The EMBO Journal (2006) 25, 4909–4920
www.embojournal.org
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2006 European Molecular Biology Organization | All Rights Reserved 0261-4189/06
THE
EMBO
JOURNAL
EBP1 regulates organ size through cell growth
and proliferation in plants
Beatrix M Horváth1,*, Zoltán Magyar2,3,
Yuexing Zhang4, Anne W Hamburger4,
László Bakó3,5, Richard GF Visser1,
Christian WB Bachem1,6 and
László Bögre2,6
1
Laboratory of Plant Breeding, Department of Plant Sciences, Graduate
School of Experimental Plant Sciences, Wageningen University and
Research Centre, Wageningen, The Netherlands, 2School of Biological
Sciences, Royal Holloway, University of London, Egham Hill, Egham,
UK, 3Institute of Plant Biology, Biological Research Centre, Szeged,
Hungary, 4Department of Pathology and Greenebaum Cancer Centre,
School of Medicine, University of Maryland, Baltimore, MD, USA and
5
Department of Plant Physiology, Umeå Plant Science Centre, Umeå
University, Umeå, Sweden
Plant organ size shows remarkable uniformity within
species indicating strong endogenous control. We have
identified a plant growth regulatory gene, functionally and
structurally homologous to human EBP1. Plant EBP1
levels are tightly regulated; gene expression is highest in
developing organs and correlates with genes involved in
ribosome biogenesis and function. EBP1 protein is stabilised by auxin. Elevating or decreasing EBP1 levels in
transgenic plants results in a dose-dependent increase
or reduction in organ growth, respectively. During early
stages of organ development, EBP1 promotes cell proliferation, influences cell-size threshold for division and
shortens the period of meristematic activity. In postmitotic
cells, it enhances cell expansion. EBP1 is required for
expression of cell cycle genes; CyclinD3;1, ribonucleotide
reductase 2 and the cyclin-dependent kinase B1;1. The
regulation of these genes by EBP1 is dose and auxin
dependent and might rely on the effect of EBP1 to reduce
RBR1 protein level. We argue that EBP1 is a conserved,
dose-dependent regulator of cell growth that is connected
to meristematic competence and cell proliferation via
regulation of RBR1 level.
The EMBO Journal (2006) 25, 4909–4920. doi:10.1038/
sj.emboj.7601362; Published online 5 October 2006
Subject Categories: cell cycle; plant biology
Keywords: cell growth; cell proliferation; EBP1; organ
growth; ribosome biogenesis
Introduction
Morphogenesis in plants is largely postembryonic, and along
with organ growth is influenced by environmental factors,
*Corresponding author. Department of Biology, Section of Molecular
Genetics, Utrecht University, Padualaan 8, 3584CH Utrecht, The
Netherlands. Tel.: þ 31 30 253 2245; Fax: þ 31 30 251 3655;
E-mail: Beatrix.Horvath@WUR.nl
6
These authors contributed equally to this work
Received: 24 February 2006; accepted: 31 August 2006; published
online: 5 October 2006
& 2006 European Molecular Biology Organization
such as light or nutrient availability (Ingram and Waites,
2006). Moreover, plant growth is intimately connected to
the capacity of source organs to produce assimilates. This
regulatory mechanism determines the yield potential for
harvested organs in agricultural crops. In potato (Solanum
tuberosum), the separation of source and sink organs
illustrates the long-distance regulation of organ growth
through the interplay of assimilates such as sucrose and
other growth factors produced in the source and sink organs
(Bologa et al, 2003).
The capacity for growth of plant organs is determined
by zones of proliferating cells, called meristems. When cells
leave the meristematic zone they begin to exit the cell cycle
and undergo differentiation that is accompanied by increases
in cell size. Cell expansion is facilitated by the loosening
of crosslinks between cell wall polymers accompanied by
water uptake to vacuoles, and frequently by endoreduplication of DNA (Sugimoto-Shirasu and Roberts, 2003).
Therefore, the timing of the transition between proliferative
growth and cell expansion/differentiation, largely determines
the final cell number and so the size potential of the organ.
Differences in organ size between closely related species,
such as rapeseed and Arabidopsis or differential responses to
environmental conditions tend to reflect cell number rather
than cell size variation (Beemster et al, 2003).
The control of cell size is best understood in yeast where
the attainment of a cell size threshold triggers the initiation
of cell division. This coordination is thought to be regulated
through the translational machinery which, in turn, is determined by the nutritional state (Jorgensen and Tyers, 2004).
In plants, remarkably uniform cell sizes in both meristematic
regions and young developing organs also indicate the
existence of an intricate regulation of cell growth and cell
division. However, their coordination is poorly understood
(Beemster et al, 2003). A mechanism for such coordination
could occur via the TCP transcription factors that coregulate
the expression of genes coding for the translational apparatus
and cell cycle genes (Ingram and Waites, 2006).
In multicellular organisms, it is debated whether the
factors determining growth impose their influence on organs
as a whole or whether they regulate growth and proliferation
at the cellular level (Tsukaya and Beemster, 2006). Moreover,
cells in multicellular organisms do not proliferate logarithmically. Their growth and proliferation requires coordination
with other parts of the organism mediated by growth and
mitogenic factors that may impinge on growth regulation
(ribosome biogenesis) and cell cycle control (DNA replication) separately (Conlon and Raff, 1999).
The nucleolus is the main site of ribosome biosynthesis. In
humans, EBP1, the ErbB-3 epidermal growth factor receptor
binding protein has been shown to be a nucleolar dsRNA
binding protein; forming part of the ribonucleoprotein complexes via association with different rRNA species (Squatrito
et al, 2004). EBP1 was also shown to associate with mature
ribosomes and to block the stress-induced phosphorylation of
The EMBO Journal
VOL 25 | NO 20 | 2006 4909
Results
Induction of gene expression during potato tuber
initiation
Potato tubers are formed at the termini of stolons as a result
of internal and external cues such as light period and
assimilate supply. We have used an in vitro system to study
the molecular mechanisms that control tuber development
and growth (Bachem et al, 1996). In the course of this work,
a transcript-derived fragment was identified (TDF1044) that
showed a transient elevation in abundance during early
stages of tuberisation (Supplementary Figure 1A). The
expression pattern of the gene represented by TDF1044 was
confirmed by microarray data (Supplementary Figure 1B;
Kloosterman et al, 2005).
