Download Title Regulation of Vascular Development by CLE Peptide

Title
Regulation of Vascular Development by CLE Peptide-receptor Systems
Yuki Hirakawa, Yuki Kondo, and Hiroo Fukuda
Department of Biological Sciences, Graduate School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Corresponding author: Dr. Hiroo Fukuda
Department of Biological Sciences, Graduate School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail [email protected]; FAX 81-3-3812-4929
Running title: Vascular development regulated by CLE peptides/receptors
Abstract
Cell division and differentiation of stem cells are controlled by
non-cell-autonomous signals in higher organisms. The plant vascular meristem is a
stem-cell tissue composed of procambial cells that produce xylem cells on one side
and phloem cells on the other side. Recent studies have revealed that a TDIF
(tracheary element differentiation inhibitory factor)/CLE41/CLE44 peptide signal
controls the procambial cell fate in a non-cell-autonomous manner. TDIF produced
in and secreted from phloem cells is perceived by TDR/PXY, a leucine-rich repeat
receptor kinase located in the plasma membrane of procambial cells. This signal
suppresses xylem cell differentiation of procambial cells and promotes their
proliferation. In addition to TDIF, some other CLE peptides play roles in vascular
development. Here, we summarize recent advances in CLE signaling governing
vascular development.
Key words: CLE peptide; LRR-RLK; stem cell; vascular development
Introduction
Plants have acquired unique morphological features and keen environmental
response systems to adapt to their specific ecological niche. The vascular system, the
long-distance transport system that links every part of the plant body, has also
developed as a component of this adaptive system. In the vascular system, there are two
conducting tissues, the phloem and xylem. The vascular meristem, called the
procambium/cambium, is located between the phloem and xylem, and gives rise to both
phloem and xylem cells in opposite sides (Figure 1).
An important function of the vascular system is the transport of water and nutrients
essential for the growth and development of developing tissues and organs. Vessels and
tracheids in the xylem transport water and ions absorbed from the soil, whereas sieve
tubes in the phloem transport sucrose and amino acids produced in photosynthetic
leaves. Vascular tissues also function as pathways for the delivery of signaling
molecules, such as cytokinins, synthesized in roots and auxin produced in meristems
and young leaves. Thus, the network of the vascular system plays an important role in
the integration of plant growth and development at the whole-plant level.
To form the complex tissues of the vascular system, local communication must
control the behavior of each cell. Plant hormones are known to act as signaling
molecules that mediate in the cell-cell communication pathways. Recently, new players,
small secretory peptides have come into the limelight in this regard.
CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) peptides are central
players of such cell-cell communication in plants (Diévart and Clark 2004;
Matsubayashi and Sakagami 2006; Jun et al. 2007; Fukuda et al. 2007; Fiers et al. 2007;
Butenko et al. 2009). It has now been shown that a tracheary element differentiation
inhibitory factor (TDIF), a CLE peptide, functions as a signaling molecule mediating
vascular cell communication to form well-organized vascular tissues (Ito et al. 2006;
Hirakawa et al. 2008). Here, we review recent advances in cell-cell signaling through
CLE peptides and their receptors in terms of the regulation of vascular tissue
development.
CLE peptides and their signaling pathway
The CLE genes are found extensively in terrestrial plants (Cock and McCormick 2001).
In the Arabidopsis thaliana genome, 32 CLE genes have been identified (Jun et al.
2007) and the rice (Oryza sativa) genome some 47 genes have been annotated
(Kinoshita et al. 2007). CLE family genes encode small proteins of less than 15 kDa
with a signal peptide at the N-terminus and a conserved 14-amino-acid domain, called
the CLE domain, at or near the C-terminus. Functional CLE peptides of CLV3, CLE2
and TDIF are secreted as peptides composed of 12 or 13 amino acids that contain the
last 12 amino acids of the CLE domain; these peptides undergo hydroxylation at two
proline residues (Ito et al. 2006; Kondo et al. 2006; Ohyama et al. 2009). Recently
Ohyama et al. (2009) identified 12-amino-acid CLE2 and 13-amino-acid CLV3 peptides
having three arabinose residues on a hydroxyproline residue as being the most active
forms. These studies were based on culture medium derived from submerged whole
plants that were overexpressing CLE2 or CLV3 genes. In contrast, functional TDIF is
not glycosylated (Ito et al. 2006; Ohyama et al. 2008). Thus, the function of CLE
peptides is regulated through various processes involving transcription, translation,
processing, hydroxylation, and glycosylation (Figure 2).
