1 Max Planck Institute for Plant Breeding, Carl von Linne Weg 10, D-50829
Cologne, Germany
2 Department of Agricultural Sciences, Imperial College London, Wye Campus, Wye,
Kent TN25 5AH, UK
Author for correspondence (e-mail:
coupland{at}mpiz-koeln.mpg.de)
Accepted 1 April 2004
![]() |
SUMMARY |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Key words: Flowering, Arabidopsis, Phloem, CONSTANS, FT, Grafting, Photoperiod
![]() |
Introduction |
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Mutants impaired in the flowering response to day length may provide a
route to identifying the transmissible substance, to explaining how its
synthesis and transport are regulated, and to defining the mechanism by which
it induces flower development. In pea, mutations that alter flowering and
impair the formation of graft transmissible substances were identified
(Beveridge and Murfet, 1996;
Weller et al., 1997
), but the
corresponding genes have not been isolated. In addition, the
INDETERMINATE gene of maize is required for flowering and encodes a
transcription factor expressed only in leaves
(Colasanti et al., 1998
),
suggesting that it may affect the synthesis or transport of the floral
stimulus. However, the molecular genetics of the photoperiodic control of
flowering is best understood in Arabidopsis, where a pathway of genes
required to activate flowering specifically in response to LDs has been
identified (Hayama and Coupland,
2003
; Yanovsky and Kay,
2003
), and their global effects on gene expression at the shoot
apex have been described (Schmid et al.,
2003
).
The Arabidopsis photoperiod pathway was initially defined by
late-flowering mutants, including gigantea (gi),
constans (co) and ft
(Koornneef et al., 1998).
Analysis of the genes impaired by these mutations demonstrated that
GI encodes a large nuclear protein of unknown function, which is
required for the activation of CO transcription
(Fowler et al., 1999
;
Park et al., 1999
;
Huq et al., 2000
;
Suarez-Lopez et al., 2001
). In
turn, CO encodes a nuclear zinc-finger containing protein that
activates the expression of FT and SUPPRESSOR OF OVEREXPRESSION
OF CO 1 (SOC1; also known as AGL20 - The Arabidopsis
Information Resource) (Putterill et al.,
1995
; Samach et al.,
2000
). FT encodes a RAF-kinase inhibitor-like protein and
SOC1 encodes a MADS-box transcription factor
(Kardailsky et al., 1999
;
Kobayashi et al., 1999
;
Samach et al., 2000
). All of
these genes are regulated by the circadian clock, and overexpression of
CO, FT or SOC1 causes extreme early flowering
(Borner et al., 2000
;
Kardailsky et al., 1999
;
Kobayashi et al., 1999
;
Lee et al., 2000
;
Onouchi et al., 2000
). This
pathway is highly conserved in rice, and presumably in other Angiosperms.
Orthologues of each of the genes were identified in rice, and OsGI,
Heading date 1 (Hd1; an orthologue of CO) and
Heading date 3a (an orthologue of FT) were shown to regulate
photoperiodic flowering by acting in a genetic pathway in the same order as
their orthologues do in Arabidopsis
(Hayama et al., 2003
;
Izawa et al., 2002
;
Kojima et al., 2002
;
Yano et al., 2000
).
Although the detection of day length occurs in the leaves of
Arabidopsis (Corbesier et al.,
1996), the tissues in which the components of the genetically
defined pathway act to regulate flowering have been difficult to assess
because of their low expression levels or general patterns of expression. Here
we show that CO, a nuclear zinc-finger protein that plays a central role in
the photoperiod-response pathway (Hayama
and Coupland, 2003
; Yanovsky
and Kay, 2003
), acts in the phloem companion cells to trigger
floral development at the apex, and controls a systemic signal that crosses
graft junctions. The mechanism by which CO acts in the phloem
involves cell-autonomous activation of its target gene FT and, based
on analysis of a GFP:CO fusion protein, does not require movement of the CO
protein. These data identify CO as a regulator of the floral stimulus, and
place the floral stimulus within the network of regulatory proteins that
control flowering in response to day length.
