Hypocotyl elongation is a highly coordinated physiological response regulated by myriad internal and external cues. evening complex that represses the expression of and expression levels in a COP1-dependent manner. By contrast expression coinciding with the initiation of hypocotyl growth in the early evening is positively correlated with the transcript abundance. These results collectively establish BBX19 as an adaptor that binds to and recruits ELF3 for degradation by COP1 and as such dynamically gates the formation of the evening complex resulting in derepression of characterized by the presence of one or two B-box motifs at the N-terminal domain alone or in combination with a C-terminal CCT (for CONSTANS CO like and TOC) domain (Khanna et al. 2009 Huang et al. 2012 Gangappa and Botto 2014 The B-box motif is critical for both transcriptional regulation and heterodimeric protein formation and the CCT domain is implicated in transcriptional regulation and in some instances nuclear CA-074 protein import (Crocco and Botto 2013 Gangappa and Botto 2014 Eight of the BBX proteins belong to structural group IV (BBX18 to BBX25) by virtue of containing two tandem repeat B-box motifs (Box1 and Box2) in the N terminus and lacking the CCT domain (Kumagai et al. 2008 Khanna et al. 2009 Gangappa and Botto 2014 Members of group IV are considered to be evolutionarily conserved functional proteins associated with photomorphogenesis and stress responses (Yamawaki et al. 2011 Among the group members BBX18 BBX19 BBX24 and BBX25 are implicated as negative regulators of photomorphogenesis whereas BBX20 BBX21 and KPSH1 antibody BBX22 are known to promote photomorphogenesis (Datta et al. 2006 2007 2008 Indorf et al. 2007 Kumagai et al. 2008 Chang et al. 2011 Wang et al. 2011 Yan et al. 2011 Fan et al. 2012 Jiang et al. 2012 Gangappa et al. 2013 BBX25 and its homolog BBX24 function through their direct interaction with ELONGATED HYPOCOTYL5 (HY5) a basic domain/leucine zipper transcription factor and with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) additively enhancing COP1 and suppressing HY5 functions (Holm and Deng 1999 Hardtke et al. 2000 Lau and Deng 2012 Gangappa et al. 2013 HY5 plays a central role in the regulation of seedling photomorphogenesis and COP1 an E3 CA-074 ubiquitin ligase is a crucial mediator of ubiquitination and subsequent targeted degradation of positive regulators of light signal transduction such as HY5 and BBX22 in dark conditions (Holm and Deng 1999 Hardtke et al. 2000 Yu et al. 2008 Lau and Deng 2012 The functioning of promoters of the photomorphogenic response BBX21 and BBX22 is reported to be through their recruitment by COP1 to nuclear speckles (nuclear bodies) and CA-074 the enhancement of HY5 activity by their physical interaction via the B-box motifs (Datta et al. 2007 2008 Chang et al. 2008 Hypocotyl elongation is a physiological response controlled by myriad internal and external cues including the convergence of light signaling and the circadian clock with maximal growth occurring at the end of night under diurnal conditions (Nozue CA-074 et al. 2007 Nusinow et al. 2011 One regulator of growth is a protein complex known as the evening complex comprising EARLY FLOWERING3 (ELF3) ELF4 and the transcription factor LUXARRHYTHMO (LUX) (Nusinow et al. 2011 ELF3 and ELF4 are nuclear proteins that regulate circadian rhythms and LUX is a single MYB domain-containing SHAQYF-type GARP transcription factor initially identified in a genetic screen for long-hypocotyl mutants (Zagotta et al. 1996 Hicks et al. 2001 Doyle et al. 2002 Khanna et al. 2003 Early in the night the evening complex represses the expression of two growth-promoting basic helix-loop-helix transcription factors (expression promotes hypocotyl elongation but at dawn when PIF4/5 proteins are targeted for degradation through their interaction with the light-activated form of phytochrome B hypocotyl growth concludes (Huq and Quail 2002 Khanna et al. 2004 As such PIF4 and PIF5 are considered to be integrators of the light and circadian clock signaling networks (Nozue et al. 2007 Recently we established that the small metabolite methylerythritol cyclodiphosphate (MEcPP) a plastidial precursor of isoprenoids produced by the methylerythritol phosphate pathway also functions as a stress-specific retrograde signal relaying environmental.