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 proteinCONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) is involved in many light-regulated responses and is responsible for the ubiquitination and degradation of several positive transcription factors in the dark (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). For example, COP1 interacts with ELONGATED HYPOCOTYL 5 (HY5), which 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). COP1 also interacts with CONSTANS-LIKE3 (COL3), which acts as a positive regulator under red light and localizes to nuclear speckles. Additionally, 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 col3 mutant has longer hypocotyls and flowers early and exhibits a reduced number of lateral branches (Datta et al., 2006). COL3 also directly interacts with B-BOX32 (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 BBX zinc finger transcription factor (TF) family, which has 32 members (Kumagai et al., 2008). This gene family is 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 (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), have been implicated in light signal transduction during early photomorphogenesis. 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.
A previous study showed that COL3 plays 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 B-BOX32 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/B-BOX11 and an HY5-COL3-COL13 regulatory chain for controlling hypocotyl growth in A. thaliana .