Download Making inroads into plant receptor kinase signalling pathways

Review
TRENDS in Plant Science
Vol.8 No.5 May 2003
231
Making inroads into plant receptor
kinase signalling pathways
Gabrielle Tichtinsky1, Vincent Vanoosthuyse2, J. Mark Cock3 and Thierry Gaude1
1
Reproduction et Développement des Plantes, UMR 5667 CNRS-INRA-ENSL-UCBL, École Normale Supérieure de Lyon,
46 allée d’Italie, F-69364 Lyon Cedex 07, France
2
Swann 508, ICMB, University of Edinburgh, King’s buildings, Mayfield Road, Edinburgh, UK EH9 3JR
3
UMR 1931 CNRS-Goëmar, Station Biologique, Place Georges Teissier, BP74, 29682 Roscoff Cedex, France
Cell-membrane-located receptor kinases play important
roles in many plant signal-transduction pathways.
Exciting progress has been made in recent years with
the characterization of four ligand–receptor systems
involved in physiological processes as diverse as selfpollen rejection, stem-cell maintenance and differentiation at the shoot meristem, the response to the
brassinosteroid hormones and the innate response to
bacterial pathogens. These new findings emphasize the
remarkably high diversity of these signalling pathways,
although some downstream components are shared.
This observation supports the idea that the wide diversification of plant receptors is associated with a high
degree of specialization, one receptor potentially regulating a single developmental process. However, the
possibility that one receptor might have a dual recognition function cannot be ruled out.
The plant receptor kinase (PRK, see Glossary), formally,
receptor-like kinase [1], gene family constitutes a large,
monophyletic group that includes .400 genes in Arabidopsis. These genes encode typical single-pass, membrane-anchored receptor kinases, with an extracellular
N-terminal domain and an intracellular C-terminal kinase
domain [2]. Although PRKs are less well characterized
than their animal counterparts, there appear to be many
features common to receptor signalling pathways in the
two kingdoms [1].
The leucine-rich repeat (LRR) subfamily of receptors
(Table 1) represents a large proportion of the PRKs
characterized so far. This family includes CLAVATA1
(CLV1), which controls stem-cell maintenance in apical
meristems [3], the brassinosteroid-insensitive 1 (BRI1)
receptor and its co-receptor BRI1-associated receptor
kinase 1 (BAK1 ), which mediate the response to brassinosteroid (BR ) hormones [4 – 6], and the flagellin-sensing 2
(FLS2) receptor, which mediates the innate response to
bacterial pathogens [7]. The S-locus receptor kinase (SRK)
is the female determinant of self-incompatibility in
Brassicaceae [8]. It belongs to the S-domain subfamily of
PRKs (Table 1). Exciting progress has been made in recent
years with the identification of specific ligands and
characterization of components of the downstream
signalling pathways of these four regulators. In this
article, we compare and contrast these four signalling
systems.
Nature of the cognate ligands
Most of the identified ligands for PRKs are peptides
(Table 1). S-locus cysteine-rich protein (SCR), for example,
is the male determinant of self-incompatibility in the
Brassicaceae. It encodes the SRK ligand, a highly
polymorphic secreted peptide [9,10]. Interestingly, a
search of the Arabidopsis genome revealed the presence
of at least 114 genes that are predicted to encode peptides
with a similar primary structure to SCR [11]. It is tempting
to postulate that some of these genes might encode ligands
for other members of the S-domain PRK subfamily [11].
CLAVATA3 (CLV3), the CLV1 ligand, is also a secreted
peptide but it shares no sequence similarity with SCR
[12,13]. As for plant defences, flagellin, a major protein
constituent of bacterial flagella, elicits plant responses
efficiently. Its most conserved peptide, flg22 , is sufficient to
elicit these responses [7,14]. Several other peptides, such
as AtGRP-3, systemin, phytosulfokine and Lat52, have
also been identified as putative PRK ligands [15 – 18].
