Download Plant meristems: CLAVATA3/ESR-related signaling in the shoot

J Plant Res (2009) 122:31–39
DOI 10.1007/s10265-008-0207-3
CURRENT TOPICS IN PLANT RESEARCH
Plant meristems: CLAVATA3/ESR-related signaling
in the shoot apical meristem and the root apical meristem
Hiroki Miwa Æ Atsuko Kinoshita Æ Hiroo Fukuda Æ
Shinichiro Sawa
Received: 6 November 2008 / Accepted: 27 November 2008 / Published online: 23 December 2008
Ó The Botanical Society of Japan and Springer 2008
Abstract The plant meristems, shoot apical meristem
(SAM) and root apical meristem (RAM), are unique
structures made up of a self-renewing population of
undifferentiated pluripotent stem cells. The SAM produces
all aerial parts of postembryonic organs, and the RAM
promotes the continuous growth of roots. Even though the
structures of the SAM and RAM differ, the signaling
components required for stem cell maintenance seem to be
relatively conserved. Both meristems utilize cell-to-cell
communication to maintain proper meristematic activities
and meristem organization and to coordinate new organ
formation. In SAM, an essential regulatory mechanism for
meristem organization is a regulatory loop between
WUSCHEL (WUS) and CLAVATA (CLV), which functions
in a non-cell-autonomous manner. This intercellular signaling network coordinates the development of the
organization center, organ boundaries and distant organs.
The CLAVATA3/ESR (CLE)-related genes produce signal
peptides, which act non-cell-autonomously in the meristem
regulation in SAM. In RAM, it has been suggested that a
similar mechanism can regulate meristem maintenance, but
these functions are largely unknown. Here, we overview
the WUS–CLV signaling network for stem cell maintenance
in SAM and a related mechanism in RAM maintenance.
We also discuss conservation of the regulatory system for
stem cells in various plant species.
S. Sawa is the recipient of the BSJ Award for Young Scientist, 2007.
H. Miwa A. Kinoshita H. Fukuda S. Sawa (&)
Department of Biological Sciences,
Graduate School of Science, University of Tokyo,
Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
e-mail: [email protected]
Keywords CLAVATA3/ESR (CLE) CLAVATA (CLV) Leucine-rich repeat-receptor like kinase (LRR-RLK) Meristem Root apical meristem (RAM) Shoot apical meristem (SAM)
Introduction
The term meristem was first defined by Carl Wilhelm von
Nägeli in his book ‘‘Beiträge zur Wissenschaftlichen
Botanik’’ (von Nägeli 1858). Meristem is a formative plant
tissue made up of cells capable of dividing and giving rise
to new cells (Scofield and Murray 2006). Because of their
essential role in higher plants, meristems have received
much attention from plant scientists for more than
150 years. The shoot apical meristem (SAM) and the root
apical meristem (RAM) are known to be two important
meristems that provide cells for postembryonic growth and
development.
The SAM is a collection of cells that continuously renew
themselves by cell division and provide cells to new
organs. Although continuous cell division and differentiation of their daughters are observed in meristematic
regions, the meristem size and the number of stem cells are
kept constant. These observations suggest that stem cell
maintenance and new organ formation are well balanced.
Recently, it has been shown that intercellular communication controls the organization and maintenance of the
SAM, as well as cell-fate specification. Thus, stem cell
maintenance requires the well-coordinated regulation of
different signals for meristem homeostasis.
SAM maintenance is regulated by two main signaling
pathways: the WUSCHEL (WUS) pathway and the SHOOT
MERISTEMLESS [STM; KN1-related homeobox (KNOX)
protein] pathway (Lenhard et al. 2002). The WUS signaling
123
32
J Plant Res (2009) 122:31–39
Fig. 1 Models of structures and signaling cascade of SAM and c
RAM. a Structure of SAM and the expression of regulatory genes.
The SAM is divided into three zones, the peripheral zone (PZ), the
central zone (CZ) and the rib zone (RZ), and three layers, L1, L2 and
L3. CLV3 is expressed in the stem cell (yellow) whereas WUS is
expressed in the organizing center (OC, red). The coordinated SAM
structure is regulated by WUS–CLV3 negative feedback loop. b
Molecular components involved in SAM maintenance. CLV3
precursor is maturated and secreted to the apoplastic space, then
received by sets of receptors, CLV1, CLV2, and SOL2/CRN. The
receptors transmit the signal to regulate WUS expression. Exogenously applied synthetic peptides presumably act through the same
pathway. c Structure of the RAM. The quiescent center (QC, red) is
surrounded by stem cells (yellow), and meristematic cells (blue) are
located in the upper area. WUSCHEL-related homeobox 5 (WOX5) is
expressed in the QC cells (inbox). The region of expression of CLE
genes in the RAM remains to be found
pathway is mediated by the intercellular signal of the
CLAVATA3/ESR (CLE) peptide and perception by leucine-rich repeat (LRR) receptor kinases. In this article we
overview SAM organization and maintenance by focusing
on CLE peptides, cognate LRR receptor-like kinases and
the WUS-related molecules, together with the similar
machinery operated in the RAM. We also review the
conservation of the CLAVATA–WUSCHEL (CLV–WUS)
signaling network in various plants.
CLV function in SAM
The anatomy of the SAM is defined in terms of layers and
radial zones (Kwiatkowska 2008). The SAM can be divided into three layers based on cell fate. The L1 layer cells
divide parallel to the surface and give rise to the epidermis.
