The pbotocontrol of cbalconc syntbase gene expression was studied by means of promoter analyses, in vitro systems, pbotoreceptor mutants and microinjections, and pbarmacological approaches. A 52 bp element of the promoter is necessary and sufficient to transfer lij»ht regulation. Chaicone synthase expression is primarily under the control of phyA and blue/lJV photoreceptors; the latter are functional even in the absence of phyA and phyB. Phytochromes seem to be soluble proteins and, within seconds of irradiation, light-dependent phosphorylations were observed in membrane-depleted cytosol preparations, indicating very early processes of signal transduction. Microinjection and pharmacological experiments reveal that, in the phyA pathway, heterotrinieric (J-proteins, edllVlP and a genistein-sensitive protein kinase are involved, whereas the UV pathway includes several trimerie G-proteins and Ca/calmodulindependent steps.