Nevertheless, not all putative PRK ligands are peptides:
BRI1 and BAK1 mediate the response to plant steroid
Glossary
ARC1: Armadillo-repeat-containing 1
BAK1: BRI1-associated receptor kinase 1
BES2: BRI-EMS-suppressor 2
BIN2: BR-insensitive 2 kinase
BL: brassinolide
BR: brassinosteroid
BRI1: brassinosteroid-insensitive 1 receptor
BZR1: brassinazole-resistant 1
EGF: epidermal growth factor
EGFR: EGF receptor
eSRK: extracellular domain of SRK
flg22: flagellin 22
FLS2: flagellin-sensing 2
KAPP: kinase-associated protein phosphatase
LRR: leucine-rich repeat
MAPK: mitogen-activated protein kinase
PRK: plant receptor kinase
Rop: Rho-GTPase-related protein
SCR: S-locus cysteine rich
SLG: S-locus glycoprotein
SRK: S-locus receptor kinase
THL: thioredoxin-h-like
Corresponding author: Thierry Gaude ([email protected]).
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Table 1. Characteristics of the four ligand –receptor systems analysed in this article
Physiological process
Receptor
Cognate ligand
Refs
Name
Subfamilya
Name
Nature
Molecular massb
Stem-cell maintenance in apical meristems
CLV1
LRR-PRK
CLV3
Secreted peptide
Response to the brassinosteroid hormones
Innate response to bacterial pathogens
Self-incompatibility
BRI1, BAK1
FLS2
SRK
LRR-PRK
LRR-PRK
S-domain
receptor
kinases
BR, BL
Flagellin
SCR
Steroid hormones
Bacterial protein
Secreted peptide,
basic and cysteine-rich
Predicted: 8.7 kDa;
observed: 25.0 kDa
480 Da
33.0 kDa
Predicted: 5.7 –8.0 kDa;
observed: 16.0 kDa
[3,12,13]
[4,6,19]
[7,30]
[8,9,62]
Abbreviations: BAK1, BRI1-associated receptor kinase 1; BL, brassinolide; BR, brassinosteroids; BRI1, brassinosteroid-insensitive 1; CLV1, CLAVATA 1; CLV3, CLAVATA 3;
FLS2, flagellin-sensing 2; LRR-PRK, leucine-rich repeat; SCR, S-locus cysteine rich; SRK, S-locus receptor kinase.
a
The subfamilies of plant receptor kinases differ in their N-terminal extracellular domains. The LRR-PRK subfamily has an extracellular domain consisting of tandemly
repeated LRRs, flanked in most cases by two conservatively spaced cysteine pairs. The S-domain subfamily is characterized by an extracellular domain similar to the S (selfincompatibility) locus glycoprotein [36].
b
Predicted molecular mass is based on the deduced amino acid sequence. The range of molecular masses indicated for SCR refers to the many allelic forms of this protein.
Observed molecular masses might correspond to dimers or complexes of ligands.
hormones (brassinosteroids), particularly brassinolide
(BL), and the bacterial Nod factors, which are lipochitin
oligosaccharides essential for symbiotic nodule development in leguminous plants, might be sensed by a LRR
PRK, the nodulation receptor kinase NORK [5,6,19,20].
Ligand –receptor binding
The analysis of the ligand-binding properties at the
cellular level provides information about the functional
sites in vivo. However, the identification of the biochemical
nature of these functional sites (i.e. whether they are
composed of a single PRK or of homo- or heterodimers, or of
bigger protein complexes) remains in most cases a major
challenge.
Functional binding sites for flg22, BL and SCR have
been characterized at the cellular level (Table 2). Binding
of 125I-labelled flg22 to suspension-cultured cells of tomato
or Arabidopsis is rapid (maximal binding reached within
15 – 20 min), stable (over at least 90 min) and irreversible
under physiological conditions [21,22]. The high stability
of the interaction might be due to either a low rate of
dissociation or stabilization of the complex. Investigation
of the cellular fate of ligand– receptor complexes after
ligand perception should provide some insight into this
putative stabilization process. In contrast to flg22 and BL
binding (Table 2), 125I-labelled SCR binding to stigma
microsomal membranes revealed two types of binding site:
an abundant low-affinity site and a less-abundant highaffinity site [10]. Interestingly, a similar feature has been
described when the animal epidermal growth factor (EGF)
binds to its receptor (EGFR), ErbB1 [23,24].