Northern analysis was carried out on tissues of different
developmental stages using the TDF1044 as a probe (Supplementary Figure 1C). Its expression is correlated with growth
and cell division activities of a range of organs. This ubiquitous expression profile suggests that this gene is functionally
not limited to tuber development, but correlates with actively
growing tissues.
TDF1044 is homologous to the human EBP1,
a conserved cell proliferation and cell growth-related
protein
In order to identify the full-length potato mRNA corresponding to TDF1044, a cDNA library derived from swelling stolon
tips was screened (Taylor et al, 1992). The isolated 1.5 kb fulllength cDNA codes for a 43 kDa protein and shows sequence
similarity (69%) to the human EBP1 (Yoo et al, 2000). The
gene was named StEBP1 accordingly. Two major groups of
ESTs were identified in the current potato TIGR-EST database;
one is showing 100% homology (TC126314), whereas the
other (TC128561) shares 87% similarity with StEBP1. It is
not clear whether the second group of ESTs are derived from
an allele within the tetraploid potato or from another gene
family member. The StEBP1 protein shows high similarity
(89%) to the Arabidopsis G2p protein encoded by a single
copy gene at locus At3g51800 (unigene10184), that we name
4910 The EMBO Journal VOL 25 | NO 20 | 2006
A
80
B
80
60
Colony number
60
40
20
1 µg 0.25 µg
CMV-10
C
0.5 µg
40
20
1 µg 0.25 µg
1 µg
CMV-10
StEBP1
D
2
CMV-10
0.5 µg
1 µg
∆-StEBP1
∆-StEBP1 StEBP1
1.5
50 kDa
RLU
the eukaryotic initiation factor 2 alpha (eIF2a) and thus
presumably sustain protein translation (Squatrito et al,
2006). Furthermore, EBP1 is a nuclear cell survival factor
that together with the protein kinases, Akt and PKC inhibit
apoptosis (Ahn et al, 2006). Surprisingly, ectopic expression
of EBP1 decreased proliferation rate and induced cellular
differentiation in cultured human breast carcinoma cell
lines (Lessor et al, 2000). As the effect on proliferation was
linked to its nucleolar localisation in human fibroblasts, it
was suggested that EBP1 may represent a new link between
ribosome biosynthesis and cell proliferation (Squatrito et al,
2004).
Studying tuber organogenesis in potato upon sucrose
induction, we have identified StEBP1, the potato homologue
of the human EBP1 gene. We show, via modulating the level
of EBP1 in potato and Arabidopsis, that this gene regulates
plant organ growth, effects the expression of different cell
cycle genes and influences RBR1 protein level. Furthermore,
we demonstrate that auxin regulates the protein stability of
EBP1 through which it may influence plant growth.
Colony number
Control of organ and cell growth by EBP1 in plants
BM Horváth et al
1
0.5
0
CMV-10 StEBP1 ∆-StEBP1
Figure 1 StEBP1 inhibits colony formation and represses E2Fmediated transcription in MCF7 human cell line. Colony formation
was inhibited when cells were transfected with varying amounts
(A) of the full-length StEBP1 and (B) the truncated D-StEBP1
plasmids and compared to the control CMV-10 construct. (C)
Expression of StEBP1 or the D-StEBP1 repressed the activity of the
E2F1 promoter-luciferase reporter when compared to the CMV-10
control. The data are shown as a ratio between the firefly and the
control cotransfected Renilla luciferase activities; relative luciferase
unit (RLU). Error bars represent standard deviations. (D) StEBP1
protein expression in transfected human cells was detected with
the FLAG-M2 monoclonal antibody. Molecular mass (50 kDa) is
indicated.
AtEBP1. The structurally and functionally conserved aminoacid sequences are shown in Supplementary Figure 2.
The functional conservation of StEBP1 was tested by
studying the effect of its overexpression on colony formation
(Figure 1A and B) and on E2F-dependent gene expression
(Figure 1C) in the MCF-7 human breast cancer cell line.
Cells were transfected with increasing concentrations (0.25,
0.5 and 1 mg) of plasmid coding for the full-length protein
(Figure 1A) or a truncated version of the protein starting from
the second in-frame ATG (Figure 1B). As a control, 1 mg of
the vector p3xFLAG-CMV10 was used. The full-length
StEBP1 significantly (Po0.05) reduced colony formation in
a dose-dependent manner when compared to the vector alone
(Figure 1A). The colony inhibition with StEBP1 is similar
to that found with the human EBP1 (Lessor et al, 2000).
Interestingly, transfection with the truncated StEBP1 resulted
in a more pronounced inhibition of colony formation
(Figure 1B).
To test whether StEBP1 can repress the expression of E2Fdependent genes, human cells were cotransfected with the
StEBP1 constructs together with an E2F1-luciferase reporter
(Cress and Nevins, 1996). The truncated version of StEBP1
was significantly more potent in suppressing E2F1 promoter
activity than the full-length version (Figure 1C). The presence
of the StEBP1 proteins was verified using Western blot
analysis (Figure 1D). From these results, we conclude that
StEBP1 is conserved both structurally and functionally with
the human EBP1 protein.
EBP1 regulates plant growth in a dose-dependent
manner
To elucidate the function of the EBP1 gene, its expression was
altered in potato and Arabidopsis plants. From around 100
& 2006 European Molecular Biology Organization
Control of organ and cell growth by EBP1 in plants
BM Horváth et al
antisense Stebp1(as) potato lines, 11 showed growth retardation. Three independent lines, chosen for further characterisation, were smaller in their final height, as well as the tuber
yield was lower than in the nontransformed control
(Supplementary Figure 3A, B).
Of 51 regenerated RNA interference lines, Stebp1(RNAi), 11
showed a similar but more pronounced phenotype than the
antisense lines. Plants such as the Stebp1-12(RNAi) (Figure
2A and E) were severely dwarfed and showed hardly any
growth on soil. Lines, such as Stebp1-13, -14 and -67(RNAi)
had a medium phenotype and were stunted and grew slower
during the entire course of their development, when compared to control plants (Figure 2A and E). Other lines, such as
Stebp1-65(RNAi) did reach a similar height to the wild type
(Figure 2E).
Quantitative real-time PCR (qRT-PCR) was carried out to
determine whether these alterations to the phenotype were
directly correlated to the reduction of the corresponding
mRNA levels. As shown in Figure 2C and Supplementary
Figure 3, the level of the StEBP1 mRNA was 1.5–2-fold less
in the Stebp1(as) lines, whereas it decreased between 8
and 12-fold compared to the endogenous wild-type level
in the different Stebp1(RNAi) lines. Comparison between
the Stebp1(as) and Stebp1(RNAi) lines should, however, be
drawn independently as the plants were grown at different
times and environmental conditions.
