Cellular localization of Arabidopsis EARLY FLOWERING3 is responsive to light quality

Circadian clocks facilitate the coordination of physiological and developmental processes to changing daily and seasonal cycles. A hub for environmental signaling pathways in the Arabidopsis (Arabidopsis thaliana) circadian clock is the evening complex (EC), a protein complex composed of EARLY FLOWERING3 (ELF3), ELF4, and LUX ARRYTHMO (LUX). Formation of the EC depends on ELF3, a scaffold protein that recruits the other components of the EC and chromatin remodeling enzymes to repress gene expression. Regulating the cellular distribution of ELF3 is thus an important mechanism in controlling its activity. Here, we determined that the cellular and sub-nuclear localization of ELF3 is responsive to red (RL) and blue light and that these two wavelengths have apparently competitive effects on where in the cell ELF3 localizes. We further characterized the RL response, revealing that at least two RL pathways influence the cellular localization of ELF3. One of these depends on the RL photoreceptor phytochrome B (phyB), while the second is at least partially independent of phyB activity. Finally, we investigated how changes in the cellular localization of ELF3 are associated with repression of EC target-gene expression. Our analyses revealed a complex effect whereby ELF3 is required for controlling RL sensitivity of morning-phased genes, but not evening-phased genes. Together, our findings establish a previously unknown mechanism through which light signaling influences ELF3 activity.