The L2 layer gives rise to mesophyll cells, and the L3
layers give rise to the central tissues of the leaf and stem
(Fig. 1a). The organization of the SAM is also divided into
three areas: the peripheral zone (PZ), the central zone (CZ),
and the rib zone (RZ) (Fig. 1a). The activity of cell division
in the CZ is low, whereas cells in the PZ divide actively
and provide cells that are needed for growth and differentiation of lateral organs.
During the past two decades, genetic analyses of a
variety of Arabidopsis thaliana (L.) Heynh. mutants have
identified a number of molecular components involved in
SAM maintenance. Among these, clv1, clv2, and clv3 are
the loss-of-function mutants that exhibit a remarkable
phenotype of shoot fasciation, enlarged floral meristems,
and increased numbers of floral organs. Two of these
mutants have mutations in membrane-associated receptorlike proteins that have LRRs in their predicted extracellular
domain; CLV1 encodes an LRR-receptor like kinase (LRRRLK), and CLV2 encodes an LRR-receptor like protein
lacking a kinase domain (Clark et al. 1997; Jeong et al.
123
1999; Fig. 1b; Table 1). CLV3 encodes a 96-amino acid
protein with a putative secretory signal peptide sequence in
its N-terminal region (Fletcher et al. 1999).
J Plant Res (2009) 122:31–39
33
Table 1 Receptor-like kinases which function in meristem maintenance (FM floral meristem)
Receptor-like
kinase
Accession
no.
Classification
Primary expression
Biological function
Reference
CLV1
At1g75820
Class XI LRR-RLK
SAM
Promotion of differentiation
at SAM and FM
Clark et al. (1997)
CLV2
At1g65380
LRR-RLP
Whole tissue
Promotion of differentiation
at SAM and FM
Jeong et al. (1999)
SOL2/CRN
At5g13290
RLK
Whole tissue
Promotion of differentiation
at SAM and FM
Miwa et al. (2008) and
Müller et al. (2008)
BAM1
At5g65700
Class XI LRR-RLK
SAM, root
Inhibition of stem cell
differentiation at SAM
DeYoung et al. (2006)
BAM2
At3g49670
Class XI LRR-RLK
SAM, root
Inhibition of stem cell
differentiation at SAM
DeYoung et al. (2006)
BAM3
At4g20270
Class XI LRR-RLK
Stele
Inhibition of stem cell
differentiation at SAM
DeYoung et al. (2006)
ERECTA
At2g26330
Class XIII LRR-RLK
SAM
Shoot organ growth
Torii et al. (1996)
OsFON1
AB182389
Class XI LRR-RLK
SAM and FM
Promotion of differentiation
at FM
Suzaki et al. (2004)
Class XI LRR-RLK
Vegetative seedling apex
Promotion of differentiation
at SAM and FM
Bommert et al. (2005)
ZmTD1
LjHAR1
AB092810
Class XI LRR-RLK
Whole tissue
Regulation of nodule
numbers
Nishimura et al. (2002) and
Krusell et al. (2002)
GmNARK
AY166655
Class XI LRR-RLK
Whole tissue
Regulation of nodule
numbers
Searle et al. (2003)
PsSYM29
AJ495759
Class XI LRR-RLK
Regulation of nodule
numbers
Krusell et al. (2002)
MtSUNN
AY769943
Class XI LRR-RLK
Whole tissue
Regulation of nodule
numbers
Schnabel et al. (2005)
ZmFEA2
AY055124
LRR-RLP
Vegetative seedling apex
Promotion of differentiation
at SAM and FM
Taguchi-Shiobara et al.
(2001)
Although it is believed that CLV3, as a small ligand,
binds to the putative CLV1/CLV2 receptor complex to
regulate stem cell identity in the SAM, the molecular
characteristics of these proteins have long been unknown
(Fig. 1b). Recently, biochemical studies provided key
findings for the CLV signaling molecules (Kondo et al.
2006; Sawa et al. 2006; Ogawa et al. 2008). In situ matrixassisted laser desorption/ionization time-of flight mass
spectrometry (MALDI-TOF MS) analysis identified a
mature form of CLV3 (MCLV3) as a 12-amino acid peptide with two hydroxy prolines (RTVPhSGPhDPLHH). The
12-amino acid sequence is located near the C-terminal end
of CLV3, and an application of the chemically synthesized
dodecapeptide results in the CLV3 overexpression-like
phenotype, suggesting that this peptide regulates meristematic identity in both the SAM and the RAM (Kondo et al.
2006). Ogawa et al. (2008) detected biochemical interactions between the LRR domain of CLV1 and MCLV3
using membrane fraction of tobacco BY-2 cells expressing
CLV1 that had an epitope tag instead of the kinase domain
(CLV1-DKD-HT). This was the first molecular evidence
that CLV3 and CLV1 function as a ligand-receptor pair in
the plant stem cell maintenance system. The interaction
between CLV3 and CLV2 still remains to be assessed.
CLV1 is a member of the large gene family, the LRRRLK family (Shiu and Bleecker 2001; Table 1). Since the
CLV1 is categorized in class XI, in accordance with the
distinctive LRRs in their putative extracellular domain,
members of the CLV1 group might be the receptors for
CLV3 homologs.
Functional CLE peptide in SAM and RAM
Database searches using the CLV3 entire sequence yielded only a poor match with the maize (Zea mays)
embryo-surrounding region (ESR) proteins in 14 amino
acids. However, when this 14-amino acid sequence was
used for database queries, a total of 31 related sequences
were identified in the Arabidopsis genome (Cock and
McCormick 2001; Sharma et al. 2003; Ito et al. 2006;
Strabala et al. 2006). These genes were named CLE
genes, and the conserved 14-amino acid sequence was
designated as the CLE domain.
