Phenolic Compounds

Flavonoids, phenolic acids, and polyphenols are the main classes of dietary phenolics (King and Young 1999). Flavonoids, which include anthocyanins, are the largest group of plant phenols and have been the subject of considerable research since they impart color to many horticultural commodities. Vinson etal. (1998) determined that vegetables high in phenolic compounds have antioxidant quality superior to that of the antioxidant vitamins A, C and E. In a survey of S. lycopersicum and S. pimpinellifolium accessions, total phenolic content was most closely associated with measures of antioxidant activity (Hanson et al. 2004). Rousseaux et al. (2005) noted large environmental interactions for fruit antioxidants and identified nine QTLs for total phenolic concentration in fruit of S. pennellii ILs.

Flavonoids Flavonoids comprise a large group of secondary plant metabolites and include anthocyanins, flavonols, flavones, catechins, and flavonones (Har-borne 1994; Harborne and Williams 2000). As food constituents, flavonoids are believed to have numerous health promoting properties because of their antioxidant and free radical scavenging activity (Shahidi and Wanasundara 1992; Ames et al. 1993; Cook and Samman 1996; Hertog et al. 1997; Knekt et al. 1997; Rice-Evans et al. 1997; Sawa et al. 1999; Commenges et al. 2000; Harborne and Williams 2000; Nijveldt et al. 2001; Pietta 2000).

In tomato, the dominant anthocyanin fruit (Aft) gene elicits elevated levels of anthocyanin in the fruit skin and outer pericarp tissues, predominantly petu-nidin, and lesser amounts of malvidin and delphinidin (Giorgiev 1972; Jones et al. 2003). The recessive atro-violacium (atv; Rick 1963) and dominant Aubergine (Abg; Rick et al. 1994a) loci also result in varying degrees of anthocyanin accumulation in fruit epidermal tissues.

Genetic diversity for flavonoids in tomato and a well-characterized biosynthetic pathway make tomato an attractive crop in which to develop cultivars with enhanced flavonoid levels. Numerous efforts have focused on manipulation of transgene expression to enhance fruit flavonoids (Muir et al. 2001; Bovy et al. 2002; Colliver et al. 2002). Whereas conventional selection schemes have relied upon color assays or quantification of flavonoid products to identify unique genotypes that may be useful in breeding programs, Willits et al. (2005) devised an alternative strategy and identified two S. chilense accessions and a single S. pennellii accession that expressed structural genes of the anthocyanin biosynthetic pathway in the fruit peel and fruit flesh. Introgression of the S. pennellii accession into tomato produced progeny that accumulated high levels of quercetin in fruit flesh and peel.

Phenolic Acids Phenolic acids form a diverse group that includes the widely distributed hydroxybenzoic and hydroxycinnamic acids. Hydroxycinnamic acid esters of caffeic acid predominate in Solanaceous species and chlorogenic acid is typically the most abundant (Molgaard and Ravn 1988). Chlorogenic and related caffeoyl esters are among the most potent free radical scavengers in plant tissues (Sawa et al. 1999; Nakatani et al. 2000) and act as antioxidants in human erythrocytes and for low-density lipoproteins in vitro (Nardini et al. 1995; Lekse et al. 2001).

Silencing of candidate genes demonstrated that hydroxycinnamoyl transferase (HQT) was the primary route for chlorogenic acid accumulation in Solanaceous species (Niggeweget al. 2004).HQTover-expression caused plants to accumulate higher levels of chlorogenic acid, with no demonstrable negative effects on levels of other soluble phenolics or lignin.

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