The flesh on the inside of an apple is white, and devoid of pigments. Why is this the case?

The accumulation of anthocyanin pigments in apple fruit is an important determinant of fruit quality. Usually restricted to the skin of apples, these pigments provide essential cultivar differentiation for consumers and are implicated in the health attributes of apple fruit (Boyer and Liu, 2004). Anthocyanins belong to the diverse group of ubiquitous secondary metabolites collectively known as flavonoids. In plants, flavonoids are believed to have a variety of functions, including defence and protection against light stress, and the pigmented anthocyanin compounds play an important physiological role as attractants in plant/animal interactions (Harborne and Grayer, 1994; Koes et al., 1994).

Differences in colour attributable to anthocyanins may be due to a number of factors including the number of hydroxyl groups on the B-ring, the sugars and acyl side groups (Harborne, 1967), the environment of the vacuole including its pH or the accumulation of specific metal ions (Brouillard, 1988), or cellular ultrastructure (Noda et al., 1994). One of the most common anthocyanin pigments is cyanidin, which, in the form of cyanidin 3-O-galactoside, is the pigment primarily responsible for red colouration in apple skin (Lancaster, 1992; Tsao et al., 2003). The enzymes operating in this biosynthetic pathway in apple have been well characterized (Honda et al., 2002; Kim et al., 2003).

In apple, pigment biosynthesis may be induced by light, particularly UV, and various stress treatments including cold (Dong et al., 1998). There is also evidence that the anthocyanin biosynthetic enzymes are coordinately induced during development of apple fruit (Lister and Lancaster, 1996). This suggests that expression of the genes encoding the biosynthetic enzymes is coordinately regulated by one or a few regulatory proteins.

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