Direct protein interaction between SCR and SRK has
been demonstrated in vitro using SCR produced in the
periplasmic space of Escherichia coli and the extracellular
domain of SRK (eSRK) produced in Nicotiana benthamiana
leaves [25]. This physical interaction was confirmed using
an in vitro binding and immunoprecipitation assay in
which synthetic radioactive-iodinated SCR was mixed
with stigma microsomal membranes containing SRK [10].
Interaction between CLV3 and CLV1, and BL and BRI1
was detected in vivo by co-immunoprecipitation from plant
extracts [26,27]. These observations do not rule out the
possibility that the interaction between the ligand and its
receptor might require additional partners. For example, a
dominant mutation, brs1, was shown to suppress weak
alleles of bri1 in genetic backgrounds in which normal
levels of brassinosteroids are produced [28]. Because BRS1
encodes a secreted carboxypeptidase, it was suggested that
BRS1 processes either the receptor itself or, more probably,
a proligand, such as a steroid-binding protein [28].
Recently, genetic data indicated that BRI1 cooperates
with BAK1 to transduce the BL signal. Both proteins also
interact in vitro and in vivo [5,6]. In previous work, when a
chimeric receptor composed of the extracellular and
transmembrane domains of BRI1 fused to the intracellular
kinase domain of Xa21 (an LRR PRK involved in the
defence response) was produced in rice cells, treatment
with BL triggered cellular defence responses. This
indicates that the extracellular and transmembrane
domains of BRI1 can recognize BL and transduce this
signal to a heterologous kinase domain [29]. Whether
BAK1 is required for BL binding is unclear. It could simply
be necessary to initiate downstream signalling leading to
the BL response. However, an endogenous rice homologue
of BAK1 might be present in the cells, allowing BL binding.
Further work is required to distinguish between these
alternatives.
Binding of flagellin or of flg22 to purified FLS2 has not
yet been demonstrated. Nevertheless, binding of flg22 to
crude plant extracts is much reduced in fls2 mutants and
binding is restored following complementation with a wildtype copy of FLS2 [30]. These data indicate that FLS2 is a
Table 2. Ligand-binding properties at the cellular level
Ligand
Binding system
Binding capacitya
Dissociation constant (Kd)
Refs
125
Suspension-cultured cells
Microsomal membranes
Microsomal membranes
2.00 pmol mg21
0.23 pmol mg21
1.00 pmol mg21
0.25 pmol mg21
1.3 –1.6 nM
7.4 –10.8 nM
32.0 nM (low-affinity site)
1.2 nM (high-affinity site)
[22]
[27]
[10]
I-flg22
3
H-BL
125
I-SCR
a
Expressed as pmol of bound ligand per mg of microsomal proteins.
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Review
TRENDS in Plant Science
crucial component of the membrane-located flg22 binding
site. This assumption is supported by an analysis of the
Arabidopsis ecotype Ws-0, which is insensitive to flg22.
Specific binding of flg22 to leaf microsomal membranes is
greatly reduced in Ws-0 compared with ecotype Landsberg
erecta [21]. The locus responsible for this difference
between ecotypes, originally referred to as FLS1, has
recently been shown to be identical to FLS2 [31]. In
support of these genetic data, affinity cross-linking of flg22
to microsomal membrane fractions allowed the identification of a single protein of ,115 kDa, which is the
predicted size of FLS2 [22].
Receptor complexes
When analysed under native conditions, PRKs are usually
associated with other proteins. Thus, two protein
complexes including CLV1 were detected in cauliflower
extracts [26,32]. The smaller one (185 kDa) includes a
second protein linked by one or more disulfide bonds.
