Description
Plants develop along apical-basal and radial axes. In Arabidopsis thaliana, the radial axis becomes evident when the cells of the 8-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 the 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 8-cell stage and cell differentiation along the radial axis. Live-cell imaging revealed that the mutant defect is already evident in the behavior of the embryo’s 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, and set proper radial axis formation during embryogenesis.
Funding information
This work was supported by the Japan Society for the Promotion of Science
(a Grant-in-Aid for Scientific Research on Innovative Areas) (JP19H05670
and JP19H05676 to M.U., JP22H04719 to K.F., and JP16H06280 [Advanced
Bioimaging Support]), a Grant-in-Aid for Scientific Research (B) (JP23H02494
to M.U. and JP20H03289 to T. Higaki), International Leading Research KEPLR
(JP22K21352 to T. Higashiyama and M.U.), the Japan Science and Technology
Agency (CREST [JPMJCR2121 to M.U., T. Higaki, and K.F.]), the Suntory Rising
Stars Encouragement Program in Life Sciences (SunRiSE; to M.U.), and the
Toray Science Foundation (20-6102 to M.U.).