Genetic analysis has established that CLV3, CLV1 (a leucine-rich repeat
receptor-like kinase, LRR-RLK) and CLV2 (a leucine-rich repeat receptor-like protein,
LRR-RLP) are on the same signaling pathway by which meristem size in the shoot
apical meristem (SAM) is controlled (Clark et al. 1997; Fletcher et al. 1999; Jeong et al.
1999). Indeed, specific binding of functional CLV3 peptides to CLV1 was demonstrated,
in vitro (Ogawa et al. 2008). Therefore, CLV1 is thought to be a receptor for the
functional CLV3 peptide. However, the function of CLV2 in this pathway remains to be
elucidated. This CLV3/CLV1 signaling pathway represses the expression of WUSCHEL
(WUS) which encodes a homeodomain transcription factor essential for stem cell fate in
SAM (Meyer et al. 1998). On the other hand, WUS enhances CLV3 expression. Thus,
SAM size is controlled via a feedback loop between CLV3 and WUS (Brand et al. 2000;
Schoof et al. 2000; Haecker and Laux 2001).
Identification of TDIF and its physiological action
A Zinnia xylogenic culture is an efficient system for analysis of xylem cell
differentiation and has been used to identify various key genes, proteins and signaling
molecules involved in xylem differentiation (Fukuda and Komamine 1980; Demura and
Fukuda 1994; Yamamoto et al. 2001; Ito and Fukuda 2002; Ohashi-Ito and Fukuda
2003; Motose et al. 2004; Fukuda 2004; Yoshida et al. 2005; Ito et al. 2006). In the
culture, isolated Zinnia mesophyll cells transdifferentiate into tracheary elements (TEs),
when cultured in medium containing both naphtaleneacetic acid as an auxin and
benzyladenine as a cytokinin. Ito et al. (2006) found that TE differentiation was
inhibited by extracellular factor(s) in cultured medium of Zinnia cells, and they named
the unknown factor(s) tracheary element differentiation inhibitory factor (TDIF).
This TDIF was present in a 20% methanol fraction taken from the cultured medium.
A bioassay with Zinnia mesophyll cell culture demonstrated that a dodeca-peptide,
which is the product of genes from the CLE family, is TDIF. This peptide consists of 12
amino acids, and first and second proline residues are hydroxylated
(HEVHypSGHypNPISN). The full-length cDNA corresponding to TDIF from Zinnia
cells encodes a protein of 132 amino acids, but only 12 from H120 to N131 match the
TDIF sequence. This result indicates that TDIF is produced through the removal of
residues M1-A119 at the N-terminus and R132 at the C-terminus. Through homology
searches, the TDIF sequence was found to be the same as the Arabidopsis C-terminal
12-amino-acid sequences of CLE41 and CLE44, and highly similar to that of CLE42.
Experiments with synthetic peptides revealed that TDIF functions at as low as 30
pM in the Zinnia xylogenic culture and a dodeca-peptide from CLE42 also functions in
suppression of TE differentiation, but the other dodeca-CLE peptides do not (Ito et al.
2006). TDIF peptide was identified in culture medium derived from submerged culture
of CLE44-overexpressed transgenic plants, which confirmed that CLE44 and probably
CLE41 produce TDIF peptide (Ohyama et al. 2008).
In situ function of TDIF has also been analyzed by addition of chemically
synthesized TDIF to submerged culture of Arabidopsis seedlings (Hirakawa et al. 2008).
TDIF caused discontinuity of xylem vessel strands in rosette leaves, although phloem
and procambium strands were normal even in vessel strand-discontinuous regions
(Figure 3A). This result indicates that TDIF specifically inhibits the differentiation from
procambial cells into TEs. This in situ function of TDIF is confirmed by the fact that
CLE41- and CLE44-overexpressed transgenic plants showed the same vessel
stand-disrupted phenotype as observed with peptide-treated plants.
Surprisingly, TDIF causes an incremental increase in vascular area as well as
production of discontinuous vessel strands. This effect is remarkable in hypocotyls.
Detailed analysis revealed that this increase in vascular area results from the promotion
of procambium cell divisions (Figure 3B). This finding is consistent with the result that
TDIF promotes cell proliferation in the Zinnia xylogenic culture (Ito et al. 2006).
