Identification of BBX proteins as rate-limiting cofactors of HY5

LONG HYPOCOTYL 5 (HY5) is a central transcription factor that acts downstream of CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) during light signalling. Here, the authors show that three B-box containing proteins (BBX) are cofactors of HY5 and modulate transcriptional responses for a subset of HY5-dependent processes. As a source of both energy and environmental information, monitoring of incoming light is crucial for plants to optimize growth throughout development(1). Concordantly, the light signalling pathways in plants are highly integrated with numerous other regulatory pathways(2,3). One of these signal integrators is the basic leucine zipper domain (bZIP) transcription factor LONG HYPOCOTYL 5 (HY5), which has a key role as a positive regulator of light signalling in plants(4,5). Although HY5 is thought to act as a DNA-binding transcriptional regulator(6,7), the lack of any apparent transactivation domain(8)makes it unclear how HY5 is able to accomplish its many functions. Here we describe the identification of three B-box containing proteins (BBX20, BBX21 and BBX22) as essential partners for HY5-dependent modulation of hypocotyl elongation, anthocyanin accumulation and transcriptional regulation. Thebbx20 bbx21 bbx22(bbx202122) triple mutant mimics the phenotypes ofhy5in the light and its ability to suppress thecop1mutant phenotype in darkness. Furthermore, 84% of genes that exhibit differential expression inbbx202122are also regulated by HY5, and we provide evidence that HY5 requires the B-box proteins for transcriptional regulation. Finally, expression of a truncated dark-stable version of HY5 (HY5(Delta N77)) together with BBX21 mutated in its VP motif strongly promoted de-etiolation in dark-grown seedlings, demonstrating the functional interdependence of these factors. In sum, this work clarifies long-standing questions regarding HY5 action and provides an example of how a master regulator might gain both specificity and dynamicity through the obligate dependence of cofactors.