Vitamin A and vitamin C are the principal vitamins in tomato fruit. Tomatoes also provide moderate levels of folate and potassium in the diet and lesser amounts of vitamin E and several water-soluble vitamins.
Vitamin A ß-carotene is the principal provitamin A carotenoid and is an essential nutrient in the human diet because of its retinoid activity (Tee 1992; Omenn et al. 1994). Epidemiological evidence indicates that increased intake of high ß-carotene containing fruit and vegetables may be associated with a reduced risk of heart disease and certain cancers (Ziegler 1989; Doll 1990; Block etal. 1992; Omenn etal. 1994). Vitamin A deficiency has been described as one of the most serious nutritional disorders of children in the world, especially in developing countries (Sommer 1997; World Health Organization 2005). Adapted germplasm with increased levels of this provitamin A carotenoid have been developed (Tomes and Quack-
Table 15. QTL that influence fruit nutritive value identified via analysis of segregating populations developed from crosses between S. lycopersicum and wild tomato species
Chromosomal locations Reference
S. lycopersicum S. habrochaites
S. neorickii S. pennellii
Ascorbic acid Total phenolics Color
S. pimpinellifolium Color
Causse et al. 2002
18 15 1,2,3,4,6,8,9,10,11 Bernacchi et al. 1998a, b;
Monforte et al. 2001; Kabelka et al. 2004; Yates et al. 2004
12 10 2, 3, 4, 6, 7, 8, 9, 10, 11, 12 Monforte et al. 2001;
Liu et al. 2003; Frary et al. 2004 1 5 3,5,10,12 Rousseaux et al. 2005
5 12 1, 3, 4, 6, 7, 8, 9, 10, 12 Fulton et al. 1997;
Monforte et al. 2001; Yates et al. 2004
Tanksley and Nelson 1996
enbush 1958; Tigchelaar and Tomes 1974; Stommel 2001; Stommel et al. 2005a).
Vitamin C Fruits and vegetables supply approximately 91% of the vitamin C in the USA food supply. Within S. lycopersicum and its wild relatives, ascorbic acid levels range from 10 to 120mg/100 g fresh weight (Lambeth et al. 1966; Hobson and Davies 1971). Depending upon cultivar, environment, fruit maturity and post-harvest treatment, ascorbic acid comprises 40 to 90% of the organic acids (Bradley 1946; Carangal etal. 1954; McClendon etal. 1959; Davies 1965). Malic acid is the principal organic acid. Stevens (1972) determined that citrate and malate concentration were controlled by single linked genes for each compound with the dominant alleles conditioning high citrate and low malate concentrations. Causse et al. (2003) reported additive inheritance for ascorbic acid content in 45 hybrids from parental lines that included ten large-fruited S. lycopersicon and three cherry-fruited (including one S. pimpinellifolium) types.
S. lycopersicum x S. peruvianum crosses have produced genotypes high in vitamin C. As noted previously, fruit of high pigment genotypes also contain increased levels of vitamin C. Although a negative relationship exists between vitamin C content and fruit size, demonstration that newer cultivars contain approximately 25% more vitamin C than those developed 20 years earlier suggests that additional incremental gains in genetic improvement of tomato vitamin C content may be achieved (Matthews et al. 1973; Burge et al. 1975).
In contrast with a well-understood ascorbic acid biosynthetic pathway in animals (Burns 1967), apath-way in plants was not proposed until 1998 (Wheeler et al. 1998). Agius et al. (2003) recently isolated the gene GalUR which encodes an NADPH-dependent D-galacturonate reductase and demonstrated that biosynthesis of ascorbic acid in strawberry fruit occurs through galacturonic acid, a component of cell wall pectins. Overexpression of GalUR in Arabidop-sis enhanced vitamin C levels two- to three-fold, thus demonstrating the potential to manipulate vitamin C levels. Analysis of S. pennellii introgression lines identified six QTLs for fruit ascorbic acid content, most of which had a negative effect on ascorbic acid concentration (Rousseaux et al. 2005). More recently, Zou et al. (2006) utilized S. pennellii introgression lines to map 15 genes involved in tomato ascorbic acid biosynthesis and metabolism.
Folate Plant sources, principally green leafy vegetables and legume seeds (Scott et al. 2000; Konings et al.
2001), are the main source of dietary folate. Folic acid dietary fortification is practiced to offset birth defects, anemia, and increased risk of vascular disease and certain cancers (Lucock 2000; Krishnaswamy and Nair 2001; Molloy and Scott 2001). The biosynthetic pathway of folate is well characterized (Hanson and Gregory 2002; Goyer et al. 2004), and thus provides good opportunity for genetic improvement of plant folate content (Zhang et al. 2003; Hossain et al. 2004). Overexpression of GTP cyclohydrolase I in fruit of tomato transformants resulted in a 3- to 140-fold increase in levels of the folate precursor, pteridine, and an average 2-fold increase in folate content (de la Garza et al. 2004). Exogenous supply of folate precursors resulted in additional 10-fold increases in folate content, suggesting that additional genetic modifications in the folate biosynthetic pathway may further boost fruit folate content.
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