CLV2, a gene acting in the CLV1– CLV3 pathway, encodes
an LRR-receptor-like protein resembling CLV1 but with
only a short cytoplasmic tail and no intracellular kinase
domain [33,34]. In clv2 mutants, the stability of CLV1 is
reduced [3], whereas the 185 kDa complex is partially
disrupted in a clv1 mutant in which the LRR structure is
altered [32]. Therefore, CLV1 and CLV2 probably interact
through their LRR domains to form the 185 kDa complex
(Fig. 1). CLV3 is not detected in the 185 kDa complex but
is present in a bigger, 450 kDa complex that also
contains kinase-associated protein phosphatase (KAPP)
and Rho-GTPase-related proteins (Rop) [26,32]. Analysis
of CLV1 complexes in mutant backgrounds revealed
that a functional CLV3 allele and the kinase activity
of CLV1 are required for assembly of the 450 kDa
complex [26]. Thus, the 450 kDa complex is probably the
active ligand– receptor complex (Fig. 1).
Like CLV1, SRK was found to form protein complexes
in vivo. In the basal state (i.e. in unpollinated stigmas),
cross-linking experiments indicated that SRK interacts
with other cellular components, in complexes of , 161 kDa
and , 233 kDa [35]. The second complex of , 233 kDa has
the expected size for an SRK dimer. Interestingly, soluble
proteins encoded by S-locus genes are secreted by
stigmatic papillar cells. They include a soluble truncated
form of SRK (eSRK), which corresponds to the SRK
extracellular domain alone, and a highly similar protein,
SLG (S-locus glycoprotein) [36]. Using co-immunoprecipitation and sucrose-gradient-based approaches, neither
eSRK nor SLG were detected in complexes including SRK
in unpollinated stigmas [35] (D. Cabrillac et al., unpublished). However, cross-linking experiments performed on
stigma proteins after binding of labelled SCR to stigma
microsomal fractions identified two radiolabelled proteins
of 120 kDa and 65 kDa, which correspond to SRK and
either eSRK or SLG, respectively [10]. The 65 kDa protein
was also co-immunoprecipitated with SRK from stigma
extracts. These results suggest that SCR can bind to both
SRK and eSRK or SLG, and that SCR binding initiates
association of SRK with the soluble stigma proteins to form
heterodimers (Fig. 1). Binding affinity data suggest that
the complex of SRK plus eSRK or SLG, might constitute
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the high-affinity binding site for SCR on the papillar cell
membrane [10]. Similarly to the self-incompatibility
recognition system, EGF binding by EGFR implies two
states of different affinities [23,37,38]. The proportion of
EGFR in the high-affinity state is modulated by its
heteromerization with ErbB2, another member of the
ErbB family [23,38]. Whether SRK behaves in a similar
way through its association with soluble proteins remains
to be confirmed.
Activation of the receptor
All the PRKs discussed in this article share a homologous
intracellular kinase domain and autophosphorylate
in vitro on serine and threonine residues when this
domain is produced in E. coli [5,30,36,39]. The physiological importance of this kinase activity is stressed by the
many PRK mutants with lesions affecting the kinase
domain [7,40,41]. In vivo, there is evidence that the binding
of ligands to PRKs induces autophosphorylation and
activation. SRK, for example, is phosphorylated in vivo
within an hour of an incompatible pollination [42]. This
autophosphorylation can also be triggered in vitro by treating
plasma-membrane fractions with SCR peptides [10]. Transphosphorylation of a kinase-inactive form by an active one
was shown to occur when both forms are co-expressed in a
membranous environment [35]. BL treatment also induces
BRI1 phosphorylation in plants previously treated with
inhibitors of brassinolide production [27]. However, the
ability of the complete BRI1 protein expressed in either
bacterial or yeast cells to autophosphorylate remains
controversial [6,43]. Experiments in which wild-type and
kinase-negative forms of BRI1 and BAK1 were expressed
in yeast suggest that activation of the kinase activity of
these receptors requires heterodimerization and transphosphorylation [6]. There are no data available for the
integral CLV1 receptor but a kinase-active form of the
cytosolic domain of CLV1 does transphosphorylate a
kinase-inactive form of the same domain in vitro [44].
Taken together, these observations indicate that activation of receptor kinases (in either a homo- or a
hetero-oligomeric configuration) by transphosphorylation might be a common feature of PRK signalling
pathways.