Therefore, TDIF should have two critical roles: first, to inhibit xylem differentiation,
and second to promote procambium proliferation in the vascular system.
Identification of a receptor for TDIF
As aforementioned, CLV1, which is a receptor for CLV3, belongs to the LRR-RLK
family. LRR-RLKs are single trans-membrane protein kinases with extracellular LRR
domains. The LRR-RLK family is a major family of plant receptor kinases, and
Arabidopsis has more than 200 members and rice has nearly 400 members in their
genome (Shiu et al. 2004). Another receptor group, the LRR-RLP, of which CLV2 is a
member, lack the kinase domain of LRR-RLK family. Most LRR-RLPs are categorized
into three super-subclades (Cf-9, RPP27, and PSKR), but CLV2 is not classified into
any of these subclades (Fritz-Laylin et al. 2005). Some LRR-RLK genes have been
implicated in development, disease resistance, and symbiosis, but for most LRR-RLKs,
neither their function nor ligand is known (Morillo and Tax 2006). However, public
database AtGenExpress and gene profiling of differentiating Zinnia cells (Demura et al.
2002) and of Arabidopsis xylogenic cells (Kubo et al. 2005) indicate that a number of
LRR-RLKs are expressed in association with vascular development. This fact suggested
that various LRR-RLKs may be involved in cell-cell communication during vascular
development (Fukuda et al. 2007).
Hirakawa et al. (2008) selected approximately 60 Arabidopsis genes that appeared to
be expressed preferentially in procambium, based on the abovementioned gene
expression data. When the T-DNA insertion mutant lines were treated with TDIF, all
except one line showed fragmented vein patterns. Three different alleles of this line
similarly exhibited TDIF-insensitive phenotypes. The causal gene for these lines
encoded an LRR-RLK belonging to the LRR subclass XI, the same subclass as CLV1
(Shiu and Bleecker 2001), and was named putative TDIF RECEPTOR (TDR). This gene
was also identified as PHLOEM INTERCALATED WITH XYLEM (PXY) in a
forward-genetic analysis (Fisher and Turner 2007).
Further biochemical analysis, using the extracellular domain of TDR expressed in
BY-2 cells, and a radiolabelled photoactivable TDIF derivative, showed the direct and
specific binding of TDIF to TDR. tdr mutants exhibited reduced procambial cell
proliferation and, sometimes, in the hypocotyl xylem vessels were located adjacent to
phloem cells without intervening procambial cells (Figure 3C). This fact can be
explained by the loss of sensitivity to endogenous TDIF in procambial cells.
Collectively, these experiments demonstrated that TDR is a receptor for TDIF.
The TDIF/TDR system involves a member of the CLE peptide ligand family and its
receptor belonging to the subclass XI of the LRR-RLK family. The CLV3/CLV1 system
also consists of a CLE peptide and a member of the LRR-RLK family XI. A
phylogenetic tree derived from sequences of the extracellular region, to which CLE
ligands would bind, displays a clade comprising 7 members, TDR/PXY, CLV1, BAM
(BARELY ANY MERISTEM)1, BAM2, BAM3, PXL (PXY-like)1, and PXL2 among
26 members of the subclass XI (Figure 4A). On the other hand, CLV3 and TDIF are far
among CLE peptides in an unrooted neighbor-joining tree of Arabidopsis dodeca-CLE
sequence (Figure 4B). Therefore the 7 members may be receptors for most of CLE
peptides in Arabidopsis. Indeed, a mature CLE2 peptide with three arabinose residues
binds to CLV1 at almost the efficacy as does the mature CLV3 (Ohyama et al. 2009).
A phloem-xylem crosstalk mediated by the TDIF/TDR signal
Where are TDIF and TDR produced and where do they function? To answer these
questions, the tissue-specific expression patterns of TDIF and TDR genes were analyzed
with promoter–GUS reporter gene fusions (pTDR::GUS, pCLE41::GUS, and
pCLE44::GUS) (Hirakawa et al. 2008). pCLE41::GUS activity was observed along
the vascular strands in all organs examined, and restricted to the phloem and the
neighboring pericycle cells in the root and hypocotyl. pCLE44::GUS expression
occurred in not only cells in the phloem, and the adjacent pericycle, but also in
endodermal cells. These findings indicated that TDIF is mainly produced in phloem
cells from where it is secreted.
