PHYTOCHROME AND UV SIGNAL-TRANSDUCTION PATHWAYS

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.