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; Wu, Cameron,
Ljung, & Spalding, 2010).
Light-grown
seedlings exhibit a developmental response termed
photomorphogenesis, resulting in
short hypocotyls and expanded green cotyledons. In 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, including B-BOX CONTAINING PROTEINs
(BBXs) and ELONGATED HYPOCOTYL 5 (HY5) (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).
BBXs family has 32 members (Kumagai et al., 2008), which are 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
and implicated in light signal transduction during early
photomorphogenesis (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). The first BBX protein 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. COP1 can interact with
BBXs proteins. For example, COP1 interact with COL3, which acts as a
positive regulator under red light and localizes to nuclear speckles.
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 col3mutant has longer hypocotyls and flowers early and exhibits a reduced
number of lateral branches (Datta et al., 2006). COL3 also directly
interacts with 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 BBXs family.
HY5 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). It mediates about one-third of genes expression throughout theArabidopsis genome, including BBX s (Lee et al., 2007). For
example, HY5 binds to the promoters of BBX30 and BBX31 ,
both of which negatively regulate light response (Heng et al., 2019).
Meanwhile, the expression of HY5 is regulated by BBXs. For
instance, BBX21, BBX22, and BBX23
directly
binds to the HY5 promoter to activate its transcription and promote
photomorphogenesis (Datta et al., 2008; Zhang et al., 2017; Job et al.,
2018), whereas BBX24, BBX25 and BBX28 repress HY5 action (Gangappa et
al., 2013; Job et al., 2018; Lin et al. , 2018). Therefore, BBXs and HY5
constitute a light signal regulatory network that was essential for
promoting photomorphogenesis.
A
previous study showed that COL3 played 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 BBX32 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 proposed a role for COL13 and an HY5-COL3-COL13 regulatory
chain for controlling hypocotyl growth in A. thaliana .