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The Mystery of FLOWERING
About 150 million years ago dinosaurs were disappearing from the Earth, and flowering plants were
appearing.
Since that time, flowering plants (angiosperms) have flourished, rapidly diversifying and spreading
into virtually every landscape on the planet. Charles Darwin referred to this abrupt origin and highly
accelerated rate of diversification of angiosperms an “abominable mystery”. (Please see ref. 1 below
for an excellent review.)
Another great botanical mystery over the past 100 years has to do with what induces plants to make
flowers.
Young (juvenile) angiosperms don’t have flowers. They make just roots, shoots and leaves. But at
some point during their lifetime, they begin to make flowers. The mystery was: what triggered
flowering?
A major clue in solving this mystery happened about 100 years ago when scientists discovered
evidence for a flower-inducing signal, produced in the leaves, that later was named florigen.
Discovering the nature of florigen has taken nearly 100 years. But recently scientists likely have not
only identified florigen but also have pieced together how it works to induce flowering.
Mystery Solved?
The latest scientific evidence supports the hypothesis that florigen is actually a protein called FT coded
for by the geneFlowering Locus T in Arabidopsis. (Please see refs. 2 & 3 below for current reviews of
the subject.)
Briefly, FT is produced in the leaves and is transported via the phloem to the shoot apical meristems
(SAM). Here FTacts like a molecular “alarm-clock”, evoking a complex genetic scenario, which
culminates in flower formation. (Please see my YouTube video for an overview of the genetics of
flowering.)
Making Plants Flower At Will?
Can scientists now induce plants to flower at will by simply spraying a “flowering hormone” on them?
The answer is: Not yet, but maybe someday real soon.
Recent experiments indicate that the first commercially-available plants you’ll likely see will be
genetically-engineered to over-express the FT gene under special conditions. (See ref. 4, for
example.)
The Flowering Locus T gene codes for the FT protein, which most biologists currently agree is indeed
florigen.
Presuming the plant is competent to flower, then the sudden appearance of lots of florigen would
probably “tip the scales” to induce flowering in responsive SAMs.
To “turn on” the Flowering Locus T genes, one could engineer them to be activated by alcohol
(ethanol), for example. So that to induce such an engineered plant to flower, one would merely need
to spray a dilute solution of alcohol (wine?) on the leaves.
Florigen
a natural complex of plant hormones that induce flowering. The term “florigen” was introduced in 1936 b
y the Soviet plantphysiologist M. Kh. Chailakhian.
The existence of the hormone was discovered in experiments on the photoperiodic regulation of floweri
ng. Under favorableconditions in terms of the optimal length of the day, a physiologically active substan
ce that stimulates flowering forms in theleaves of adult plants. Entering the stem buds from the leaves t
hrough living tissues, the substance induces the formationof flowers. Experiments with grafts of plants h
ave established that florigen moves from a flowering-plant donor to avegetativeplant receptor, fostering the flowering of the latter. In the same experiments it was made clear that florig
en is notspecies-specific: it induces flowering in various species and photoperiodic groups of plants.
However, the mechanism of florigen formation and the hormone’s properties have been insufficiently stu
died, and thecomplete composition of florigen has not been established. Most widely accepted is Chaila
khian’s hypothesis (1958) thatflorigen consists of two components: gibberellins and the hypothetical ant
hesins (from the Greek word for flowering—
anthesis). A plant flowers only when both components are present. The gibberellins induce the formatio
n of flower stems,and the anthesins induce the formation of the flowers. The flowering of shortday plants is retarded on long days becausethere is only weak synthesis of anthesins under those cond
itions. The flowering of longday plants is retarded on a short dayowing to an insufficiency of gibberellins. During a favorable photop
eriod, both kinds of plants synthesize the lackingcomponent parts of florigen.
