1. Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037, USA

Correspondence to: Joanne Chory Correspondence and requests for materials should be addressed to J.C. (Email: chory@salk.edu). The GenBank accession code for PFT1 is AY170377.

Abstract

The transition to flowering in plants is regulated by environmental factors such as temperature and light¹. Plants grown under dense canopies or at high density perceive a decrease in the ratio of red to far-red incoming light. This change in light quality serves as a warning of competition, triggering a series of responses known collectively as the 'shade-avoidance syndrome'. During shade avoidance, stems elongate at the expense of leaf expansion, and flowering is accelerated^{2, 3}. Of the five phytochromes—a family of red/far-red light photoreceptors—in Arabidopsis, phytochrome B (phyB) has the most significant role in shade-avoidance responses^{4, 5}, but the mechanisms by which phyB regulates flowering in response to altered ratios of red to far-red light are largely unknown. Here we identify PFT1 (PHYTOCHROME AND FLOWERING TIME 1), a nuclear protein that acts in a phyB pathway and induces flowering in response to suboptimal light conditions. PFT1 functions downstream of phyB to regulate the expression of FLOWERING LOCUS T (FT), providing evidence for the existence of a light-quality pathway that regulates flowering time in plants.