Immunohistochemical analysis, using an anti-TDIF antibody, also revealed the
localization of mature TDIF peptide in the apoplasm surrounding two phloem precursor
cells (Figure 5). In contrast, pTDR::GUS was expressed specifically in the procambium
in the root and hypocotyl. This result is consistent with procambial cell-specific
expression of TDR mRNA in xylogenic cultures (Kubo et al. 2005). Based on these
findings, we propose that TDIF functions in a non-cell-autonomous manner (Figure 6A).
TDIF is produced in and secreted from phloem cells and perceived by TDR located in
the plasma membrane of procambial cells. This signal enhances division of procambial
cells and suppresses the differentiation of procambial cells into xylem cells. This means
that there must be crosstalk between phloem and xylem formation, mediated by a
phloem-derived xylem-differentiation inhibitor, TDIF.
TDIF might make a gradient from the phloem to the xylem via the procambium
(Figure 6B). This gradient might act as positional information for the control of cell
division and differentiation within the procambium. That is, procambial cells near the
phloem perceive a relatively high concentration of TDIF and are maintained to be
undifferentiated and in a proliferative state. A greater distances from the phloem, TDIF
signal becomes weak, which allows procambial cells to acquire the potential to
differentiate into xylem cells. Indeed, exogenously supplied TDIF causes not only
procambial cell proliferation but also ectopic xylem cells (Hirakawa et al. 2008). This
might result from a rapid decrease in the TDIF gradient resulting from an increase in the
number of intervening procambial cells.
Other CLE peptides that control vascular development
Of the 32 CLE genes that have been identified in Arabidopsis, most genes have yet to be
characterized in terms of function. Eighteen CLE genes were overexpressed and their
gain-of-function phenotypes were observed (Strabala et al. 2006). The overexpression
of 11 genes, including CLV3, showed a growth arrest phenotype at the level of the SAM
equivalent to the wus mutant, suggesting similar functions for these CLE peptides. The
redundant function of CLE peptides has also been indicated by rescue of the clv3
phenotype by various CLE genes (Hobe et al. 2003; Ni and Clark 2006). In contrast,
CLE42- and CLE44-overexpression produced shrub-like dwarf plants that lacked apical
dominance, whose phenotype was not observed in the other CLE-overexpressing plants.
Therefore, this phenotype might be directly or indirectly related to vascular
development. Finally, there may be CLE peptides other than CLE41/CLE44 and CLE42,
that are involved in vascular development.
Whitford et al. (2008) reported that overexpression of some CLE genes (CLE6,
CLV3 and CLE19), although they could not affect vascular development alone,
cooperate with TDIF to enhance procambium proliferation. These CLE genes are known
to reduce the size of the SAM and root apical meristem (RAM). In clv1 and clv2
mutants, these CLE peptides do not cause a reduction in meristem size, but they can
enhance TDIF-induced procambium proliferation. These results establish that CLE6,
CLV3 and CLE19 peptides cooperate with TDIF to enhance procambial cell
proliferation through receptors different from CLV1 and CLV2. Therefore, it seems
reasonable to suggest that one CLE ligand may bind to a range of receptors.
Furthermore, based on results from competition analysis with mature CLV3 and CLE2
peptides, a particular LRR-RLK may well recognize some different CLE ligands
(Ohyama et al., 2009).
An earlier report indicated that, in Arabidopsis, transcripts for some of the CLE
genes (CLE10, CLE26, CLE27) increase during in vitro xylogenesis (Kubo et al. 2005).
Brassica napus CLE19 gene is expressed in a few pericycle cells facing the protoxylem
poles as well as in organ primordia (Fiers et al. 2004). We have also found that various
CLE genes are preferentially expressed in vascular tissues and that some CLE peptides
function at different stages of procambium and xylem development (Kondo,
unpublished results). Therefore, CLE peptides other than TDIF may play roles in
vascular development. Sophisticated regulation by various CLE ligands and receptors
may control the well-organized formation of vascular tissues.
Intracellular signal transduction from CLE peptide / receptor system
Recent studies have revealed that a novel receptor-like kinase, CRN/SOL2, is involved
in meristem maintenance, thus indicatings that receptor-like kinases other than CLV1
can function in CLE signaling (Müller et al. 2008; Miwa et al. 2008). This extracellular
domain-lacked kinase may function with the kinase domain-lacked CLV2 through an
interaction via their transmembrane domains (Müller et al. 2008). This finding suggests
that the stem cell population in the SAM may be regulated by a complicated signaling
perception pathway through the numerous receptors, including CLV1, CLV2 and
CRN/SOL2. Also, in vascular development, numerous receptors as well as several CLE
peptides may produce a complex cell-cell communication network (Figure 7).