Leaf size and morphology were altered both in the
Stebp1(as) and the Stebp1(RNAi) lines. Wild-type potato
has a compound leaf structure (Figure 2F and G). Leaves of
the Stebp1(RNAi) lines have a reduced number of leaflet
pairs, more comparable to younger leaves in the control
plants. The total surface area of the leaves was reduced
compared to the wild type, although the individual leaflets
from the top nodes were somewhat larger (Figure 2F and G
and Figure 4A). The wild-type leaf lamella is smooth,
whereas in contrast, the morphology of the individual leaflet
in the silenced lines was convex with edges curling downwards, giving a folded, compact structure (Figure 2G, i-13).
At later stages of development, the tops of the plants were
foreshortened and deformed resulting in zigzag internodal
stem growth with unopened curled leaves turning towards
the stem (Figure 2F, i-13). The size of individual tuber
and tuber yield per plant were also reduced and tuber
morphology was abnormal (Figure 2H and I).
Plants with elevated expression of StEBP1 showed normal
development, but reached a greater final height compared to
the control (Figure 2B and E). Overexpressed StEBP1 protein
in the different Stebp1(oe) lines was detected on Western
blots via the myc-epitope (Figure 2D). The increase in StEBP1
transcript level is shown in Supplementary Figure 3D.
EBP1 was also silenced and overexpressed in Arabidopsis.
Eighteen out of 80 kanamycin-resistant T1 Atebp1(RNAi)
lines displayed distorted growth, 10 of these did not reach
maturity and only three produced homozygous T1 seeds. In a
similar way, we also obtained three homozygous Atebp1(oe)
lines. As shown in Figure 3A, the reduction of endogenous
AtEBP1 mRNA resulted in smaller plants, whereas the increase in EBP1 led to enlarged plant size compared to the
wild-type Columbia control. Images of three parallel plants
from each transgenic line and measurements of the canopy
area convincingly show the size differences (Supplementary
Figure 4). At the seedling stage, a delay in leaf initiation and
& 2006 European Molecular Biology Organization
distorted leaf shape were characteristic of the silenced lines
(Figure 3B). Using qRT-PCR, we found a four-fold reduction
of AtEBP1 level in a representative Atebp-1(RNAi) line
(Figure 3C) whereas the Atebp1(oe) lines contained significantly higher levels of both EBP1 mRNA and protein than the
control (Figure 3D and E, respectively). These results are in
good agreement with the data obtained for the potato Stebp1
transgenic lines.
StEBP1 regulates both cell number and cell size in
developing leaves
Leaves positioned along the potato stem represent consecutive developmental stages, and therefore provide a suitable
experimental system to study the developmental regulation of
organ growth. The size of the leaflets in the wild-type plant
gradually increases from the sixth to the 12th node. In the
Stebp1(RNAi) lines, leaflets at the sixth node were slightly
larger than the equivalent control. At subsequent developmental stages, leaf growth ceased between leaf nodes 8 and
12 in lines Stebp1-13(RNAi) and Stebp1-67(RNAi), whereas
leaves in Stebp1-65(RNAi) continued to grow, although their
size stayed behind the equivalent wild type (Figure 4A).
To determine the basis of the observed organ size differences, cell size was measured in the leaves of these lines
(Figure 4, Supplementary Table 2). The cell size increased
gradually between sixth and 12th leaves in the wild type.
In the sixth nodal leaf of the RNAi lines, the cell size was
B30% larger. In 8–12th node leaves, the size of the pavement
cells remained smaller in the Stebp1-13(RNAi) and Stebp167(RNAi) lines compared to the wild type, but was similar in
line Stebp1-65(RNAi), correlating with the degree of silencing
(Figure 2C). In summary, in young developing leaves, cell
size becomes larger in the Stebp1 silenced lines, whereas at
later stages, during expansion growth, the cell size falls
behind the wild type. Using statistical analysis, the differences in cell size were significant here and in subsequent data
sets. Numerical presentation of the data is summarised
in Supplementary Table 2.
In order to better understand the relationship between leaf
and cell size, an index for the total cell number in the
investigated leaflet was calculated by dividing the leaflet
surface area with the average cell size. We were aware
of the fact that the size of cells in different areas of the leaves
is variable, but we standardised our sampling procedure as
much as possible. The total cell number per leaflet gradually
increased with the developmental stages in the wild type. The
comparison of several data sets revealed that the total cell
number per leaflet generally reaches a plateau of around 106
cells per leaflet. We thus assume that after this number,
no more cell division occurs. Leaf growth then continues
through cell expansion (Figure 4A and B). In contrast, in
Stebp1-13(RNAi) and Stebp1-67(RNAi) lines there was no
increase in the total cell number between sixth and 12th
leaves, indicating that leaf growth in these lines is attributed
to cell expansion across all the developmental stages. In line
Stebp1-65(RNAi), the total cell number initially increased
from leaves sixth to eighth, after which it levelled off.
Interestingly, when Stebp1-13(RNAi) was grown under
greenhouse conditions, these plants showed similar stability
in the total cell number, but an abnormal cell expansion led
to a larger leaf size (Supplementary Figure 5). The phenomenon of counteracting the block in cell proliferation by cell
The EMBO Journal
VOL 25 | NO 20 | 2006 4911
Control of organ and cell growth by EBP1 in plants
BM Horváth et al
expansion is known as compensation (Horiguchi et al, 2006).
Our experiments show that compensation can be environmentally dependent.
4912 The EMBO Journal VOL 25 | NO 20 | 2006
During differentiation, pavement cells gain a more complex shape, becoming longitudinally expanded featuring a
lobed structure. The shape complexity was quantified by
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Control of organ and cell growth by EBP1 in plants
BM Horváth et al
calculating the ‘shape factor’ (4p area/perimeter2), which is
defined as 1 for a perfect circle and decreases for more
complex shapes. During development, wild-type cells adopt
more complex shapes as they differentiate (Figure 4A and B).
Pavement cells in the Stebp1(RNAi) lines were arrested in cell
division and although they expanded, the complexity of their
shape did not increase (Figure 4C and D, sixth and 12th
leaves, respectively).
