Identification of BBX proteins as rate-limiting co-factors of HY5

As a source of both energy and environmental information, the importance for plants to monitor the incoming light is evidenced by the highly integrated nature of the light signal transduction pathway with numerous other pathways throughout plant development. One of these signal integrators is the bZIP transcription factor HY5 which holds a key role as a positive regulator of light signalling in plants. Although HY5 is described to act as a DNA-binding transcriptional regulator, the lack of any apparent transactivation domain makes it unclear how HY5 is able to accomplish its many functions. We describe the identification of three B-box containing proteins (BBX20, 21 and 22) as essential partners for HY5 dependent modulation of hypocotyl elongation, anthocyanin accumulation and transcriptional regulation. The bbx202122 triple mutant mimics the phenotypes of hy5 in the light and its strong suppression of the cop1 phenotype in darkness. Furthermore, RNA-seq experiments show that 84% of genes differentially regulated in bbx202122 are also HY5 regulated, consistent with the B-box proteins and HY5 acting interdependently. Overall design: Investigation of transcriptomic changes in bbx202122 and hy5 Arabidopsis seedlings grown for 4 days in red light, in triplicate, using RNA-seq analysis.