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 .