Fig. 3. Partitioning of the apical domain into shoot apical meristem and cotyledons. (A) Transition-stage and early heart-stage Arabidopsis embryos showing the direction of auxin transport (arrows) and the partitioning of the embryo into cotyledonary (green) and meristem (orange) domains. (B) Cross section (as shown in A) through the apical domain of a wild-type embryo, showing the region that will develop into the shoot apical meristem (dark orange), the intercotyledonary zones (light orange), the adaxial domain (top) of the cotyledon (dark green) and the abaxial domain (bottom) of the cotyledon (light green). The meristem and intercotyledonary zones have low auxin levels and high CUC levels, whereas the opposite holds for the cotyledon primordia. (C) Cross section of the apical domain from a cup-shaped cuc1 cuc2 cotyledon mutant. No separation is made between the cotyledons and no shoot apical meristem is made in the central region of the embryo. (D) Cross section of the apical domain from a pin pid embryo. These embryos lack the PIN1 auxin transporter. They also lack the PID kinase that appears to be responsible for positioning PIN1 on the apical side of the cell. Lack of high auxin levels in the cotyledon primordia in these embryos would allow CUC accumulation throughout the apical domain, thus preventing cotyledon outgrowth.