Floral Induction and Florigen

Inductive processes play an important role in the development of multicellular organisms. From a developmental biologist's perspective, induction can be broadly defined as the effect on the developmental pathway of one group of cells by a substance displayed by or emitted from another. In plants, the phenomenon commonly referred to as floral induction fits this broad definition of an inductive process: cells within the growing tip of the plant switch from a vegetative to a reproductive mode of development in response to a signal emitted from cells in another location, for instance the leaves. Unlike the classic type of induction described in animals, the inductive signal involved in flowering can travel over a long distance, from its source in the leaves to the growing tip, and in this respect the inductive signal behaves more like a hormone or group of hormones. The growing tip of a plant shoot is a population of stem cells, referred to as the shoot apical meristem (SAM), that gives rise to the tissues of the above ground parts of the plant. In the first phase of the plant life cycle, the SAM forms primordia that divide and differentiate into leaves. Upon floral induction, the SAM forms primordia that develop into flowers. In order to optimize seed production and to ensure that seed set occurs during the appropriate season, plants have evolved a complex set of regulatory pathways to control when this floral transition occurs. These regulatory pathways are of two general types. One type involves pathways designed to sense environmental cues such as changes in daylength, temperature, or soil moisture. For example, many plant species have evolved photoperiod-responsive pathways (some species are induced to flower by long days, whereas other species flower earlier in response to short days). The second class of pathways are those that involve the state of development. For example, many plant species must pass through a juvenile phase during which they are not competent to initiate flowering. These developmental pathways are often referred to as autonomous to indicate that they are not sensitive to environmental cues. The extent to which environmental-response pathways versus autonomous pathways control flowering varies among plant species. Even within a species there are often photoperiod-responsive and -nonresponsive types that exist naturally or as a result of induced mutations. In fact the analysis of a spontaneous mutation that converted a variety of toba