Agriculture Reference
In-Depth Information
Table 6.2
Other components of the
CLAVATA
pathway
Protein-phospatase KAPP
The KINASE ASSOCIATED PROTEIN PHOSPATASE (KAPP) is recruited to the active
450-kDa CLV1 receptor complex (Trotochaud
et al.
, 1999) where it binds to phosphorylated
CLV1 (Williams
et al.
, 1997; Stone
et al.
, 1998). As increased levels of KAPP protein in
transgenic plants trigger an enlargement of the shoot apical meristem, it is likely that KAPP
functions as a negative regulator of CLV signalling by dephosphorylating CLV1 in the
activated complex (William
et al.
, 1997).
ROP,asmall GTPase
Another component of the 450-kDa CLV1 complex is a small GTPase belonging to the Rho
subfamily, called ROP (Trotochaud
et al.
, 1999). It is thought that ROP either binds directly or
via a smaller linker protein to the phosphorylated kinase domain of CLV1. Although the
downstream components of the CLV signalling pathway are not known, it seems likely, by
analogy with RLK associated with innate immunity, that a mitogen-activated protein kinase
(MAPK) signalling cascade might be involved. Activation of the MAPK pathway may be
mediated by ROP (Asai
et al.
, 2002).
SHEPHERD
SHEPHERD
(
SHD
) mutants are fasciated, but also display reduced pollen tube growth and
disorganisation of the root apical meristem (Ishiguro
et al.
, 2002). The
shd
mutant suppresses
the effects of constitutive CLV signalling, showing that it functions in the CLV signalling
pathway.
SHD
encodes a GRP94-like chaperone protein, which is likely to reside in the
endoplasmic reticulum, where it may promote folding of all three CLV proteins or associated
proteins (Ishiguro
et al.
, 2002).
POLTERGEIST
Mutations in
POLTERGEIST
(
POL
) suppress the
clv
mutant phenotype (Yu
et al.
, 2000),
suggesting that
POL
is a negative regulator of the
CLV
pathway. However, genetic interactions
with
wus
suggest
POL
works redundantly with
WUS
to promote stem cell identity (Yu
et al.
,
2003).
POL
encodes a nuclear-localised protein phosphatase 2C (PP2C) that has a broad
domain of expression and may function in other pathways (Yu
et al.
, 2003).
of
CLV1
is also dependent on WUS activity and at least one other factor (see below;
Gallois
et al.
, 2002).
The extensive molecular and genetic characterisation of the CLV-WUS pathway
has thus provided insight into the complex signalling involved in maintaining the
integrity of the meristem (see Fig. 6.2B). The pathway is essentially a negative
feedback loop in which the
WUS
-expressing organising centre generates a non-cell
autonomous signal that, in conjunction with other factors (see below and Table 6.2),
promotes stem cell identity and
CLV3
expression in the overlying layers of the central
zone. The secreted CLV3 ligand either diffuses or is actively transported through the
apoplast of the meristem where it activates the putative CLV1/2 receptor complex in
underlying cells. Activation of the CLV pathway ultimately leads to transcriptional
repression of
WUS
, which in turns limits the size of the organising centre and the
strength of the stem-cell-promoting signal.
This model does not explain why CLV3 fails to activate CLV signalling in the
organising centre, a region expressing
CLV1
.Asectopic expression of
CLV3
in this
region causes
WUS
repression (Brand
et al
., 2000), there must be some mechanism
that prevents it from occurring normally. A recent study has shown that the putative