123
34
The CLE family encodes small proteins characterized by
the conserved CLE domain at the C-terminal region and a
hydrophobic signal peptide at the N-terminal region. Most
CLE genes are transcribed in multiple tissues during development (Sharma et al. 2003; Strabala et al. 2006), suggesting
that these proteins may function as signaling molecules in
various aspects of morphogenesis. However, no loss-offunction cle mutant other than clv3 has been reported to date.
Tracheary element (TE) differentiation inhibitory factor
(TDIF) has been isolated as an inhibitor of TE differentiation
in a Zinnia elegans L. xylogenic culture system (Fukuda
1997) and is known to function as a dodecapeptide with two
hydroxyproline residues (HEVPhSGPhNPISN) (Ito et al.
2006). Interestingly, the active form of TDIF corresponds to
a functional CLV3 peptide; both are dodecapeptides derived
from the CLE domain, and the fourth and seventh prolines
are hydroxylated. This suggests that a typical length and
amino acid modification are conserved in some CLE proteins. So far, TDIF and CLV3 are the only CLE proteins that
show a specific function.
Although the function of CLE proteins is mostly
unclear, gain-of-function analyses help us to speculate on
their function. Strabala et al. (2006) examined overexpression phenotypes of CLV3 and 17 CLE genes and
identified ten CLE genes that arrest SAM growth and
seven CLE genes that inhibit root growth. Additionally,
synthetic peptide treatment assays using conserved 12amino acid sequences of 26 CLE peptides, corresponding
to putative mature forms of the 31 Arabidopsis CLE gene
products, revealed that ten CLE peptides, including CLV3,
have a strong effect on the SAM, and 19 CLE peptides,
including CLV3, regulate the RAM identity (Kinoshita
et al. 2007; Fig. 1b, c). These data suggest that CLE
proteins are, in part, redundant in the SAM and RAM.
Because the clv3 mutant exhibits no phenotype in the
RAM (Clark et al. 1995), it is possible that other CLE
genes are expressed in the RAM and redundantly regulate
its stem cell identity.
Post-translational regulation of CLE peptide is suggested to be important for CLE function. For example,
overexpression of the CLE1 gene induces short root phenotypes (Strabala et al. 2006), whereas application of a
synthetic CLE1 peptide does not induce any visible phenotypes (Kinoshita et al. 2007), suggesting that posttranslational modification of the CLE protein may be
essential for CLE1 function in plants.
Receptor kinases that act in the SAM and RAM
Besides CLV1 and CLV2, the receptor-like kinases that
function in SAM maintenance in Arabidopsis are CORYNE (CRN)/SUPPRESSOR OF LLP1 2 (SOL2), BARELY
123
J Plant Res (2009) 122:31–39
ANY MERISTEM 1 (BAM1), BAM2, BAM3, and ERECTA
(Table 1).
The sol2 mutant was isolated as a suppressor of the
CLE19 overexpression phenotype in RAM development
(Casamitjana-Martı́nez et al. 2003), and crn was identified
as a suppressor of the CLV3 overexpression phenotype in
SAM development (Müller et al. 2008). Molecular genetic
studies on SOL2 and CRN showed that both SOL2 and
CRN encode the same receptor-like kinase with a short
extracellular domain (Miwa et al. 2008; Müller et al.
2008). In shoot development, the crn/sol2 mutant shows
an enlarged SAM and is defective in floral organ development. This phenotype is similar to that of clv mutants,
and, therefore, the action of CRN/SOL2 is implicated in
the same signaling pathway of CLV–WUS. Genetic studies
have revealed that wus is epistatic with crn/sol2, suggesting that CRN/SOL2 acts upstream of WUS (Müller
et al. 2008). Spatial expression analysis by mRNA in situ
hybridization revealed that the CLV3 and WUS expression
domains were expanded in the crn/sol2 mutant. The
expression of CLV3 and WUS also expanded in the clv
mutants, in comparison with that in the wild-type.
Therefore, it is postulated that CLV and CRN/SOL2 act
closely to repress the WUS expression in the SAM. When
carpel number is used as an indicator of the CLV signaling activity, the clv1 and crn mutations have an
additive effect on carpel number, but the clv2 crn double
mutant has carpel number similar to each single mutant.
This suggests that CRN and CLV2 act together but
independently of CLV1. Taken together, these results
suggest that CRN/SOL2 mediates CLV signaling, together
with CLV2, and balances cell proliferation and differentiation in the SAM in parallel with CLV1 (Müller et al.
2008). The function of CRN/SOL2 and CLV2 in RAM
maintenance has been implicated by the application of
synthetic 12-amino acid peptides on seedlings. In a root
growth inhibition assay with the 26 CLE peptides, sol2
showed different levels of resistance to the various peptides, and the spectrum of peptide resistance was quite
similar to that of clv2 (Miwa et al. 2008). RAM consumption caused by various CLE peptides depends mostly
on CLV2 and CRN/SOL2, indicating that these two
receptor proteins work together for the same CLE signaling pathway in the RAM. The effect of the CLE
peptides on root growth is not affected by the clv1
mutations. This suggests that CLV1 is not involved in the
CLE signaling pathway in roots or gene(s) with functions
redundant to CLV1 operating in the roots (Miwa et al.
