Floral Induction and Florigen

that they are not sensitive to environmental cues. The Milo J. Aukerman and Rick M. Amasino Department of Biochemistry extent to which environmental-response pathways versus autonomous pathways control flowering varies 420 Henry Mall University of Wisconsin among plant species. Even within a species there are often photoperiod-responsive and -nonresponsive types Madison, Wisconsin 53706-1569 that exist naturally or as a result of induced mutations. In fact the analysis of a spontaneous mutation that converted a variety of tobacco that normally flowers indeInductive processes play an important role in the development of multicellular organisms. From a developpendently of daylength toa variety (Maryland Mammoth) that required exposure to short days for flowering facilimental biologist’s perspective, induction can be broadly defined as the effect on the developmental pathway of tated the discovery of photoperiodism in plants (Garner and Allard, 1920). one group of cells by a substance displayed by or emitted from another (Slack, 1991). In plants, the phenomeStudies of photoperiod-responsive plants led to the development of the florigen hypothesis: leaves perceive non commonly referred to as floral induction fits this broad definition of an inductive process: cells within the inductive photoperiods and subsequently produce a signal that is translocated from the leaves to SAMs to growing tip of the plant switch from a vegetative to a reproductive mode of development in response to a initiate the floral transition (Chailakhyan, 1936). This signal has been referred to as florigen although the chemisignal emitted from cells in another location, for instance the leaves. Unlike the classic type of induction described cal nature of the signal is not known. Although the term florigen implies a single compound (a unique flowering in animals, the inductive signal involved in flowering can travel over a long distance, from its source in the leaves hormone), the flowering signal may be a mixture of several components (Bernier et al., 1993). Regardless of to thegrowing tip, and in this respect the inductive signal behaves more like a hormone or group of hormones. the nature of the signal, there is much evidence that translocatable signals regulate flowering (Zeevaart, 1984). The growing tip of a plant shoot is a population of stem cells, referred to as the shoot apical meristem For example, applying inductive photoperiods only to leaves causes flowering at the SAM. Moreover, a flow(SAM), that gives rise to the tissues of the above ground parts of the plant. In the first phase of the plant life cycle, ering signal can be transmitted through a graft union from a photoperiodically induced shoot or even from a the SAM forms primordia that divide and differentiate into leaves (Figure 1). Upon floral induction, the SAM leaf to a noninduced graft partner (Figure 2). It has also been known for some time that the control of flowering forms primordia that develop into flowers. In order to optimize seed production and to ensure that seed set by translocatable signals is not restricted to species that respond to photoperiod. Grafting studies demonstrate occurs during the appropriate season, plants have evolved a complex set of regulatory pathways to control that flowering signals can be transmitted from a photoperiod-responsive to a photoperiod-insensitive plant when this floral transition occurs. These regulatory pathways are of two general types. One type involves path(Lang et al., 1977) or between photoperiod-insensitive plants (McDaniel et al., 1996). It is tempting to speculate ways designed to sense environmental cues such as changes in daylength, temperature, or soil moisture. For that there is a universal flowering signal used by all higher plants, but grafting studies do not provide strong example, many plant species have evolved photoperiod-responsive pathways (some species are induced evidence for this because grafting is only possible between related species, and it is expected that the nature to flower by long days, whereas other species flower earlier in response to short days). The second class of of flowering signals would be conserved in related species. pathways are those that involve the state of development. For example, many plant species must pass Whereas much progress has been made in identifying the molecules involved in inductive processes in anithrough a juvenile phase during which they are not competent to initiate flowering. These developmental pathmals, the molecular basis of floral induction in plants has remained a mystery. A molecular genetic approach ways are often referred to as autonomous to indicate