Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 154

is sensitive to levels of CLV3, raising the possibility that products of both genes
physically interact (Clark et al. , 1995).

6.4.2 The CLAVATA signalling pathway
All three CLV loci have been cloned and their molecular identity indicates they are
components of a signalling pathway. CLV1 encodes a receptor-like kinase (RLK)
with an extracellular domain containing leucine-rich repeats (LRR), a transmem-
brane domain and an intracellular serine/threonine kinase domain (Clark et al. ,
1997). The protein encoded by CLV2 is related to CLV1, having both an LRR and
transmembrane domains, but lacking a kinase domain (Jeong et al. , 1999). LRR is
a motif that is commonly found in protein-binding domains, suggesting that these
RLKs are likely to interact with protein ligands. Based on genetic interactions (see
above), the likely ligand for CLV1 is the product of the CLV3 locus, which encodes a
small secreted protein of 96 amino acids (Fletcher et al. , 1999). As expected, two of
the CLV genes are expressed specifically within the shoot apical and floral meristems
(Clark et al. , 1997; Fletcher et al. , 1999), with CLV3 accumulating predominantly
in the L1 and L2 layers of the central zone and CLV1 in the underlying L3 layer
(Fig. 6.2A). CLV2 is apparently expressed in the shoot apex as well as in other tissues
of the plant, which is consistent with it having a broader role in development (Jeong
et al. , 1999). The molecular identities of the CLAVATA loci imply that cell-to-cell
signalling between the layers of the meristem is involved in regulating the size of
the central zone. Elegant biochemical and genetic studies have provided insight into
how this signalling pathway functions.
Purification of CLV1 from the Arabidopsis meristem shows that it is present in
both 185- and 450-kDa protein complexes (Trotochaud et al. , 1999). The 450-kDa
complex appears to be the active form, requiring the presence of both CLV2 and
CLV3 for its formation (Jeong et al. , 1999; Trotochaud et al. , 1999). Absence of both
forms of the complex in clv2 mutants is consistent with CLV2 forming a heterodimer
with CLV1, a complex that is predicted to be 185 kDa in size. Surprisingly, CLV1 is

associated with a larger 600-kDa complex in clv2 mutants, indicating that CLV1 may
also interact with other receptor-like proteins (Jeong et al. , 1999). The mild meristem
defects of clv2 mutants may result from these larger CLV1-containing complexes
functioning, albeit partially, in CLV signalling. The finding that mutations in the
LRR domain of CLV1 are semi-dominant, and condition a more severe meristem
defect than loss-of-function clv1 alleles, can also be explained if CLV1 forms com-
plexes with other RLKs. According to this model, the severity of the missense muta-
tions is caused by non-functional CLV1 proteins forming an inactive complex with
these RLKs, effectively eliminating their function from the meristem (Dievart et al. ,
2003).
Several lines of evidence indicate that CLV3 is the likely ligand for the putative
CLV1/CLV2 receptor complex. Periclinal chimaeras that have a wild-type L1 layer
and clv3 mutant L2 and L3 layers form normal meristem, consistent with CLV3
functioning non-cell-autonomously (Fletcher et al. , 1999). Also, immunological

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