Fig. 1. Arabidopsis embryo development. Quadrant (A,F), octant (B,G), dermatogen (C,H) globular (D,E,I,J), and postglobular (K-T) stages of phenotypically normal (A-E,K-O) and amp1-13 (F-J,P-T) embryos. Staging of amp1 mutants was based on comparison with phenotypically normal (amp1/+ or +/+) embryos from the same silique. Abnormalities in preglobular amp1 mutant embryos are restricted to basal-cell derivatives, whereas the arrangement of apical derivatives (brackets in G-I) remains unaltered. Abnormal suspensor cell divisions (F) lead to a massive three-dimensional cell arrangement (I,J), clearly recognizable as part of the embryo proper by the triangular stage (L,Q). Note the presence of a basally extended epidermal layer (I) and the presence of additional cell tiers in the embryo proper in amp1 mutants (numbered in I). Note the emergence of cotyledon primordia from basal positions and of an oversized SAM at triangular (P), heart (Q-S) and torpedo (T) stage in amp1 mutants. (U) Quantification of cell tier numbers (as illustrated in I) for wild-type and amp1 globular-stage embryos (n values between 42 and 51; error bars indicate s.e.m.). (V,W) Formation of a second embryo (outlined cells) and of twin-seedlings from single seeds in amp1-13 mutant seeds. Arrows in W point to two separate roots. (X-AA) Bent-cotyledon-stage embryos of wild-type (X), amp1-9 (Y), amp1-9 mpG92 (Z) and mpG92 (AA) genotype. The embryo in Z represents a largely normalized individual from a spectrum of embryonic phenotypes. All images except W are cleared whole-mounts viewed with DIC optics. ut, upper tier; lt, lower tier. Scale bars: 20 µm in A-J,V; 50 µm in K-T,X-AA.