HD-ZIP IV genes are essential for embryo initial cell polarization in the radial axis initiation in Arabidopsis

Plants develop along apical-basal and radial axes. In Arabidopsis thaliana, the radial axis becomes evident when the cells of the eight-cell proembryo divide periclinally, forming inner and outer cell layers. Although changes in cell polarity or morphology likely precede this oriented cell division, the initial events and the factors regulating radial axis formation remain elusive. Here, we report that three transcription factors belonging to class IV homeodomain-leucine zipper (HD-ZIP IV) family redundantly regulate radial pattern formation: HOMEODOMAIN GLABROUS11 (HDG11), HDG12, and PROTODERMAL FACTOR2 (PDF2). The hdg11 hdg12 pdf2 triple mutant failed to undergo periclinal division at the eight-cell stage and cell differentiation along the radial axis. Live-cell imaging revealed that this failure in radial axis formation can be traced back to the behavior of the embryo initial cell (apical cell), which is generated by zygote division. In the wild type, the apical cell grows longitudinally and then radially and its nucleus remains at the bottom of the cell, where the vertical cell plate emerges. By contrast, the mutant apical cell elongates longitudinally and its nucleus releases from its basal position, resulting in a transverse division. Computer simulations based on the live-cell imaging data confirmed the importance of the geometric rule (the minimal plane principle and nucleus-passing principle) in determining the cell division plane. We propose that HDG11, HDG12, and PDF2 promote apical cell polarization, i.e., radial cell growth and basal nuclear retention, as the initial event of radial axis formation during embryogenesis.