Box 1. The anthocyanin biosynthetic pathway
Anthocyanins are synthesized through a branch of the flavonoid
biosynthetic pathway (Figure Ia) (reviewed in Refs [11,12]). Genes
involved in the pathway can be grouped into two different classes:
those encoding the enzymes that catalyse the different reaction
steps (structural genes) and those regulating the expression of the
structural genes (regulatory genes). The enzymatic reactions of
flavonoid biosynthesis and most of the corresponding structural
genes have been identified and cloned in several species, including
cultivated tomato plants, where many of them have been mapped
[32]; most of the relative EST (expressed sequence tag) sequences
are deposited in GenBank or in the Sol Genomic Network database
(http://www.sgn.cornell.edu).
Regulatory genes encode TFs and other regulatory proteins,
which interact with the promoter regions of the structural genes,
enabling precise temporal and spatial coordination of their transcription
and of the resulting production of anthocyanins. Several
studies indicate that two main classes of TFs are involved in
anthocyanin production: R2R3 MYB-type and bHLH-type TFs [31].
Recent models indicate that these physically interact and, with the
participation of another protein with a ‘WD40’ repeat (WDR), form a
MYB–bHLH–WDR (MBW) transcription complex that is directly
responsible for the activation of the structural gene transcription