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DEVELOPMENT AND HORMONE ACTION

Acceleration of Flowering during Shade Avoidance in Arabidopsis Alters the Balance between FLOWERING LOCUS C-Mediated Repression and Photoperiodic Induction of Flowering^1,[W],[OA]

Graduate Program in Cellular and Molecular Biology, University of Wisconsin, Madison, Wisconsin 53706 (A.C.W.); Fundación Instituto Leloir, 1405 Ciudad de Buenos Aires, Argentina (B.S., P.D.C.); Consejo Nacional de Investigaciones Científicas y Técnicas, 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. Finally, our analysis of the contribution of PHYTOCHROME AND FLOWERING TIME1 (PFT1) to shade-mediated rapid flowering has led us to suggest a new model for the involvement of PFT1 in light signaling.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Richard M. Amasino (amasino{at}biochem.wisc.edu).

^[W] The online version of this article contains Web-only data.

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.125468

^* Corresponding author; e-mail amasino{at}biochem.wisc.edu.

Received June 27, 2008; accepted September 2, 2008; published September 12, 2008.

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