The artificial treatment of plants with gibberellins stimulates flowering in longday plants, which lack that hormone, whereasit does not influence the flowering of shortday ones, whose development is limited by insufficient anthesins. Hence, thehypothesis of the bicómpo
nent composition of florigen is further supported. In a number of cases the flowering of twovegetative pl
ants with different requirements as to length of day has been achieved by uniting the component parts
offlorigen contained in them through grafting.
Florigen (or flowering hormone) is the hypothesized hormone-like molecule responsible for
controlling and/or triggering flowering in plants. Florigen is produced in the leaves, and acts in
the shoot apical meristemof 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.
Contents
[hide]



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1Mechanism
o 1.1Initiation
o 1.2Translocation
o 1.3Flowering
2Research history
3References
4External links
Mechanism[edit]
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 distinguish day length from night length.[1]
The current model suggests the involvement of multiple different factors. Research into florigen is
predominately centred on 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.[2] 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.
Initiation[edit]
In Arabidopsis thaliana, the signal is initiated by the production of messenger RNA (mRNA) coding
a transcription factor called CONSTANS (CO). CO mRNA is produced approximately 12 hours after
dawn, a cycle regulated by the plant's biological clock.[3] This mRNA is then translated into CO
protein. However CO protein is stable only in light, so levels stay low throughout short days and are
only able to peak at dusk during long days when there is still a little light.[4][5] CO protein
promotes transcription of another gene called Flowering Locus T (FT). By this mechanism, CO
protein may only reach levels capable of promoting FT transcription when exposed to long days.
Hence, the transmission of florigen—and thus, the induction of flowering—relies on a comparison
between the plant's perception of day/night and its own internal biological clock.[2]
Translocation[edit]
The FT protein resulting from the short period of CO transcription factor activity is then transported
via the phloem to the shoot apical meristem.[6][7]
Flowering[edit]
At the shoot apical meristem, the FT protein interacts with a transcription factor (FD protein) to
activate floral identity genes, thus inducing flowering.[8][9] Specifically, arrival of FT at the shoot apical
meristem and formation of the FT/FD heterodimer is followed by the increased expression of at least
one direct target gene, APETALA 1 (AP1),[8] along with other targets, such as SOC1 and several
SPL genes, which are targeted by a microRNA.[10]
Research history[edit]
This section
requires expansion.(April 2009)
Florigen was first described by Soviet Armenian plant physiologist Mikhail Chailakhyan, who in 1937
demonstrated that floral induction can be transmitted through a graft from an induced plant to one
that has not been induced to flower.[11] Anton Lang showed that several long-day plants and biennials
could be made to flower by treatment with gibberellin, when grown under a non-flower-inducing (or
non-inducing) photoperiod. This led to the suggestion that florigen may be made up of two classes of
flowering hormones: Gibberellins and Anthesins.[12] It was later postulated that during non-inducing
photoperiods, long-day plants produce anthesin, but no gibberellin while short-day plants produce
gibberellin but no anthesin.[11] However, these findings did not account for the fact that short-day
plants grown under non-inducing conditions (thus producing gibberellin) will not cause flowering of
grafted long-day plants that are also under noninductive conditions (thus producing anthesin).
As a result of the problems with isolating florigen, and of the inconsistent results acquired, it has
been suggested that florigen does not exist as an individual substance; rather, florigen's effect could
be the result of a particular ratio of other hormones.[13][14] However, more recent findings indicate that
florigen does exist and is produced, or at least activated, in the leaves of the plant and that this
signal is then transported via thephloem to the growing tip at the shoot apical meristem where the
signal acts by inducing flowering. In Arabidopsis thaliana, some researchers have identified this
signal as mRNA coded by the FLOWERING LOCUS T (FT) gene, others as the
resulting FT protein.[15] First report of FT mRNA being the signal transducer that moves from leaf to
shoot apex came from the publication in Science Magazine. However, in 2007 other group of
scientists made a breakthrough saying that it is not the mRNA, but the FT Protein that is transmitted
from leaves to shoot possibly acting as "Florigen".[16] The initial article[17] that described FT mRNA as
flowering stimuli was retracted by the authors themselves.