In plants, seasonal changes in day length are perceived in leaves, which initiate long-distance signaling that induces flowering at the shoot apex. The identity of the long-distance signal has yet to be determined. In Arabidopsis , activation of FLOWERING LOCUS T ( FT ) transcription in leaf vascular tissue (phloem) induces flowering. We found that FT messenger RNA is required only transiently in the leaf. In addition, FT fusion proteins expressed specifically in phloem cells move to the apex and move long distances between grafted plants. Finally, we provide evidence that FT does not activate an intermediate messenger in leaves. We conclude that FT protein acts as a long-distance signal that induces Arabidopsis flowering.

The transition from vegetative to reproductive growth is controlled by day length in many plant species. Day length is perceived in leaves and induces a systemic signal, called florigen, that moves through the phloem to the shoot apex. At the shoot apical meristem (SAM), florigen causes changes in gene expression that reprogram the SAM to form flowers instead of leaves. Analysis of flowering of Arabidopsis thaliana placed the CONSTANS/FLOWERING LOCUS T (CO/FT ) module at the core of a pathway that promotes flowering in response to changes in day length. We describe progress in defining the molecular mechanisms that activate this module in response to changing day length and the increasing evidence that FT protein is a major component of florigen. Finally, we discuss conservation of FT function in other species and how variation in its regulation could generate different flowering behaviors.

Floral development at the Arabidopsis shoot apical meristem occurs in response to environmental cues that are perceived in different tissues. Photoperiod is detected in the vascular tissue of the leaf (phloem) and promotes production of a systemic signal that induces flowering at the meristem. Vernalization, the response to winter temperatures, overcomes a block on photoperiodic floral induction. In Arabidopsis, this block is caused by inhibitors of flowering that comprise several related MADS-box transcription factors, the most prominent of which is FLC. We show that FLC delays flowering by repressing production in the leaf of at least two systemic signals, one of which is controlled by the RAF kinase inhibitor- In Arabidopsis, reproductive development is initiated by the formation of floral primordia on the flanks of the shoot apical meristem. The time at which flowering occurs is governed by environmental cues such as day length and temperature, and is influenced by endogenous signals related to the age of the plant. Classical physiological experiments demonstrated that environmental signals that influence flowering are perceived in different tissues. For example, day length is detected in expanded leaves, and in response to exposure to day lengths that trigger flowering, a systemic signal termed the floral stimulus travels through the phloem to the shoot apical meristem, where it induces floral development (Knott 1934;Zeevaart 1976;Corbesier and Coupland 2005). Vernalization, the acquisition of competence to flower that results from exposure to extended periods of low temperatures that mimic winter conditions, typically relieves a block to the photoperiod pathway (Lang 1965).Here we describe an analysis of the tissues in which key Arabidopsis genes involved in vernalization act to control flowering and demonstrate the significance of transcriptional regulation in the phloem and the meristem for the vernalization response.Genetic analyses of the control of flowering in Arabidopsis thaliana identified four major floral promotion pathways Boss et al. 2004). Environmental responses studied genetically in Arabidopsis include photoperiodic control of flowering, induction of flowering by response to long days (LD) of spring or early summer, and vernalization, which is the promotion of flowering by extended exposures to low temperatures that mimic winter conditions. The responses to day length (or photoperiod) and vernalization are mediated by pathways that specifically control responses to