Stomata are innovations of land plants that allow regulated gas exchange. Stomatal precursor cells are produced by asymmetric cell division, and once formed, signal their neighbors to inhibit the formation of stomatal precursors in direct contact. We report a gene of Arabidopsis thaliana, EPIDERMAL PATTERNING FACTOR 1 (EPF1) that encodes a small secretory peptide expressed in stomatal cells and precursors and that controls stomatal patterning through regulation of asymmetric cell division. EPF1 activity is dependent on the TOO MANY MOUTHS receptor-like protein and ERECTA family receptor kinases, suggesting that EPF1 may provide a positional cue interpreted by these receptors. Although multicellularity evolved independently in animals and plants (Baldauf 2003), both utilize asymmetric cell division for creating new cell lineages during development (Scheres and Benfey 1999;Betschinger and Knoblich 2004). In plants, stomatal development offers an excellent, tractable system to study asymmetric cell division. Stomata consist of two guard cells and a pore between them for gas exchange. The guard cells are produced through a series of asymmetric and symmetric cell divisions that not only produce the differentiated cells, but control the overall density and pattern of stomata on the organ surface (Bergmann 2003). In most dicot leaves, stomata follow a "one-cell-spacing" rule in which two stomata are separated by at least one intervening nonstomatal epidermal cell (Sachs 1991). This spacing is hypothesized to be important for efficient gas exchange (Nadeau and Sack 2002b). The first morphologically discernible stomatal precursor cell is the meristemoid, which is the smaller, triangular-shaped daughter cell produced by asymmetric cell division. The larger daughter cell may differentiate into a nonstomatal epidermal cell (pavement cell) or it may undergo its own asymmetric cell division, creating a satellite meristemoid. A meristemoid has a self-renewing capability and can continue asymmetric divisions, but eventually converts into a guard mother cell (GMC), which then divides symmetrically to form two guard cells that constitute a stoma. The major enforcer of the one-cell-spacing pattern appears to be a signal sent from stomata and precursors (guard cells, GMCs, and mature meristemoids) to undifferentiated neighbor cells that influences the plane of cell division (Geisler et al. 2000).A number of key regulators of the asymmetric cell divisions that ensure the one-cell-spacing rule and control sto- . Because TMM mutations appear to affect only stomatal development, whereas the ER family is important for a broader range of biological processes (Shpak et al. 2004), it may be that a presumed TMM/ER family complex would rely on TMM for specificity of the positional information and the kinasecontaining ER family proteins to transmit signals to downstream elements. One potential downstream target is YDA, a mitogen-activated protein kinase kinase kinase (MAPKKK) that has been shown to act as a switch between stomatal and pavemen...