The intracellular signaling pathway downstream of CLE signals is partly resolved in
the SAM. Analysis using a transient CLV3 induction system has shown that the CLV3
signal rapidly, but transiently, represses WUS expression, which after a few hours leads
to a reduction of endogenous CLV3 expression (Müller et al. 2006). The signaling
pathway between perception of CLV3 and control of WUS expression is still largely
unknown (Carles and Fletcher 2003). In contrast, a target of WUS has been reported.
WUS directly represses the expression of Type-A ARR (Arabidopsis Response
Regulators) genes which are negative regulators of cytokinin signaling and act as
positive regulators of SAM maintenance (Leibfried et al. 2005). Recently, it has also
been established that localized perception of cytokinin regulates the expression of WUS
(Gordon et al. 2009). Hence, the CLV3 and cytokinin signals cooperate to control SAM
maintenance.
In the vascular system, the CLE peptide-enhanced procambial proliferation is
positively regulated by auxin, and negatively regulated by NPA (1-naphtylphthalamic
acid) which is an inhibitor of auxin transport (Whitford et al. 2008). This result suggests
that auxin is necessary for enhancement of procambium proliferation by CLE peptides.
Cellular proliferation in the procambium/cambium is also controlled by cytokinin and
its signaling mediators in Arabidopsis and Poplar (Mähönen et al. 2006;
Matsumoto-kitano et al. 2008; Nieminen et al. 2008; Hejátko et al. 2009). These
findings suggest that, in general, CLE peptides may regulate the fate of stem cells
through crosstalk with signaling pathways mediated by other plant hormones, such as
auxin and cytokinin.
Future perspectives
First and foremost, the relationship between various CLE peptides and their
receptors should be clarified biochemically. Another important target for future research
is the downstream signaling pathway after the perception of the CLE peptides. This will
involve identifying the intracellular phosphor-relay machinery as well as the key
transcription factor(s) that function downstream of TDIF/TDR. Here, it is important to
reiterate that TDIF/TDR acts at two different events; promotion of procambial cell
division and suppression of xylem differentiation. It will be interesting to learn whether
a single or two separate downstream signaling pathways are responsible for these two
events. Crosstalk between the CLE peptide and plant hormones is also essential. It will
be a challenge to identify the molecular mechanism by which these hormonal signals
are integrated to control behavior at the single cell level. Regarding production of CLE
peptides, it will be crucial to identify the enzymes responsible for cleavage of the
peptide precursors and for modification of amino acid residues, such as
proline-hydroxylation and glycosylation.
Finally, phloem controls xylem development through TDIF. However, this system is
unbalanced. Intercellular signaling derived from the xylem should also control phloem
development. Identification of such xylem-derived signals involved in controling
phloem development will further clarify our overall understanding of vascular
organization.
Acknowledgements
This work was supported in part by Grants-in-Aid from the Ministry of Education,
Science, Sports and Culture of Japan (19060009) to HF, from the Japan Society for the
Promotion of Science (20247003 to HF, JSPS Research Fellowships for Young
Scientists to YH), and from Bio-oriented Technology Research Advancement Institution
(BRAIN) to HF.
References
Brand U, Fletcher JC, Hobe M, Meyerowitz EM, Simon R (2000). Dependence of
stem cell fate in Arabidopsis on a feedback loop regulated by CLV3 activity. Science
289, 617-619.
Butenko MA, Vie AK, Brembu T, Aalen RB, Bones AM (2009). Plant peptides in
signalling: looking for new partners. Trends Plant Sci. 14, 255-263.
Carles CC, Fletcher JC (2003). Shoot apical meristem maintenance: the art of a
dynamic balance. Trends Plant Sci. 8, 394-401.
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.
Demura T, Fukuda H (1994). Novel vascular cell-specific genes whose expression is
regulated temporally and spatially during vascular system development. Plant Cell 6,
967-981.
Demura T, Tashiro G, Horiguchi G, Kishimoto N, Kubo M, Matsuoka N et al.
(2002). Visualization by comprehensive microarray analysis of gene expression
programs during transdifferentiation of mesophyll cells into xylem cells. Proc. Natl.
Acad. Sci. U.S.A. 99, 15794-15799.
Diévart A, Clark SE (2004). LRR-containing receptors regulating plant development
and defense. Development 131, 251–261.
