Regulation of ovule development.

Ovule development in Arabidopsis and other plants has been the focus of classic and molecular genetic analyses in recent years. This has been an exciting time to be involved in plant developmental biology, because many genetic pathways and regulatory mechanisms have been elucidated. Continued studies of ovule mutants have contributed to and benefited from these advances. Ovule development involves the same basic processes necessary for the formation of other plant organs, such as primordium initiation and specification, directed cell division and expansion, and asymmetric growth and differentiation. However, ovules differ from other plant structures in their reproductive function and apparent evolutionary origin from sporangiophores (Herr, 1995), allowing the basic developmental processes to be studied in a unique context. Researchers today are using both the wealth of botanical knowledge and new molecular insights to achieve a more comprehensive understanding of ovule morphogenesis and evolution. Recent reviews by Grossniklaus and Schneitz (1998) and Gasser et al. (1998) describe progress in the analysis of several ovule mutants. Here, we extend these earlier reviews with the latest information on the roles and nature of genes involved in the formation of the placenta as the site of ovule initiation, ovule identity, patterning of the ovule primordium, and control of integument morphogenesis. We focus primarily on recent results in Arabidopsis and indicate when results from other species are being discussed. Ovules are the site of processes essential for sexual plant reproduction, including the formation of the megagametophyte, fertilization, embryogenesis, and finally, the formation of the persistent propagule—the seed. Arabidopsis ovules are initiated as small, finger-like primordia from regions (the placentas) of the internal surface of the carpels (Robinson-Beers et al., 1992). The inner and outer integuments arise from the surface of each ovule primordium, with their region of origin defining the chalaza, which separates the apical nucellus from the funiculus (Figures 1A and 1B). The two integuments grow to cover and enclose the nucellus, leaving a small opening, the micropyle (Figure 1C). The funiculus, or stalk, provides a conduit for nutrients to the developing ovule and embryo and partly determines the position of the micropyle. The integuments are required to house the embryo sac, contribute to ovule positioning, and later, form the protective seed coat. The nucellus provides the cellular initial for the differentiation of amegasporocyte, which undergoesmeiosis and mitosis to produce a seven-celled megagametophyte, the embryo sac (Figure 1D) (Webb and Gunning, 1990; Mansfield et al., 1991). Molecular genetic analyses with genes important for ovule development that have contributed to recent advances revealing the genetic pathways and regulatory mechanisms involved in plant development are the focus of this review.