ELF3 modulates resetting of the circadian clock in Arabidopsis

The Arabidopsis early flowering 3 (elf3) mutation causes arrhythmic circadi an output in continuous light, but there is some evidence of clock function in darkness. Here, we show conclusively that normal circadian function occ urs with no alteration of period length in elf3 mutants in dark conditions and that the light-dependent arrhythmia observed in elf3 mutants is pleiotr opic on multiple outputs normally expressed at different times of day, Plan ts overexpressing ELF3 have an increased period length in both constant blu e and red light; furthermore, etiolated ELF3-overexpressing seedlings exhib it a decreased acute GASP response after a red light pulse, whereas the nul l mutant is hypersensitive to acute induction. This finding suggests that E LF3 negatively regulates light input to both the clock and its outputs. To determine whether ELF3's action is phase dependent, we examined clock reset ting by using light pulses and constructed phase response curves. Absence o f ELF3 activity causes a significant alteration of the phase response curve during the subjective night, and constitutive overexpression of ELF3 resul ts in decreased sensitivity to the resetting stimulus, suggesting that ELF3 antagonizes light input to the clock during the night. The phase of ELF3 f unction correlates with its peak expression levels in the subjective night. ELF3 action, therefore, represents a mechanism by which the oscillator mod ulates light resetting.