We analysed leaves of Stebp1(oe) lines in order to learn
whether StEBP1 is sufficient to drive cell and organ growth.
These lines had a larger leaflet surface area when compared
to the corresponding leaflets in the controls (Figure 4B).
Surprisingly, the cell size of the overexpression lines in the
youngest sixth leaf was around half of the size of the
wild-type cells whereas at later stages cell sizes surpassed
the wild type, in parallel with the accelerated organ
growth. The total cell number per leaflet reached a higher
level at an earlier stage than in the wild type, indicating
that the switch between meristematic and expansion growth
is shifted to an earlier developmental stage (Figure 4B).
Overexpression of the StEBP1 also brought forward differentiation (Figure 4B and D). In summary, elevated StEBP1 level
leads to an increase in the number of cells at early stages of
leaf development, ceasing later on in development when
further organ growth occurs via a boost in cell expansion
and differentiation.
Figure 3 Altered expression level of EBP1 results in changes in growth habit in Arabidopsis transgenic lines. (A) Silencing of the AtEBP1
causes growth retardation in Atebp-1(RNAi), referred as Ati-1, whereas overexpression of the StEBP1 in Atebp-12(oe) line, labelled as AtOE12
leads to larger plants compared to the control Columbia. The plants were 3 weeks old. (B) Silencing of AtEBP1 delays leaf initiation and alters
leaf morphology, as shown by representatives of Atebp1-1(RNAi), and Atebp1-5(RNAi) lines compared to the wild type photographed 10 days
after germination. (C) Silencing of the AtEBP1 expression is shown for the representative line Atebp1-1(RNAi) and (D) the elevated expression
of StEBP1 is shown in the overexpression lines (AtOE-12, AtOE-19, AtOE-43). RNA for qRT-PCR was isolated from the second to fourth leaves of
the same set of plants shown in (A). The expression level of EBP1 is standardised to the level of the Arabidopsis actin2 gene. The relative
expression is given as a ratio compared to the endogenous EBP1 transcript, set to unit 1 in the control. (E) The presence of EBP1 in Atebp1(oe)
lines (AtOE-12, AtOE-19, AtOE-43) was confirmed with Western blot analysis using myc antibody. Molecular mass (55 kDa) is indicated. Error
bars represent standard deviations. Asterisk indicates a cross-reacting protein band with the myc antibody. As a loading control, amido-black
staining of the corresponding membrane is shown.
Figure 2 Alteration in expression of the StEBP1 effects growth in S. tuberosum transgenic lines. (A) Silencing of the StEBP1 inhibited growth in
height of the Stebp1(RNAi) lines; i-12, i-13, i-14, i-67 whereas (B) overexpression of StEBP1 in Stebp1(oe) lines, OE-3, OE-5, OE-16 led to
increased height compared to the control wild-type (wt) under greenhouse conditions. (C) Silencing of StEBP1 mRNA level in the antisense
Stebp1 lines (as-23, as-78, as-81) and Stebp1(RNAi) lines (i-12, i-13, i-14, i-65, i-67) was determined by qRT-PCR. The level of expression is
shown as a ratio compared to the wild type given the value of 1. (D) The elevated level of StEBP1 was detected using the myc-antibody in
protein extracts of the overexpression lines (OE-3, OE-5 and OE-16). Molecular mass (55 kDa) is indicated. (E) Potato plants with strong (i-12),
intermediate (i-13) and a weak phenotype (i-65) of the Stebp(RNAi) lines are compared to the wt and to the Stebp1(oe) line (OE-5) grown in the
climate chamber. (F) The morphology of the apical area at different developmental stages and (G) the leaf morphology of the Stebp1-13(RNAi)
is compared to the control (wt). (H) Average weight of tubers per plant in the Stebp(RNAi) lines. (I) Tuber yield for four representative plants of
wt and i-13 are shown. Error bars represent standard deviations. Bars on F and I ¼ 10 cm.
& 2006 European Molecular Biology Organization
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Control of organ and cell growth by EBP1 in plants
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Figure 4 Leaf size, cell size, total cell number per leaflet (cell number index) and cell shape factor in the Stebp1 transgenic lines.
(A) Stebp1(RNAi) lines (i-13, i-65, i-67) and (B) Stebp1(oe) lines (OE-3 and OE-5) compared to the wild type (wt). Columns represent values
for each successive leaflet from the 6th, 8th, 10th and 12th nodes and coloured in shades of grey. The error bars refer to standard deviations of
the values except in the cell number index, where the standard error was calculated. (C) Representative images from the adaxial epidermal cell
layer of the sixth leaflet and (D) of the 12th leaflet from the Stebp1-13(RNAi), wild type and Stebp1-5 (oe) are shown for illustration.
Bar ¼ 100 mM.
EBP1 regulates RBR1 protein level and the expression
of cell cycle regulators in an auxin-dependent manner
To understand how cell division is arrested during leaf
development when the StEBP1 mRNA level is reduced, the
expression of critical cell cycle regulators of the G1 to S
and G2 to M transitions were followed. QRT-PCR was carried
out using primers for the potato homologues of the
Arabidopsis CYCD3;1 (Dewitte et al, 2003), RNR2 (ribonucleotide reductase) (Chaboute et al, 2000), CDKB1;1 (Boudolf
et al, 2004) genes. Two independent lines, Stebp1-13 and
-67(RNAi) and a control plant were chosen to detect the
expression in the apex (pool of the meristem, the first and
second nodal leaves) and in leaves from the sixth and
tenth nodes. The expression of different genes was quantified
as the difference in the cycle numbers (DCT) in the qRT-PCR
4914 The EMBO Journal VOL 25 | NO 20 | 2006
experiments between the gene of interest and the constitutive Ubiquitine gene (Figure 5A). In the apex of young,
developing plants, no significant difference was detected in
the expression level of the CDKB1;1 and RNR2 genes, whereas
the level of CYCD3;1 mRNA was somewhat lower in the
Stebp1(RNAi) plants compared to the control (Figure 5A).
However, in young developing leaves, the abundance of
all the three cell cycle regulators was reduced compared
to the control (Figure 5A). Although the expression of these
cell cycle genes naturally diminishes as leaves develop, their
relative levels in the Stebp1(RNAi) lines remained below
the control even in the fully developed leaves (Figure 5A).
Thus, the effect of StEBP1 gene silencing reduces the expression of cell cycle regulators in a developmentally dependent
manner.
