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.