2008). Structurally, the CRN/SOL2 protein has a cytoplasmic kinase domain and a short extracellular domain,
whereas CLV2 has an extracellular LRR domain and a
short intracellular domain. One explanation is that CRN/
SOL2 and CLV2 act cooperatively in the same receptor
J Plant Res (2009) 122:31–39
complex, and that the kinase domain of CRN/SOL2
complements the lack of an intracellular domain in CLV2.
Phylogenetic analysis identified BAM1, BAM2, and
BAM3 receptor kinases, which are classified into the same
monophyletic group with CLV1 (DeYoung et al. 2006;
Table 1). Although none of these single mutants shows a
phenotype, the bam1 bam2 double mutants and the
bam1 bam2 bam3 triple mutants show reductions in shoot
meristem size. This phenotype is opposite to that of the
clv1 mutant, suggesting the opposite role of BAM proteins
in stem cell regulation (DeYoung et al. 2006). On the other
hand, weak phenotypes of clv1 null alleles were enhanced
by the bam mutations, resulting in enlarged SAM formation (DeYoung and Clark 2008). Taking these findings
together, the authors suggested that the BAM receptors
function to sequester unknown CLE peptide ligands produced in the PZ and that the effects of BAM1 and BAM2
on stem cell homeostasis are integral to the CLV signaling
pathway and do not represent a separate pathway regulating meristem development (DeYoung and Clark 2008).
ERECTA encodes an LRR-RLK, which functions in
SAM development (Torii et al. 1996). The er mutants are
defective in shoot organ development, showing a phenotype of short and thick inflorescence stem and increased
numbers of flower buds, and blunt, short, and wider siliques. When clv1 alleles are crossed to other Arabidopsis
accessions, enhancement of the clv1 phenotype is observed
in the progeny crossed by Ler accession. This suggests
functional overlap between CLV1 and ER in SAM
homeostasis (Diévart et al. 2003).
In the SAM, some RLKs have been shown to function
differently to regulate meristem maintenance, but the
molecular mechanism for how each receptor-like kinase
integrates the CLE signals remains to be solved. Although
no obvious root phenotypes have been identified in these
receptor-like kinase mutants, this does not rule out a
potential role for these genes in regulating root meristem
function. Root phenotype might be too subtle to be
detected, or other LRR-RLK proteins might function in the
RAM redundantly. A genome-wide collection of receptorlike protein gene transferred DNA (T-DNA) insertion
mutants has been reported (Wang et al. 2008), and this
collection would be helpful for further analyses to determine the roles of receptor genes in root development.
WUS and WOX5 as master regulators
in the SAM and RAM
WUS is a member of the WUSCHEL-related homeobox
(WOX) family homeodomain transcription factor and acts
as a master regulator to specify stem cell identity (Fig. 1a,
b; Laux et al. 1996; Mayer et al. 1998). One important role
35
of WUS in the regulation of stem cells is to activate CLV3
in a non-cell-autonomous manner through unknown intercellular signaling factors (Lenhard and Laux 2003). CLV3
is secreted from the L1 and L2 layers to suppress WUS
expression non-cell-autonomously (Fig. 1a, b), giving rise
to a negative feedback loop. This feedback regulation in
SAM maintenance allows the SAM to balance stem cell
division in the central zone and cell differentiation in the
peripheral zone (Schoof et al. 2000). When WUS is
expressed ectopically in roots, leaves are developed in the
RAM region, suggesting that WUS is sufficient to redirect
root cells to other developmental pathways such as leaf
cells (Gallois et al. 2004).
The regulation system for stem cell maintenance seems
partly conserved in shoot and roots. The WUS-expressing
organization center cells in the SAM correspond to the
quiescent center (QC) cells in roots. The QC cells, surrounded by stem cells, express the WUS homolog, WOX5
(Fig. 1c). A loss-of-function mutation in the WOX5 gene
causes terminal differentiation, with enlarged cells at the
QC and in columella stem cells. Conversely, WOX5 overexpression causes repression of differentiation in the
columella cells and overproduces the columella initial
cells. When WOX5 is expressed under the WUS promoter
in the wus mutant, phenotypes in an indeterminate inflorescence meristem are restored. In contrast, WUS
expression under the WOX5 promoter restores the QC and
columella abnormalities in the wox5 mutant. These results
suggest that WOX5 and WUS are interchangeable in stem
cell control, and WUS and WOX5 function in stem cell
maintenance in shoots or roots respectively (Sarkar et al.
2007).
Recent studies have identified several signaling components involved in the WUS signaling pathway. BRCA1associated RING domain 1 (BARD1) encodes a protein
containing two tandem BRCA1 C-terminal (BRCT)
domains, which function in phosphorylation-dependent
protein–protein interactions, and a RING domain, thought
to be involved in DNA repair. Severe SAM defects in the
bard1 mutant have been observed, and the bard1 wus
double mutant shows the wus mutant phenotype. Direct
interaction between BARD proteins and the WUS promoter
region has been shown by chip analysis. Together with
molecular genetic analyses that showed that BARD gene
overexpression induced the wus mutant phenotypes,
BARD1 is suggested to be responsible for the regulation of
WUS gene expression in the CLV signaling pathway (Han
et al. 2008).
OBERON1 (OBE1) and OBE2 encode homeodomain
finger proteins. CLV3, WUS, and WOX5 gene expressions
are dramatically reduced in the obe1 obe2 double mutant.
The obe1 obe2 wus triple mutant phenotype is similar to
that of the obe1 obe2 double mutant. Thus, OBE1 and
123
36
OBE2 are suggested to be responsible for plant cells
reaching an appropriate state for the establishment and
maintenance of the meristem by the action of meristem
genes, rather than by the specification of the apical meristems directly (Saiga et al. 2008).
