A Short History of florigen, • - The identity of florigen remains one of the major mysteries in plant biology. • In 1865, Sachs observed that leaves in the light produce flower-forming substances, which are present in small quantities and lead to the formation of flowers. • The floral stimulus, or florigen, is formed in the leaves as a response to an inductive photoperiod and translocated through the phloem to the apical meristem. • Grafting experiments (Zeevaart, 1976) have shown that this floral stimulus is graft transmissible and can be interchangeable between different species and plants of different photoperiodic response types (SDP and LDP).
What Could Florigen Be? • - The original assumption was that the phloem only transported assimilates in the form of carbohydrates. • Results in recent years have shown that, apart from sugars, the phloem contains small molecules, peptides and proteins (Fisher et al. 1992; Sakuth et al. 1993; Marentes and Grusak 1998; Xoconostle-Cazares et al. 1999; Kehr et al. 1999), and nucleic acids (Kühn et al. 1997; Ruiz-Medrano et al. 1999). • The contents of the phloem are now known to be so complex that phloem transport has been called the "superinformation highway" of plants (Lucas 2000).
Early Flowering Mutant • - Arabidopsis is a facultative long-day plant: long days stimulate the transition to flowering, but the plants will eventually flower under short day conditions. • CONSTANS mediates between photoperiod perception and the transition to reproductive growth, and transcript levels accumulate under inductive, long-day conditions (Suarez-Lopez et al., 2001). • - Arabidopsis plants homozygous for mutant alleles of CO flower late under long-day conditions, whereas plants overexpressing CO flower early (Putterill et al., 1995).
FLOWERING LOCUS T (FT) is a downstream target of CO, and a correlation between FT and CO expression patterns argues that CO does not directly promote gene expression in cells in which CO, itself, is not expressed (Takada and Goto, 2003). • Overexpression of FT in several tissues, including phloem-limited expression, results in early flowering in Arabidopsis, suggesting that FT also acts upstream of florigen (An et al., 2004). • FT, a small protein of 23 kDa, shares homology with RAF-kinase-inhibitor proteins in mammals (Kardailsky et al., 1999; Kobayashi et al., 1999). FT is thus smaller than the green fluorescent protein (GFP; 26 kDa) which is able pass through the plasmodesmata between companion cells and sieve elements, and move long-distance via the phloem (Imlau et al., 1999). • If florigen is a macromolecule, then FT’s small size and potential role in modulating phosphorylation make it an intriguing candidate for the phloem-mobile floral stimulus.
Researchers at the Umea Plant Science Centre in Sweden and the Universite Joseph Fourier in France have identified the messenger RNA for the flowering locus T (FT) gene as the elusive “florigen” signaling compound, appears to be this trigger for initiation of flowering. The group constructed transgenic plants expressing the FT gene and the marker gene GUS under control of a heat-shock inducible promoter (Hsp) from soybean as well as the FT promoter (pFT). When microdissected plant tissues, whole plants or individual leaves were subjected to heating, gene expression levels could be monitored throughout the plants. In this way they were able to follow expression levels of FT in various tissues and confirm movement of the FT-induced signal from leaf to apex tissues. - Huang, T., Bohlenius, H., Eriksson, S., Parcy, F., Nilsson, O., (2005) “The mRNA of the Arabidopsis Gene FT Moves front Leaf to Shoot Apex and Induces Flowering” Science 309(5741): - Abe,M. Kobayashi, Y., Yamamoto, S., Daimon, Y., Yamaguchi, A., Ikeda, Y., Ichinoki, H., Notaguchi, M., Goto, K., Araki, T.., (2005)“FD, a bZIP Protein Mediating Signals from the Floral Pathway Integrator FT at the Shoot Apex” Science, 309(5737): 1052-1056. - Wigge, P.A., Kim, M.C., Jaeger, K.E., Busch, W., Markus Schmid,M., Lohmann, J.U., Weigel, D., (2005)“Integration of Spatial and Temporal Information During Floral Induction in Arabidopsis”Science 309(5737): 1056-1059. - Hoffmann-Benning, S., Zeevaart, J.A.D. “Searching for Florigen” Plant Physiology Online 24:2 (2003) http://www.plantphys.net/article.php?ch=e&id=288 - Ayre, B.G., “Florigen and a Genetic Approach to Long-Distance Signaling Through the Phloem” Plant Physiology Online 24:3 (2005) http://www.plantphys.net/article.php?ch=e&id=291 - Miguel A. Blázquez (2005) “The Right Time and Place for Making Flowers” Science 309 (5737): 1024-1025. Plant “Florigen” factor identified.