The ELF3 zeitnehmer regulates light signalling to the circadian clock

The circadian system regulates 24-hour biological rhythms' and seasonal rhy thms, such as flowering(2). Long-day flowering plants like Arabidopsis thal iana, measure day length with a rhythm that is not reset at lights-off(3), whereas short-day plants measure night length on the basis of circadian rhy thm of light sensitivity that is set from dusk(2). early flowering 3 (elf3) mutants of Arabinopsis are aphotoperiodic(4) and exhibit light-conditional arrhythmia(5,6). Here we show that the elf3-7 mutant retains oscillator fu nction in the light but blunts circadian gating of CAB gene activation, ind icating that deregulated phototransduction may mask rhythmicity. Furthermor e, elf3 mutations confer the resetting pattern of short-day photoperiodism, indicating that gating of phototransduction may control resetting. Tempera ture entrainment can bypass the requirement for normal ELF3 function for th e oscillator and partially restore rhythmic CAB expression. Therefore, ELF3 specifically affects light input to the oscillator, similar to its functio n in gating CAB activation, allowing oscillator progression past a light-se nsitive phase in the subjective evening. ELF3 provides experimental demonst ration of the zeitnehmer ('time-taker') Concept(7,8).