FT protein movement contributes to long-distance signaling in floral induction of Arabidopsis.

Abstract

In plants, seasonal changes in day length are perceived in leaves, which initiate long-distance signaling that induces flowering at the shoot apex. The identity of the long-distance signal has yet to be determined. In Arabidopsis, activation of FLOWERING LOCUS T (FT) transcription in leaf vascular tissue (phloem) induces flowering. We found that FT messenger RNA is required only transiently in the leaf. In addition, FT fusion proteins expressed specifically in phloem cells move to the apex and move long distances between grafted plants. Finally, we provide evidence that FT does not activate an intermediate messenger in leaves. We conclude that FT protein acts as a long-distance signal that induces Arabidopsis flowering.

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DOI: 10.1126/science.1141752
, 1030 (2007);
316
Science
et al.Laurent Corbesier,
ArabidopsisSignaling in Floral Induction of
FT Protein Movement Contributes to Long-Distance

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27. The change in Coulomb failure stress is defined as
DCFS = Dss+ mDsn, where Dssis the change in shear
stress, Dsnis the change in effective normal stress, and
m is the coefficient of effective internal friction (28). We
use m = 0.4.
28. G. C. P. King, R. S. Stein, J. Lin, Bull. Seismol. Soc. Am.
84, 935 (1994).
29. S. Owen et al., Geophys. Res. Lett. 27, 2757 (2000).
30. S. Jonsson et al., Geophys. Res. Lett. 26, 1077
(1999).
31. Y. Fukushima, V. Cayol, P. Durand, J. Geophys. Res. 110,
B03206 (2005).
32. We thank NASA’s Earth Science program and the NSF’s
Geophysics program for funding, the Alaska satellite
facility for conducting the data acquisition, and the
Hawaii Volcano Observatory for their support. This work is
based on Radarsat imagery, a satellite operated by the
Canadian Space Agency. Two reviewers provided
constructive comments. Center for Southeastern Tropical
Advanced Remote Sensing (CSTARS) contribution #11.
Supporting Online Material
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Material and Methods
Table S1
Figs. S1 to S4
References
17 January 2007; accepted 3 April 2007
10.1126/science.1140035
FT Protein Movement Contributes to
Long-Distance Signaling in Floral
Induction of Arabidopsis
Laurent Corbesier,1Coral Vincent,1* Seonghoe Jang,1* Fabio Fornara,1Qingzhi Fan,2Iain Searle,1
Antonis Giakountis,1Sara Farrona,1Lionel Gissot,1Colin Turnbull,2George Coupland1†
In plants, seasonal changes in day length are perceived in leaves, which initiate long-distance
signaling that induces flowering at the shoot apex. The identity of the long-distance signal has yet
to be determined. In Arabidopsis, activation of FLOWERING LOCUS T (FT) transcription in leaf
vascular tissue (phloem) induces flowering. We found that FT messenger RNA is required only
transiently in the leaf. In addition, FT fusion proteins expressed specifically in phloem cells move to
the apex and move long distances between grafted plants. Finally, we provide evidence that FT
does not activate an intermediate messenger in leaves. We conclude that FT protein acts as a
long-distance signal that induces Arabidopsis flowering.
P
erception of day length takes place in
the leaf, whereas flowers are formed by
the shoot apical meristem at the apex of
the shoot (1, 2). A long-distance signal, called
florigen or the floral stimulus, has been dem-
onstrated to be transmitted through the phloem
vascular system from the leaves to the meristem,
although the identity of this signal has remained
unclear since the 1930s. Molecular-genetic ap-
proaches in Arabidopsis have defined a regulatory
pathway that promotes flowering in response to
long days (LDs) and have suggested how this
pathway responds to day length (3–5). UnderLDs,
the CONSTANS (CO) transcriptional regulator
activates transcription of FLOWERING LOCUS T
(FT) in the vascular tissue of leaves (6–8). FT
encodes a small protein with similarity to RAF-
kinase inhibitors that acts at the meristem to-
gether with the transcription factor FD to activate
transcription of the floral meristem identity gene
APETALA1 (7, 9–11). FT is expressed in the
leaves in response to photoperiod, but FT protein
1Max Planck Institute for Plant Breeding Research, Carl von
Linne Weg 10, D-50829 Cologne, Germany.2Division of
Biology, Imperial College London, Wye Campus, Wye, Kent
TN25 5AH, UK.
*These authors contributed equally to this work.