Regulation of kinase activity
Analysis of animal receptor kinases has highlighted
the importance of regulating kinase activity as a
means to prevent inappropriate activation of downstream signal transduction pathways. The plant phosphatase KAPP interacts in vitro with the kinase
domain of many PRKs including those of SRK [45],
CLV1 [44 – 46] and FLS2 [30]. In the case of CLV1 and
SRK, interaction does not occur with mutant forms
altered in their kinase activity [44,46] (V. Vanoosthuyse
et al., unpublished), suggesting that the interaction
depends on the phosphorylation status or activity of
the receptor. KAPP is also phosphorylated by CLV1
and SRK, and can dephosphorylate both receptors [44]
(V. Vanoosthuyse et al., unpublished). The role of KAPP
in PRK regulation was investigated by producing
transgenic plants with up- or downregulated KAPP
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Vol.8 No.5 May 2003
(a)
(b)
Inactive
complex
Active
complex
Down-regulated
complex
Active
Sterol binding complex
peptide?
CLV2 CLV1 CLV3
BRI1
BAK1
BR
X
SS
SS
SS
SS
?
KAPP
POL
SS
ROP
?
MAPK
cascade?
BIN2
Stem cell
identity genes
BES2 and BZR1
BR response
genes
WUS
(c)
Active
complex
FLS2
Downregulated
complex
flg22
(d)
Inactive
complex
eSRK or
SLG
SRK
Active
complex
Down-regulated
complex
SCR
THL
KAPP
ARC1
MAPK
cascade
MEKK1
MKK4/5
MPK3/6
Defence response
genes
Self-incompatibility
response
TRENDS in Plant Science
Fig. 1. Models of signal transduction through four plant receptor kinases (PRKs). (a) CLV3–CLV1 signalling. The inactive CLAVATA complex is composed of disulfide-bound
CLAVATA 1 (CLV1) and CLAVATA 2 (CLV2) proteins. Binding of the ligand CLAVATA 3 (CLV3), perhaps in association with an unknown X factor, induces receptor oligomerization and transphosphorylation followed by phosphorylation of kinase-associated protein phosphatase (KAPP). Rho-GTPase-related protein (Rop), (which might interact
directly or via an unknown intermediate with the CLV1– CLV2 complex), might be responsible for signal transduction downstream of CLV1. Repression of WUSCHEL (WUS)
expression enables progression of meristem stem cells toward differentiation. Genetic evidence indicates that dephosphorylation by KAPP downregulates the activated
receptor complex. The POLTERGEIST (POL) protein functions as a negative regulator of CLV1 signalling. (b) BR– BRI1– BAK1 signalling. Binding of brassinosteroids (BR),
perhaps in association with a sterol binding protein, activates heterodimers of brassinosteroid-insensitive 1 receptor (BRI1) and BRI1-associated receptor kinase 1 (BAK1),
leading to mutual transphosphorylation. Signal transduction from the activated receptor complex leads to inactivation of the BR-insensitive 2 (BIN2) kinase probably via
one or more unknown intermediates. The reduced BIN2 activity in turn leads to an increase in unphosphorylated BRI-EMS-suppressor 2 (BES2) and brassinazole-resistant 1
(BZR1), which move into the nucleus and trigger at least some of the cell responses to BR. (c) flg22– FLS2 signalling. Binding of flagellin 22 (flg22) to flagellin sensing 2
(FLS2) triggers kinase activation. It is not known whether this involves FLS2 oligomerization. FLS2 activation, in turn, triggers a mitogen-activated protein (MAP) kinase cascade, putatively through signal-transduction intermediates. KAPP also interacts with FLS2, probably to downregulate its activity. (d) SCR –SRK signalling. In the basal
(unpollinated) state, S-locus receptor kinase (SRK) is present as a monomer and as a homodimer and is maintained in an inactive state by interacting with thioredoxin-hlike (THL). After self-pollination, the ligand, S-locus cysteine-rich (SCR), binds as a dimer either to an SRK homodimer or to a hetetodimer of SRK with extracellular SRK
(eSRK) or S-locus glycoprotein (SLG). SCR binding is thought to induce receptor oligomerization and transphosphorylation, followed by phosphorylation of armadillorepeat-containing 1 (ARC1) and KAPP. ARC1 is a positive effector of the self-incompatibility response. KAPP probably dephosphorylates SRK and consequently downregulates the activated SRK complex. Red stars indicate protein phosphorylation.