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Control of organ and cell growth by EBP1 in plants
BM Horváth et al
5
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StEBP1
Figure 5 Altered level of StEBP1 affects the expression of cell-cycle-related genes. (A) The expression levels of CYCD3, CDKB1:1 and RNR2
mRNAs were determined by qRT-PCR in the apex (meristem, first and second leaves sampled together) and in the sixth and 10th leaves taken
from the Stebp1-13(RNAi), Stebp1-67(RNAi) and the wild-type control. The expression levels are given as a difference in cycle numbers during
qRT-PCR of the genes of interest and Ubiquitin (DCT). (B) The expression levels of these cell-cycle-related transcripts in the Stebp1-5(oe) line
and the wild type were similarly determined as in (A) in the meristem sampled together with the first leaf primordium (M þ 1), and in the
second, third, fifth and ninth leaves. Regulation of the promoters of (C) CYCD3;1, (D) CDKB1;1, (E, F) RNR2 by altered EBP1 expression in
transfected Arabidopsis cells cultured either in the presence of auxin ( þ ) or in its absence (). StEBP1 stands for the transfected StEBP1
expression construct under the control of CaMV promoter and iAtEBP1 labels the silencing construct below the graphs. Promoter activities
were determined through their fusion construct to the GUS reporter. The inset in (F) illustrates the increasing amount of c-myc-tagged StEBP1
protein as a result of the elevation in concentration of the transfected plasmid, detected by the c-myc antibody. Molecular mass of the protein
is 55 kDa. The error bars represent standard deviations.
The expression of these cell cycle regulators showed the
opposite tendency in the Stebp1-5(oe) line (Figure 5B). To
detect subtle changes during early development, we examined expression in the meristem sampled together with the
first leaf and in the leaves from nodes second, third, fifth and
ninth. The CYCD3;1 expression in the meristem increased
approximately 60%, compared to the wild type, whereas at
later stages it followed the same level as the control. The
expression level of the CDKB1;1 increased 2–3 times during
early leaf development whereas in leaf 9 it was equal to the
corresponding wild-type level. An elevated expression of
the RNR2 was present in all leaves of the Stebp1-5 (oe) line
that were analysed.
To follow the transcriptional control of the genes CYCD3;1,
CDKB1;1 and RNR2, their promoter regions were cloned and
fused to a GUS-GFP reporter. The activity of these promoters
was analysed by measuring b-glucuronidase (GUS) activity
after transfection into Arabidopsis protoplasts derived from a
suspension culture. As the growth hormone, auxin positively
regulates the cell cycle, these experiments were carried out in
either the absence or the presence of auxin. When no auxin
was added, the promoters had no, or a very low residual
activity. With the addition of the hormone, the activity rose
to different levels depending on the promoter used (Figure
5C–E). These results show that exogenously applied auxin
stimulates the expression of these cell cycle regulated
promoters in cultured cells.
To investigate the effect of elevated StEBP1 expression a
construct of StEBP1 under the control of 35S CaMV promoter
was cotransfected with plasmids carrying CYCD3;1, CDKB1;1
& 2006 European Molecular Biology Organization
and RNR2 promoters fused to the GUS reporter gene (Figure
5C–F). In all cases, in the presence of auxin and an excess
of StEBP1, a further increase of GUS activity was found when
compared to applying auxin alone. In contrast, StEBP1 overexpression did not alter the promoter activities when cells
were cultured in the absence of auxin (Figure 5C–F).
Subsequently, the requirement of AtEBP1 in the auxindependent activation of cell cycle promoters was tested. For
this, the expression of the AtEBP1 was silenced using the
AtEBP1(RNAi) construct. We found that silencing of AtEBP1
completely abolished the activation of RNR2 promoter by
auxin (Figure 5E), indicating that AtEBP1 is required for
the auxin-dependent promoter activity. To analyse the dose
dependency, quantity of StEBP1 was varied by transfecting
increasing amounts (5, 10 and 15 mg) of StEBP1 plasmid with
the RNR2-promoter construct into cells cultured with auxin
(Figure 5F). This transfection series resulted in a corresponding accumulation of StEBP1 protein (Figure 5F, inset) and a
proportional activation of the promoter activity (Figure 5F).
As auxin was required for EBP1 to regulate the expression
of cell cycle promoters, we examined its effect on EBP1
protein level. We found that EBP1 protein accumulated in
the presence of auxin, whereas it was hardly detectable in
cells cultured for 2 days in its absence, despite of being
expressed under the control of the constitutive 35S CaMV
promoter (Figure 6A). To study post-translational regulation, cells were cultured for a shorter 16 h period either in
the presence or absence of auxin, and subsequently the de
novo synthesis of proteins was blocked using cycloheximide
(CHX). At this time point (T0), the difference in EBP1 levels
The EMBO Journal
VOL 25 | NO 20 | 2006 4915
Control of organ and cell growth by EBP1 in plants
BM Horváth et al
Figure 6 EBP1 protein level is regulated by auxin (2,4-D) through its stability and affects the quantity of RBR1 protein. (A) Arabidopsis cells
transfected with the Myc-StEBP1 construct and cultured for 2 days in the presence ( þ ) or absence () of 2,4-D. (B) Arabidopsis cells
transfected with the Myc-StEBP1 construct and cultured for 16 h in the presence or absence of 2,4-D after which CHX was added (T0) and
sampled for the indicated times (CHX þ 2,4-D, or CHX2,4-D, respectively). StEBP1 was detected on Western blots in (A, B) with the c-myc
antibody. Molecular mass (55 kDa) is indicated. (C) Transfection of Arabidopsis cells with 0, 5 and 10 mg Myc-StEBP1 plasmid (, þ , þ þ )
together with 5 mg RBR1 construct ( þ ). StEBP1 was detected by the c-myc antibody and RBR1 with the RBR1-specific antibody. (D) Silencing of
AtEBP1 in the Atebp (RNAi) lines (Ati-1, Ati-5, and Ati-39) effects the endogenous levels of RBR1 and CDKB1;1. The RBR1 is detected using
RBR1-antibody (molecular mass B125 kDa) and CDKB1;1 by the CDKB1;1-antibody (molecular mass B35 kDa). Loading control is as in
Figure 3.
in the presence or absence of auxin was smaller than after
48 h (Figure 6A and B). In cells cultured with auxin, the
EBP1 level remained unchanged for the course of the CHX
experiment, whereas without auxin EBP1 decreased with an
estimated half-life of 1 h (Figure 6B). In summary, EBP1
protein level alters in accordance with presence of the growth
promoting hormone, auxin.
