Activated photoreceptors initiate at least two signaling cascades in Arabidopsis. First, phytochromes in their active conformation interact with PHYTOCHROME INTERACTING FACTORS (PIF

INTRODUCTION Light is an important informational cue with which to regulate many stages of plant growth and development. To sense the ambient light conditions, plants have evolved multiple photoreceptors, which include the red/far-red light (R/FR)-sensing phytochromes and the UV-A/blue light-sensing cryptochromes and phototropins (Chen et al., 2004; Whitelam and Halliday, 2007). Activated photoreceptors initiate at least two signaling cascades in Arabidopsis. First, phytochromes in their active conformation interact with PHYTOCHROME INTERACTING FACTORS (PIF proteins), most of which are subsequently phosphorylated and degraded in the proteasome. PIF proteins activate the normal dark response of seedlings and their degradation is, therefore, necessary for photomorphogenesis (Leivar and Quail, 2010). Second, phytochromes and cryptochromes are thought to inhibit the COP1/SPA complex, which acts as an E3 ubiquitin ligase in darkness to ubiquitylate transcription factors required for light signaling (Yi and Deng, 2005). The COP1/SPA complex consists of two essential components, COP1 and members of the fourmember SPA protein family, which act in concert with the CULLIN4-DDB1 complex (Chen et al., 2010; Laubinger et al., 2004; Zhu et al., 2008). cop1 and spa1 spa2 spa3 spa4 quadruple mutants fail to degrade the substrate transcription factors and, therefore, show features of light-grown seedlings even when grown in complete darkness (Kang et al., 2009; Laubinger et al., 2004; Osterlund et al., 1999; Zhu et al., 2008). Though primarily active in darkness, the COP1/SPA complex is also important for fine-tuning de-etiolation of light-grown seedlings because spa1, spa3 and spa4 mutant seedlings show exaggerated photomorphogenesis in the light (Hoecker et al., 1998; Laubinger and Hoecker, 2003). Furthermore, cop1 and spa1 spa2 spa3 spa4 mutant leaves are much smaller than wild-type leaves (Deng and Quail, 1992; Laubinger et al., 2004). cop1 and spa mutants also fail to delay flowering in short days (Laubinger et al., 2004; McNellis et al., 1994). The COP1/SPA complex controls photoperiodic flowering by regulating the stability of the floral inducer CONSTANS (Jang et al., 2008; Laubinger et al., 2006; Liu et al., 2008). The four SPA genes exhibit redundant but also distinct functions throughout plant development. Nevertheless, the SPA1 gene provides the most significant contribution among the four SPA genes. SPA1 is important for seedling growth in the light and in darkness, leaf expansion and photoperiodic flowering (Fittinghoff et al., 2006; Laubinger et al., 2004; Laubinger et al., 2006). Light initiates developmental responses through non-cellautonomous signaling (Bou-Torrent et al., 2008; Montgomery, 2008). Here, the photoperiodic induction of flowering is the beststudied example. Day length is perceived by leaves, and then a mobile signal, the FT protein, moves through the phloem to induce flowering at the shoot apex (Corbesier et al., 2007; Jaeger and Wigge, 2007; Mathieu et al., 2007). Consistent with this, CO, the transcription factor that activates FT transcription, acts in the Development 138, 1851-1862 (2011) doi:10.1242/dev.061036 © 2011. Published by The Company of Biologists Ltd