HANABA TARANU (HAN) encodes a GATA-3-like
transcription factor and functions in meristem formation in
a non-cell-autonomous manner (Zhao et al. 2004). HAN
expression in vascular tissues and cells separating the
meristem from organ primordial cells controls the number
and correct positioning of WUS-expressing cells (Tucker
and Laux 2007).
ULTRAPETALA1 (ULT1) encodes a SAND-domain
transcription factor that restricts SAM activity by negatively regulating WUS (Carles et al. 2004, 2005). ULT1 and
a close homolog, ULT2, are expressed in embryonic shoot
apical meristems and in developing stamens, carpels and
ovules. The ult1 wus double mutant shows additive phenotypes, and ULT1 is suggested to have WUS-independent
functions in maintaining SAM activity, converging with
the CLV pathway primarily at the point of limiting the
lateral expansion of the WUS-expressing cell population
(Carles et al. 2005).
Mutations in the homeobox gene, STIP/WOX9 reduce
CLV3 and WUS expression in the SAM, resulting in the
wus mutant-like phenotypes. Genetic analysis has revealed
that the loss of STIP function completely suppresses the
clv3 phenotype. On other hand, overexpression of STIP
enhances clv3 mutant phenotypes, implying that STIP is a
positive regulator that functions in the WUS pathway (Wu
et al. 2005).
Conservation of CLV signaling pathway
in various plants
Recent bioinformatics analysis has identified many CLE
genes from various plant species in addition to Arabidopsis. Database searches revealed 15 CLE genes from Glycine
max, 16 from Medicago truncatula, 26 from Populus
trichocarpa, and 47 from Oryza sativa (Kinoshita et al.
2007; Oelkers et al. 2008; Sawa et al. 2008). CLE genes
have also been found in Chlamydomonas reinhardtii and
Physcomitrella patens, suggesting that the CLE peptide
signaling pathway is conserved in plants (Oelkers et al.
2008). Besides plants, the cyst nematode Heterodera glycines has been shown to contain one CLE gene that may
function in the process of infecting host plants (Wang et al.
2001).
Genetic studies of rice and maize have shown that the
CLV signaling pathway is conserved in monocotyledons. In
rice, mutation in either FLORAL ORGAN NUMBER 1
(FON1) or FON2 causes enlarged floral meristems, leading
123
J Plant Res (2009) 122:31–39
to an increase in the number of floral organs (Nagasawa
et al. 1996; Suzaki et al. 2004, 2006; Table 1). FON1
encodes an LRR receptor kinase, which is the rice counterpart of CLV1. FON2 and FON2-LIKE CLE PROTEIN1
(FCP1)/OsCLE402 encode small secretary proteins containing a CLE domain, closely related to CLV3 (Chu et al.
2006; Suzaki et al. 2006, 2008). Unlike FON1 and
FON2, which regulate the maintenance of the flower and
inflorescence meristem, FCP1 appears to regulate the
maintenance of the vegetative SAM and RAM (Suzaki
et al. 2008). These two CLE proteins may have diversified
functions to regulate the different types of meristems in
rice. In maize, the thick tassel dwarf 1 (td1) mutant and the
fasciated ear 2 (fea2) mutant show massive over-proliferation of female inflorescence meristems, resulting in the
fasciated ear. These mutants exhibit the abnormality in
floral meristems and an increased number of floral organs
(Taguchi-Shiobara et al. 2001; Bommert et al. 2005).
Whereas the clv1 mutant shows defects in all aerial meristems, td1 and fea2 do not show any defects in meristem
development in the vegetative phase. Genetic studies show
that TD1 and FEA2 encode orthologous proteins of CLV1
and CLV2, respectively (Taguchi-Shiobara et al. 2001;
Bommert et al. 2005; Table 1). TD1 expression is observed
in leaf primordia and in leaves, but it is absent in vegetative
meristems, and its expression pattern is different from
CLV1 expression in Arabidopsis. In the reproductive stage,
TD1 gene expression is observed in the outer layers of the
inflorescence meristem, whereas, in Arabidopsis, CLV1
gene expression is observed in the inner layers.
These results indicate that the CLV signaling pathways
in meristem maintenance systems are relatively similar but
that there are some diversified molecular mechanisms
between eudicotyledonous and monocotyledonous plants.
In legumes CLV1 orthologs have been identified, such as
HYPERNODULATION ABERRANT ROOT 1 (HAR1) in
Lotus japonicus, NODULE AUTOREGULATION RECEPTOR KINASE (NARK) in soybean, SUPERNUMERIC
NODULES (SUNN) in Medicago truncatula, and SYM29 in
pea (Krusell et al. 2002; Nishimura et al. 2002; Searle et al.
2003; Schnabel et al. 2005; Table 1). Mutations in these
genes induce increased numbers of nodules of root endosymbiosis with soil bacteria, but these mutants do not show
apparent abnormalities in their shoot meristems. This
suggests that, even within eudicotyledons, there is a
divergence of CLV1 function in meristem maintenance and
regulation of nodule formation.
The downstream target of the CLV signaling pathway is
the WUS transcription factor in Arabidopsis. WUS gene
expression is negatively regulated by CLV signaling and
promotes stem cell proliferation in Arabidopsis. So far,
WUS orthologs have been identified in rice (OsWUS),
maize (ZmWUS1 and ZmWUS2), Antirrhinum majus
J Plant Res (2009) 122:31–39
(ROSRATA), Petunia hybrida (TERMINATOR), citrus
(CsWUS), and in Brachypodium distachyon (BdWUS)
(Stuurman et al. 2002; Kieffer et al. 2006; Nardmann and
Werr 2006; Nardmann et al. 2007; Tan and Swain 2007).
