Previous attempts by others to enhance flavonoid content by
manipulating structural or regulatory genes found that a major
limiting factor along the flavonoid biosynthetic pathway was the
lack of expression of the CHI gene in the tomato peel, likely caused
by a mutation in the promoter (Gonzali and others 2009). Reintroduction of the expression of the CHI gene in cultivated tomato fruit can be achieved by interspecific crosses with wild tomato species or ectopic expression of CHI isolated from other species (Muir and others 2001; Willits and others 2005). A single CHI gene suffices to increase flavonol content in tomato peel (Colliver and others 2002). Although the chalcone synthase (CHS), CHI, flavanone-3-hydroxylase (F3H), and flavonol synthase (FLS) genes were ectopically expressed simultaneously, the flavonol content of the tomato with the 4 genes was not significantly increased compared with tomatoes in which only the CHI gene was expressed.