Summary: Acceleration of Flowering during Shade Avoidance in
Arabidopsis Alters the Balance between FLOWERING
LOCUS C-Mediated Repression and Photoperiodic
Induction of Flowering1[W][OA]
Amanda C. Wollenberg, BaŽrbara Strasser, Pablo D. CerdaŽn, and Richard M. Amasino*
Graduate Program in Cellular and Molecular Biology, University of Wisconsin, Madison, Wisconsin 53706
(A.C.W.); FundacioŽn Instituto Leloir, 1405 Ciudad de Buenos Aires, Argentina (B.S., P.D.C.); Consejo Nacional
de Investigaciones CientiŽficas y TeŽcnicas, Ciudad de Buenos Aires, Argentina (P.D.C.); and Department of
Biochemistry, University of Wisconsin, Madison, Wisconsin 53706 (R.M.A.)
The timing of the floral transition in Arabidopsis (Arabidopsis thaliana) is influenced by a number of environmental signals.
Here, we have focused on acceleration of flowering in response to vegetative shade, a condition that is perceived as a decrease
in the ratio of red to far-red radiation. We have investigated the contributions of several known flowering-time pathways to
this acceleration. The vernalization pathway promotes flowering in response to extended cold via transcriptional repression
of the floral inhibitor FLOWERING LOCUS C (FLC); we found that a low red to far-red ratio, unlike cold treatment, lessened
the effects of FLC despite continued FLC expression. A low red to far-red ratio required the photoperiod-pathway genes
GIGANTEA (GI) and CONSTANS (CO) to fully accelerate flowering in long days and did not promote flowering in short days.
Together, these results suggest a model in which far-red enrichment can bypass FLC-mediated late flowering by shifting the
balance between FLC-mediated repression and photoperiodic induction of flowering to favor the latter. The extent of this shift
was dependent upon environmental parameters, such as the length of far-red exposure. At the molecular level, we found that
far-red enrichment generated a phase delay in GI expression and enhanced CO expression and activity at both dawn and dusk.