Consisting of only four morphological parts, the Arabidopsis ovule is a relatively simple structure that lends itself to the study of genetic regulation of pattern formation and organogenesis in plants (for review, see Gasser and Robinson-Beers 1993;Reiser and Fischer 1993;Angenent and Colombo 1996;Gasser et al. 1998;Schneitz et al. 1998b). Although initiating as a radially symmetrical primordium, the developing ovule subsequently exhibits differences between the side toward the apex of the carpel (adaxial) and the side toward the base of the carpel (abaxial)-resulting in a bilaterally symmetrical structure. Asymmetric development is most apparent in the outer integument (one of two sheathing structures) and in the funiculus (supporting stalk). The outer integument grows extensively only on the abaxial side of the ovule, and the funiculus curves in the abaxial direction. Two genetic loci involved in asymmetric development of the outer integument have been described. In addition to effects on flower development (Schultz et al. 1991;Bowman et al. 1992), the SUPERMAN (SUP) gene is essential for suppressing adaxial growth of the outer integument (Gaiser et al. 1995), and mutations in this gene lead to nearly equal outer integument growth on both sides of the ovule. In contrast, inner no outer (ino) mutations can lead to an absence of outer integument growth on both sides of the ovule primordium, implicating INO as a positive regulator of integument growth or a determinant of polarity (Gaiser et al. 1995;Baker et al. 1997;Schneitz et al. 1997).In angiosperms, bilateral symmetry is also a common characteristic of leaves, floral organs, and often whole flowers. In these structures, bilateral symmetry results from significant differences in development between the adaxial (toward the shoot apex) and abaxial (away from the shoot apex) sides. Recently, several genes involved in establishing these abaxial-adaxial patterns have been identified. The cycloidea (cyc) and dichotoma (dich) genes of Antirrhinum majus are expressed on the adaxial sides of flowers, where they specify adaxial floral development (Luo et al. 1996). In contrast, phantastica (phan), which appears to be essential for identity of the adaxial leaf surface in this same species, is expressed throughout the leaf (Waites et al. 1998), and must, therefore, require additional asymmetrically distributed factors for its activity.In Arabidopsis thaliana, vegetative structures of the phabulosa-1d mutant are radially symmetrical, apparently as a result of adaxialization, and PHABULOSA may be one determinant of abaxial cell fate in this species (McConnell and Barton 1998). A recently described family of Arabidopsis genes, the YABBY genes, encoding putative transcription factors (Bowman and Smyth 1999;Kumaran et al. 1999;Sawa et al. 1999;Siegfried et al. 1999), participate in determination of abaxial identity in a variety of organs.

Pollen tube guidance precedes the double fertilization of flowering plants. Here, we report the identification of a small maize protein of 94 amino acids involved in short-range signaling required for pollen tube attraction by the female gametophyte. ZmEA1 is exclusively expressed in the egg apparatus, consisting of the egg cell and two synergids. Chimeric ZmEA1 fused to green fluorescent protein (ZmEA1:GFP) was first visible within the filiform apparatus and later was localized to nucellar cell walls below the micropylar opening of the ovule. Transgenic down-regulation of the ZmEA1 gene led to ovule sterility caused by loss of close-range pollen tube guidance to the micropyle.

Recent developments of tools for targeted genome modification have led to new concepts in how multiple traits can be combined. Targeted genome modification is based on the use of nucleases with tailor-made specificities to introduce a DNA double-strand break (DSB) at specific target loci. A re-engineered meganuclease was designed for specific cleavage of an endogenous target sequence adjacent to a transgenic insect control locus in cotton. The combination of targeted DNA cleavage and homologous recombination–mediated repair made precise targeted insertion of additional trait genes (hppd, epsps) feasible in cotton. Targeted insertion events were recovered at a frequency of about 2% of the independently transformed embryogenic callus lines. We further demonstrated that all trait genes were inherited as a single genetic unit, which will simplify future multiple-trait introgression.

Ovules are the direct precursors of seeds and thus play central roles in sexual plant reproduction and human nutrition. Extensive classical studies have elucidated the evolutionary trends and developmental processes responsible for the current wide variety of ovule morphologies. Recently, ovules have been perceived as an attractive system for the study of genetic regulation of plant development. More than a dozen regulatory genes have now been identified through isolation of ovule mutants. Characterization of these mutants shows that some aspects of ovule development follow independent pathways, while other processes are interdependent. Some of these mutants have ovules resembling those of putative ancestors of angiosperms and may help in understanding plant evolution. Clones of several of the regulatory genes have been used to determine expression patterns and putative biochemical functions of the gene products. Newly constructed models of genetic regulation of ovule development provide a framework for interpretation of future discoveries.