Continuous organ formation from the shoot apical meristem requires the integration of two functions: a set of undifferentiated, pluripotent stem cells is maintained at the very tip of the meristem, while their daughter cells

Postembryonic development of higher plants is characterized by the continuous formation of organs from the shoot apical meristem (SAM) (Steeves and Sussex, 1989). The SAM serves two main functions: in the central zone, a population of undifferentiated, pluripotent stem cells is maintained, and in the peripheral zone, lateral organ primordia are initiated. While all cells of the meristem dome remain undifferentiated until they are incorporated into organ primordia, only a specialized subset functions as long-term stem cells from which all cells of the shoot and its lateral organs are ultimately derived (Satina et al., 1940; Stewart and Dermen, 1970). These stem cells are located in three cell tiers at the very apex and coincide with the domain where the CLAVATA3 (CLV3) gene is expressed (Fletcher et al., 1999). Genetic analysis in Arabidopsis has identified two major regulators of SAM formation and maintenance, the homeobox genes WUSCHEL (WUS) and SHOOTMERISTEMLESS (STM). In wus mutants the apical stem cells are unable to selfmaintain (Laux et al., 1996; Mayer et al., 1998), whereas ectopic WUS expression can abolish organ formation at the SAM and induce expression of the putative stem cell marker CLV3 (Schoof et al., 2000). During embryogenesis, WUS mRNA can first be detected in the four inner apical cells of the 16-cell stage embryo and later becomes restricted to a small central cell group underneath the presumed stem cells in the outermost three cell layers. Thus, WUS expression appears to define an organizing centre whose activity establishes an apical group of long-term stem cells. WUS expression is under negative control by the CLAVATA genes (CLV1, CLV2 and CLV3), which encode components of a presumed receptor-kinase signal transduction pathway (Clark et al., 1997; Jeong et al., 1999; Fletcher et al., 1999). In clv mutants, the SAM enlarges progressively by the accumulation of stem cells (Clark et al., 1993; Clark et al., 1995; Fletcher et al., 1999), and this enlargement appears to be a consequence of ectopic WUS expression in more apical and lateral cells in clv mutant SAMs (Schoof et al., 2000). This has led to a model in which stem cell maintenance is regulated by a negative feedback loop mediated by the WUS and CLV3 genes, with the organizing centre signalling to the apical neighbours to specify them as stem cells, which in turn signal back to restrict the size of the organizing centre (Brand et al., 2000; Schoof et al., 2000). Loss-of-function mutations in the SHOOTMERISTEMLESS (STM) gene, which encodes a homeodomain protein of the KNOTTED class (Long et al., 1996) also result in a lack of a self-maintaining meristem. Instead of forming a SAM, the cells in the apex of stm mutant embryos appear to differentiate 3195 Development 129, 3195-3206 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV0437