†To whom correspondence should be addressed. E-mail:
coupland@mpiz-koeln.mpg.de
Fig. 1. Regulation of FT
mRNA in leaves during
flowering. (A) Flowering
time of wild-type Ler and
ft-7 plants grown for 2
weeks under SD and ex-
posed to three inductive
LDsbeforereturntoSDs.
(B) Expression of FT
mRNA during 7 days
comprising one SD fol-
lowed by three LDs and
then three subsequent
SDs. FT mRNA expression
in the SD-grown controls
is also shown. RNA was
tested every 4 hours. The
inserted three LDs are
shaded. Below the graph,
bars show the duration of
day (white) and night
(black) for the shift exper-
iment (top) and the con-
trol experiment (bottom).
(C) Endogenous FT mRNA
[FT 3′ untranslated region
(UTR)] and FT:GFP mRNA
(GFP) expression in 14-
day-old Ler, 35S:FT:GFP,
and SUC2:FT:GFP plants.
(D)LeafnumberatfloweringofCO:CO:GR,co-2plantstreated(+DEX)ornottreated(–DEX)withdexamethasone.Plantsweregrownfor2weeksinSDconditionsandthen
shiftedtoLDsfor4days.DexamethasonewasappliedduringtheLDtreatment.(EandF)FTmRNAexpressionintreated(E)andnontreated(F)leavesofCO:CO:GRplants.
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acts in the meristem to promote gene expres-
sion, suggesting that a product of FT may be
transported to the meristem as the floral stimulus
(6, 7, 9). Experiments indicating that FT mRNA
comprises the transmissible signal have recently
been retracted (12). Furthermore, the floral
stimulus, but no detectable mRNA of genes sim-
ilar to FT, crossed the junction between grafted
tomatoplants(13).Weexaminedtherequirement
for FT expression in the leaves during floral
induction and explored the possibility that FT
protein comprises the floral stimulus.
First, we tested whether stable induction of FT
expression in the leaves of Arabidopsis is required
forflowering.Perillaleavesexposedtoappropriate
photoperiods produce the floral stimulus perma-
nently(14,15).Shortday(SD)–grownArabidopsis
plants exposed to three LDs and then returned to
SDs flowered much earlier than plants exposed
only to SDs (16) [Fig. 1A and supporting online
material (SOM) text]. FT expression rises during
the first LD after a shift from SDs (17). We tested
whether this increase is stable by analyzing ex-
pression ofCO and FT mRNAevery 4hoursfor7
days, covering the shift from SDs to LDs and back
to SDs (Fig. 1B and fig. S1A). In control plants
grownonlyinSDs,FTmRNAabundanceremained
low (Fig. 1B). In contrast, in plants exposed to
three LDs, FT mRNA abundance was increased
in each of the three LDs. However,after return to
SDs, FT mRNA levels fell after 1 day to the low
levelcharacteristicofSD-grownplants(Fig.1B).
Therefore, in these conditions, FT mRNA expres-
sionisnotstablymaintainedafterexposuretoLDs.
However, expression of endogenous FT mRNA
was increased in the leaves of plants in which FT
was substantially overexpressed from a transgene
(Fig.1C).WeconcludedthatFTmRNAexpression
at wild-type levels in the leaves for 3 days is suf-
ficienttostablyinducefloweringattheshootapical
meristem and that under these conditions FT ex-
pression in the leaves is not maintained.
In some plants, leaves that have not been ex-
posed to inductive day lengths can be indirectly
induced to form the floral stimulus. For example,
graftingaplantexposedtoinductivedaylengthsto
a second noninduced plant can cause the second
plant to produce the floral stimulus (2, 14). To test
whether FT expression is induced indirectly in
leaves ofArabidopsis,weconstructed a fusion of
theCOpromotertoageneencodingatranslational
fusion between CO and the rat glucocorticoid re-
ceptor binding domain (CO:CO:GR), and we in-
troduced this into the co-2 mutant. In these
plants, CO activity is induced by addition of the
steroid dexamethasone (dex) only under LDs,
during which the CO mRNA accumulates in the
light (18–20). Application of dex to a single leaf
induced flowering and increased the amount of
FT mRNA in the leaves to which dex was added
(Fig. 1, D to F, and fig. S1C). However, no
difference in FT mRNA abundance was detected
between the untreated leaves of plants treated with
dex and similar leaves from untreated plants (Fig.
