The role of blue light in regulating both circadian rhythmicity and flowering is also supported by studies with an Arabidopsis flowering-time mutant: elf3 (early flowering 3) (Hicks et al. 1996; Zagotta et al. 1996). The elf3 mutant flowers earlier than the wild type under a variety of growth conditions and is insensitive to photoperiod; that is, it is day-neutral.

Transformation of elf3 with the luciferase reporter construct revealed a defect in circadian rhythmicity. The mutant displayed no circadian rhythmicity when grown in continuous light. Significantly, the elf3 mutant also has a long hypocotyl when grown in constant white light, similar to the hy mutants, and is less responsive than the wild type to all wavelengths of light, but especially to blue light. This finding suggests that the circadian clock is intact in the mutant, but that the transduction of light signals to the clock is impaired (Hicks et al. 1996; Zagotta et al. 1996).

The fact that the elf3 mutant is more impaired in its response to blue light than in its response to red light further suggests that a blue-light photoreceptor plays a role in the photoperiodic control of flowering.