Fiers M, Hause G, Boutilier K, Casamitjana-Martinez E, Weijers D, Offringa R et
al. (2004). Mis-expression of the CLV3/ESR-like gene CLE19 in Arabidopsis leads to a
consumption of root meristem. Gene 327, 37-49.
Fiers M, Ku LK, Liu CM (2007). CLE peptide ligands and their roles in establishing
meristems. Curr. Opin. Plant Biol. 10, 39-43.
Fisher K, Turner S (2007). PXY, a receptor-like kinase essential for maintaining
polarity during plant vascular-tissue development. Curr. Biol. 17, 1061-1066.
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.
Fritz-Laylin LK, Krishnamurthy N, Tör M, Sjölander KV, Jones JD (2005).
Phylogenomic analysis of the receptor-like proteins of rice and Arabidopsis. Plant
Physiol. 138, 611-623.
Fukuda H, Komamine A (1980). Establishment of an Experimental System for the
Study of Tracheary Element Differentiation from Single Cells Isolated from the
Mesophyll of Zinnia elegans. Plant Physiol. 65, 57-60.
Fukuda H (2004). Signals that control plant vascular cell differentiation. Nat. Rev. Mol.
Cell Biol. 5, 379-391.
Fukuda H, Hirakawa Y, Sawa S (2007). Peptide signaling in vascular development.
Curr. Opin. Plant Biol. 10, 477-482.
Gordon SP, Chickarmane VS, Ohno C, Meyerowitz EM (2009). Multiple feedback
loops through cytokinin signaling control stem cell number within the Arabidopsis
shoot meristem. Proc. Natl. Acad. Sci. U.S.A. 106, 16529-16534.
Haecker A, Laux T (2001). Cell-cell signaling in the shoot meristem. Curr. Opin. Plant
Biol. 4, 441-446.
Hejátko J, Ryu H, Kim GT, Dobesová R, Choi S, Choi SM et al. (2009). The
histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS HISTIDINE
KINASE2 and 3 regulate vascular tissue development in Arabidopsis shoots. Plant Cell
21, 2008-2021.
Hirakawa Y, Shinohara H, Kondo Y, Inoue A, Nakanomyo I, Ogawa M et al. (2008).
Non-cell-autonomous control of vascular stem cell fate by a CLE peptide/receptor
system. Proc. Natl. Acad. Sci. U.S.A. 105, 15208-15213.
Hobe M, Müller R, Grünewald M, Brand U, Simon R (2003). Loss of CLE40, a
protein functionally equivalent to the stem cell restricting signal CLV3, enhances root
waving in Arabidopsis. Dev. Genes Evol. 213, 371-381.
Ito J, Fukuda H (2002). ZEN1 is a key enzyme in the degradation of nuclear DNA
during programmed cell death of tracheary elements. Plant Cell 14, 3201-3211.
Ito Y, Nakanomyo I, Motose H, Iwamoto K, Sawa S, Dohmae N et al. (2006).
Dodeca-CLE peptides as suppressors of plant stem cell differentiation. Science 313,
842-845.
Jun JH, Fiume E, Fletcher JC (2008). The CLE family of plant polypeptide signaling
molecules. Cell. Mol. Life Sci. 65, 743-755.
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.
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 et al. (2006). A
plant peptide encoded by CLV3 identified by in situ MALDI-TOF MS analysis. Science
313, 845-848.
Kubo M, Udagawa M, Nishikubo N, Horiguchi G, Yamaguchi M, Ito J et al. (2005).
Transcription switches for protoxylem and metaxylem vessel formation. Genes Dev. 19,
1855-1860.
Leibfried A, To JP, Busch W, Stehling S, Kehle A, Demar M et al. (2005).
WUSCHEL controls meristem function by direct regulation of cytokinin-inducible
response regulators. Nature 438, 1172-1175.
Matsubayashi Y, Sakagami Y (2006). Peptide hormones in plants. Annual review of
plant biology 57, 649-674.
Matsumoto-Kitano M, Kusumoto T, Tarkowski P, Kinoshita-Tsujimura K,
Václavíková K, Miyawaki K et al. (2008). Cytokinins are central regulators of cambial
activity. Proc. Natl. Acad. Sci. U.S.A. 105, 20027-20031.
Mayer KF, Schoof H, Haecker A, Lenhard M, Jürgens 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 in Arabidopsis. Plant Cell Physiol.
