Plants and microorganisms provide a primary natural source of essential vitamins for human diets. These micronutrients are necessary for human health and development. There has been a quest over the last decade to discover genes and biochemical processes which control an individual plant's production of these health-promoting micronutrients. They are generally classified into fat and water soluble vitamins, depending on their biochemical properties. The group of fat soluble vitamins consists of provitamin A (typically a- or p-carotene), vitamins D (calciol), E (tocopherols and tocotrienols) and K1 (phylloquinone). The water soluble class of vitamins include vitamin B1 (thiamine), B2 (riboflavin, nicotinamide, folate and pantothenate), B6 (pyridoxal), B12 (cobalamine), C (ascorbate) and H (biotin). A dietary sufficiency of these essential micronutrients is not typically a major hurdle for residents of wealthy nations. They can be obtained from a balanced diet or via vitamin and mineral supplements. However, people from developing nations rely upon only a few staple crops, such as rice, maize, wheat and cassava, all of which are poor sources of essential nutrients. Even if people consume large quantities of these foods, they are still at high risk of malnutrition.
Biofortification is a sustainable means to enrich the nutrient content of crops. The goal is to provide a balanced group of micronutrients in staple crops, as opposed to adding them by fortification during processing in products, such as bread flour. Target crops include wheat and rice enriched with iron, vitamin A (prevents blindness), vitamin E (antioxidant activities) and folate (a B-vitamin which prevents congenital disorders such as spina bifida and facial deformities). There are a number of wheat, rice and maize varieties available to plant breeders and, when equipped with the appropriate molecular tools, they can select for varieties containing optimal levels of some micronutrients for bread wheat, pasta and noodles. For particular micronutrients in specific species, genetic modification is essential—for example, the production of golden rice which enhances provitamin A content (Paine et al. 2005). Fruits and vegetables also have the potential to be fortified with bone-building calcium and the nutrient content can be enhanced though genetic modification. The reason for all this scientific effort being targeted at fruits, vegetables and grains is that they can provide a rich source of a number of vitamins and minerals essential to human health (reviewed in Davies 2007).
The health benefits associated with vitamins, carotenoid metabolites and other micronutrients, as well as their modification in plants have been reviewed extensively (Herbers 2003; Fraser and Bramley 2004; Krinsky and Johnson 2005; DellaPenna and Pogson 2006; Davies 2007; Tanaka and Ohmiya 2008; Giuliano et al. 2008). The nutraceutical industry has targeted carotenoids mainly on the basis of their antioxidant properties, some having provitamin A activity, and their potential role in limiting age-related macular degeneration of the eye leading to blindness. Five carotenoids are manufactured synthetically on an industrial scale, namely lycopene, p-carotene, canthaxanthin, zeaxanthin and astaxanthin. These nutraceuticals are used in a range of food products and cosmetics, such as vitamin supplements, and health products, and as feed additives for poultry, livestock, fish and crustaceans, which are essential for the animals' growth, health and reproduction (reviewed in Del Campo et al. 2007; Jackson et al. 2008). The following section reviews only the medical benefits and molecular approaches for biosynthesis associated with (1) lutein, zeaxanthin and prevention of macular degeneration of the eye, (2) p-carotene and the biosynthesis of provitamin A, and (3) antioxidant properties of other xanthophylls and xanthophyll derivatives.
7.3.1 Zeaxanthin, Lutein and Prevention of Macular Degeneration
Both lutein and zeaxanthin are necessary for efficient photoprotection in plants and have been implicated in protecting against age-related blindness due to macular degeneration (AMD) in humans (reviewed in Coleman and Chew 2007). Macular degeneration is the leading cause of blindness in the developed world and results in the loss of central and detailed vision especially in the elderly over 60 years of age. Within the central macula, zeaxanthin is the dominant carotenoid, whereas in the peripheral retina, lutein predominates. These pigments are able to absorb blue light which damages photoreceptors and pigmentary epithelium. Due to their antiox-idative properties, they can reduce changes in membrane permeability via quenching reactive oxygen species and free radicals. A higher dietary intake of lutein and zeaxanthin was independently associated with decreased likelihood of having neovascular age-related macular degeneration, geographic atrophy, and large or extensive intermediate drusen, while other nutrients such as vitamin A, a-tocoph-erol and vitamin C did not show a similar association (SanGiovanni et al. 2007). Zeaxanthin is the pigment which gives corn its characteristic colour. Some yellow fruits and vegetables and almost any green vegetable are considered a good source of lutein and may have some zeaxanthin. These may include kale, spinach, turnip greens, collard greens, romaine lettuce, broccoli, zucchini, garden peas and brussels sprouts.
A number of biotechnological approaches are underway to establish algal production systems to replace industrial synthesis of lutein and zeaxanthin (Del Campo et al. 2007). The unicellular green alga Dunaliella salina is currently being cultivated as a source of p-carotene by the natural products industry in Australia and Israel. A novel mutant (zeal) of the halotolerant unicellular green alga D. salina is impaired in the zeaxanthin epoxidation reaction, thereby lacking a number of the beta-branch xanthophylls. The zeal mutant lacks neoxanthin, vio-laxanthin and antheraxanthin, but constitutively accumulates zeaxanthin with no adverse effects upon photosynthesis or growth of the zeal strain (Jin et al. 2003).
The staple crop potato, which accumulates high concentrations of lutein and violaxanthin, was altered genetically by suppressing the potato zeaxanthin epox-idase gene, which blocked the conversion from zeaxanthin to violaxanthin and increased tuber concentrations of zeaxanthin (four- to 130-fold) and slightly enhanced antheraxanthin without affecting the level of lutein. Surprisingly, manipulation of this enzyme also resulted in elevated transcript levels of the first step in the carotenoid pathway and a concomitant two to threefold increase in a-tocopherol (vitamin E; Romer et al. 2002). In addition to the potential health benefits displayed by zeaxanthin, it may limit lipid peroxidation and enhance the maintenance of membrane fluidity and thermostability (Tardy and Havaux 1997; Gruszecki et al. 1999). Therefore, manipulation of zeaxanthin and lutein may not only improve the nutritional value of the foods, but may also improve plant vigour.
Lutein is the most abundant carotenoid found in plants and its levels are remarkably stable, with no known natural mutants being identified to date. Mutant screening has previously identified lutein deficient mutants in Arabidopsis (ccrl, ccr2, lut2 and lutl) which exhibit altered non-photochemical quenching and light harvesting complexes (Pogson et al. 1996, 1998; Park et al. 2002; Cazzonelli et al. 2009a). There are examples of plant species which contain higher concentrations of a-carotene (Fig. 7.1; e.g. Coffea canephora), the precursor to lutein (Simkin et al. 2008), and it is likely that variations in eLCY amino acid composition may be responsible for modifying the eLCY and/or pLCY enzyme activity, thereby causing an increased flux of carotenoids through the lutein branch of the pathway (Cunningham and Gantt 2001; Howitt et al. 2009). As a result, natural variation in the amount of lutein amongst different wheat varieties and novel molecular markers (linked with phytoene synthase and epsilon cyclase biosynthetic genes) have been developed, which may now provide the potential to breed new varieties to produce grains rich in lutein (Howitt et al. 2009). Alternatively, further insight into chromatin modifying enzymes (e.g. SDG8; Fig. 7.1) which alter the accessibility of transcription factors to the CRTISO promoter, and affect not only plant development but also lutein composition, may provide the switch to increase antioxidant levels in staple food crops (Cazzonelli et al. 2009a).
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