Flowering in Arabidopsis thaliana is promoted by long-day (LD) photope
riods such that plants grown in LD flower earlier, and after the produ
ction of fewer leaves, than plants grown in short-day (SD) photoperiod
s. The early-flowering 3 (elf3) mutant of Arabidopsis, which is insens
itive to photoperiod with regard to floral initiation has been charact
erized. elf3 mutants are also altered in several aspects of vegetative
photomorphogenesis, including hypocotyl elongation. When inhibition o
f hypocotyl elongation was measured, elf3 mutant seedlings were less r
esponsive than wild-type to all wavelengths of light, and most notably
defective in blue and green light-mediated inhibition. When analyzed
for the flowering-time phenotype, elf3 was epistatic to mutant alleles
of the blue-light receptor encoding gene, HY4. However, when elf3 mut
ants were made deficient for functional phytochrome by the introductio
n of hy2 mutant alleles, the elf3 hy2 double mutants displayed the nov
el phenotype of flowering earlier than either single mutant while stil
l exhibiting photoperiod insensitivity, indicating that a phytochrome-
mediated pathway regulating floral initiation remains functional in el
f3 single mutants. In addition, the inflorescences of one allelic comb
ination of elf3 hy2 double mutants form a terminal flower similar to t
he structure produced by tfl1 single mutants. These results suggest th
at one of the signal transduction pathways controlling photoperiodism
in Arabidopsis is regulated, at least in part, by photoreceptors other
than phytochrome, and that the activity of the Arabidopsis infloresce
nce and floral meristem identity genes may be regulated by this same p
athway.