The Arabidopsis B-BOX Protein BBX25 Interacts with HY5, Negatively Regulating BBX22 Expression to Suppress Seedling Photomorphogenesis  

Abstract

ELONGATED HYPOCOTYL5 (HY5) is a basic domain/leucine zipper (bZIP) transcription factor, central for the regulation of seedling photomorphogenesis. Here, we identified a B-BOX (BBX)-containing protein, BBX25/SALT TOLERANCE HOMOLOG, as an interacting partner of HY5, which has been previously found to physically interact with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1). BBX25 physically interacts with HY5 both in vitro and in vivo. By physiological and genetic approaches, we showed that BBX25 is a negative regulator of seedling photomorphogenesis. BBX25 and its homolog BBX24 regulate deetiolation processes and hypocotyl shade avoidance response in an additive manner. Moreover, genetic relationships of bbx25 and bbx24 with hy5 and cop1 revealed that BBX25 and BBX24 additively enhance COP1 and suppress HY5 functions. BBX25 accumulates in a light-dependent manner and undergoes COP1-mediated degradation in dark and light conditions. Furthermore, a protoplast cotransfection assay showed that BBX24 and BBX25 repress BBX22 expression by interfering with HY5 transcriptional activity. As HY5 binds to the BBX22 promoter and promotes its expression, our results identify a direct mechanism through which the expression of BBX22 is regulated. We suggest that BBX25 and BBX24 function as transcriptional corepressors, probably by forming inactive heterodimers with HY5, downregulating BBX22 expression for the fine-tuning of light-mediated seedling development.

UV-B-Responsive Association of the Arabidopsis bZIP Transcription Factor ELONGATED HYPOCOTYL5 with Target Genes, Including Its Own Promoter

“…FHY3 binds to an FHY3 binding site and HY5 to an ACGT-containing element in the COP1 promoter, both of which seem to be required for the UV-B responsiveness of COP1 . Combinatorial regulation exerted by the interaction of other transcription factors with HY5/HYH is widely present, with negative or positive regulatory effects (Shin et al, 2007;Andronis et al, 2008;Holtan et al, 2011;Huang et al, 2012;Jiang et al, 2012;Singh et al, 2012;Gangappa et al, 2013aGangappa et al, , 2013bJing et al, 2013;Abbas et al, 2014). Moreover, HY5 interacts with the chromatin-remodeling factor PICKLE (PKL), recruiting PKL to promoters of target genes repressing the H3K27me3-repressive histone mark (Jing et al, 2013).…”

Molecular characterisation of flowering genes in Pongamia Pinnata

“…This finding is in agreement with other reports of conserved BBX proteins domain in rice and other plant species including green algae, dicots and woody perennial tree (Huang et al, 2012b;Crocco and Botto, 2013;Yuceer et al, 2002). Moreover, it also demonstrates that the B-box domain and the CCT motif are highly conserved and crucial for their roles.…”

“…In Arabidopsis, CO acts as a transcription factor that binds DNA via a unique sequence motif present in the FT promoter, activating its expression. The conserved CCT domain in CO is required for its binding to the unique DNA motif present in the FT promoter, while the B-box zinc finger domain plays a role in protein-protein interaction and transcription regulation (Datta et al, 2007;Datta et al, 2008;Tiwari et al, 2010;Qi et al, 2012;Gangappa et al, 2013).…”

“…The CCT domain was selected as it plays an important role in the binding of CO to the FT promoter (Tiwari et al, 2010;Gangappa et al, 2013), while the GAF and PHY domain, part of the N-terminal domains in PHYB, is important for its photosensing function . These domains were also found to be highly conserved between its orthologue genes, as shown in Section 3.3.3, (Figure 3.10) and Section 4.3.3, (Figure 4.16).…”

“…The BBX proteins in Arabidopsis are categorised into five groups based on their structure and domain organisations (Crocco and Botto, 2013;Gangappa and Botto, 2014). The highly conserved B-boxes domain is found in several classes of proteins and has been reported to be involved in transcriptional regulation and proteinprotein interactions both within and outside of the protein family (Torok and Etkin, 2001;Datta et al, 2007;Qi et al, 2012;Gangappa et al, 2013). The CCT domain present in the C-terminus region comprises 42 to 43 amino acids and has an important role in the transport of nuclear protein and also transcriptional regulation (Robson et al, 2001;Laubinger et al, 2006;Yan et al, 2011).…”