Fig. 8. A model for the patterning of the apical region of the embryo. Solid arrows indicate activation. Dotted arrows indicate an effect on spatial expression. PIN1 promotes auxin transport and creates a specific auxin distribution. The auxin distribution activates the expression of CUC2 and influences the spatial expression pattern of CUC1. MP regulates CUC gene expression in response to auxin signals. MP could also contribute to auxin distribution through promoting auxin transport. CUC1 and CUC2 redundantly promote cotyledon separation as well as expression of STM, the latter of which in turn promote SAM formation. STM also contributes to cotyledon separation through regulating spatial expression of CUC1 and CUC2 at the late embryo. In addition, STM promotes the CUC1 activity from early embryogenesis on.