Florigen, often called the universal flowering signal, is the substance responsible for triggering the transition from a plant’s vegetative growth stage to its reproductive stage. This small, mobile signal allows the plant’s leaves, which perceive environmental cues, to communicate the correct time to flower to the distant shoot tip. Identifying and characterizing florigen, which is conserved across nearly all flowering species, was a major accomplishment in modern plant biology.
Environmental Triggers for Florigen Production
Plants use environmental signals to determine the optimal time to reproduce. The primary signals that regulate the production of florigen are photoperiodism and vernalization. Photoperiodism is the plant’s response to the relative length of day and night, perceived in the leaves by specialized photoreceptor proteins.
Plants are categorized into groups like long-day plants, which flower when the night period is shorter than a specified threshold, and short-day plants, which require a night period longer than a threshold. The leaves translate these light signals into the genetic activation of the florigen gene.
Vernalization involves the requirement of prolonged exposure to cold temperatures before flowering can occur. This mechanism prevents plants from flowering prematurely in the autumn, ensuring flowering is reserved for the following spring. This long-term cold exposure leads to epigenetic changes that suppress floral inhibitor genes, which in turn allows for the eventual production of florigen.
The Long Quest for the Flowering Signal
The concept of a mobile flowering signal existed for nearly a century before its molecular identity was confirmed. Early experiments in the 1930s established that the flowering stimulus was perceived exclusively in the leaves, while flower formation happened at the shoot tip. This spatial separation implied that a chemical messenger must travel from the leaves to the distant growing points.
The Soviet plant physiologist Mikhail Chailakhyan coined the term “florigen” in 1937. Grafting experiments provided the strongest evidence for its existence, demonstrating that a leaf from a flowering-induced plant, when grafted onto a non-induced plant, could cause the recipient plant to flower. This phenomenon worked even across different plant types, suggesting the signal was universal across species.
For decades, florigen remained a scientific mystery because researchers could prove its effect but could not isolate the compound itself. Multiple substances, including various hormones, were investigated and ruled out because they could not fully account for the long-distance signaling observed in grafting experiments. Molecular genetics eventually provided the tools needed to uncover its true nature.
Florigen’s Molecular Identity and Travel
Modern research has identified florigen not as a classical plant hormone, but as a small protein encoded by the FLOWERING LOCUS T (FT) gene. The FT protein is highly conserved across flowering plant species, reinforcing the observation that florigen is a universal signal. The FT gene is transcribed primarily in the leaves, specifically within the phloem companion cells, which are part of the plant’s vascular system.
Once the FT gene is activated by environmental signals, the resulting FT protein is synthesized in these leaf cells. This protein then enters the phloem, the vascular tissue responsible for transporting sugars and nutrients. The phloem allows the FT protein to move rapidly from the leaves to the distant growing points, or meristems.
The FT protein travels in the phloem stream to the shoot apical meristem (SAM), the dome of cells at the plant’s apex where new leaves and stems originate. This movement is essential because the SAM is the location where the developmental changes leading to flower formation occur.
Activating the Flowering Process
Upon arrival at the shoot apical meristem, the FT protein is unloaded from the phloem and enters the meristem cells. The protein functions as a signaling factor that must interact with other molecules present only in the meristem. Specifically, the FT protein binds to a local transcription factor called FLOWERING LOCUS D (FD).
This interaction is often mediated by a third protein, a 14-3-3 protein, to form a three-part structure known as the Florigen Activation Complex (FAC). The FAC is the molecular switch that initiates the flowering program. Once formed, this complex moves into the cell nucleus, where it can begin to regulate gene expression.
The complex targets and switches on floral identity genes, such as APETALA1 (AP1) and LEAFY (LFY). These genes are master regulators that reprogram the meristem cells, permanently changing their fate from producing leaves and stems to generating the sepals, petals, stamens, and carpels of a flower. This genetic cascade causes the transformation of the SAM into a floral meristem.