self-perpetuating stem cell system in the SAM is responsible for the repetitive formation of leaves and secondary shoots during vegetative development, and the same system is responsible for the repetitive generation of flowers during later

The shoot apical meristem (SAM) is the source of all aerial parts of plants. In Arabidopsis, a self-perpetuating stem cell system in the SAM is responsible for the repetitive formation of leaves and secondary shoots during vegetative development, and the same system is responsible for the repetitive generation of flowers during later reproductive development. This later SAM that develops flowers instead of shoots is termed ‘inflorescence meristem’ (IM), in which flowers are initiated at the periphery in a spiral pattern (Fig, 1A,B). A meristem (whether SAM or IM) can be divided into three zones: the central zone at the apex of the meristem where cell division is infrequent, the peripheral zone that surrounds the central zone where cell divisions are relatively rapid, and the rib meristem (or rib zone) beneath the central zone, where divisions are also rapid (Fig. 1C; Steeves and Sussex, 1989; Meyerowitz, 1997; Clark, 1997). The central zone harbors the stem cells that divide slowly to generate daughter cells to the surrounding peripheral zone, where these daughter cells are incorporated into differentiating leaf, shoot or flower primordia. The Arabidopsis CLAVATA genes CLV1, CLV2 and CLV3 regulate a critical aspect of SAM development. Loss-offunction clv mutations cause progressive SAM enlargement due to accumulation of undifferentiated cells in the central zones (Clark et al., 1993, 1995; Kayes and Clark, 1998; Fletcher et al., 1999). Thus, CLV genes either function to repress cell division in the central zone or to promote the transition of cells from an undifferentiated state toward organ formation and differentiation. CLV1 encodes a putative receptor kinase, while CLV3 encodes the putative ligand for the CLV1 receptor (Clark et al., 1997; Fletcher et al., 1999). CLV2, also a receptor-like protein, functions in the same pathway as CLV1 and CLV3 in regulating meristem development (Kayes and Clark, 1998; Jeong et al., 1999). SHOOT MERISTEMLESS (STM) from Arabidopsis encodes a homolog of maize homeobox gene KNOTTED1 (KN1) (Vollbrecht et al., 1991; Long et al., 1996) and acts to oppose the function of CLV (Clark et al., 1996; Endrizzi et al., 1996). These and other molecular genetic studies are beginning to reveal the molecular mechanism for SAM formation and maintenance. Another important aspect of meristem development is the regulation and execution of basic cell division processes. Since plant cells are surrounded by a rigid wall, the development and maintenance of the meristem requires regulated cell divisions with respect to the rate and orientation of division. For example, meristems in most flowering plants can be divided into three layers, L1, L2 and L3 (Fig. 1C; Poethig, 1989; Kerstetter and 2207 Development 127, 2207-2217 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV0289