Florigen (or flowering hormone) is the term used to describe the hypothesized hormone-like molecules responsible for controlling and/or triggering flowering in plants. Florigen is produced in the leaves and acts in the shoot apical meristem of buds and growing tips. It is known to be graft-transmissible and even functions between species. However, despite having been sought since the 1930s, the exact nature of florigen is still a mystery.
Central to the hunt for florigen is an understanding of how plants use seasonal changes in day length to mediate flowering, a mechanism known as photoperiodism. Plants which exhibit photoperiodism may be either ‘short day’ or ‘long day’ plants, which in order to flower require short days or long days respectively. Although plants in fact determine day length from night length.
The current model suggests the involvement of multiple different factors. Research into florigen is predominately centred around 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.