The SCs in SAM are accurately marked by
CLV3 expression (
Fletcher et al., 1999). As a small 96-AA extracellular protein, CLV3 was proposed to act as a peptide ligand to interact with CLV1 and CLV2 receptors (
Fletcher et al., 1999;
Lenhard and Laux, 2003). Another CLE member, CLE40, shares little sequence identity to CLV3 except CLE motif (
Hobe et al., 2003). The peptide identity was verified firstly through the
in vitro function assay using a 14-AA synthetic peptide (CLV3p) corresponding to the conserved CLE motif among CLV3, CLE19 and CLE40 (
Fiers et al., 2005). In Arabidopsis genome, there are at least 34 CLE members that share a common feature of small proteins less than 10 kD, with a signal peptide at their N-terminals and a conserved 14-AA CLE box at their C-terminals (
Cock and McCormick, 2001).
In vivo complementation experiments showed that the non-conserved central domain and C-terminal tails can be deleted without affecting CLV3 functions (
Fiers et al., 2006). Although the endogenous CLV3 peptide has never been isolated from wild type plants, using
CLV3 overexpression cell lines, Kondo et al. (
2006) identified a 12-AA peptide corresponding to CLV3p, with two proline residues modified by hydroxylation. Later, another group proposed that the endogenous CLV3 has 13-AA, with three arabinoses covalently linked to one of two hydoxylated prolines (
Ohyama et al., 2009) (Fig. 4). Since the unmodified peptides are functional, it is likely that these modifications may enhance the stability of the peptides. CLV3 peptides bind directly to the extracellular domain of CLV1 (
Ogawa et al., 2008). Mutation of the
CLV3 ortholog of rice,
FON4, leads to an increased number of floral organ, suggesting a functional conservation between dicots and monocots (
Chu et al., 2006). The suppressor screen using
RCH1::CLE19 overexpression plants also identified a
SOL1 locus that encodes a Zn-dependent caboxylpeptidase (
Casamitjana-Martínez et al., 2003). Most likely, SOL1 is involved in removing the C-terminal AAs from CLV3 and CLE19 (Fig. 4).