The pattern of cell division is very regular in Arabidopsis embryogenesis, enabling seedling structures to be traced back to groups of cells in the early embryo. Recessive mutations in the FASS gene alter the pattern of cell division

Embryogenesis generates the primary plant body of the seedling which is fairly uniform among flowering plants (Steeves and Sussex, 1989). How this body organisation is established in the plant embryo has become a focus of genetic analysis, using Arabidopsis as a model (Jürgens et al., 1991). A large-scale screen for mutations affecting the seedling body organisation identified genes that appear to play specific roles in one of three different aspects: apical-basal pattern, radial pattern or shape (Mayer et al., 1991). Mutations in those genes were shown to alter embryogenesis before or at the heart stage when the primordia of seedling structures become morphologically recognisable. In Arabidopsis and other crucifers, the embryo grows by oriented cell divisions such that the seedling structures can be traced back to groups of cells in the early embryo (Schulz and Jensen, 1968; Tykarska, 1976, 1979; Mansfield and Briarty, 1991; Jürgens and Mayer, 1994). Such regularity of cell division pattern is not common to all flowering plant embryos (Natesh and Rau, 1984), and thus it is not obvious how it relates to embryonic pattern formation. However, recent studies of embryonic pattern mutants in Arabidopsis have provided evidence that oriented cell divisions reflect processes underlying pattern formation. Mutations in the GNOM gene render the normally asymmetric division of the zygote nearly symmetric, resulting in an enlarged apical daughter cell which also divides abnormally; subsequent cell divisions are highly irregular, yielding a seedling with a grossly perturbed apicalbasal pattern (Mayer et al., 1993). Mutations in the MONOPTEROS gene alter the cell division pattern of the basal region of the embryo from the early-globular stage, producing seedlings without hypocotyl and root (Berleth and Jürgens, 1993). Mutations in the FASS (FS) gene were shown to drastically change the shape of the seedling without altering its body pattern; furthermore, fs embryos did not display the characteristic cell shapes or cell arrangements by which the primordia of seedling structures are normally recognised at the heart stage (Mayer et al., 1991). These observations raised the possibility that the seedling body is generated by two distinct processes in the Arabidopsis embryo, pattern formation and morphogenesis, and that the FS gene might only be involved in the latter but not the former; the term ‘morphogenesis’ is used to cover cell activities, such as oriented cell divisions and cell shape changes, that generate body shape (for definition of the terms see e.g. Slack, 1991). To test this idea, we have carried out a detailed genetic and developmental analysis, and the results indicate that the FS gene is continuously required for the elaboration of cell shape and for correct orientation of cell walls 2967 Development 120, 2967-2978 (1994) Printed in Great Britain © The Company of Biologists Limited 1994