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). 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. In this study, we demonstrated that COL13 , 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 of COL13 or knockdown of its transcript resulted in a shorter or longer hypocotyl, respectively, (Fig. 2).
As 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), HY5 mediates about one-third of genes expression throughout theArabidopsis genome (Lee et al., 2007). This study revealed that HY5 bound to the promoter of COL3 and upregulate its transcription (Fig. 4b), indicating that COL3 also acted as a downstream target of HY5. Interestingly, COL3 bind to the COL13promoter and positively regulate its expression (Fig.4b-g), suggesting that COL3 are positive regulators of COL13. Thus, these findings suggest that HY5, COL3, and COL13 constitute a hypocotyl regulatory pathway.
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). For example, COL3 belong to the BBX family and COL3 interact with BBX32 to regulate flowering (Tripathi et al., 2017). In this study, we demonstrated that COL13 could 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). Therefore, we proposed a possible COP1-dependent COL3-COL13 feedback pathway as a hypothesis to optimize the HY5-COL3-COL13 regulatory pathway (Fig. 7b). This feedback pathway could enrich the regulation network in hypocotyl elongation.