49, 1752-1757.
Morillo SA, Tax FE (2006). Functional analysis of receptor-like kinases in monocots
and dicots. Curr. Opin. Plant Biol. 9, 460-469.
Motose H, Sugiyama M, Fukuda H (2004). A proteoglycan mediates inductive
interaction during plant vascular development. Nature 429, 873-878.
Mähönen AP, Bishopp A, Higuchi M, Nieminen KM, Kinoshita K, Törmäkangas K
et al. (2006). Cytokinin signaling and its inhibitor AHP6 regulate cell fate during
vascular development. Science 311, 94-98.
Müller R, Borghi L, Kwiatkowska D, Laufs P, Simon R (2006). Dynamic and
compensatory responses of Arabidopsis shoot and floral meristems to CLV3 signaling.
Plant Cell 18, 1188-1198.
Müller R, Bleckmann A, Simon R (2008). The receptor kinase CORYNE of
Arabidopsis transmits the stem cell-limiting signal CLAVATA3 independently of
CLAVATA1. Plant Cell 20, 934-946.
Ni J, Clark SE (2006). Evidence for functional conservation, sufficiency, and
proteolytic processing of the CLAVATA3 CLE domain. Plant Physiol. 140, 726-733.
Nieminen K, Immanen J, Laxell M, Kauppinen L, Tarkowski P, Dolezal K et al.
(2008). Cytokinin signaling regulates cambial development in poplar. Proc. Natl. Acad.
Sci. U.S.A. 105, 20032-20037.
Ogawa M, Shinohara H, Sakagami Y, Matsubayashi Y (2008). Arabidopsis CLV3
peptide directly binds CLV1 ectodomain. Science 319, 294.
Ohashi-Ito K, Fukuda H (2003). HD-zip III homeobox genes that include a novel
member, ZeHB-13 (Zinnia)/ATHB-15 (Arabidopsis), are involved in procambium and
xylem cell differentiation. Plant Cell Physiol. 44, 1350-1358.
Ohyama K, Ogawa M, Matsubayashi Y (2008). Identification of a biologically active,
small, secreted peptide in Arabidopsis by in silico gene screening, followed by
LC-MS-based structure analysis. Plant J. 55, 152-160.
Ohyama K, Shinohara H, Ogawa-Ohnishi M, Matsubayashi Y (2009). A
glycopeptide regulating stem cell fate in Arabidopsis thaliana. Nat. Chem. Biol. 5,
578-580.
Schoof H, Lenhard M, Haecker A, Mayer KF, Jürgens 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.
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.
Shiu SH, Karlowski WM, Pan R, Tzeng YH, Mayer KF, Li WH (2004).
Comparative analysis of the receptor-like kinase family in Arabidopsis and rice. Plant
Cell 16, 1220-1234.
Strabala TJ, O'donnell PJ, Smit AM, Ampomah-Dwamena C, Martin EJ, Netzler
N et al. (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.
Whitford R, Fernandez A, De Groodt R, Ortega E, Hilson P (2008). Plant CLE
peptides from two distinct functional classes synergistically induce division of vascular
cells. Proc. Natl. Acad. Sci. U.S.A. 105, 18625-18630.
Yamamoto R, Fujioka S, Demura T, Takatsuto S, Yoshida S, Fukuda H (2001).
Brassinosteroid levels increase drastically prior to morphogenesis of tracheary elements.
Plant Physiol. 125, 556-563.
Yoshida S, Kuriyama H, Fukuda H (2005). Inhibition of transdifferentiation into
tracheary elements by polar auxin transport inhibitors through intracellular auxin
depletion. Plant Cell Physiol. 46, 2019-2028.
Figure legends
Figure 1. Radial pattern of the plant vascular system.
(A) A vascular bundle is composed of the phloem (green), the procambium (yellow),
and the xylem, which has tracheary elements (blue) and xylem parenchyma cells (red).
(Reproduced from Fukuda 2004, with permission.)
(B) Intervening procambial cells proliferate through periclinal cell divisions to give rise,
in opposite directions, to phloem and xylem cells.
Figure 2. Biosynthetic pathway for a CLE peptide.
The CLE gene encodes a small protein, which contains the N-terminal signal peptide (sp,
grey) and the conserved CLE domain at or near the C-terminus (CLE, blue). Translated
prepro-CLE protein is modified through processing, hydroxylation, and glycosylation to
give rise to a mature CLE peptide.
