Introduction
Light is one of the most important environmental cues influencing the
early stages of post-germination plant development
(Kami, Lorrain, Hornitschek, &
Fankhauser, 2010; Olle & Viršile, 2013;
G. Wu, Cameron, Ljung, & Spalding,
2010).
Light-grown
seedlings exhibit a developmental response termed
photomorphogenesis, resulting in
short hypocotyls and expanded, green cotyledons. By contrast, dark-grown
seedlings are characterized by long hypocotyls and unexpanded, etiolated
cotyledons; this process is called
skotomorphogenesis
(Josse & Halliday, 2008;
McNellis & Deng, 1995;
Smith, 2000). As a central light signal
repressor, the RING finger protein CONSTITUTIVE PHOTOMORPHOGENIC1(COP1) is involved in many light-regulated responses and is responsible
for the ubiquitination and degradation of several positive transcription
factors in the dark(Dornan et al., 2004;
Duek, Elmer, van Oosten, & Fankhauser,
2004; Lau & Deng, 2012;
Osterlund, Hardtke, Wei, & Deng, 2000;
Seo, Watanabe, Tokutomi, Nagatani, &
Chua, 2004; Seo et al., 2003). For
example, COP1 interacts with ELONGATED HYPOCOTYL 5 (HY5), which is a
positive regulator under far-red, red, blue, and UV-B light
conditions(Ang et al., 1998;
Delker et al., 2014;
Hardtke et al., 2000); COP1 also
interacts with
CONSTANS-LIKE3
(COL3), which acts as a positive regulator in red light and localizes to
nuclear speckles. In addition, the col3 mutant partially
suppresses the cop1 mutation, suggesting that COL3 acts
genetically downstream of COP1(Datta,
Hettiarachchi, Deng, & Holm, 2006).
The loss-of-function col3 mutant has longer hypocotyls, flowers
early and shows a reduced number of lateral
branches(Datta et al., 2006). COL3 also
directly interacts with B-BOX32 (BBX32), which is regulated by the
circadian clock, to mediate
flowering(Tripathi, Carvallo, Hamilton,
Preuss, & Kay, 2017). Interestingly, both COL3 and BBX32 belong to the
BBX zinc finger transcription factor (TF) family, which has 32
members(Kumagai et al., 2008). This gene
family is divided into five groups based on whether their respective
proteins contain one or two BBX motifs and whether or not they possess a
CCT domain(Khanna et al., 2009). BBX
family members, some of which have been characterized
(Cheng & Wang, 2005;
Graeff et al., 2016;
Li et al., 2014;
Park et al., 2011;
Preuss et al., 2012;
Wang, Guthrie, Sarmast, & Dehesh, 2014;
Xu, Jiang, Li, Holm, & Deng, 2018;
Xu et al., 2016;
Yang et al., 2014), are commonly
implicated in light signal transduction during early photomorphogenesis.
The first BBX protein to be identified in Arabidopsisthaliana was CONSTANS
(CO)(Putterill, Robson, Lee, Simon, &
Coupland, 1995). In addition to CO, 16 other CO-Like (COL)
proteins have been identified, which contain one or two B-box domains at
the N-terminus and a CCT domain at the C
terminus(Cheng & Wang, 2005). However,
most of their functions remain unclear.
A
previous study showed that COL3 plays multiple roles in plant
development (e.g. flowering, hypocotyl elongation and
lateral
root formation)(Datta et al., 2006).
Although COL3 is known to interact with B-BOX32 to regulate
flowering(Tripathi et al., 2017), there
has been little research on how COL3 regulates hypocotyl elongation and
the respective downstream pathways are uncharacterized. In the present
study, we propose a role for COL13/B-BOX11 and a possible COP1-dependent
COL3-COL13 feedback pathway in regulating the hypocotyl growth inA. thaliana .