UC San Diego

The formation of the apical-basal axis during Arabidopsis thaliana embryogenesis is a crucial step that directs the overall body plan of the seedling and directs the establishment of stem cell populations at either end of the embryo that give rise to all of the adult structures. The establishment of the apical-basal axis occurs with the first division of the zygote and remains plastic until the transition stage, where apical-basal polarity becomes fixed. The molecular mechanisms and genes responsible for the formation and fixation of the apical-basal axis remain largely unknown. This dissertation describes the advances made toward the elucidation of two major pathways that contribute to the formation and fixation of the apical- basal axis. Transcriptional repression plays a major role in developmental processes in general and this dissertation describes the role of the TOPLESS (TPL) gene in repression of the basal embryonic gene program in the developing apical domain of the embryo. We provide a framework for the function of TPL in transcriptional repression through chromatin remodeling. Furthermore, we show that the PLETHORA (PLT) family of transcription factors are direct targets for TPL mediated repression and necessary for the shoot to root transformation seen in the tpl-1 mutant. Secondly, using the tpl-1 mutant as a tool for forward genetics, the HD-ZIP Class III transcription factor family members are identified as determinants of apical fate in the globular stage embryo. Additionally, the HD-ZIP III genes are shown to have a mutually antagonistic relationship with the PLT gene family. Lastly, we show that the HD-ZIP III genes are true apical determinants and able to cause a homeotic conversion of the basal pole to a second apical pole