The ELF4-ELF3-LUX Complex Links the Circadian Clock to Diurnal Control of Hypocotyl Growth

Abstract

The circadian clock is required for adaptive responses to daily and seasonal changes in environmental conditions ^{1- 3} . Light and the circadian clock interact to consolidate the phase of hypocotyl cell elongation to dawn under diurnal cycles in Arabidopsis thaliana ^{4- 7} . Here we identify a protein complex (Evening Complex) composed of EARLY FLOWERING 3 (ELF3), EARLY FLOWERING 4 (ELF4) and the transcription factor LUX ARRHYTHMO (LUX) that directly regulates plant growth ^{8- 12} . ELF3 is both necessary and sufficient to form a complex between ELF4 and LUX, and the complex is diurnally regulated, peaking at dusk. ELF3, ELF4 and LUX are required for the proper expression of the growth-promoting transcription factors PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PIF5 under diurnal conditions ^{4, 6, 13} . LUX targets the complex to the promoters of PIF4 and PIF5 in vivo. Mutations in PIF4 and/or PIF5 are epistatic to the loss of the ELF4-ELF3-LUX complex, suggesting that regulation of PIF4 and PIF5 is a critical function of the complex. Therefore, the Evening Complex underlies the molecular basis for circadian gating of hypocotyl growth in the early evening.