The unfolding drama of flower development: recent results from genetic and molecular analyses

There has been an explosion of information about flower development recently, largely because of genetic and molecular studies in Arabidopsis thaliana and Antirrhihum majus. A number of homeotic genes have been identified that regulate flower development, and models have been proposed for the specification of meristem and floral organ identities. Molecular cloning of many of these genes has allowed the testing of specific predictions of the models but also has led to modifications of a floral organ identity model. Furthermore, several of the floral genes contain a conserved region, the MADS box, which encodes a domain with striking sequence similarity with known transcription factors from human and yeast. Additional MADS box genes have been isolated from several plants; these genes are likely to play important regulatory roles during flower development. The genetic and molecular studies have uncovered many of the components of a complex network of regulatory proteins that directs flower development. Further characterization of these and other yet to be discovered components promises to contribute a great deal to our understanding of the mechanisms controlling flower development. Flowering plants, like other land plants, have vegetative organs such as roots, stems, and leaves, which absorb nutrients and water from the soil, transport them to other parts of the plant, and synthesize organic compounds using the sun's energy. In addition, flowering plants produce elaborate reproductive structures, the flowers, which, following fertilization, become fruits and bear seeds. From the seasoned gardener to the casual observer, from the naturalist to the florist shopper, people have always been fascinated by the enormous variety of flowers, ranging from 2 mm to > 10 cm in length, covering the whole visual spectrum with their colors, and differing in the arrangement of flowers and the symmetry within a flower. How do flowers develop? What genes regulate this complex process? In recent years, rapid advances have been made in addressing these questions, largely as a result of genetic and molecular studies in two distantly related flowering plants, Arabidopsis thaliana, a relative of cauliflower and cabbage, and the snapdragon, Antirrhinum majus. Postembryonic plant development is repetitive, because of the reiterative nature of the morphogenesis initiated from the meristems, which are groups of undifferentiated progenitor cells. The meristem at the apex of the plant [or the tip of a steml is called the apical meristem. While the cells at the very summit of the domeshaped apical meristem divide and maintain the meristem, the cells at the periphery of the apical meristem divide to give rise to additional meristems (e.g., of branches) or organ primordia, which are groups of cells with specified fates and which develop into different organs, such as leaves. The cells just below the apical meristem divide and differentiate to form the stem itself. Many plants, such as trees, have a recognizable dominant stem [trunk), with the primary apical meristem, and additional branches, each with a meristem at the tip. Other plants lack an obvious dominant stem and a primary meristem, and have a bushy morphology. In Arabidopsis, the vegetative apical meristem produces leaves arranged in a spiral arrangement with very short distances between successive leaves, forming a rosette (Fig.la). In contrast, the Antirrhinum vegetative apical meristem produces pairs of leaves opposite tO each other, and each pair is at right angle with the previous pair (Fig. lb; Coen and Meyerowitz 1991). Flower development requires several steps (Meyerowitz et al. 1991}. The first step is floral induction, which establishes a reproductive meristem(s). The reproductive meristem is often called an inflorescence meristem because it gives rise to a series of flowers, called an inflorescence. The number of flowers in an inflorescence varies between species and ranges from several (determinate inflorescence) to indefinite (indeterminate inflorescence). Both Arabidopsis and Antirrhinum have indeterminate inflorescences (Fig. la, b). The second step is the formation of a floral meristem (Fig. lc), which is homologous to a branch meristem, for a flower can be regarded as a very short stem with specialized organs. In some plants, floral induction results in the formation of a floral meristem directly, and a single flower develops at the end of a stem. In Arabidopsis, the primary inflorescence meristem gives rise to, in a spiral, first a small number of cauline leaf primordia (Fig. la), each with an adjacent secondary inflorescence meristem, and then a large (in-