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Abstract
The diverse architecture of many plant species belies their similar developmental origin; each was derived from a single fertilized egg cell. Subsequent divisions of this cell and its daughters, produces a simplified version of the adult plant, comprised of an apical and basal meristem interconnected by a central vascular cylinder. But how is the spatial arrangement of the meristems decided upon, and what comprises the molecular network regulating this process? In this dissertation I detail my efforts towards elucidating the molecular mechanisms regulating embryonic polarity in Arabidopsis thaliana. Previously, it was shown that the GATA transcription factor HANABA TARANU is required for positioning the inductive boundary at which the root initiates; han mutants display an apical shift in gene expression domains normally delimited by the proembryo boundary, yet often recover to form complete seedlings. I show that their recovery is due to the action of two closely related genes, HAN-LIKE 1 (HANL1) and HANL2. Loss of all three HAN genes produces embryos that are initially similar to han single mutants, but over time deteriorate and arrest as blimp-shaped structures lacking a discernible shoot or root apical meristem, and vascular precursors. Molecular markers of embryonic polarity, such as WUS and PLT1, remain unchanged in these embryos, yet many regulators of organogenesis or meristem maintenance, such as FIL, TMO5, and CLV3 are never expressed. Strikingly, genes normally confined to the basal side of the proembryo boundary, such as WOX5 and WOX8, which are normally transcribed in the precursors of the root quiescent center and distal root, expand into sub-surface and surface cell layers. I interpret this effect as partial radialization of the apical/basal pattern. HAN family genes are characterized by a short, deeply conserved amphiphilic stretch at their N-terminus, called the HAN domain. Through a number of approaches, I show that this domain is required for normal activity, and functions to mediate interactions between HAN and transcriptional co-repressors of the TPL family. These results suggest that HAN genes, which are transcribed in the proembryo, may function to directly antagonize the expression of basal or suspensor transcripts in this domain.