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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).
Mass
Sequence
Nonflowering
Sequence similarities (%)
2162
GPPKYXGSASEIA
Reduced
No similarities found
Detected
A.t. 2-oxo-glutarate/malate translocator-like protein AF370494
(60%)
Rice pyrophosphatase ovp1, AB012765 (60%)
Trillium maturase, AB017379 (60%)
Phophorylmutases in bacteria
Not determined
Potato leaf EST425039, BE921270 (72%) with similarity to
plastocyanin from, e.g., Mercurialis perennis
Modified
Possibly glycosylated
Similarity to protein kinases
KAVIVPILANLVLVHyp
5075/5167
KAAIVPGLVSLIVVP
MAAILPDLATQVLP
FAVIVEIPFSLRLVP
LDVTLGSDDGGLVFIPQN
1860
3388
LDVLLGGDDGSLAFIPGN
LDVLLGSDDGGLAFVPNN
VYVHGN
FTTGSGHNS
(From Hoffmann-Benning et al., Planta 216:140—147, 2002)
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, longday 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.