expression. A floral phenotype reminiscent of clv1 loss of
function alleles was observed in plants overexpressing
KAPP [44,46]. Moreover, reducing the level of KAPP
expression suppressed the clavata phenotype of the
weak clv1 – 1 mutant allele [46]. These results suggest
that KAPP probably acts as a negative regulator of
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CLAVATA signalling, probably by dephosphorylating
CLV1. KAPP-overexpressing plants also mimic fls2
mutants in their response to treatment with flg22, and
have a much lower flg22 binding capacity [30]. Taken
together, these data allow us to suggest a general model in
which KAPP binds to activated, phosphorylated PRKs, is
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TRENDS in Plant Science
in turn phosphorylated by PRKs and then downregulates
them by dephosphorylating the kinase domain (Fig. 1).
A possible mechanism for this downregulation has been
suggested by recent work on another PRK, AtSERK1 [47].
AtSERK1 interacts with KAPP in a phosphorylationdependent manner and in vivo protein –protein interaction experiments indicate that this interaction occurs in
intracellular vesicles. This raises the possibility that
KAPP mediates PRK downregulation by modulating
receptor endocytosis.
Recently, we discovered two novel SRK-interacting
proteins using a yeast two-hybrid screen (V. Vanoosthuyse
et al., unpublished). These interactors (calmodulin and a
sorting-nexin-like protein) bind to the kinase domain of
SRK and of several other PRKs in vitro. Remarkably,
calmodulin and sorting nexin have both been shown to
interact with the kinase domain of EGFR [48,49]; sorting
nexin is involved in EGFR internalization [49]. Although
the physiological relevance of the interactions of calmodulin or sorting-nexin-like protein with PRK remains to be
elucidated, we can presume a conserved regulatory
function of these proteins in plant cell signalling.
Another protein, thioredoxin-h-like (THL) has been
shown to act as a negative regulator of SRK. In vitro, THL
inhibits the kinase activity of recombinant SRK from
microsomal fractions of insect cells [42]. This inhibition is
released following the addition of self-pollen proteins,
presumably as a result of specific SCR binding to SRK [42].
We suppose that thioredoxins maintain SRK in an inactive
state in vivo in the absence of its ligand (Fig. 1).
Downstream signalling events
In general, signal transduction downstream of receptor
kinases involves cell effectors that interact with and are
phosphorylated by the cytoplasmic domain of the receptor.
Armadillo (ARM)-repeat-containing protein 1 (ARC1) is an
SRK-interacting partner that satisfies both criteria [50].
Transgenic plants with reduced levels of ARC1 transcripts
showed a partial breakdown of self-incompatibility, indicating that ARC1 is a positive effector of self-incompatibility signalling [51]. Interestingly, ARC1 belongs to a
subclass of U-box- and ARM-repeat-containing proteins
that are encoded by at least 18 genes in Arabidopsis [52].
Although the U-box and ARM repeats are present in some
yeast and animal proteins, no clear homologues of the
ARC1 subclass of genes are detected in these organisms,
suggesting that ARC1 belongs to a plant-specific class of
signalling proteins.
The signalling pathway downstream of flg22 and FLS2
has largely been unravelled using an in vivo system
consisting of mesophyll protoplasts in which defence
reactions were induced by exposure to flg22 [53]. A specific
mitogen-activated protein kinase (MAPK) cascade is
involved, including AtMEKK1, AtMAPKK4/AtMAPKK5
and AtMAPK3/AtMAPK6. This MAPK cascade stimulates
the expression of early (within 30 min of flg22 induction)
transcribed genes. It is not known whether MEKK1 is a
direct substrate for FLS2 or whether additional signalling
intermediates are involved. A possible intermediate is the
plant-specific protein AtPhos43, which is specifically
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phosphorylated in cultured cells within 4 min of activation
of the flg22/FLS2 pathway [54].
