in human nutrition [33]. Epidemiological evidence has indicated that vitamin E supplementation results in decreased risk for cardiovascular disease and cancer, aids in immune function, and prevents or slows a number of degenerative diseases associated with aging, such as cataracts, arthritis, and disorders of the nervous system, caused by cumulative damage to tissues mediated
Biosynthesis of tocopherols
Tocopherols are synthesized from precursors of two pathways: the isoprenoid pathway, which provides the hydrophobic tail, and that of homogentisic acid formation, which provides the head group (Figure 2). The biosynthetic pathway in plants was elucidated three decades ago using classical biochemical studies 38, 39. Many details remained obscured, however, due to lack of molecular description. Recently, significant progress was made at the molecular level regarding the synthesis and accumulation
Genetic manipulation of α-tocopherol
Deciphering the biosynthetic pathway of α-tocopherol and the availability of cloned genes for key enzymes have made metabolic engineering of the pathway possible. One can predict rate limiting steps whose up-regulation could increase the flux through the pathway to reach relatively higher amounts of tocopherols in a given plant tissue. Availability of HGA and the phytol sidechain are potentially important in this respect. Manipulating the enzymes HPPDase, phytyl/prenyl transferase and GGDP
Conclusions
Current understanding of the carotenoids and tocopherols biosynthesis in plants enables the genetic manipulation of these pathways. Modifying existing pathways has achieved alteration in composition of tocopherols and diversion of carotenoids to a novel high-value species. Attempts to increase the concentration of these compounds, however, have yielded so far only limited results.
Acknowledgements
I thank Dean DellaPenna for helpful suggestions and careful reading of the manuscript section on vitamin E. Research in my laboratory is supported by Grant 578/97 from the Israel Science Foundation and by the Israel Ministry of Science.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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