The SPA Quartet: A Family of WD-Repeat Proteins with a Central Role in Suppression of Photomorphogenesis in Arabidopsis

Abstract

The Arabidopsis thaliana proteins suppressor of phytochrome A-105 1 (SPA1), SPA3, and SPA4 of the four-member SPA1 protein family have been shown to repress photomorphogenesis in light-grown seedlings. Here, we demonstrate that spa quadruple mutant seedlings with defects in SPA1, SPA2, SPA3, and SPA4 undergo strong constitutive photomorphogenesis in the dark. Consistent with this finding, adult spa quadruple mutants are extremely small and dwarfed. These extreme phenotypes are only observed when all SPA genes are mutated, indicating functional redundancy among SPA genes. Differential contributions of individual SPA genes were revealed by analysis of spa double and triple mutant genotypes. SPA1 and SPA2 predominate in dark-grown seedlings, whereas SPA3 and SPA4 prevalently regulate the elongation growth in adult plants. Further analysis of SPA2 function indicated that SPA2 is a potent repressor of photomorphogenesis only in the dark but not in the light. The SPA2 protein is constitutively nuclear localized in planta and can physically interact with the repressor COP1. Epistasis analysis between spa2 and cop1 mutations provides strong genetic support for a biological significance of a COP1-SPA2 interaction in the plant. Taken together, our results have identified a new family of proteins that is essential for suppression of photomorphogenesis in darkness.

CUL4 forms an E3 ligase with COP1 and SPA to promote light-induced degradation of PIF1

“…The plates were kept at 4°C in the dark for 3 to 4 days to stratify and then exposed to 3 h of white light at room temperature before being placed in the dark for 21 h. The plates were incubated at 21°C for an additional 3 days in the dark or light as indicated. Arabidopsis thaliana wild type and various mutants (pif1, cop1-4, cop1-6, spa1, spa2, spa12, spa123, spa124, spaQ, cul1-6, cul3a, cul3b, cul3ab, cul4-1 and cul4cs) in the Col-0 background were used 6,32,[40][41][42][43][44][45][46][47][48] . For generation of different spaQpif1 mutant, pif1 and spaQ were crossed to obtain the F1 seeds.…”

Biochemical Characterization ofArabidopsisComplexes Containing CONSTITUTIVELY PHOTOMORPHOGENIC1 and SUPPRESSOR OF PHYA Proteins in Light Control of Plant Development

“…Previous studies demonstrated that the transcript levels of SPA1, SPA3, and SPA4 are upregulated by various light signals, but the mRNA level of SPA2 is not altered in response to light (Hoecker et al, 1999;Laubinger et al, 2004;Fittinghoff et al, 2006). Here, we examined the protein expression patterns of these SPA genes under different light conditions.…”

“…Mutation in each individual SPA protein causes a weak or no phenotype defect, while the quadruple spa mutant displays a constitutive photomorphogenic phenotype similar to that of a strong cop1 mutant allele (cop1-5). Systematic phenotypic analysis of various combinations of spa mutants indicated that the four SPA genes confer partially redundant but not completely overlapping functions in mediating light responses at both seedling and adult stages (Laubinger and Hoecker, 2003;Hoecker et al, 2004;Laubinger et al, 2004Laubinger et al, , 2006Ishikawa et al, 2006).…”

PHYTOCHROME INTERACTING FACTOR1 Enhances the E3 Ligase Activity of CONSTITUTIVE PHOTOMORPHOGENIC1 to Synergistically Repress Photomorphogenesis in Arabidopsis

“…Seeds of wild-type (Col-0) Arabidopsis thaliana and various mutants (pif1, pif3, pif4, pif13, pif34, pif45, pif134, pif135, pif145, pif345, pifq, cop1-4, cop1-6, spa123, and hy5-215) in the Col-0 background were used (McNellis et al, 1994;Oyama et al, 1997;Laubinger et al, 2004;Leivar et al, 2008b). For the generation of different cop1, spa123, pif, and hy5-215 mutant combinations, cop1 and spa123 were first crossed with pif1, pif3, and pif4 to generate cop1-4 pif1, cop1-6 pif1, cop1-6 pif3, cop1-6 pif4, and spa123 pif1.…”

“…Among these COP/DET/FUS proteins, COP1 is a RING-type E3 ligase that is evolutionarily conserved from plants to vertebrates (Deng et al, 1992;Osterlund et al, 2000;Bianchi et al, 2003;Dornan et al, 2004;Yi and Deng, 2005;Lau and Deng, 2012). COP1 plays a central role in repressing photomorphogenesis in the dark by forming multiple complexes with SUPPRESSOR OF PHYTOCHROME A-105 (SPA1-4) family members in a tissue-and developmental stage-specific manner (Deng et al, 1992;Hoecker et al, 1999;Osterlund et al, 2000;Laubinger et al, 2004;Zhu et al, 2008;Lau and Deng, 2012). These complexes ubiquitylate and degrade positively acting transcription factors such as LONG HYPOCOTYL5 (HY5)/ LONG AFTER FAR-RED LIGHT1 (LAF1)/LONG HYPOCOTYL IN FAR-RED1 (HFR1) to repress photomorphogenesis in the dark (Hoecker et al, 1999;Saijo et al, 2003;Seo et al, 2003;Jang et al, 2005;Yang et al, 2005;Zhu et al, 2008).…”