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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
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