Separation of shoot and floral identity

The aerial parts of Arabidopsis are ultimately derived from the primary shoot apical meristem which is established during embryogenesis (Sussex, 1989; Evans and Barton, 1997). Over the plant life cycle, a series of growth phases reflect the activity of this meristem (Poethig, 1990; Schultz and Haughn, 1993; Ratcliffe et al., 1998). Following germination, the shoot apical meristem generates leaf primordia, which bear axillary shoot meristems. The duration of this vegetative phase (V) depends on environmental conditions and is controlled via an extensive network of flowering-time genes (Koornneef et al., 1991; Martinez-Zapater et al., 1994; Simon et al., 1996; Ruiz-Garcia et al., 1997; Nilsson et al., 1998). These genes regulate when the shoot switches to reproductive development and becomes an inflorescence (I). During a first-inflorescence phase (I1), 23 cauline leaf primordia are produced, before a secondinflorescence phase (I2) in which floral meristems are generated. During I2, it is necessary for the shoot meristem to retain a distinct identity from the floral meristems it produces. A key question concerns how this separation is achieved (Shannon and Meeks-Wagner, 1993; Okamuro et al., 1993). The distinction between shoot and floral meristems is maintained by two complementary sets of genes. First, floral fate depends upon the action of meristem identity genes such as LEAFY (LFY), APETALA 1 (AP1), and CAULIFLOWER (CAL) (Mandel et al., 1992; Weigel et al., 1992; Bowman et al., 1993; Gustafson-Brown et al., 1994; Kempin et al., 1995; Mandel and Yanofsky, 1995; Weigel and Nilsson; 1995). Secondly, a group of genes, including TERMINAL FLOWER 1 (TFL1), prevent the shoot from becoming a flower by retarding progression through all growth phases (Ratcliffe et al., 1998). The opposing functions of TFL1 and floral meristem identity genes is reflected in their complementary expression patterns and phenotypic effects. In wild type, TFL1 and the floral meristem identity genes are expressed in separate domains (Fig. 1). Both types of genes are most strongly expressed during inflorescence development, with TFL1 in the centre of the apex and floral meristem identity genes on its periphery (Mandel et al., 1992; Weigel et al., 1992; Kempin et al., 1995; Bradley et al., 1997). A similar separation is also observed between low levels of TFL1 and LFY expression during vegetative growth (Bradley et al., 1997; Blazquez et al., 1997, 1998; Hempel et al., 1997). If the activity of floral meristem identity genes is reduced, flowers develop with various shoot-like characteristics. (Irish and Sussex 1990; Schultz and Haughn 1991, 1993; Mandel et al., 1992; Huala and Sussex, 1992; Weigel et al., 1992; Bowman et al., 1993; Shannon and Meeks-Wagner, 1993). In extreme cases, such as the lfy;ap1;cal triple mutant, inflorescence nodes comprise secondary shoots with subtending leaves, and flower-like structures are rarely made (Bowman et al., 1993). Compared to wild type, therefore, the lfy;ap1;cal shoot apex switches from the V to the I phase at about the normal time, but flowers of the I2 phase are replaced by shoots. Conversely, when LFY or AP1 are constitutively expressed from a 35S CaMV promoter, all growth phases are shortened and shoot meristems are converted into flowers (Mandel and Yanofsky, 1995; Weigel and Nilsson, 1995). 1109 Development 126, 1109-1120 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV3950