adventitious SAM formation are the formation of SAMS from the petioles of isolated leaves in African violets and from Arabidopsis roots cultured in vitro

Almost all of the tissue in a mature plant is ultimately derived from specialized groups of cells, called meristems, that reside at the tips of roots and shoots. Shoot apical meristems (SAMs) are responsible for elaborating the above ground portions of the plant, which include stems, leaves and flowers, whereas root apical meristems generate the extensive underground network of roots necessary for water and mineral uptake. The shoot apical meristem consists of a small mound of densely cytoplasmic, undifferentiated, dividing cells. Based on several criteria, the cells in the meristem can be classified as stem cells (Potten and Loeffler, 1990). Among these are the ability to proliferate, to replace themselves, to give rise to a variety of differentiated cell types, and to regenerate a new meristem if damaged (Sussex, 1952). The shoot apical meristem of angiosperm plants is organized into two regions the tunica and the corpus (Schmidt, 1924). The tunica consists of one or more cell layers that cloak the underlying corpus. The Arabidopsis vegetative meristem consists of two tunica layers overlying a shallow corpus (Vaughn, 1955; Medford et al., 1992). The organization of the meristem into these two distinct regions is a consequence of differences in the orientation of cell division. Cell divisions in the tunica are principally anticlinal1, thus maintaining the layered arrangement of the tunica. The cells in the corpus on the other hand may divide anticlinally, periclinally or obliquely resulting in a jumbled arrangement of cells. Although the tunica and corpus have been recognized as anatomically distinct regions since the early part of this century (Schmidt, 1924) the significance of this organization to meristem function, if any, is unknown. Shoot apical meristems of angiosperm plants form under three distinct sets of conditions. The first of these is during embryogensis when the primary shoot apical meristem is formed. The second condition is in the axils of leaves where lateral shoot apical meristems are formed. The third is adventitiously, i.e. from a portion of the plant that when unperturbed does not give rise to a SAM. Examples of adventitious SAM formation are the formation of SAMS from the petioles of isolated leaves in African violets and from Arabidopsis roots cultured in vitro (Valvekens et al., 1988). Previous studies of the mechanism of meristem initiation have focused on the growth factors that regulate adventitious meristem formation in vitro. Numerous studies on many different species have shown that a high auxin:cytokinin ratio promotes root development and a low auxin:cytokinin ratio promotes shoot development (e.g. Skoog and Miller, 1957). However, the basis of this effect and the mechanism by which the initiation and identity of meristems is regulated in vivo are still unknown. Although embryogenesis has been described in a wide variety of plants, little attention has been paid to the initiation of the shoot apical meristem. In order to understand the mechanism of meristem initiation we have char823 Development 119, 823-831 (1993) Printed in Great Britain © The Company of Biologists Limited 1993