Stem cells of the shoot apical meristem (SAM) give rise to all aerial structures of the plant, including stems, leaves, and flowers. The homeodomain transcription factor WUSCHEL (WUS), produced in the organizing center of the SAM, promotes stem cell maintenance, and signaling by the peptide CLAVATA3 (CLV3) represses WUS to restrict the stem cell niche. CLV3 signaling through two receptor complexes, one containing the leucine-rich repeat (LRR) receptor-like kinase (RLK) CLV1 and another containing the LRR receptor-like protein CLV2, which lacks a kinase domain. In a screen for mutants that were insensitive to application of a synthetic CLV3 peptide, Kinoshita et al. identified receptor-like protein kinase 2 (RPK2), another LRR RLK also known as TOADSTOOL 2, as a third transducer of CLV3 signaling. rpk2 mutants exhibited phenotypes similar to but weaker than those of clv1 and clv2 mutants—an enlarged SAM and increased number of floral organs and carpels. RPK2 was expressed in the inflorescence meristem, floral meristem, and floral organ primordia, and overexpression of RPK2 inhibited SAM growth and reduced the number of floral organs, with the strength of the phenotype correlating with the amount of overexpression. In genetic interaction experiments using null alleles of clv1, clv2, and rpk2, plants carrying mutations in any two genes exhibited greater SAM overproliferation than did any single mutant and increased carpel numbers, and the triple mutant showed the greater SAM expansion, similar to that observed in clv3 mutants. As previously reported, and consistent with genetic data suggesting crosstalk between the CLV1 and CLV2 receptor complexes, CLV1 and CLV2 physically interacted in a transient expression assay. However, no interaction was detected between RPK2 and CLV1- or CLV2-containing complexes, which suggests that RPK2 defines a third, independent route through which CLV3 signaling restricts WUS expression. The presence of at least three CLV3 signaling pathways highlights the importance of regulating stem cell proliferation for proper SAM homeostasis.
A. Kinoshita, S. Betsuyaku, Y. Osakabe, S. Mizuno, S. Nagawa, Y. Stahl, R. Simon, K. Yamaguchi-Shinozaki, H. Fukuda, S. Sawa, RPK2 is an essential receptor-like kinase that transmits the CLV3 signal in Arabidopsis. Development 137, 3911–3920 (2010). [Abstract] [Full Text]