The flavonoid biosynthetic pathway has been analysed
in different S. lycopersicum lines [16], including S. lycopersicum
v. cerasiforme, which is the subspecies thought to
be the most probable wild ancestor of domesticated tomatoes
[17]. A major constraint found in the flavonoid biosynthetic
pathway is the lack of expression of the chalcone
isomerase (CHI) gene (Box 1, Figure Ia) in the peel of the
fruit, which is probably caused by a mutation in a fruitspecific
element of the promoter [16]. This could explain
the high level of naringenin chalcone in this tissue of
tomatoes, because this is the main substrate of the CHI
enzyme (Box 1, Figure Ia). The biosynthetic pathway, by
contrast, seems to be constitutively switched off in the flesh
of the fruit, where none of the structural genes analysed
are expressed [16]. Reintroducing the expression of CHI in
cultivated tomato fruit, by interspecific crosses with wild
tomato species [16] or heterologous expression of CHI