![]() |
Materials and methods |
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Both the ANT::LhG4 and the CLV1::LhG4 activator lines
were obtained from Dr T. Laux (University of Freiburg, Germany) in
Ler (Schoof et al.,
2000). The structure of the activator constructs was described by
Schoof et al. (Schoof et al.,
2000
). They were introgressed into the co-2 mutant to
generate homozygous activator lines.
Plasmid constructions
To allow the site-specific CRE recombinase to excise 35S:GUS,
direct repeats of the loxP sites were inserted flanking the gene. Two
pairs of oligonucleotides containing the repeats with accompanying restriction
sites were synthesised (sequences available on request). The complementary
primers were annealed and cloned in a pUC derivative carrying 35S::GUS.
loxR was cloned in the EcoRI site 5' of 35S::GUS
and loxH in the HindIII site 3' of the marker gene,
generating pGUSLOX. 35S::GUS flanked by the loxP sites was
inserted in the EcoRI site in the CO intron using a 4.3 kb
sub-clone of CO (Onouchi et al.,
2000). A fusion of the CRE gene to the heat-shock
promoter was kindly provided by Dr E. Meyerowitz
(Sieburth et al., 1998
).
To construct the Op::GUS-Op::CO tandem reporter plasmid, the GUS-coding sequence was inserted into pUBOP (gift of I. Moore, University of Oxford). The Op::GUS fragment was then inserted into pGreen0029 to yield plasmid pGreen0029-Op::GUS. Similarly, the full-length CO cDNA was cloned into pUBOP. The Op::CO fragment was then purified and cloned into pGreen0029-Op::GUS generating pGreen0029-Op::GUS-Op::CO. Details of the cloning are available upon request. This reporter construct was introduced into co-2 and lines that segregated 3:1 for the T-DNA identified in the T2 generation. Homozygous lines were identified in the T3 generation. These plants flowered at the same time as co-2 mutants, so Op::CO did not promote flowering prior to activation. Expression of GUS and CO was transactivated in crosses with activator lines.
The AtSUC2, AtKNAT1, AtSTM, AtUFO and AtML1 promoters were PCR amplified from Columbia genomic DNA using specific primers with GATEWAY tails. The forward primers contain the AttB1 tail (5'-GGGGACAAGTTTGTACAAAAAAGCAGGCT), reverse primers contain the AttB2 tail (5'-GGGGACCACTTTGTACAAGAAAGCTGGGT). Specific sequences for each primer pair were:
SUC2-F, 5'-AAAATCTGGTTTCATATTAATTTCA-3';
SUC2-R, 5'-ATTTGACAAACCAAGAAAGTAAGA-3';
KNAT1-F, 5'-GATCTAGAGCCCTAGGATTTGA-3';
KNAT1-R, 5'-ACCCAGATGAGTAAAGATTTGAG-3';
STM-F, 5'-GTGTGTTTGATTCGACTTTTGT-3';
STM-R, 5'-CTTCTCTTTCTCTCACTAGTA-3';
UFO-F, 5'-GAATTCTCTGTTTTAATTGCCCCA-3';
UFO-R, 5'-TTAGCTGAAAAATGAAAAGA-3';
ML1-F, 5'-AAGCTTATCAAAGAAAAAACAAGA-3'; and
ML1-R, 5'-AACCGGTGGATTCAGGGAGTTTCT-3'.
