Fruit color and vitamin A

Tomato fruit color is primarily determined by the concentration of the red carotenoid, lycopene, and its precursors. Because these carotenoid pigments are the source of provitamin A as well as of the other major nutritional antioxidants in tomato, their levels have significant consequences for both the appearance and nutritional value of fresh and processed tomatoes. Modification of the carotenoid composition of tomato fruit has been achieved by transgenic modification of the activity of a key enzyme in the carotenoid biosynthetic pathway, phytoene synthase (Fraser et al., 1995) that converts phytoene to lycopene. Suppression of phytoene synthase gene Psy(1) expression significantly reduced carotene and xanthophyll as well as abscisic acid (ABA) in ripe fruit compared to control fruit. The resulting yellow fruit were very similar to naturally occurring phytoene synthase mutant fruit (Fray and Grierson, 1993). Plants constitutively over expressing Psy1 are dwarf, apparently because gibberellin biosynthesis is reduced, and have less chlorophyll in their leaves (Fray et al., 1995). A second phytoene synthase gene Psy(2) also is expressed in ripening fruit but apparently is not involved in carotenoid synthesis in this tissue (Fraser et al., 1999).

Recently, in an effort to increase the vitamin A content of tomato, the expression of a bacterial phytoene desaturase in tomato has been reported (Romer et al., 2000). The Erwinia uredovora crtI gene was expressed constitutively in tomato plastids. The total carotene composition of these plants was unaltered although the proportion of β-carotene increased moderately. The lycopene produced in the transgenic plants was cyclized by the induction of two endogenous lycopene cyclases to increase the β-carotene content about two-fold in these orange fruit. Unlike retinol or vitamin A, β-carotene or pro vitamin A is non-toxic and can be stored by after human ingestion. Thus, this transgenic modification provides provides an effective nutritional supplement of about 40% of the recommended daily consumption of vitamin A in a single fruit.