1F). Therefore, no detectable indirect activation
of FT mRNA expression occurs in Arabidopsis
leaves under the inductive conditions used in
thisexperiment,andactivation ofFTinasingle
leaf is sufficient to induce flowering.
Next, we compared the spatial distribution of
FT mRNAandprotein,exploitingtransgenicplants
expressing FT and FT fusion proteins from het-
erologous promoters exclusively in the phloem
companioncells,whereCOandFTareexpressedin
wild-typeplants(6,21).Theuseofwell-characterized
Fig. 2. Analysis of FT:GFP protein distribution in SUC2:FT:GFP ft-7. (A) Flowering time expressed as total
leaf number (rosette and cauline) of representative transformants grown in LDs and compared with Ler and
ft-7. (B) Western blot analysis showing expression of the intact FT:GFP fusion protein in SUC2:FT:GFP ft-7
plants. SUC2:GFP Ler and Ler were used as positive and negative controls, respectively. The Comassie-
stained gel acts as loading control. (C and D) In situ hybridization of apices of SUC2:FT:GFP ft-7 plants
grown for 8 extended short days (ESDs) (C) and 10 ESDs (D) and probed with a chimeric RNA fragment
spanning the junction between FT and GFP in FT:GFP. The hybridization signal is restricted to the mature
phloem (arrowheads). (E) In situ hybridization of a 12-ESD-old SUC2:CO co-2 apex probed with FT. (F to H)
Confocal analysis of the distribution of the GFP fluorescence produced by the FT:GFP fusion protein in the
apical region of SUC2:FT:GFP ft-7 transgenic plants. Images on the right show GFP signals separated from
background emissions. (F) Six-day-old vegetative plant and [(G) and (H)] 10-day-old plant that is induced to
flower [fluorescence is detected in the provascular tissue and at the base of the shoot apical meristem
(SAM); arrowhead]. In (H), a leaf primordium flanking the SAM was removed to facilitate visualization. Lp,
leaf primordium; IM, inflorescence meristem. Scale bars, 50 mm in (C) to (E), (G), and (H); 25 mm in (F).
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heterologous promoters prevented difficulties as-
sociatedwiththelowabundanceofFTmRNAin
the vascular tissue of wild-type plants (6, 10, 11).
ThepromoteroftheSUCROSETRANSPORTER2
(SUC2)geneofArabidopsisisactivespecificallyin
the phloem companion cells (22), whereas the
promoter of the KNAT1 gene is active in the shoot
apical meristem, and expression of FT from these
promoters causes early flowering of co-2 mutants
(6). A gene fusion comprising FT and GREEN
FLUORESCENT PROTEIN (GFP) was con-
structed and expressed from the SUC2, FT, and
KNAT1 promoters.IntroductionofSUC2:FT:GFP,
KNAT1:FT:GFP,andFT:FT:GFPintoft-7mutants
causedtheseplantstoflowermuchearlierthanft-7,
although slightlylaterthanSUC2:FTft-7 or FT:FT
ft-7 (Fig. 2A and fig. S2). Protein was extracted
from seedlings of SUC2:FT:GFP and SUC2:GFP
plants and probed with a GFP antibody. The fu-
sionproteinwaspresentinSUC2:FT:GFPplants,
and importantly no free GFP protein was detected
(Fig. 2B). Taken together, these results indicate
that FT:GFP promotes flowering, although it is
slightly less active than the wild-type FT protein.
ThespatialdistributionofFT:GFPproteinand
mRNA were then compared in SUC2:FT:GFP
plants. FT:GFP and FT mRNAs were strongly
detected in the mature phloem tissue where the
SUC2 promoter is active, but no mRNA was de-
tectedintheshootapicalmeristemorprotophloem
(Fig.2,CtoE).ThedistributionofFT:GFPprotein
was then tested by confocal microscopy. In 6-day-
old plants, which had not undergone the transition
to flowering, FT:GFP was detected in the vascular
tissue of the shoot (Fig. 2F). In 10-day-old plants,
which were about to undergo the floral transition
and had not yet formed floral primordia, FT:GFP
wasalsodetectedintheprovasculatureattheshoot
apex and at the base of the shoot apical meristem
(Fig.2,GandH).FT:GFPwasdetectedinprovas-
culature and apical tissues in which FT:GFP
mRNA was not detected (compare Fig. 2, D and
G). These results suggest that FT:GFP protein
moves from the phloem companion cells to the
meristem (SOM text). Such movement could oc-
curthroughsymplasticunloadingfromthephloem
into the apical meristem region (23).