The 35S promoter was PCR amplified from the pBI121 plasmid using the specific primer sequences 35S-F (5'-AGGTCCCCAGATTAGCCTT-3') and 35S-R (5'-TCCCCCGTGTTCTCTCCAA-3'). The ROLC and TobRB7 promoters were gifts from A. Heyer and I. Moore, respectively, and were also PCR amplified from the corresponding plasmids with the following primer pairs:
rolC-F 5'-GAAAAAGGCAAGTGCCAGGGCC-3' and rolC-R 5'-TACCCCATAACTCGAAGCATCC-3'; and
TobRB7-F 5'-CCCCTTATTGTACTTCAATTA-3' and TobRB7-R 5'-TTTCCAAGTTTCACATAACCT-3'.
All PCR products were introduced into the GATEWAYTM pDONR207 (Invitrogen) vector through BP reactions, generating promoter entry clones. The GATEWAYTM vector conversion fragment rfA was fused upstream of GUS in pGPTV-BAR, or CO and FT cDNA in pGreen0229, to generate the binary destination vectors. Different promoter fusions were produced by LR reactions.
A 2451 base pair CO promoter fragment purified from plasmid pBCOPL was fused in frame to the GUS ORF to yield plasmid pCOGUSL. A 4.61 kb fragment containing the CO::GUS fusion was purified from pCOGUSL and cloned into the binary vector pSLJ1714 to yield plasmid pSLCOGUSL.
Heat-shock induction of CRE-mediated recombination
For heat shock, co-2 transgenic plants containing
CO(35S::GUS) and HS::CRE were exposed to a temperature of
39°C for 1-3 hours. Heat shock of developing embryos was performed by
exposing plants with siliques at different days after pollination to cycles of
1 hour at 39°C 1 hour at room temperature, up to a maximum of 3 hours at
39°C. Immediately after heat shock, plants were transferred to standard
growth conditions or imbibed seeds were sown on soil. Experiments involving
the generation of mosaic plants to determine the activity of CO were
performed using F1 embryos/seeds from crosses between plants
homozygous for co-2 HS::CRE CO(35S::GUS) or co-2 HS::CRE.
The resulting F1 carries one copy of the CO(35S::GUS)
construct, facilitating the detection of GUS-negative sectors after
excision.
Plant transformation
All plasmids, except pSLCOGUSL, were introduced into Agrobacterium
strain GV3101(pMP90) (Koncz and Schell,
1986) and transformed into Ler or co-2 plants by
floral dip (Clough and Bent,
1998
). Plasmid pSLCOGUSL was introduced into
Agrobacterium strain C58C1(pGV2260) and transformed into
Ler.
Grafting experiments
Y-grafts (two shoots on a single root system) were constructed as described
by Turnbull et al. (Turnbull et al.,
2002). Seedlings were grown initially on half-strength MS salts,
then transferred to compost (Levingtons F2S/vermiculite 4:1). Temperature was
23°C, with a light level of approx 120 µmol m-2
s-1 for 16 hours (LD) or 8 hours (SD).
Photoperiod induction across Y-grafts was tested using grafted wild-type Col plants grown for 70 days under SDs. One shoot was exposed to 7 LDs (the donor shoot) while keeping the other under continuous SDs by covering with a blackened foil cap for all except 8 hours per day. The shoots under SDs (termed receivers) were partially defoliated to enhance their sink strength. After the LD treatment, plants were returned to SDs. Flowering of both shoots was assessed 17 days after the start of LD treatment. Defoliation controls indicated this manipulation did not retard the flowering of plants induced under LDs.
Sucrose transport across Y-graft unions was measured by applying [U-14C]sucrose (3.7 MBq, 1.5 nmol) to a single mature leaf on one shoot. After 2 hours, both shoots were dissected, and the radioactivity in leaf, stem and root segments was analysed by scintillation counting of ethanol extracts. Data were expressed relative to the total radioactivity recovered at sites away from the fed leaf.
Graft rescue of flowering time in co mutants was tested by Y-grafting co-2 shoots onto wild-type Ler plants under LDs. Grafts with weak co-2 shoots were excluded. Controls included self-grafts of co-2, and ungrafted co-2 plants.
