Discussion
Light regulates
photomorphogenesis
in plants. A large number of genes that are involved in such
photomorphogenesis processes have been identified as light receptors
(Datta et al., 2006; Kircher et al., 2002; Peter H. Quail, 2002), signal
transduction factors (Gangappa et al., 2013; Osterlund et al., 2000)or
degradation proteins (Crocco, Holm, Yanovsky, & Botto, 2010; Crocco et
al., 2015; Delker et al., 2014). One of the immediate questions is how
these genes act in a network to mediate various light-related
phenotypes. It has been shown that multiple pathways are interlinked to
form a gene network of photomorphogenesis (Lau & Deng, 2012; Lee, Park,
Ha, Baldwin, & Park, 2017). Among these pathways, it is worth
mentioning the ones formed by a subset of family genes termed the COL
genes (Cheng & Wang, 2005). These family of genes plays multiple roles
in plant development (Datta et al., 2006; Graeff et al., 2016; Muntha et
al., 2018; Tripathi et al., 2017; Wang et al., 2014). As an effort
toward COL networking, we investigated the relationship betweenCOL3 and COL13 and provided evidence that these two COLs
and HY5 were connected together to form an HY5-COL3-COL13 regulatory
chain that controls hypocotyl elongation in Arabidopsis(Fig.
7b). In addition, we also proposed a possible
COP1-dependent
COL3-COL13 feedback pathway to optimize this regulatory pathway (Fig.
7b).
Hypocotyl elongation is a genetically well-controlled process that
responds to light. In Arabidopsis , several key genes are required
for hypocotyl growth. Among these, COP1 is a negative regulator
(McNellis, von Arnim, & Deng, 1994), whereas HY5 and COL3 are
considered to be positive (Datta et al., 2006; Hardtke et al., 2000). A
previous study showed that COL3 plays a role in flowering and hypocotyl
elongation (Datta et al., 2006), and COL3 is known to interact with
B-BOX32 to regulate flowering (Tripathi et al., 2017). However, there
has been no research on how COL3 regulates hypocotyl elongation. To
explore how the COL family genes, COL3 in particular, function in the
regulation of hypocotyl elongation, it will be facilitated by
identifying the downstream genes. In this study, we demonstrated thatCOL13 , whose RNA accumulated to a high level in the hypocotyl
(Fig. 1), was one more positive regulator in the regulation of hypocotyl
elongation under red-light conditions. For example, overexpression ofCOL13 or knockdown of its transcript resulted in a shorter or
longer hypocotyl, respectively, (Fig. 2). To further define and
characterize COL13 , we analyzed the genetic interactions betweencol13 and col3. Seedlings of the col13 andcol3 mutants showed reduced inhibition of hypocotyl elongation
under red light (Fig. 3). Analysis of col3 col13 double mutants
and COL13 transgenic plants revealed that COL3 is epistatic toCOL13 concerning hypocotyl elongation (Fig. 3). Given thatcol3 hy5 double mutants behaved like the hy5 mutation
(Datta et al., 2006), we hypothesized that there is an HY5-COL3-COL13
regulatory chain for controlling hypocotyl growth. As expected,
our data showed that HY5 targeted the promoter of COL3 and COL3
directly bound to the promoter of COL13 (Fig. 4a-c), indicating
that HY5, COL3, and COL13 constitute a hypocotyl regulatory pathway.
CONSTANS-LIKE genes belong to the BBX family. Given that
BBX
family members are commonly involved in photomorphogenesis and that they
can interact with other BBX proteins to regulate plant growth (Tripathi
et al., 2017; Wang et al., 2014), COL3 may interact with other BBX
proteins (for example. COL13/B-BOX11) to regulate plant development
under light conditions. Indeed, we provided evidence that COL13 can
interact with COL3 (Fig. 6). Furthermore, we found that the expression
of COL13 promoted the interaction between COP1 and COL3 (Fig. 7a). To
our knowledge, COP1 is responsible for the degradation of several
positive TFs, such as COL3, in the dark (Datta et al., 2006; Dornan et
al., 2004; Duek et al., 2004; Lau & Deng, 2012; Osterlund et al., 2000;
Seo et al., 2004; Seo et al., 2003). Increasing the binding activity of
COP1 and COL3 would lead to the degradation of COL3. As a result, there
would be less COL3 to activate the expression of COL13 (Fig. 7b). The
COP1-dependent COL3-COL13 feedback pathway could enrich the regulation
network in hypocotyl elongation.