The function of WUS in maintaining the SAM appears to
be conserved, at least in eudicotyledons, because both the
Petunia ter mutant and the Antirrhinum roa mutant show
shoot meristem termination, which is similar to that of the
Arabidopsis wus mutant. In contrast, the spatial expression
pattern and ectopic expression of OsWUS suggest that this
gene is not involved in the promotion of stem cell proliferation (Nardmann and Werr 2006; Hirano, personal
communication). In the root, the WUS-type homeobox
gene QHB is highly expressed in the center of the cells in
the RAM in rice (Kamiya et al. 2003). Ectopic expression
of QHB leads to the development of multiple shoots from
ectopic SAMs with malformed leaves, suggesting that the
WUS-type homeobox gene is involved in the specification
and maintenance of stem cells in the RAM, by a mechanism similar to that for WUS in the SAM.
In conclusion, the regulatory mechanisms mediated by
CLE peptides and LRR-RLKs seem conserved in the SAM
and RAM in various eudicotyledonous and monocotyledonous plants. It has been indicated that the transcriptional
regulation of meristem homeostasis by the WOX gene
family is basically conserved in both shoot and root apical
meristems in various vascular plants. However, recent
genetic analyses of rice mutants has also indicated that
there may be species-specific signaling pathways that
control meristem maintenance independently of the CLV/
FON pathway. Further studies by genetic and biochemical
analyses will provide new opportunities to broaden our
understanding of the signaling components involved in
plant meristem maintenance.
Acknowledgment We would like to thank Shigeyuki Betsuyaku
and Yuki Kobayashi for helping us to prepare the figure. We appreciate Hiroyuki Hirano’s critical reading of this manuscript. This work
was supported by the Sumitomo Foundation; the Fuji Foundation; a
grant-in aid for Creative Scientific Research; a grant-in-aid for Young
Scientists (19677001) from the Japan Society for the Promotion of
Science; a grant-in-aid for Scientific Research for Priority Areas from
the Ministry of Education, Culture, Sports, Science (19060009 to
H.F., 20061004, and 19060016) and Technology, and a Program of
Basic Research Activities for Innovative Biosciences from the Biooriented Technology Research Advancement Institution.
References
Bommert P, Lunde C, Nardmann J, Vollbrecht E, Running M,
Jackson D, Hake S, Werr W (2005) thick tassel dwarf1 encodes a
putative maize ortholog of the Arabidopsis CLAVATA1 leucinerich repeat receptor-like kinase. Development 132:1235–1245
Carles CC, Lertpiriyapong K, Reville K, Fletcher JC (2004)
The ULTRAPETALA1 gene functions early in Arabidopsis
37
development to restrict shoot apical meristem activity and acts
through WUSCHEL to regulate floral meristem determinacy.
Genetics 167:1893–1903
Carles CC, Choffnes-Inada D, Reville K, Lertpiriyapong K, Fletcher
JC (2005) ULTRAPETALA1 encodes a SAND domain putative
transcriptional regulator that controls shoot and floral meristem
activity in Arabidopsis. Development 132:897–911
Casamitjana-Martı́nez E, Hofhuis HF, Xu J, Liu CM, Heidstra R,
Scheres B (2003) Root-specific CLE19 overexpression and the
sol1/2 suppressors implicate a CLV-like pathway in the control
of Arabidopsis root meristem maintenance. Curr Biol 13:1435–
1441
Chu H, Qian Q, Liang W, Yin C, Tan H, Yao X, Yuan Z, Yang J,
Huang H, Luo D, Ma H, Zhang D (2006) The FLORAL ORGAN
NUMBER4 gene encoding a putative ortholog of Arabidopsis
CLAVATA3 regulates apical meristem size in rice. Plant Physiol
142:1039–1052
Clark SE, Running MP, Meyerowitz EM (1995) CLAVATA3 is a
specific regulator of shoot and floral meristem development
affecting the same processes as CLAVATA1. Development
121:2057–2067
Clark SE, Williams RW, Meyerowitz EM (1997) The CLAVATA1
gene encodes a putative receptor kinase that controls shoot and
floral meristem size in Arabidopsis. Cell 89:575–585
Cock JM, McCormick S (2001) A large family of genes that share
homology with CLAVATA3. Plant Physiol 126:939–942
Deyoung BJ, Clark SE (2008) BAM receptors regulate stem cell
specification and organ development through complex interactions with CLAVATA signaling. Genetics 180:895–904
DeYoung BJ, Bickle KL, Schrage KJ, Muskett P, Patel K, Clark SE
(2006) The CLAVATA1-related BAM1, BAM2 and BAM3
receptor kinase-like proteins are required for meristem function
in Arabidopsis. Plant J 45:1–16
Diévart A, Dalal M, Tax FE, Lacey AD, Huttly A, Li J, Clark SE