Histochemical analysis of GUS expression
Analyses were carried out on plants grown on soil under LDs. After heptane
treatment, samples were processed as described by Sieburth and Meyerowitz
(Sieburth and Meyerowitz,
1997). For histological analysis, samples were dehydrated through
an ethanol series into Histoclear (National Diagnostics), and embedded in
Paramat Extra (Gurr®, BDH). Eight µm sections and whole seedlings were
viewed after deparaffinisation under bright field on a Leica microscope.
In situ hybridisation
Methods of digoxigenin labeling of mRNA probes, tissue preparation and in
situ hybridisation were as already described
(Bradley et al., 1993) with
small modifications. Protease treatment was not performed with Pronase but
with Proteinase K [1 µg.ml-1 in 100 mM Tris (pH 8), 50 mM EDTA]
at 37°C for 30 minutes, and the post-hybridisation washes were preformed
in 0.1xSSC.
Probes used to detect the CO and FT transcripts were prepared from p21CO containing the full-length CO cDNA and from pD301 containing 450 base pairs of the 5' FT cDNA, respectively.
Analysis of CO and FT mRNA abundance
At day 15, emerging true leaves of 100 plants per sample were collected
from soil-grown plants 16 hours after dawn, cotyledons were discarded. RNA was
analysed by RT-PCR. For synthesis of cDNA, 3 µg of total RNA was primed
using dT15 primer. cDNA was diluted to 150 µl with water, and 3
µl of diluted cDNA was used for PCR. CO was amplified using
primers CO53 and COoli9 as described
(Suarez-Lopez et al., 2001).
FT was amplified using primers FT-RTPCR-F
(5'-AGAAGACTTTAGATGGCTTCTT-3') and FT-RTPCR-R
(5'-TTATCGCATCACACACTATATAAG-3'). UBQ10 was amplified
(Blazquez and Weigel, 1999
) and
used as a control to normalise the amounts of cDNA. For CO, FT and
UBQ, 17, 20 and 17 cycles were used, respectively. PCR products were
separated on agarose gels, transferred to filters and hybridised with
radioactively labelled probes. Images were visualised using a Phosporimager
(Molecular Dynamics), band intensities were quantified using ImageQuant
software and values were normalised to UBQ10.
GFP fluorescence images
Leaves, leaf epidermal cells and vascular tissues of the 7- to 10-day-old
SUC2::GFP, SUC2::GFP:CO and CO::GFP:CO seedlings grown on MS
agar under LD were analysed using a Zeiss LSM 510 Meta confocal laser scanning
microscope. Images were collected using a 5x lens (for whole leaf
image), a 40x lens (for leaf epidermis) and a 63x oil-immersion
lens (for vascular tissues), as described by Valverde et al.
(Valverde et al., 2004). GFP
signal (cyan) was separated from background (black and blue) using the
emission fingerprinting Linear Unmixing function.
![]() |
Results |
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|
|
|
Direct expression of CO from the AtSUC2 and rolC
promoters complemented the co-2 mutation. The AtSUC2
promoter is expressed specifically in the companion cells of the phloem and
not in the meristem or in young leaf primordia
(Fig. 4A)
(Imlau et al., 1999;
Stadler and Sauer, 1996
), and
the rolC promoter is expressed specifically in the phloem
(Booker et al., 2003
).
Transgenic co-2 mutant plants carrying AtSUC2::CO or
rolC::CO exhibited extremely early flowering, indicating that
CO expression in the phloem was sufficient to trigger flowering and
that expression in the meristem or leaf primordia was not required
(Fig. 4A-C). These transgenes
caused early flowering under both LDs and SDs, as was previously shown for
35S::CO transgenic plants.
|
In CO::GUS plants staining was also detected throughout the young
leaf primordia (Fig. 1B), and
therefore CO was expressed in these organs using a fusion of the
AINTEGUMENTA (ANT) promoter
(Elliott et al., 1996;
Klucher et al., 1996
) to
LhG4 to drive CO expression in ANT::LhG4 Op::GUS-Op::CO
co-2 plants. These plants flowered late, at a similar time to
co-2 mutants, suggesting that CO was not able to drive early
flowering when expressed in leaf primordia
(Fig. 4A,C). Finally, fusion of
the ML1 promoter, which is expressed specifically in the epidermis
(Abe et al., 2001
), or the
TobRB7 promoter, which is expressed in the root
(Yamamoto et al., 1991
), to
CO did not complement the co-2 mutation
(Fig. 1B and data not shown).
