Identification of BBX proteins as rate-limiting cofactors of HY5.

As a source of both energy and environmental information, monitoring of incoming light is crucial for plants to optimize growth throughout development1. Concordantly, the light signalling pathways in plants are highly integrated with numerous other regulatory pathways2,3. 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 plants4,5. Although HY5 is thought to act as a DNA-binding transcriptional regulator6,7, the lack of any apparent transactivation domain8 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. The bbx20 bbx21 bbx22 (bbx202122) triple mutant mimics the phenotypes of hy5 in the light and its ability to suppress the cop1 mutant phenotype in darkness. Furthermore, 84% of genes that exhibit differential expression in bbx202122 are 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(ΔN77)) 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.

Degradation of the transcription factors PIF4 and PIF5 under UV-B promotes UVR8-mediated inhibition of hypocotyl growth in Arabidopsis.

Inhibition of hypocotyl growth is a well-established UV-B-induced photomorphogenic response that is mediated by the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8). However, the molecular mechanism by which UVR8 signaling triggers inhibition of hypocotyl growth is poorly understood. The bZIP protein ELONGATED HYPOCOTYL 5 (HY5) functions as the main positive regulatory transcription factor in the UVR8 signaling pathway, with HY5-HOMOLOG (HYH) playing a minor role. However, here we demonstrate that hy5 hyh double mutants maintain significant UVR8-dependent hypocotyl growth inhibition. We identify UVR8-dependent inhibition of the activities of bHLH transcription factors PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 as part of the UVR8 signaling pathway, which results in inhibition of hypocotyl growth. The UVR8-mediated repression of several hypocotyl elongation-related genes is independent of HY5 and HYH but largely associated with UVR8-dependent degradation of PIF4 and PIF5, a process that consequently diminishes PIF4/5 target promoter occupancy. Taken together, our data indicate that UVR8-mediated inhibition of hypocotyl growth involves degradation of PIF4 and PIF5. These findings contribute to our mechanistic understanding of UVR8-induced photomorphogenesis and further support the function of PIFs as integrators of different photoreceptor signaling pathways.

Arabidopsis RUP2 represses UVR8-mediated flowering in noninductive photoperiods.

Plants have evolved complex photoreceptor-controlled mechanisms to sense and respond to seasonal changes in day length. This ability allows plants to optimally time the transition from vegetative growth to flowering. UV-B is an important part intrinsic to sunlight; however, whether and how it affects photoperiodic flowering has remained elusive. Here, we report that, in the presence of UV-B, genetic mutation of REPRESSOR OF UV-B PHOTOMORPHOGENESIS 2 (RUP2) renders the facultative long day plant Arabidopsis thaliana a day-neutral plant and that this phenotype is dependent on the UV RESISTANCE LOCUS 8 (UVR8) UV-B photoreceptor. We provide evidence that the floral repression activity of RUP2 involves direct interaction with CONSTANS, repression of this key activator of flowering, and suppression of FLOWERING LOCUS T transcription. RUP2 therefore functions as an essential repressor of UVR8-mediated induction of flowering under noninductive short day conditions and thus provides a crucial mechanism of photoperiodic flowering control.

Regulation of MYB and bHLH transcription factors: a glance at the protein level.

In complex, constantly changing environments, plants have developed astonishing survival strategies. These elaborated strategies rely on rapid and precise gene regulation mediated by transcription factors (TFs). TFs represent a large fraction of plant genomes and among them, MYBs and basic helix-loop-helix (bHLHs) have unique inherent properties specific to plants. Proteins of these two TF families can act as homo- or heterodimers, associate with proteins from other protein families, or form MYB/bHLH complexes to regulate distinct cellular processes. The ability of MYBs and bHLHs to interact with multiple protein partners has evolved to keep up with the increased metabolic complexity of multi-cellular organisms. Association and disassociation of dynamic TF complexes in response to developmental and environmental cues are controlled through a plethora of regulatory mechanisms specifically modulating TF activity. Regulation of TFs at the protein level is critical for efficient and precise control of their activity, and thus provides the mechanistic basis for a rapid on-and-off switch of TF activity. In this review, examples of post-translational modifications, protein-protein interactions, and subcellular mobilization of TFs are discussed with regard to the relevance of these regulatory mechanisms for the specific activation of MYBs and bHLHs in response to a given environmental stimulus.