The post-embryonic development of above-ground tissues in plants is dependent upon the maintenance and differentiation of stem cells at the shoot meristem. The Arabidopsis WUSCHEL (WUS) transcription factor establishes an organizing center within the shoot meristem that is essential for specification of stem-cell identity in overlying cells. The CLAVATA (CLV)signaling pathway, including the CLV1 receptor-kinase, promotes the differentiation of stem cells by limiting the WUS expression domain,yet the mechanism of CLV signaling is largely unknown. Previously, we have shown that mutations in two protein phosphatases, POLTERGEIST (POL) and PLL1,partially suppress clv mutant phenotypes. Here, we demonstrate that POL and PLL1 are integral components of the CLV1 signaling pathway. POL and PLL1 are essential for stem-cell specification, and can also block stem-cell differentiation when overexpressed. We provide extensive evidence that POL and PLL1 act downstream of CLV signaling to maintain WUS expression and that they regulate WUS at a transcriptional level. Our findings suggest that POL and PLL1 are central players in regulating the balance between stem-cell maintenance and differentiation, and are the closest known factors to WUS regulation in the shoot meristem.

Despite the presence of more than 400 genes that encode receptor-like kinases (RLKs) in the Arabidopsis thaliana genome, very little is known about the range of biological processes that they control, or the mechanisms by which they function. This review focuses on the most recent findings from studies of several leucine-rich-repeat (LRR) class RLKs in A. thaliana , and their implications for our understanding of plant receptor function and signaling. We compare the biological functions of plant and animal LRR-containing receptors, and the potential commonalities in the signaling mechanisms employed.