Several components that function downstream of the
BRI1 receptor have recently been discovered by genetic
screens. BR-insensitive 2 (bin2) is a semidominant
mutation conferring brassinolide insensitivity, suggesting
that BIN2 acts as a negative regulator of BRI1 signalling
[55]. BIN2 encodes a SHAGGY/GSK-3 protein kinase,
which in its dominant mutant form (bin2-D) has an
increased kinase activity [55]. The two mutations
BRI-EMS-suppressor 1 (bes1-D) and brassinazole-resistant
1 – 1D (bzr1–1D) suppress phenotypes of BR-signalling
mutants [56,57]. This suggests a function of BZR1 and
BES1 as positive regulators of the signalling cascade.
BZR1 and BES1 (which is identical to BZR2) encode
proteins that belong to a small plant-specific protein
family [56,57]. They both accumulate in nuclei in response
to BL treatment, as early as 30 min after addition of BL. In
the nucleus, BES1 accumulates predominantly as an
unphosphorylated form. Interestingly, in a bin2-D/BIN2
background (i.e. in plants with enhanced BIN2 kinase
activity), the accumulation of the dephosphorylated form
of BES1 in response to BL is greatly reduced [57]. Taken
together these data suggest a model [57] in which, in the
absence of a BL signal, BES1 is phosphorylated by BIN2
and remains in the cytoplasm, where it is probably
degraded. In the presence of brassinosteroids, BIN2 is
inhibited and the unphosphorylated form of BES1
accumulates in the nucleus, where it modifies the
expression of BL target genes. Like BES1, BZR1 has
conserved phosphorylation sites that match the consensus
site for SHAGGY/GSK-3 protein kinase phosphorylation
and might function in a similar way [56]. In addition to the
BRI1 –BAK1 pathway described above, there is also
evidence for alternative pathways involving a Hþ-ATPase,
MAPKs and calcium-dependent protein kinases downstream of BRI1 [58,59]. Recent data suggest that the
tomato homologue of BRI1 is a dual ligand receptor for
systemin and brassinosteroids [60]. If this dual recognition
function is confirmed, it would be particularly interesting
to investigate how the same receptor can trigger two
different downstream signalling pathways.
In contrast to the other three signalling systems
described above, the CLAVATA signal-transduction pathway remains largely unknown. Genetic data indicate that
stem-cell maintenance is regulated through a regulation
loop involving WUSCHEL (a homeobox gene) and the locus
POLTERGEIST, which encodes a protein phosphatase 2C
[3,61]. Moreover, it has been suggested that Rop, which is a
component of the activated CLV1 receptor complex, might
act in a manner analogous to the related Ras protein in
animals and activate a downstream MAPK pathway [32].
Conclusion
Recently, significant advances have been made in the
molecular characterization of signal-transduction pathways mediated by PRKs by studying a limited number of
model systems. Several basic features of cell signalling,
such as dimerization of receptors following ligand binding,
receptor transphosphorylation and the existence of receptor complexes, appear to be common to both plants and
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animals. However, there are marked differences between
the two kingdoms. The large number of PRKs supports the
idea of a high degree of specialization, even though recent
data suggest that some PRKs might have a dual
recognition function. PRK signalling pathways also seem
to be remarkably diverse, with few downstream components being shared by more than one receptor. However,
this impression might be partly because of the fragmentary nature of current data and it is likely that common
features (such as MAPK cascades) will emerge as each
pathway is analysed in more detail and as additional
pathways are characterized. The elucidation of many
mechanistic aspects of PRK signalling, such as the
identification of novel cytosolic signalling components,
the mechanism of turnover of activated receptor complexes
and the nature of cross-talk between signalling pathways,
remain challenges for the coming years.
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Phloem 2003
International Meeting on Phloem Transport 2003
31 August–5 September 2003
Bayreuth, Germany
TOPICS: – Source–sink relationships in nutrient allocation; phloem development and differentiation;
phloem as a pathway for signal substances; phloem interactions with pathogens and herbivores.
For more information contact: Professor Ewald Komor, Plant Physiology, University Bayreuth, D-95440
Bayreuth, Germany
e-mail: [email protected] – http://www.phloem2003.de/
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