The promoters of the cell cycle genes examined are
thought to be regulated by the E2F transcription factors
and repressed by the RBR1 pathway (Chaboute et al, 2000;
Boudolf et al, 2004). Therefore, we tested RBR1 protein
level in Arabidopsis suspension cells and transgenic plants
with altered EBP1 level (Figure 6C and D, respectively). We
found that the overexpression of EBP1 in Arabidopsis cells
dose-dependently reduced the endogenous RBR1 protein
(Figure 6C). In contrast, the Atebp1(RNAi) plants had a
consistently elevated RBR1 protein accumulation that was
paralleled with the reduction of the CDKB1;1 (Figure 6D).
The negative regulation of RBR1 level by EBP1 could provide
the mechanism where modulated EBP1 level influences cell
cycle promoters, and thus cell proliferation in leaves.
Discussion
EBP1 is a conserved nucleolar protein
In human cells, the EBP1 protein was shown to be part of
ribonucleoprotein complexes binding to rRNA precursors and
small nucleolar RNA species in the nucleoli, and to mature
rRNAs of ribosomes. HsEBP1 was suggested to regulate the
production and assembly of translational machinery and to
regulate translation in response to cellular stresses (Squatrito
et al, 2004, 2006). The plant EBP1 protein shows a remarkably high structural conservation to the human counterpart
and contains all the essential sequences shown to be required
4916 The EMBO Journal VOL 25 | NO 20 | 2006
for the nuclear and nucleolar transport and retention, and for
efficient binding to rRNA species in humans. Proteomics of
the Arabidopsis nucleolus also identified AtEBP1 as a nucleolar protein (Pendle et al, 2005). We have screened for StEBP1
interacting proteins using a stolon-derived yeast two-hybrid
cDNA library and found numerous putative interactors that
relate to ribosome biogenesis (Horvath, unpublished results).
These proteins include components of the 60S ribosomal
subunit, such as the proteins L6, L7 and the elongation factor
EF1A. Putative homologues of the StEBP1 interacting
partners were also identified as part of the HsEBP1 complex
in human cells (Squatrito et al, 2004). The nucleolar localisation and the interaction with ribosome biogenesis factors
suggest a conserved function for EBP1 in regulation of protein
synthesis and cell growth.
Expression of EBP1 correlates with active growth and
protein biosynthesis
We identified Stebp1 as an early tuberisation-induced transcript. However, its expression is not restricted to the developing tuber, but is present in all actively growing organs.
Analysis of publicly available microarray data for expression
of the Arabidopsis homologue, AtEBP1, also demonstrates
a convincing connection with cell growth and cell division.
AtEBP1 expression is rapidly induced by sucrose and correlates with the proliferation of cell cultures (Zimmermann
et al, 2004; Menges et al, 2005). Furthermore, we have
found that genes coregulated with AtEBP1 code for proteins
implicated in protein synthesis, such as nucleolin, fibrillarin,
ribosomal proteins and translational initiation factors
(Supplementary Table 1), suggesting that EBP1 is part of a
regulatory network previously described as the ribosomal
biogenesis regulon in yeast (Jorgensen and Tyers, 2004).
& 2006 European Molecular Biology Organization
Control of organ and cell growth by EBP1 in plants
BM Horváth et al
EBP1 is a dose-dependent regulator for organ size
To learn more about EBP1 function, we modulated its
expression in potato and Arabidopsis. In both systems, we
found that growth was stunted in parallel with a reduction in
EBP1 transcript, affecting organs including roots, leaves and
tubers in the case of potato. In contrast, plants with elevated
levels of EBP1 were generally normal, but had increased
growth habit and leaf size.
The organ size is the result of the constituent cell numbers
and sizes (Ingram and Waites, 2006). The timing of the
transition from proliferative growth to differentiation largely
determines the final cell numbers and thus the organ size
(Beemster et al, 2006). This transition occurs in potato leaves
at nodal positions 8–10, a region beyond which cell number
ceases to increase further. In lines with strong silencing
of Stebp1 expression, cell number is already slightly
decreased at the sixth node, and most importantly, stagnates
at this reduced level in leaves at 8–12th nodes. Thus, StEBP1
appears to regulate the developmental end point of cell proliferation. CycD3;1 is a known cell cycle regulator that inhibits
the exit from proliferation and the entry into differentiation
(Dewitte et al, 2003). CycD3;1 in complex with CDKA, phosphorylates RBR1 and releases the repression of genes required
for cell proliferation. RBR1 was shown to regulate cell production through stem cell function in Arabidopsis root meristem
(Wildwater et al, 2005). Our findings that a normal StEBP1
level is required for sustained CycD3;1 expression in the apical
meristem and that EBP1 dose dependently down regulates the
abundance of RBR1 level provides a mechanism how EBP1
may regulate cell proliferation and organ growth.
StEBP1 overexpression does not influence the final cell
number, but brings forward the developmental stage at which
the final cell number is reached. In the overexpression lines
of Argos (Hu et al, 2003) and Aintegumenta (Mizukami and
Fischer, 2000), the increase in cell number and organ size is
proportional, the period of cell proliferation and cell growth is
remaining to be linked and extended. In the CycD3;1 overexpression line, the meristematic activity is also prolonged
leading to a greatly increased cell number but cell proliferation becomes dissociated from cell growth resulting in smaller cells and stunted plants. In all these cases, however, the
cell number exceeds the control limit, which contrasts with
our results on EBP1 overexpression, suggesting that EBP1
controls organ size by a different regulatory pathway from
Argos, Aintegumenta and CycD3;1. CycD2;1 overexpression
accelerates plant growth and development through simultaneous stimulation of cell growth, cell proliferation and
development and thus has similarities to EBP1 (Cockcroft
et al, 2000). Cyclin D in Drosophila is known to be a growth
driver (Datar et al, 2000). The similarity between CycD2 and
EBP1 further extends to their expression, where both rapidly
respond to growth signals, such as sucrose.
