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 .