COP1 SUPPRESSOR 4 promotes seedling photomorphogenesis by repressing <i>CCA1</i> and <i>PIF4</i> expression in <i>Arabidopsis</i>.

Abstract

CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and DE-ETIOLATED 1 (DET1) are founding components of two central repressor complexes of photomorphogenesis that trigger the degradation of a larger number of photomorphogenic-promoting factors in darkness. Here, we identify COP1 SUPPRESSOR 4 (CSU4) as a genetic suppressor of the <i>cop1-6</i> mutation. Mutations in <i>CSU4</i> largely rescued the constitutively photomorphogenic phenotype of <i>cop1-6</i> and <i>det1-1</i> in darkness. Loss of CSU4 function resulted in significantly longer hypocotyl in the light. Further biochemical studies revealed that CSU4 physically interacts with CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and negatively regulates its transcriptional repression activity toward its targets. CSU4 represses the expression of <i>CCA1</i> in the early morning and of <i>PHYTOCHROME INTERACTING FACTOR 4</i> (<i>PIF4</i>) in the early evening. Our study suggests that CSU4 acts as a negative regulator of CCA1 via physically associating with CCA1, which in turn, likely serves to repress expression of <i>CCA1</i> and <i>PIF4</i> to promote photomorphogenesis.