StEBP1 limits cell size in proliferating cells
The second cellular parameter that influences organ size is
cell size. This is uniquely regulated in meristems during cell
division and in postmitotic differentiating cells via organspecific cell enlargement. Reduced levels of StEBP1 leads to
increased cell size in the RNAi lines, whereas overexpression
of StEBP1 results in reduced cell size in developing young
leaves. Two alternative explanations can be given for the
effect on cell size increase in the RNAi lines. Either the
& 2006 European Molecular Biology Organization
cells stop dividing at an earlier stage and then expand for a
longer period or cells divide at a larger cell size threshold. We
favour the later explanation as in the EBP1(oe) lines in young
leaves cell sizes are reduced. Furthermore, altered EBP1
levels oppositely affects cell sizes in young and old leaves,
indicating two distinct mechanisms, one possibly acting on
cell division, the other on cell expansion.
The homeostasis of cell size in proliferating cells is a
dynamic balance between cell growth and division. This
process is best understood in budding yeast, where, in
order to pass Start in the G1 phase, several requirements
must be fulfilled such as growth to a critical cell size, nutrient
sufficiency and attainment of a critical translational rate.
There is a powerful nutrient repression of Start, by which
the critical cell size threshold can be reset at each cell division depending on the nutrient availability (Jorgensen and
Tyers, 2004). In plants, however, EBP1 stimulates rather than
represses cell division allowing cells to divide at a lower cell
size. Recently, it was found that genes coding for translational
components and cell cycle genes are coregulated through the
TCP transcription factors, suggesting a positive connection
between cell growth and cell division (Li et al, 2005). Cell
division is spatially restricted to meristems and temporally
controlled by developmental and environmental cues. An
example of this is the sprouting of lateral shoots from buds,
a process that is under developmental and environmental
control, regulated by auxin and dependent on the simultaneous upregulation of protein translation and cell cycle genes
by the TCP transcription factors (Tatematsu et al, 2005).
EBP1 is required for the full activation of cell cycle genes
The connection between protein translation and cell proliferation is further substantiated by our results, showing that
EBP1 is required for the auxin-dependent activation of genes
involved in three separate phases of the cell cycle, the G1
phase-specific CycD3;1, the S-phase-specific RNR2 and the
mitosis-specific CDKB1;1. StEBP1 overexpression stimulated
these promoter activities dependent on the presence of auxin,
whereas reduced levels of StEBP1 abolished their activation.
The promoters of these genes contain E2F-binding elements
(Chaboute et al, 2000; Boudolf et al, 2004). We have found
that elevated EBP1 levels dose-dependently downregulated
the endogenous RBR1 protein that could lead to the release of
the E2F-dependent transcription of these cell cycle genes and
thus, to increased proliferation and reduced cell size.
Auxin is a plant hormone that plays a pivotal role in plant
growth and development. Previously, we have found that
auxin regulates cell proliferation by influencing the stability
of the E2FB transcription factor (Magyar et al, 2005). Here we
show that auxin also has an effect on StEBP1 stability, its halflife was drastically reduced when cells were cultured without
auxin. Thus, auxin may promote plant growth by simultaneously influencing the stability of both translational regulators such as EBP1 and cell cycle regulators such as E2FB.
It is also possible that EBP1 coregulates protein synthesis
and cell cycle through the RB-E2F pathway. Cavanaugh et al
(1995) have shown that differentiation of a human cell line is
accomplished by the accumulation of the RB protein in the
nucleolus, correlating with changes in the ribosomal RNA
synthesis. RB associates directly with the transcription factor
upstream binding protein, the key regulator of rRNA synthesis and with TFIIIB, involved in transcription driven by
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Control of organ and cell growth by EBP1 in plants
BM Horváth et al
RNA polymerase III. It was shown that HsEBP1 through
its C-terminal end binds RB protein, and contains a nuclear
localisation signal in the same region (Xia et al, 2001).
Although all these motives are conserved in the plant EBP1,
we were unable to detect in vivo interaction with RBR1 in
plant cells in pull-down experiments (Magyar, unpublished
result). Consequently, the question whether EBP1 regulates
proliferation and cell-growth-related gene expression by
direct binding to RBR1 remains open.
EBP1 positively regulates expansion growth of leaf cells
In potato, we found that leaves from the 10th to –12th nodes
continue to grow by cell expansion. This process is compromised in Stebp1(RNAi) plants leading to decreased cell size,
whereas overexpression of EBP1 drives further cell expansion, giving rise to larger cells with proportionally larger
leaves. Thus, elevated levels of EBP1 conversely regulates
cell size during meristematic and expansion growth, decreasing the size at which cells divide whereas positively influencing cell expansion. Similarly, in Drosophila the cellular
response to CycD-Cdk4-driven growth varied according to
cell type. In undifferentiated proliferating wing imaginal cells,
CycD-Cdk4 caused accelerated cell division whereas in differentiating postmitotic eyes and salivary gland cells, CycDCdk4 caused cell enlargement (Datar et al, 2000).
During cell expansion, pavement cells on the adaxial
leaf surface gain a gradually more complex lobed structure
(Fu et al, 2005). In the Stebp1(RNAi) lines, cells remain ovate
whereas elevated StEBP1 levels resulted in more complex cell
shape, indicating that in parallel to cell expansion, cell shape
is also effected by EBP1 levels. Enhanced growth through
EBP1 initially promotes cell proliferation but simultaneously
advances cell differentiation, thus the total cell number
within organs is unchanged.
Despite its obvious importance, only a handful of genes
that have an impact on plant growth or fruit size are known.
Thus, the understanding of basic growth-control mechanisms, the identification and characterisation of genes controlling plant body size, development, growth kinetics and
growth habits is an ongoing practical need. Discovering the
mechanisms how EBP1 regulates organ growth and plant cell
size, advances our knowledge in this field.
Materials and methods
Expression analysis in plant tissues
RNA isolation from axillary buds grown in vitro and potato plant
tissues for Northern analysis was carried out as described
previously (Bachem et al, 1996). Primers with two selective
nucleotides TaqI (TC) and AseI (GC) were used for cDNA-AFLP
(Bachem et al 1996). During qRT-PCR in potato, Ubiquitin, and in
Arabidopsis, Actin2 transcript was used for normalisation. Potato
primers to cell-cycle-related sequences were designed based
on the sequence of A. thaliana. The list of the Arabidopsis genes,
their potato homologues and all primer pairs are listed in the
Supplementary data. For qRT-PCR, total RNA was isolated from
2–2.5-month-old potato plants, from leaves described in the Results
and from Arabidopsis transgenic lines, from the pool of the 2nd–4th
nodal leaves using the RNA-Easy Plant Mini kit from both plant
systems (Qiagen, Hilde, Germany). QRT-PCR was carried out
as described by Kloosterman et al (2005). The reactions were
repeated in triplicate at least twice with independent cDNAs.