The misexpression data therefore indicate that CO acts specifically
in the phloem to promote flowering of Arabidopsis.
CO activates FT cell-autonomously in SUC2::CO plants
CO promotes the expression of downstream genes
(Samach et al., 2000),
particularly FT, which encodes a RAF-kinase-inhibitor-like protein
(Kardailsky et al., 1999
;
Kobayashi et al., 1999
).
Expression of FT from the viral CaMV35S promoter corrects the
late-flowering phenotype of co-2 mutants
(Kardailsky et al., 1999
;
Kobayashi et al., 1999
).
Whether the mechanism by which CO activates flowering from the phloem
companion cells involves FT was therefore examined. The abundance of
FT and CO mRNA 16 hours after dawn was examined in LD-grown
wild-type, SUC2::CO and 35S::CO plants by RT-PCR
(Fig. 5A). CO mRNA
abundance was much higher in 35S::CO and SUC2::CO than in
wild-type, with 35S::CO showing the highest levels of CO
mRNA. FT mRNA levels were also elevated in 35S::CO and
SUC2::CO plants, with SUC2::CO showing the higher levels.
This supports the idea that expression of CO in the phloem from the
SUC2 promoter causes increased FT expression, as was
previously shown for 35S::CO
(Samach et al., 2000
).
Furthermore, the higher level of FT expression in SUC2::CO
than 35S::CO plants suggests that specific expression of CO
in the phloem may be more effective in activation of FT than general
expression from the 35S promoter.
|
Genetic analysis indicates that CO activates flowering from the phloem through FT
Gain- and loss-of-function genetic experiments were used to determine
whether the activation of flowering by CO in the phloem involves
FT (Fig. 6).
Introduction of the ft-7 mutation into AtSUC2::CO plants
significantly delayed flowering (Fig.
6), indicating that the extreme early flowering induced by
expression of CO in the phloem requires FT activity.
However, these plants still flowered earlier than ft-7 mutants
(Fig. 6), thus in the phloem CO
does not exclusively function through FT activation.
|
In SUC2::GFP:CO or CO::GFP:CO plants, GFP:CO fusion protein is detected in the phloem and not in other leaf cells
The non-cell-autonomous induction of floral development by CO when
expressed in the phloem may be explained by movement of the protein into other
cells, as has been described for GFP
(Truernit et al., 1996) and
several plant transcription factors (Lucas
et al., 1995
; Nakajima et al.,
2001
; Sessions et al.,
2000
). To test the possibility that CO moves from the phloem
companion cells, the location of a GFP:CO fusion protein was tested when
expressed from the AtSUC2 or CO promoters. The
AtSUC2::GFP:CO and CO::GFP:CO transgenes fully complemented
the co-2 mutation. GFP fluorescence was then examined by confocal
microscopy. In control AtSUC2::GFP plants, GFP fluorescence was
detected in the vascular tissue, and also in the mesophyll and epidermal
layers of the leaf (Fig. 5C), indicating that GFP can move freely from the companion cells, as previously
demonstrated (Truernit et al.,
1996
). By contrast, in AtSUC2::GFP:CO plants,
fluorescence was detected only in the vascular tissue of the leaf
(Fig. 5C). In addition, at the
apex of AtSUC2::GFP:CO seedlings, GFP fluorescence was detected in
the vascular tissue, but not in the meristem
(Fig. 5D). Therefore, at the
level of detection of this experiment, GFP:CO protein remains in the phloem
and does not move to adjacent leaf cells, or to the meristem. Similarly, in
CO::GFP:CO plants, GFP:CO was detected only in the vascular tissue
(Fig. 5C). The localisation of
GFP:CO protein to the phloem is consistent with the CO-mediated activation of
FT expression in the leaves of AtSUC2::CO plants only
occurring in the phloem (Fig.
