Signal transduction in the photocontrol of chalcone synthase gene expression*

The photocontrol of chalcone synthase gene expression was studied by means of promoter analyses, in vitro systems, photoreceptor mutants and microinjections, and pharmacological approaches. A 52 bp element of the promoter is necessary and sufficient to transfer light regulation. Chalcone synthase expression is primarily under the control of phyA and blue/UV photo receptors; 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 phosphorrylations were observed in membrane-depleted cytosol preparations, indicating very early processes of signal transduction. Microinjection and pharmacological experiments reveal that, in the phyA pathway, heterotrimeric G-proteins, cGMP and a genistein-sensitive protein kinase are involved, whereas the UV pathway includes several trimeric G-proteins and Ca/calmodulin-dependent steps.