Florigen

Mechanism

For a plant to begin flowering, it must undergo changes in its shoot apical meristem (SAM). However, there are multiple environmental factors affecting the plant even before it begins this process — in particular, light. It is through "the evolution of both internal and external control systems that enables plants to precisely regulate flowering so that it occurs at the optimal time for reproductive success." The way the plant determines this optimal time is through day-night periods through the use of photoperiodism. Although it was originally thought that the accumulation of photosynthetic products controlled the flowering of plants, two men by the names of Wightman Garner and Henry Allard proved it was not. They instead found that it was a matter of day length rather than the accumulation of the products within the plants that affected their flowering abilities.

Flowering plants fall into two main photoperiodic response categories:

1. "Short-day plants (SDPs) flower only in short days (qualitative SDPs), or their flowering is accelerated by short days (quantitative SDPs)"
2. "Long-day plants (LDPs) flower only in long days (qualitative LDPs), or their flowering is accelerated by long days (quantitative LDPs)"

These types of flowering plants are differentiated by the whether the day has exceeded some duration - usually calculated by 24-hour cycles - known as the critical day length. It is also important to note that there is no absolute value for the minimum day length as it varies greatly amid species. Until the correct amount of day length is reached, the plants ensure no flowering results. They do so through adaptations like preventing immature plants from responding to inadequate day lengths. Plants also have the ability to prevent the response of the photoperiodic stimulus until a certain temperature is reached. Species like winter wheat that rely on just that. The wheat require a cold period before being able to respond to the photoperiod. This is known as vernalization or overwintering.

This ebb-and-flow of flowering in plants is essentially controlled by an internal clock known as the endogenous oscillator. It is thought that these internal pacemakers "are regulated by the interaction of four sets of genes expressed in the dawn, morning, afternoon, and evening hours [and that] light may augment the amplitude of the oscillations by activating the morning and evening genes." The rhythms between these different genes are generated internally in the plants, starts with the leaves, but requires an environmental stimulus such as light. The light essentially stimulates the transmission of a floral stimulus (florigen) to the shoot apex when the correct amount of day-length is perceived. This process is known as photoperiodic induction and is a photoperiod-regulated process that is also dependent on the endogenous oscillator..

The current model suggests the involvement of multiple different factors. Research into florigen is predominately centred on the model organism and long day plant, Arabidopsis thaliana. Whilst much of the florigen pathways appear to be well conserved in other studied species, variations do exist. The mechanism may be broken down into three stages: photoperiod-regulated initiation, signal translocation via the phloem, and induction of flowering at the shoot apical meristem.

Initiation

In Arabidopsis thaliana, the signal is initiated by the production of messenger RNA (mRNA) coding a transcription factor called CONSTANS (CO). CO mRNA is produced approximately 12 hours after dawn, a cycle regulated by the plant's circadian rhythms, and is then translated into CO protein. However CO protein is stable only in light, so levels stay low throughout short days and are only able to peak at dusk during long days when there is still some light. CO protein promotes transcription of another gene called Flowering Locus T (FT). By this mechanism, CO protein may only reach levels capable of promoting FT transcription when exposed to long days. Hence, the transmission of florigen—and thus, the induction of flowering—relies on a comparison between the plant's perception of day/night and its own internal biological clock.

Translocation

The FT protein resulting from the short period of CO transcription factor activity is then transported via the phloem to the shoot apical meristem.

Flowering

Florigen is a systemically mobile signal that is synthesized in leaves and the transported via the phloem to the shoot apical meristem (SAM) where it initiates flowering. In Arabidopsis, the FLOWERING LOCUS T (FT) genes encode for the flowering hormone and in rice the hormone is encoded by Hd3a genes thereby making these genes orthologs. It was found though the use of transgenic plants that the Hd3a promoter in rice is located in the phloem of the leaf along with the Hd3a mRNA. However, the Hd3a protein is found in neither of these places but instead accumulates in the SAM which shows that Hd3a protein is first translated in leaves and then transported to the SAM via the phloem where floral transition is initiated; the same results occurred when looked at Arabidopsis. These results conclude that FT/Hd3a is the florigen signal that induces floral transition in plants.

Upon this conclusion, it became important to understand the process by which the FT protein causes floral transition once it reaches the SAM. The first clue came with looking at models from Arabidposis which suggested that a bZIP domain containing transcription factor, FD, is somehow interacting with FT to form a transcriptional complex that activates floral genes. Studies using rice found that there is an interaction between Hd3a and OsFD1, homologs of FT and FD respectively, that is mediated by the 14-3-3 protein GF14c. The 14-3-3 protein acts as intracellular florigen receptor that interacts directly with Hd3a and OsFD1 to form a tri-protein complex called the florigen activation complex (FAC) because it is essential for florigen function. The FAC works to activate genes needed to initiate flowering at the SAM; flowering genes in Arabidopsis include AP1, SOC1 and several SPL genes, which are targeted by a microRNA and in rice the flowering gene is OsMADS15 (a homolog of AP1).

Antiflorigen

Florigen is regulated by the action of an antiflorigen. Antiflorigens are hormones that are encoded by the same genes for florigen that work to counteract its function. The antiflorigen in Arabidopsis is TERMINAL FLOWER1 (TFL1) and in tomato it is SELF PRUNING (SP).