5A), and indicates that CO protein acts in the phloem companion
cells to induce flowering.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The spatial regulation of CO function
Analysis of CO::GUS plants detected GUS expression throughout
young leaf primordia, in the vascular tissue of mature leaves and cotyledons,
as well as in the phloem and the protoxylem of stems. Weaker staining was also
detected in the shoot apical meristem, as was indicated by in situ
hybridisation (Simon et al.,
1996). CO expression in the phloem of mature tissues is
consistent with a recent report (Takada
and Goto, 2003
), although we also detected GUS staining more
widely (Fig. 1).
Recently, CO was proposed to be part of the mechanism by which
Arabidopsis distinguishes long and short days, through a combination
of circadian-clock regulation and direct responsiveness to exposure to light
(Suarez-Lopez et al., 2001;
Yanovsky and Kay, 2002
;
Imaizumi et al., 2003
;
Valverde et al., 2004
).
Furthermore, classical grafting experiments suggested that the perception of
day length occurs in the leaf, which is consistent with CO acting in phloem
cells to promote flowering. This conclusion may have significance beyond
flowering-time control, as heterologous expression of CO in potato
delayed tuberisation, and this effect was graft transmissible
(Martinez-Garcia et al.,
2002
).
The mechanism by which CO promotes flowering in SUC2::CO plants appears to involve cell-autonomous activation of FT in the phloem. In other tissues, such as the meristem and L1 layer, CO expression did not induce flowering, but FT expression did. This suggests that CO may only activate FT in the phloem, which is supported by the higher abundance of FT mRNA in SUC2::CO than 35S::CO plants. Alternatively, in tissues other than the phloem, activation of FT by CO may occur at a lower level than by direct fusion of FT to specific promoters, and below a threshold level required to induce flowering.
Some plant transcription factors move between plant cells
(Lucas et al., 1995;
Nakajima et al., 2001
;
Wu et al., 2003
). However, the
activation of FT specifically in the phloem, and the presence of
GFP:CO only in these cells within the leaves and stems of
SUC2::GFP:CO and CO::GFP:CO transgenic plants, suggest that
CO protein does not move from the phloem. The zinc fingers of CO most resemble
B-boxes that were described in several animal proteins, and which act as
protein-protein interaction domains
(Robson et al., 2001
). Thus
the presence of CO within a larger protein complex may prevent movement of the
protein from the phloem companion cells, as was previously proposed for MADS
box proteins in floral primordia (Wu et
al., 2003
). Similarly, there is no evidence that CO contains
specific sequences that would enable its translocation between cells, as have
been identified for transcription factors such as the maize protein KNOTTED
(Lucas et al., 1995
).
The role of FT downstream of CO
The position of FT downstream of CO in the photoperiod
response pathway was demonstrated genetically and by the analysis of
FT expression in co mutant or 35S::CO backgrounds
(Kardailsky et al., 1999;
Kobayashi et al., 1999
;
Samach et al., 2000
). Our data
demonstrate that in plants in which CO is expressed specifically in
the phloem, FT is required for the extreme early flowering induced by
CO, and is specifically activated by CO in phloem cells. FT is
probably also activated by CO in the phloem of wild-type plants, although its
mRNA abundance is below the level of detection.
