Florigen is a mobile signaling molecule that directs a plant to transition from vegetative growth to its reproductive phase, producing flowers. This signal helps plants coordinate their flowering time with favorable environmental seasons. Understanding this internal communication sheds light on how plants regulate their life cycles.
The Historical Hunt for a Flowering Signal
Early in the 20th century, scientists observed that plants respond to the relative length of day and night, a phenomenon termed photoperiodism. Researchers, including Wightman Garner and Henry Allard in 1920, found that day length, not light intensity or nutrient availability, influenced flowering. These observations suggested that leaves, which perceive light, send a signal to the distant growing tips where flowers form.
Further evidence for a mobile flowering signal emerged from classic grafting experiments in the 1930s. Scientists demonstrated that if a leaf from a plant exposed to flowering conditions was grafted onto a non-flowering plant, the recipient plant would begin to flower. This indicated a transmissible substance moved from the induced leaf to the shoot apical meristem. Soviet botanist Mikhail Chailakhyan coined the term “florigen” in 1936 to describe this hypothetical flower-inducing substance.
Identifying the Florigen Molecule
For decades, florigen remained a theoretical concept, its molecular identity elusive. Advancements in molecular biology and genetic research, particularly using Arabidopsis thaliana, pinpointed the specific molecule. This breakthrough occurred around 2005-2007, revealing the FLOWERING LOCUS T (FT) protein as the primary component of florigen.
The FT protein is highly conserved across a vast range of plant species, from Arabidopsis to rice and trees. This widespread conservation underscores its role as the flowering signal throughout the plant kingdom.
The Mechanism of Action
The process begins in the leaves, where the FT protein is produced in specialized phloem companion cells in response to day-length cues. For instance, in long-day plants like Arabidopsis, increasing day length triggers FT production.
The FT protein is transported from the leaves to the shoot apical meristem (SAM), the growing tip of the plant, through the phloem. This transport occurs quickly, with FT protein reaching the shoot apex within 12 hours of induction.
Upon arrival at the shoot apical meristem, the FT protein interacts with FLOWERING LOCUS D (FD) to form a complex. This FT-FD complex functions as a transcription factor, binding to the regulatory regions of specific genes. This binding activates genes responsible for changing the meristem’s developmental program. Genes such as APETALA1, LEAFY, and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) are switched on, reprogramming the meristem to produce flower structures instead of new leaves or stems.
Agricultural and Horticultural Applications
Understanding florigen’s identity and mechanism provides tools for modern agriculture and horticulture. By manipulating its expression or transport, growers can precisely control when crops flower. This capability is useful for synchronizing harvests, which can streamline operations and increase overall yields.
Knowledge of florigen allows crops to be grown outside their traditional geographical or seasonal ranges. For example, greenhouse operations can induce flowering in ornamental plants like poinsettias or chrysanthemums for holidays, regardless of natural day length. Plant breeders also use this information to develop varieties with desirable flowering times, optimizing crop performance for various climates and cultivation systems.