The phytochromes are the best studied plant photoreceptors, controllin
g a wide variety of responses at both whole plant and single cell leve
ls. Three signal transduction pathways, dependent on cGMP and/or calci
um, have been found to be utilized by phytochrome to control the expre
ssion of genes required for chloroplast development (e.g., CAB and FNR
) and anthocyanin biosynthesis (e.g., CHS). In particular, cGMP is a s
econd messenger positively regulating CHS gene expression whilst calci
um and calmodulin act as negative regulators. In addition to phytochro
me regulation of CHS we have begun to examine the signal transduction
pathways utilized by UV photoreceptors. In contrast to phytochrome-med
iated responses, results indicate a role for calcium and calmodulin as
positive regulators of CHS gene expression in UV light.