Ambient temperature response establishes ELF3 as a required component of the core Arabidopsis circadian clock

Circadian clocks synchronize internal processes with environmental cycles to ensure optimal timing of biological events on daily and seasonal time scales. External light and temperature cues set the core molecular oscillator to local conditions. In Arabidopsis, EARLY FLOWERING 3 (ELF3) is thought to act as an evening-specific repressor of light signals to the clock, thus serving a zeitnehmer function. Circadian rhythms were examined in completely dark-grown, or etiolated, null elf3-1 seedlings, with the clock entrained by thermocycles, to evaluate whether the elf3 mutant phenotype was light-dependent. Circadian rhythms were absent from etiolated elf3-1 seedlings after exposure to temperature cycles, and this mutant failed to exhibit classic indicators of entrainment by temperature cues, consistent with global clock dysfunction or strong perturbation of temperature signaling in this background. Warm temperature pulses failed to elicit acute induction of temperature-responsive genes in elf3-1. In fact, warm temperature-responsive genes remained in a constitutively "ON" state because of clock dysfunction and, therefore, were insensitive to temperature signals in the normal time of day-specific manner. These results show ELF3 is broadly required for circadian clock function regardless of light conditions, where ELF3 activity is needed by the core oscillator to allow progression from day to night during either light or temperature entrainment. Furthermore, robust circadian rhythms appear to be a prerequisite for etiolated seedlings to respond correctly to temperature signals.