Figure 8.

Overview of successive stages of maize embryogenesis and model for the ZmPIN1-mediated auxin transport during maize embryogenesis. Medial longitudinal sections of proembryo, transition, coleoptilar, and L1 and L2 stage maize embryos are shown, adapted from Bommert and Werr (2001). The localization of ZmPIN1 proteins in embryo plasma membrane is reported in red, and arrows indicate the polar auxin fluxes deduced by our results. After the first division of the zygote, a series of cell divisions in unpredictable planes gives rise, at the proembryo stage, to the small embryo proper and the larger suspensor. During the transition stage, adaxial/abaxial polarity is manifested by outgrowth of the scutellum at the abaxial side of the embryo. By the late transition stage, the SAM is evident as a group of cytoplasm-rich cells on the adaxial side. At the coleoptilar stage, the coleoptile becomes evident histologically above the SAM. Activity of the SAM is detected at leaf stage 1 (L1) by emergence of the first leaf primordium opposite the coleoptile. Note that the shoot-root axis is oriented in an oblique angle relative to the apical-basal polarity of the proembryo. The second leaf is initiated opposite the first leaf. The RAM is protected by the coleorhiza. clh, Coleorhiza; col, coleoptile; ep, embryo proper; I1, incipient primordium; P1 and P2, leaf primordia; scu, scutellum; su, suspensor.