Figure 3. Roles played by TDIF and TDR in Arabidopsis vascular development.
(A) Exogenous application of TDIF results in discontinuity of xylem vessel strands
(blue-outline), but not of phloem (yellow) or procambium (red-outline) strands in
Arabidopsis rosette leaves.
(B) Exogenous application of TDIF promotes procambial cell proliferation in the
hypocotyl vascular bundle. Red dots indicate procambial cells.
(C) Three different mutant alleles of TDR/PXY, which encodes a single-transmembrane
LRR-RLK, show reduced procambial cell proliferation and xylem cells unusually
adjacent to phloem cells in vascular bundles. Ph and Xy indicate phloem and xylem
cells, respectively. (Reproduced from Hirakawa et al. 2008, with permission.)
Figure 4. Phylogenetic trees of the LRR-RLK XI and CLE peptide families
(A) Extracellular domains of LRR-RLK subclass XI genes were used to construct the
tree. CLV1 and TDR belong to a clade composed of seven members. (Reproduced from
Hirakawa et al. 2008, with permission.)
(B) Twelve-amino-acid CLE sequences corresponding to the TDIF peptide sequence
were used to construct the tree. (Reproduced from Ito et al. 2006, with permission.)
Figure 5. Localization of TDIF to the apoplasm of procambial cells.
Immunohistochemical stain using an anti-TDIF antibody showed the localization of
mature TDIF peptide in the apoplasm surrounding two phloem precursor cells (Ph). Pc
and Xy (pink-outline) indicates procambial cells and xylem precursor cells, respectively.
(Reproduced from Hirakawa et al. 2008, with permission.)
Figure 6. Models depicting the non-cell-autonomous TDIF/TDR signaling pathway.
(A) TDIF secreted from phloem cells (green) is perceived through the TDR/PXY
receptor on procambial cells (yellow). This non-cell-autonomous signal promotes cell
division of procambial cells and suppresses differentiation of the procambial cells into
xylem cells (red).
(B) A concentration gradient of TDIF from the phloem (Ph) towards the xylem (Xy),
through the procambium (Pc), is proposed to act as positional information for xylem
specification and stem-cell maintenance.
Figure 7. A model of the CLE peptide signal transduction pathway.
A number of CLE peptide/LRR receptor pairs may produce a complex cell-cell
communication network. An Intracellular signaling pathway(s) may integrate these
signals, in some case even with plant hormonal signals, to control the cell fate and tissue
formation.
A
B
Phloem cells Procambial cells Xylem cells
CLE gene
Hydoxylation
Glycosylation
Processing
CLE peptide
OH
OH
sp
CLE domain
A
C
LRR
sp
tm
kinase
tdr-3
tdr-1 tdr-2
tdr-1
B
control
Ph
Xy
+TDIF
Ph
Xy
Ph
Xy
Ph
A
B
At2g26330
CLE11
CLE13
CLE12
CLE8
At4g20140
1000
At5g44700
At1g17230
1000
At2g33170
1000
CLE27
CLE25
CLE26
At5g63930
At1g34110
997
At3g24240
1000
758
1000
At4g26540
96
At5g56040
CLE40
(TDR/PXY)
At1g08590
(PXL1)
At4g28650 (PXL2)
At1g75820 (CLV1)
At4g20270 (BAM3)
At3g49670 (BAM2)
1000
At5g65700 (BAM1)
At5g61480
884
1000
968
681
999
494
863
CLE45
CLV3
At5g48940
990
96
CLE14
CLE9/10
CLE46
88
At5g49660
475
851
1000
994
CLE22
At1g09970
At3g19700
59
At5g65710
967
At1g28440
971
At4g28490
1000
0.1
CLE42
CLE41/44
80
At1g72180
819
68
CLE1/3/4
CLE2
CLE5/6
CLE7
At1g17750
At1g73080
0.1
CLE20
CLE16
CLE17
CLE18
CLE19
CLE21
TDIF
TDIF
Ph
Pc
Xy
Pc
Ph
A
TDIF
Phloem Cells
TDIF
B
Ph
Pc
Pc
Pc
Xy
differentiation
differentiation
Procambial Cells
with TDR/PXY
division
division
differentiation
High
Xylem Cells
Low
TDIF conc.
CLE peptides
LRR receptors
OH
OH
OH
OH
Plant hormones
Target TFs
Cell differentiation, proliferation
Vascular development