(2003) CLAVATA1 dominant-negative alleles reveal functional
overlap between multiple receptor kinases that regulate meristem
and organ development. Plant Cell 15:1198–1211
Fletcher JC, Brand U, Running MP, Simon R, Meyerowitz EM (1999)
Signaling of cell fate decisions by CLAVATA3 in Arabidopsis
shoot meristems. Science 283:1911–1914
Fukuda H (1997) Tracheary element differentiation. Plant Cell
9:1147–1156
Gallois JL, Nora FR, Mizukami Y, Sablowski R (2004) WUSCHEL
induces shoot stem cell activity and developmental plasticity in
the root meristem. Genes Dev 18:375–380
Han P, Li Q, Zhu YX (2008) Mutation of Arabidopsis BARD1 causes
meristem defects by failing to confine WUSCHEL expression to
the organizing center. Plant Cell 20:1482–1493
Ito Y, Nakanomyo I, Motose H, Iwamoto K, Sawa S, Dohmae N,
Fukuda H (2006) Dodeca-CLE peptides as suppressors of plant
stem cell differentiation. Science 313:842–845
Jeong S, Trotochaud AE, Clark SE (1999) The Arabidopsis CLAVATA2 gene encodes a receptor-like protein required for the
stability of the CLAVATA1 receptor-like kinase. Plant Cell
11:1925–1934
Kamiya N, Nagasaki H, Morikami A, Sato Y, Matsuoka M (2003)
Isolation and characterization of a rice WUSCHEL-type
homeobox gene that is specifically expressed in the central
cells of a quiescent center in the root apical meristem. Plant J
35:429–441
Kieffer M, Stern Y, Cook H, Clerici E, Maulbetsch C, Laux T, Davies
B (2006) Analysis of the transcription factor WUSCHEL and its
functional homologue in Antirrhinum reveals a potential mechanism for their roles in meristem maintenance. Plant Cell
18:560–573
123
38
Kinoshita A, Nakamura Y, Sasaki E, Kyozuka J, Fukuda H, Sawa S
(2007) Gain-of-function phenotypes of chemically synthetic
CLAVATA3/ESR-related (CLE) peptides in Arabidopsis thaliana and Oryza sativa. Plant Cell Physiol 48:1821–1825
Kondo T, Sawa S, Kinoshita A, Mizuno S, Kakimoto T, Fukuda H,
Sakagami Y (2006) A plant peptide encoded by CLV3
identified by in situ MALDI-TOF MS analysis. Science 313:
845–858
Krusell L, Madsen LH, Sato S, Aubert G, Genua A, Szczyglowski K,
Duc G, Kaneko T, Tabata S, de Bruijn F, Pajuelo E, Sandal N,
Stougaard J (2002) Shoot control of root development and
nodulation is mediated by a receptor-like kinase. Nature
420:422–426
Kwiatkowska D (2008) Flowering and apical meristem growth
dynamics. J Exp Bot 59:187–201
Laux T, Mayer KF, Berger J, Jurgens G (1996) The WUSCHEL gene
is required for shoot and floral meristem integrity in Arabidopsis.
Development 122:87–96
Lenhard M, Laux T (2003) Stem cell homeostasis in the Arabidopsis
shoot meristem is regulated by intercellular movement of
CLAVATA3 and its sequestration by CLAVATA1. Development 130:3163–3173
Lenhard M, Jürgens G, Laux T (2002) The WUSCHEL and
SHOOTMERISTEMLESS genes fulfil complementary roles in
Arabidopsis shoot meristem regulation. Development 129:3195–
3206
Mayer KF, Schoof H, Haecker A, Lenhard M, Jurgens G, Laux T
(1998) Role of WUSCHEL in regulating stem cell fate in the
Arabidopsis shoot meristem. Cell 95:805–815
Miwa H, Betsuyaku S, Iwamoto K, Kinoshita A, Fukuda H, Sawa
S (2008) The receptor-like kinase SOL2 mediates CLE
signaling pathway in Arabidopsis. Plant Cell Physiol
49:1752–1757
Müller R, A Bleckmann, Simon R (2008) The receptor kinase
CORYNE of Arabidopsis transmits the stem cell-limiting signal
CLAVATA3 independently of CLAVATA1. Plant Cell 20:934–
946
Nagasawa N, Miyoshi M, Kitano H, Satoh H, Nagato Y (1996)
Mutations associated with floral organ number in rice. Planta
198:627–633
Nardmann J, Werr W (2006) The shoot stem cell niche in
angiosperms: expression patterns of WUS orthologues in rice
and maize imply major modifications in the course of mono- and
dicot evolution. Mol Biol Evol 23:2492–2504
Nardmann J, Zimmermann R, Durantini D, Kranz E, Werr W (2007)
WOX gene phylogeny in Poaceae: a comparative approach
addressing leaf and embryo development. Mol Biol Evol
24:2474–2484
Nishimura R, Hayashi M, Wu GJ, Kouchi H, Imaizumi-Anraku H,
Murakami Y, Kawasaki S, Akao S, Ohmori M, Nagasawa M,
Harada K, Kawaguchi M (2002) HAR1 mediates systemic
regulation of symbiotic organ development. Nature 420:426–
429
Oelkers K, Goffard N, Weiller GF, Gresshoff PM, Mathesius U,
Frickey T (2008) Bioinformatic analysis of the CLE signaling
peptide family. BMC Plant Biol 8:1–15
Ogawa M, Shinohara H, Sakagami Y, Matsubayashi Y (2008)
Arabidopsis CLV3 peptide directly binds CLV1 ectodomain.