To test for movement of FT:GFP protein over
longer distances, transgenic SUC2:FT:GFP ft-7
plants were grafted to ft-7 mutants. Sugars and
other contents of the phloem sieve elements are
transported from mature leaves down to the root
and upward to the shoot apex. First, the aerial parts
of SUC2:FT:GFP seedlings were grafted to ft-7
roots. After grafting, FT:GFP protein was detected
across the graft junction and in the vasculature of
the ft-7 root stock, which represents a strong sink
for contents of the phloem (Fig. 3, A and B). No
FT:GFP mRNA could be detected in these root
stocks by reverse transcription polymerase chain re-
action after 40 cycles of amplification (Fig. 3C). A
SUC2:FT:GFP shootwas then grafted as a donor to
Fig. 3. Grafting of
SUC2:FT:GFP ft-7 plants
to ft-7 mutants. (A to C)
Root grafting: Distribu-
tion of the FT:GFP fu-
sion protein and FT:GFP
mRNA. Confocal analy-
sis of the distribution of
FT:GFPfusionproteindem-
onstrates that the protein
is able to cross a graft
junction (A) and can be
detected in the vascular
bundles of the ft-7 root
stock (B). The images on
the right in (A) and (B)
show GFP signals sep-
arated from background
emissions. (C) FT cDNA
amplification from the
roots of SUC2:FT:GFP ft-7
donor plants, ft-7 root
stock (labeled receiver)
and ft-7 controls. No
difference was detected
between the ft-7 root
stocks and ft-7 controls.
(D) Flowering time of
ft-7 mutants grafted to
SUC2:FT:GFP or to ft-7
donors. (E and F) Shoot
grafting: Distribution of
the FT:GFP fusion protein in the apical region of the SUC2:FT:GFP ft-7 donor (E) and grafted ft-7 receiver
(F). The fusion protein can be detected in the vasculature of the donor and receiver (arrowheads).
Fig. 4. Expression of FT:GFP in the minor veins
alters gene expression patterns but does not induce
flowering. (A to D) Confocal images of leaves
expressing GAS1:FT:GFP:GFP ft-7. The GFP signal is
detected in the minor veins [arrows in (A) and (B)]
but not in the petiole (C) or the midrib (D). (E)
Flowering time of GAS1:FT ft-7 and GAS1:FT:GFP
ft-7 as compared with Ler and ft-7 grown in LDs.
(F) FUL expression in leaves of the same plants.
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an ft-7 shoot receiver. These receiver shoots flow-
ered slightly earlier than receiver shoots on control
grafts (Fig. 3D and fig. S3), as observed previously
for grafts of wild-type plants to ft-7 mutants (24),
and FT:GFP protein was clearly detected in the
vascular tissue of the shoot receiver (Fig. 3, E and
F). The grafting experiments support long-distance
movement of FT:GFP protein in the phloem.
Two general models could explain the role of
FT in floral induction. The first proposes that a
product of FT expressed in the leaves moves to
the meristem and initiates flowering through the
activation of flowering-time genes such as SUP-
PRESSOR OF OVEREXPRESSION OF CON-
STANS 1 (SOC1) (7, 25, 26). Our data support
movement of the protein. The second model
suggests that FTexpression in the leaves activates
a second messenger, which is transmitted to the
apex and induces flowering, perhaps through
activation of FT genes or genes similar to FT in
the meristem. We refer to this second model as a
relay model: FT protein could move along with a
second messenger but not comprise a signal.
We used transgenic plants expressing FT and
FT:GFP from additional phloem promoters to test
the relay model. The GALACTINOL SYNTHASE
(GAS1)promoterisactivespecificallyinthephloem
companion cells of the minor veins of leaves (27)
andnotinthecompanioncellsoftheshootormajor
veinsoftheleaf.GAS1:COpromotesearlyflower-
ingofco-1mutants(28).WeconstructedGAS1:FT,
GAS1:FT:GFP, and GAS1:FT:GFP:GFP trans-
genes and introduced these into ft-7 mutants. In
plants expressing the fusion proteins, GFP was de-
tected only in the minor veins of the leaves (Fig. 4,
A to D). GAS1:FT complemented the ft-7 muta-
tion, and the transgenic plants flowered earlier
than did wild-type plants (Fig. 4E). However,
GAS1:FT:GFP ft-7 plants were as late flowering
as ft-7 mutants (Fig. 4E). Nevertheless, FT:GFP is
biochemicallyactiveintheleavesofGAS1:FT:GFP
plants. Expression of FRUITFULL (FUL) mRNA
is increased in the leaves of transgenic Arabidopsis
plants that express high levels of FT mRNA (29).