Misexpression experiments indicated that FT activates flowering
when expressed specifically in a wide range of tissues. This may be
physiologically significant, as FT is regulated by several flowering
pathways, as well as by the photoperiod pathway
(Blazquez et al., 2003;
Cerdan and Chory, 2003
;
Halliday et al., 2003
;
Mouradov et al., 2002
;
Simpson and Dean, 2002
). The
pattern of FT expression in wild-type plants has not been described, and the
tissues in which most flowering-time pathways act to promote flowering have
not been defined and may therefore activate FT expression in tissues
other than the phloem.
FT is a member of a small Arabidopsis gene family that
includes TERMINAL FLOWER 1 (TFL1), and is related to
CENTRORADIALIS (CEN) of Antirrhinum and SELF
PRUNING (SP) of tomato
(Bradley et al., 1997;
Kardailsky et al., 1999
;
Kobayashi et al., 1999
;
Pnueli et al., 1998
). These
proteins, named CETS (CEN, TFL1, FT), share homology to the
RAF-kinase-inhibitor proteins of mammals
(Kardailsky et al., 1999
;
Pnueli et al., 2001
), and the
structure of the CEN protein is related to that of RAF-kinase inhibitors
(Banfield and Brady, 2000
). The
mechanism of action of these proteins was explored by identifying proteins
that interact with SP in the yeast two-hybrid system
(Pnueli et al., 2001
). A
NIMA-like kinase, bZIP transcription factors and a 14-3-3 protein that
interact with SP were identified, and led to the suggestion that CETS proteins
act as adapters in a variety of signalling pathways. How these functions
relate to the floral promotive activity of FT is unknown.
The non-cell autonomy of the effect of FT on flowering may be due
to movement of FT protein between cells, or to the activation of intercellular
signalling processes downstream of FT. FT is only 23 kDa
(Kardailsky et al., 1999;
Kobayashi et al., 1999
),
smaller than GFP and below the size exclusion limit of plasmodesmata
(Imlau et al., 1999
),
suggesting that it may move freely through plant tissues. Therefore the
activation of FT in the phloem may precede movement of the protein to
the meristem or other tissues. This would be consistent with the observation
that FT will activate flowering when expressed in a wide range of
cell types. Although the floral stimulus is usually not considered to be a
protein, classical grafting experiments do not exclude this possibility
(Perilleux and Bernier, 2002
).
However, our data are also consistent with other possibilities, including that
FT regulates synthesis of a mobile, small molecule capable of inducing
flowering. The target of the FT-derived signal in the meristem is
unknown, but genetic experiments suggested a close correlation between
FT and activation of the floral meristem identity gene
APETALA1 (Ruiz-Garcia et al.,
1997
).
Finally, although FT plays a major role in the induction of
flowering downstream of CO, the flowering time of SUC2::CO
ft-7 plants demonstrates that FT is not essential for SUC2::CO
to promote early flowering. CO must therefore regulate flowering by both
FT-dependent and FT-independent processes. These FT-independent processes
might involve other genes previously shown to be upregulated by overexpression
of CO from the 35S promoter
(Samach et al., 2000).
Perspectives
Taken together, the grafting and misexpression data indicate that a
systemic signal, analogous to the floral stimulus, induces flowering of
Arabidopsis in response to LDs, and that this is activated by CO in
the phloem companion cells and transmitted through the phloem. FT activates
flowering when expressed in many tissues, and may move readily to a critical
group of cells in which it promotes flowering or act in almost any tissue to
promote the formation of a downstream mobile signal. However, the mechanism by
which CO activates flowering from the phloem also involves FT-independent
processes, suggesting that CO regulates more than one systemic signal. The
identification of CO as a regulator of systemic signals that induce flowering
will facilitate the definitive identification of these signals, and the
elucidation of the signalling mechanisms underlying this process.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
Footnotes |
---|
Present address: Institute of Molecular Biology of Barcelona (CSIC), Jordi
Girona 18-26, 08034 Barcelona, Spain
Present address: Departmento Biotecnología, INIA, Carretera de la
Coruña km 7, Madrid 28040, Spain
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