Science 319:294
Saiga S, Furumizu C, Yokoyama R, Kurata T, Sato S, Kato T, Tabata
S, Suzuki M, Komeda Y (2008) The Arabidopsis OBERON1 and
OBERON2 genes encode plant homeodomain finger proteins and
are required for apical meristem maintenance. Development
135:1751–1759
Sarkar AK, Luijten M, Miyashima S, Lenhard M, Hashimoto T,
Nakajima K, Scheres B, Heidstra R, Laux T (2007) Conserved
123
J Plant Res (2009) 122:31–39
factors regulate signalling in Arabidopsis thaliana shoot and root
stem cell organizers. Nature 446:811–814
Sawa S, Kinoshita A, Nakanomyo I, Fukuda H (2006) CLV3/ESRrelated (CLE) peptides as intercellular signaling molecules in
plants. Chem Rec 6:303–310
Sawa S, Kinoshita A, Betsuyaku S, Fukuda H (2008) A large family
of genes that share homology with CLE domain in Arabidopsis
and rice. Plant Signal Behav 3:337–339
Schnabel E, Journet EP, de Carvalho-Niebel F, Duc G, Frugoli J
(2005) The Medicago truncatula SUNN gene encodes a CLV1like leucine-rich repeat receptor kinase that regulates nodule
number and root length. Plant Mol Biol 58:809–822
Schoof H, Lenhard M, Haecker A, Mayer KF, Jurgens G, Laux T
(2000) The stem cell population of Arabidopsis shoot meristems
in maintained by a regulatory loop between the CLAVATA and
WUSCHEL genes. Cell 100:635–644
Scofield S, Murray JA (2006) The evolving concept of the meristem.
Plant Mol Biol 60:V–VII
Searle IR, Men AE, Laniya TS, Buzas DM, Iturbe-Ormaetxe I,
Carroll BJ, Gresshoff PM (2003) Long-distance signaling in
nodulation directed by a CLAVATA1-like receptor kinase.
Science 299:109–112
Sharma VK, Ramirez J, Fletcher JC (2003) The Arabidopsis CLV3like (CLE) genes are expressed in diverse tissues and encode
secreted proteins. Plant Mol Biol 51:415–425
Shiu SH, Bleecker AB (2001) Receptor-like kinases from Arabidopsis
form a monophyletic gene family related to animal receptor
kinases. Proc Natl Acad Sci U S A 98:10763–10768
Strabala TJ, O’Donnell PJ, Smit AM, Ampomah-Dwamena C, Martin
EJ, Netzler N, Nieuwenhuizen NJ, Quinn BD, Foote HC, Hudson
KR (2006) Gain-of-function phenotypes of many CLAVATA3/
ESR genes, including four new family members, correlate with
tandem variations in the conserved CLAVATA3/ESR domain.
Plant Physiol 140:1331–1344
Stuurman J, Jaggi F, Kuhlemeier C (2002) Shoot meristem maintenance is controlled by a GRAS-gene mediated signal from
differentiating cells. Genes Dev 16:2213–2218
Suzaki T, Sato M, Ashikari M, Miyoshi M, Nagato Y, Hirano HY
(2004) The gene FLORAL ORGAN NUMBER1 regulates floral
meristem size in rice and encodes a leucine-rich repeat receptor
kinase orthologous to Arabidopsis CLAVATA1. Development
131:5649–5657
Suzaki T, Toriba T, Fujimoto M, Tsutsumi N, Kitano H, Hirano HY
(2006) Conservation and diversification of meristem maintenance mechanism in Oryza sativa: function of the FLORAL
ORGAN NUMBER2 gene. Plant Cell Physiol 47:1591–1602
Suzaki T, Yoshida A, Hirano HY (2008) Functional diversification of
CLAVATA3-related CLE proteins in meristem maintenance in
rice. Plant Cell 20:2049–2058
Taguchi-Shiobara F, Yuan Z, Hake S, Jackson D (2001) The fasciated
ear2 gene encodes a leucine-rich repeat receptor-like protein that
regulates shoot meristem proliferation in maize. Genes Dev
15:2755–2766
Tan FC, Swain SM (2007) Functional characterization of AP3, SOC1
and WUS homologues from citrus (Citrus sinensis). Physiol Plant
131:481–495
Torii KU, Mitsukawa N, Oosumi T, Matsuura Y, Yokoyama R,
Whittier RF, Komeda Y (1996) The Arabidopsis ERECTA gene
encodes a putative receptor protein kinase with extracellular
leucine-rich repeats. Plant Cell 8:735–746
Tucker MR, Laux T (2007) Connecting the paths in plant stem cell
regulation. Trends Cell Biol 17:403–410
von Nägeli C (1858) Beiträge zur Wissenschaftlichen Botanik. Erstes
Heft. von Wilhelm Engelmann, Leipzig
Wang X, Allen R, Ding X, Goellner M, Maier T, de Boer JM, Baum
TJ, Hussey RS, Davis EL (2001) Signal peptide-selection of
J Plant Res (2009) 122:31–39
cDNA cloned directly from the esophageal gland cells of the
soybean cyst nematode Heterodera glycines. Mol Plant Microbe
Interact 14:536–544
Wang G, Ellendorff U, Kemp B, Mansfield JW, Forsyth A, Mitchell
K, Bastas K, Liu CM, Woods-Tör A, Zipfel C, de Wit PJ, Jones
JD, Tör M, Thomma BP (2008) A genome-wide functional
investigation into the roles of receptor-like proteins in Arabidopsis. Plant Physiol 147:503–517
39
Wu X, Dabi T, Weigel D (2005) Requirement of homeobox gene
STIMPY/WOX9 for Arabidopsis meristem growth and maintenance. Curr Biol 15:435–440
Zhao Y, Medrano L, Ohashi K, Fletcher JC, Yu H, Sakai H,
Meyerowitz EM (2004) HANABA TARANU is a GATA
transcription factor that regulates shoot apical meristem and
flower development in Arabidopsis. Plant Cell 16:2586–2600
123