FUL mRNA levels were higher in GAS1:FT ft-7
andGAS1:FT:GFPft-7thaninwild-typeplantsand
ft-7 mutants (Fig.4F).Thus FT:GFP isactive in the
leaves of GAS1:FT:GFP plants, but in contrast to
GAS1:FTorSUC2:FT:GFP,thisconstructdoesnot
promote flowering. The larger FT:GFP protein
may move less effectively to the meristem from
theminorveinsthanfromthelargerveinsinwhich
SUC2 is also active, or downloading from the
companion cells to the minor veins may be dif-
ferentially regulated compared with downloading
tomajorveins.Thus,FT:GFPactivityintheleaves
of GAS1:FT:GFP plants was not sufficient to pro-
moteflowering,arguingfordirectmovementofan
FT product to the meristem.
We conclude (i) that during floral induction of
Arabidopsis, transient expression of FT in a single
leaf is sufficient to induce flowering and (ii) that in
response to FT expression, a signal moves from
the leaves to the meristem. This signal is unlikely
to be a second messenger activated by FT in the
leaves given that GAS1:FT:GFP is active in leaves
but does not promote flowering (Fig. 4). In con-
trast, we propose that FT protein is transported
through the phloem to the meristem. Our data
provide evidence for movement of FT:GFP from
the phloem companion cells of SUC2:FT:GFP
plants to the meristem that correlates with flower-
ing, and of FT:GFP protein across graft junctions,
consistent with the detection of proteins similar to
FT in the phloem of Brassica napus plants (30).
The data in the Report by Tamaki et al. (31) dem-
onstrate that this function of FTis highly conserved
in rice. The presence of a wide range of different
proteins in phloem sap suggests that long-distance
movementofproteinsisthebasisofothersignaling
processes in plants (23), in addition to the shorter-
distancemovementofproteinsbetweenneighboring
cells(32)andpreviousindicationsoftheimportance
of long-distance mRNA movement (33, 34).
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Supporting Online Material
www.sciencemag.org/cgi/content/full/1141752/DC1
Materials and Methods
SOM Text
Figs. S1 to S3
References
26 February 2007; accepted 6 April 2007
Published online 19 April 2007;
10.1126/science.1141752
Include this information when citing this paper.
Hd3a Protein Is a Mobile
Flowering Signal in Rice
Shojiro Tamaki, Shoichi Matsuo, Hann Ling Wong, Shuji Yokoi,* Ko Shimamoto†
Florigen, the mobile signal that moves from an induced leaf to the shoot apex and causes
flowering, has eluded identification since it was first proposed 70 years ago. Understanding the
nature of the mobile flowering signal would provide a key insight into the molecular mechanism of
floral induction. Recent studies suggest that the Arabidopsis FLOWERING LOCUS T (FT) gene is a
candidate for encoding florigen. We show that the protein encoded by Hd3a, a rice ortholog of FT,
moves from the leaf to the shoot apical meristem and induces flowering in rice. These results
suggest that the Hd3a protein may be the rice florigen.
T
he flowering time of plants is determined by
a number of environmental factors (1–3),
among which day length (photoperiod) is a
major factor (4). On the basis of the day length,
which promotes flowering, plants are grouped into
two major classes: long-day (LD) and short-day
(SD) plants. Arabidopsis is a LD plant and rice is
a SD plant. FT is a major floral activator (5, 6),
which is expressed in the vascular tissue of leaves
(7, 8). FT protein interacts with a transcription
factor FD, which is expressed only in the shoot
apical meristem (SAM) (9, 10). The difference in
expression site implies that FT protein must move
to the SAM to interact with FD for flower in-
duction. Therefore, FT is a primary candidate for
encoding florigen (11), a mobile flowering signal.
Laboratory of Plant Molecular Genetics, Nara Institute of
Science and Technology, 8916-5 Takayama, Ikoma 630-
0101, Japan.
*Present address: Faculty of Agriculture, Iwate University,
Morioka 020-8550, Japan.
†To whom correspondence should be addressed. E-mail:
simamoto@bs.naist.jp
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