HY5 is a point of convergence between cryptochrome and cytokinin signalling pathways in Arabidopsis thaliana

Blue-light-dependent photomorphogenesis in Arabidopsis is regulated principally by the cryptochrome flavin-type photoreceptors, which control hypocotyl growth inhibition, cotyledon and leaf expansion, and the expression of light-regulated genes. Interestingly the plant hormone cytokinin induces similar responses when added exogenously to germinating seedlings, suggesting a link between cryptochrome and cytokinin signalling pathways. In this work we explore the relationship between cryptochrome and cytokinin signalling pathways in the promotion of photomorphogenesis. The effect of exogenously added cytokinins on hypocotyl growth inhibition occurs in the dark, and is largely independent and additive to that of cryptochromes in blue light, via distinct signalling pathways. By contrast, cytokinin-dependent stimulation of anthocyanin accumulation occurs only in light, and interacts with the signalling pathway downstream of cryptochrome 1 (CRY1) at the level of transcript accumulation of anthocyanin biosynthetic genes. Mutants in elongated hypocotyl 5 (hy5), a downstream intermediate in the CRY1 signalling pathway, show a reduced induction of anthocyanin accumulation in blue light by cytokinins, similar to that observed for cryptochrome (cry1) mutants. Furthermore cytokinins are shown to increase levels of HY5 protein accumulation, suggesting that cytokinins may function by reducing HY5 degradation by COP1 (constitutively photomorphogenic 1). As both cryptochrome and cytokinin signalling pathways increase HY5 protein levels, and as HY5 binds to the promoters of anthocyanin biosynthetic enzymes to stimulate gene expression, it is concluded that the regulation of HY5 protein stability represents a point of convergence between cryptochrome and cytokinin signalling pathways.