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

Abstract

SummaryBlue-light-dependent photomorphogenesis in Arabidopsis is regulated principally by the cryptochrome flavintype 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) 2 at the level of transcript accumulation of anthocyanin biosynthetic genes. Mutants in elongated hypocotyl 5 (hy5) 3 , 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.

BBX32, an Arabidopsis B-Box Protein, Functions in Light Signaling by Suppressing HY5-Regulated Gene Expression and Interacting with STH2/BBX21

“…Lee et al (2007) suggested that HY5 functions high in a regulatory hierarchy, regulating a branch of the transcriptional cascade involved in lightmediated development. Recent studies have shown that HY5 plays a role at points of convergence between light and phytohormonal signaling pathways (Cluis et al, 2004;Vandenbussche et al, 2007;Chen et al, 2008). HY5 transcript was detectable in all organs of mature plants, and the effects of hy5 mutation on the activation of light-responsive promoters fused to a GUS reporter could be detected in the leaves, stems, and roots of older plants (Oyama et al, 1997;Chattopadhyay et al, 1998).…”

DNA-Binding Study Identifies C-Box and Hybrid C/G-Box or C/A-Box Motifs as High-Affinity Binding Sites for STF1 and LONG HYPOCOTYL5 Proteins

“…Microarray analyses have shown that many auxinresponsive and auxin-signaling genes are misexpressed in hy5 mutants, an indication that the genes encoding auxin-signaling components are one group of the HY5 downstream genes (Cluis et al, 2004;Sibout et al, 2006). HY5 is also involved in cytokinin signaling (Vandenbussche et al, 2007). Cytokinin treatment results in similar growth responses to blue light, such as the development of leaves and chloroplasts, stimulation of anthocyanin production, and the inhibition of hypocotyl growth elongation (Chory et al, 1994).…”

“…Cytokinin treatment results in similar growth responses to blue light, such as the development of leaves and chloroplasts, stimulation of anthocyanin production, and the inhibition of hypocotyl growth elongation (Chory et al, 1994). It has been proposed that cytokinins increase the level of HY5 by reducing the degradation mediated by COP1 (Vandenbussche et al, 2007).…”

The Role of Brassinosteroids in Shoot Gravitropism

“…One-half-strength Murashige and Skoog medium (Duchefa) was prepared as described before (Vandenbussche et al, 2007). EBL and mannitol were from Sigma, Brz was purchased from TCI, Suc from VWR, and Glc from Merck.…”

“…Seeds were surface sterilized as described before (Vandenbussche et al, 2007) and sown on plates containing the respective media. The plates were kept for 48 h in darkness at 4°C and then transferred to white light and 22°C for 6 h to stimulate germination.…”