In C. elegans, mutations in genes for several cuticular collagens, for factors involved in X-chromosome dosage compensation, and for a subtilisin-like serine protease result in deformed gross morphology and altered cuticle formation

The outer surfaces of land plants and animals are characterized by an epidermis, which may produce an exocytoskeletal ‘cuticle’ or epidermal cytoskeleton that plays a protective role. Various components and structures are utilized by different organisms for this purpose. For example, the major structural components of the plant cuticle, which was originally isolated as a transparent thin film from cabbage leaves (Brongniart, 1834), are derivatives of lipids (cutins) (Baker and Martin, 1963; Kolattukudy, 1970; Martin and Juniper, 1970; Kolattukudy, 1980a; Kolattukudy, 1980b; Holloway, 1982), whereas in Caenorhabditis elegans they are proteins (cuticle collagens) [reviewed by Johnstone (Johnstone, 2000)]. In C. elegans, mutations in genes for several cuticular collagens, for factors involved in X-chromosome dosage compensation, and for a subtilisin-like serine protease result in deformed gross morphology and altered cuticle formation and/or function (Johnstone, 2000; Thacker et al., 1995). In vertebrates, several factors that mediate extracellular signals, such as bone morphogenetic proteins (BMPs), their activators that are subtilisin-like serine proteases (called subtilisin-related proprotein convertases in vertebrates) such as furin and/or PC6, and their receptors are involved in the normal differentiation of the epidermis (Graff, 1997; Cui et al., 1998). The epidermis of higher plants plays important roles in development, water retention, defence against pathogens and gas exchange. In Arabidopsis thaliana, the epidermis is derived from an outer cell layer, or protoderm of the embryo proper that is formed in the 16-cell embryo proper stage (Goldberg et al., 1994). The environment surrounding the embryo changes dramatically when the seed germinates: the embryo, enclosed by the endosperm and seed coat during seed development, comes in contact with the outer environment for the first time. Thus, embryos must prepare for successful germination, the seed must sense suitable conditions to germinate, and the seedling must respond to changing environmental conditions (Dure III, 1975; Howell, 1998). For survival, the surface functions of the epidermis should be present, at least in part, during embryo development and, indeed, a cuticle layer is apparent on the surface of the developing Arabidopsis (Bowman and Mansfield, 1993; Rodkiewicz et al., 1994), Citrus (Bruck and Walker, 1985), maize (Van Lammeren, 1986), Capsella (Rodkiewicz et al., 1994) and Stellaria (Rodkiewicz et al., 1994) embryos. However, little is known about the genes involved in epidermal differentiation and/or epidermal function during embryogenesis in higher plants. The fiddlehead (fdh) mutant and several other mutants of A. thaliana and maize are characterized by their striking postgenital organ fusion in leaves and/or flower organs (Lolle 4681 Development 128, 4681-4689 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV0386