Update on Development The Control of Flowering Time and Floral Identity in Arabidopsis 1

The reproductive success of plant varieties is often dependent on their flowering time being adapted to the environment in which they grow. This adaptation involves the regulation of flowering by environmental stimuli such as temperature and day length. Classic grafting experiments performed in several species including perilla and tobacco showed that day length is detected in the leaves and a signal is transmitted from there to the shoot apex (King and Zeevaart, 1973; Lang et al., 1977). Widely used early flowering ecotypes of Arabidopsis such as Columbia and Landsberg erecta flower within 3 weeks under LD conditions but not until at least 5 weeks under SD conditions. The shoot apical meristem of Arabidopsis plants grown for 30 d under SD conditions cease producing leaf primordia and start producing flower primordia within a few hours of being shifted to LD conditions (Hempel and Feldmann, 1994). In response to this photoperiodic change, alterations in cell division rates change the shape of the shoot apical meristem, and the primordia produced on the flanks of the meristem form flowers rather than leaves. The rapidity with which the first flowers develop after plants are shifted from SD to LD conditions led Hempel and Feldmann (1995) to propose that in Arabidopsis the signal from the leaves can act directly on existing primordia to alter their identity. The development of chimeric organs showing characteristics of both leaves and flowers at the last node formed prior to the induction of flower primordia also supports the idea that the floral stimulus acts directly on the primordium to confer floral identity (Hempel and Feldmann, 1995). As well as acting directly to influence primordium development, transient exposure of plants to LD conditions causes them to become irreversibly committed to flowering even after their return to SD conditions. Scanning electron micrographs of shoot apices from plants exposed to 8 d of LD conditions show no visible signs of floral development, but plants shifted back to SD conditions still flower as if grown continuously under LD conditions (Bradley et al., 1997). Therefore, exposure to LD conditions causes either persistent expression of the floral stimulus even after plants are shifted back to SD conditions, or a change in the identity of the shoot meristem such that it is stably committed to form floral primordia. The first possibility is suggested by recent experiments with maize and impatiens, which emphasize the continued requirement of leaves for the meristem to form flowers. Experiments with excised shoot apices of maize plants suggest that the presence of four to six leaves is required for the meristem to become committed to form flowers. Excised apices that retain one or two leaves behave like meristems of very young plants and form tassels only after producing the same number of leaves as plants germinated from seed, whereas excised apices that retain four to six young leaves frequently form tassels after producing fewer new leaves than plants grown from seed (Irish and Nelson, 1991; Irish and Jegla, 1997). In impatiens, continued production of an inductive signal from the leaves is also required to prevent reversion to the vegetative state (Pouteau et al., 1997). A systematic genetic approach to identifying genes involved in the transition to flowering has been taken with Arabidopsis (Koornneef et al., 1998a) and pea (Weller et al., 1997). Genes that promote the flowering of Arabidopsis were identified as mutations that delay flowering time, and genetic variation causing similar phenotypic effects was recovered by crossing different ecotypes. Alleles causing late flowering extend the duration of vegetative growth and therefore increase the number of leaves formed before the development of flowers. Floral meristem identity genes or floral initiation process genes confer floral identity upon undifferentiated primordia (Schultz and Haughn, 1993; Weigel, 1995a). Mutations in these genes cause primordia that would develop as flowers in wild-type plants to form structures with shoot-like characteristics. One of the roles of floral meristem identity genes is to activate the expression of organ identity genes that act later in flower development (Weigel and Meyerowitz, 1993). The roles of organ identity genes during flower development, and how the spatial pattern of their expression within the developing flower is regulated have been reviewed previously (Ma, 1994; Weigel and Meyerowitz, 1994). In this Update we focus on recent advances in understanding the genetic control of flowering time and floral meristem identity in Arabidopsis and on how genes in1 M.P. was supported by the European Molecular Biology Organization and the Biotechnology and Biological Science Research Council. * Corresponding author; e-mail couplnd@bbsrc.ac. uk; fax 44 – 1603–505725. Abbreviations: GR, glucocorticoid receptor; LD, long-day; SD, short-day. Plant Physiol. (1998) 117: 1–8