Relative quantification of the target RNA expression level and
standard deviation was performed using the comparative Ct method
according to the User Bulletin #2 (ABI PRISM 7700 Sequence
Detection System, December 1997, Applied Biosystems).
4918 The EMBO Journal VOL 25 | NO 20 | 2006
Methods used in human cell culture
The full-length and the truncated version of StEBP1 were cloned
into the mammalian expression vector (p3xFLAG-CMV-10, Sigma)
under the control of the CMV promoter. The colony forming assay
with human breast carcinoma cell line MCF-7 (Lessor et al 2000)
and E2F1 reporter assays were carried out according to Zhang et al
(2005). All transfection experiments were carried out in triplicate
wells and repeated three times.
Vectors, amplified fragments and cloning
Cloning of the tobacco RNR2 promoter was carried out as described
by Chaboute et al (2000). To clone the CYCD3;1 promoter genomic
DNA of A. thaliana var. Columbia was amplified. StEBP1 cDNA was
used to clone the antisense—and in a tail–head/head–tail orientation the silencing constructs. In the overexpression construct, the
coding region was translationally fused at its N-terminus to the
2xmyc sequence coming from the pBS SK-2xmyc vector (Magyar
et al, 2005). To clone AtEBP1 for silencing, the sequence related
to the clone NM180348 was amplified. The StEBP1(as) was cloned
into pBI121vector. Otherwise, pENTRTM /D-TOPOs Cloning Kit
(Invitrogen) was used and fragments were recombined into the
GATEWAY binary-vector system (Karimi et al, 2002).
Generation and analysis of transgenic plants
Transformation of the potato (var. Karnico) and Arabidopsis Col-0
was carried out using A. tumefaciens cocultivation. In total, around
150 transgenic lines carrying the silencing constructs and 50
overexpression lines were analysed in consecutive years in the
greenhouse in 3–5 repeats. Growth in height was followed on plants
with a single stem in 2-week intervals. Lines with altered phenotype
were re-examined under highly controlled conditions (160 mmol/
m2/s light intensity, 16 h light/8 h dark photoperiod, 181C/161C
light/dark). In Arabidopsis several independent, homozygous T1
transgenic lines were used in further studies.
Transient expression in Arabidopsis protoplast and GUS
assays
Protoplasts were prepared as described by Magyar et al (2005).
Around 106 protoplasts were transformed with plasmid DNA (5 mg)
and incubated either in the presence (1 mM) or absence of auxin for
16–24 h unless otherwise indicated. To determine the GUS enzyme
activity, the b-glucuronidase (GUS) Reporter Gene Activity Detection Kit (Marker Gene Technologies, Inc., The University of Oregon,
Oregon, USA) was used. All experiments were carried out in
triplicate and performed independently at least twice.
Epidermal peels and cell measurement
The youngest, not yet fully developed compound leaf (sixth node
leaf); young, fully developed leaves (eighth and 10th leaf) and an
older, fully developed leaf (12th leaf) were collected from plants
grown under controlled conditions. Cell measurements were taken
from the adaxial epidermal cell layer of the first opposing leaflet
pair of the compound leaf. Similar region of the lamina was
analysed for every measurement. Tissues for cell size measurements were prepared according to Taylor et al (2003) and were
observed under a DMLB microscope (Leica, Wetzlar, Germany)
fitted with differential interference contrast optics. Cells were
photographed from at least five different positions of the blade
section and on average 100 cells were analysed. Cell outlines were
traced and parameters such as cell area, perimeter and shape factor
were calculated with the public domain image analysis program
ImageJ (version 1.33; http://rsb.info.nih.gov/ij/). This program
was also used to measure the total canopy of Arabidopsis transgenics. For statistical analysis to compare mean cell size, the REML
procedure in Genstat was used, giving an overall Wald test for any
mean being different. Pair-wise comparisons were made by least
significant differences based on the estimated standard errors. The
average cell area and the leaf surface area were taken to calculate
the cell number index.
Protein extraction and protein gel blot analysis
Immuno-blotting was performed as described previously (Magyar
et al, 2005). The Myc-StEBP1 protein was determined by immunoblot analysis with monoclonal c-myc antibody (9E10) purchased
from Roche Diagnostics. The polyclonal antiCDKB1;1 antibody was
used in 1:2000 dilution as described previously (Magyar et al,
2005). A cDNA fragment encoding the C-terminal 236 amino acids
& 2006 European Molecular Biology Organization
Control of organ and cell growth by EBP1 in plants
BM Horváth et al
of the AtRBR1 was amplified and cloned in pET28a (Novagen). For
protein production, the plasmid was transformed into Escherichia
coli strain BL21DE3 Rosetta (Novagen) and the expressed hexahistidine-tagged RBR C-terminal polypeptide was purified under
denaturing conditions on Ni2 þ -NTA beads (Qiagen). Protein was
renatured by stepwise dialysis, concentrated and used to immunise
hens (Agrisera, Sweden). IgY was isolated from the egg yolks with
the Eggcellent kit (Pierce) and used for Western blots at a dilution
of 1:7000.
Transfected protoplasts were cultured for 16 h in the absence or
presence of auxin (1 mM) and afterwards treated with CHX (100 mM)
to block protein synthesis. Total proteins were extracted and
analysed in equal amounts (10 mg).
Supplementary data
Supplementary data are available at The EMBO Journal Online
(http://www.embojournal.org).
Acknowledgements
We thank Bjorn Kloosterman, Jose M Perez-Perez and Ben Scheres
for valuable discussions, Robert Hall for critical reading of the
manuscript, Paul Passarinho and GeneTwister for the use of their
microscopic facilities, Marcos Malosetti for statistical analysis,
Richard Immink for technical advice during yeast two-hybrid
experiments, Dirkjan Huigen, Marjan Bergervoet, Isolde Pereira
and Gerda Prins van Engelenhoven for technical support. The
CDKB1;1 promoter was a kind gift from L De Veylder, the stolon
yeast two-hybrid library from Sophia Biemelt. We are thankful to
László Ökrész for bioinformatics, Piero Morandini for performing
the coregulation bioinformatic analysis for AtEBP1 and to Elisabeth
Hatzimasoura for the transformation of Arabidopsis plants. BMH
was supported by the Technology Foundation STW of the
Netherlands, project number WBI 4923. The work in the laboratory
of LB was funded by the EU framework 5, GVE program and by
BBSRC.
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