Arabidopsis SPA proteins regulate photoperiodic flowering and interact with the floral inducer CONSTANS to regulate its stability

Abstract

The four-member SPA protein family of Arabidopsis acts in concert with the E3 ubiquitin ligase COP1 to suppress photomorphogenesis in dark-grown seedlings. Here, we demonstrate that SPA proteins are, moreover, essential for photoperiodic flowering. Mutations in SPA1 cause phyA-independent early flowering under short day (SD) but not long day (LD) conditions, and this phenotype is enhanced by additional loss of SPA3 and SPA4 function. These spa1 spa3 spa4 triple mutants flower at the same time in LD and SD, indicating that the SPA gene family is essential for the inhibition of flowering under non-inductive SD. Among the four SPA genes, SPA1 is necessary and sufficient for normal photoperiodic flowering. Early flowering of SD-grown spa mutant correlates with strongly increased FT transcript levels, whereas COtranscript levels are not altered. Epistasis analysis demonstrates that both early flowering and FT induction in spa1 mutants is fully dependent on CO. Consistent with this finding, SPA proteins interact physically with CO in vitro and in vivo, suggesting that SPA proteins regulate CO protein function. Domain mapping shows that the SPA1-CO interaction requires the CCT-domain of CO, but is independent of the B-box type Zn fingers of CO. We further show that spa1 spa3 spa4 mutants exhibit strongly increased CO protein levels, which are not caused by a change in COgene expression. Taken together, our results suggest, that SPA proteins regulate photoperiodic flowering by controlling the stability of the floral inducer CO.

NO FLOWERING IN SHORT DAY (NFL) is a bHLH transcription factor that promotes flowering specifically under short-day in Arabidopsis

“…Previously, flowering-time mutants that are affected only under SD conditions have been reported. For example, phyB, dnf, spa1 and cop1 flower early, whereas vil1, vil2 and a gain-of-function mutation in IAA7/AXR2 (axr2-1) flower late specifically under SD conditions (Ishikawa et al, 2006;Kim and Sung, 2010;Laubinger et al, 2006;Mai et al, 2011;McNellis et al, 1994;Morris et al, 2010;Reed et al, 1993;Sung et al, 2006). However, vil1, vil2 and axr2-1 mutants display quantitative differences in flowering time compared with the wt and still flower under SD conditions (Kim and Sung, 2010;Mai et al, 2011;Sung et al, 2006).…”

“…vil1 and vil2 mutants flower later only under SD conditions, but do eventually flower. However, spa1, cop1 and cul4cs mutants display early flowering only under SD conditions (Chen et al, 2010;Ishikawa et al, 2006;Jang et al, 2008;Laubinger et al, 2006;McNellis et al, 1994;Ranjan et al, 2011). SPA1 physically interacts with COP1, and COP1-SPA1 associates with CUL4, forming an E3 ubiquitin ligase, which promotes CO degradation through ubiquitin-mediated proteolysis (Chen et al, 2010;Jang et al, 2008;Laubinger et al, 2006).…”

COP1-Mediated Degradation of BBX22/LZF1 Optimizes Seedling Development in Arabidopsis

“…Because BBX22 does not physically interact with COP1 (Datta et al, 2008), identifying any proteins assisting in the targeted degradation of BBX22 by COP1 is of great interest. Possible candidates are early flowering 3 (ELF3) and suppressor of phyA-105, which are COP1-interacting partners and function to facilitate COP1-mediated degradation of GIGANTEA and HY5/CONSTANS, respectively, in the dark (Saijo et al, 2003;Laubinger et al, 2006;Yu et al, 2008). Also, reduced expression of CULLIN4 enhanced the cop phenotype in cop1-4 (Chen et al, 2010), cop10, and det1 (Chen et al, 2006).…”

PSEUDO RESPONSE REGULATORs stabilize CONSTANS protein to promote flowering in response to day length

“…The blue light photoreceptors cryptochrome 1 (cry1) and cryptochrome 2 (cry2) and the far‐red photoreceptor phytochrome A (phyA) are required to stabilize CO protein (Valverde et al , 2004; Zuo et al , 2011). A protein complex of an E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) and SUPPRESSOR OF PHYA1 (SPA1), which are repressors of A. thaliana photomorphogenesis, mediates between the photoreceptors and CO protein stabilization (Laubinger et al , 2006; Jang et al , 2008; Liu et al , 2008; Zuo et al , 2011; Lau & Deng, 2012; Sarid‐Krebs et al , 2015). During the night, the COP1/SPA1 protein complex physically interacts with CO in the nuclei to promote its proteasomal degradation, whereas during the day, the phyA and cry photoreceptors promote COP1 accumulation in the cytoplasm thus allowing CO to accumulate in the nucleus (Osterlund & Deng, 1998; Laubinger et al , 2006; Jang et al , 2008; Liu et al , 2008; Zuo et al , 2011; Sarid‐Krebs et al , 2015).…”