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John Schiefelbein

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Primary Appointment: LSA Molec./Cell./Develop. Bio
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 DESCRIPTION OF RESEARCH
  Multicellular organisms possess diverse cell types that are organized in particular patterns. In our laboratory, we seek to understand how cells acquire their distinct identities and form appropriate patterns within tissues. Our model system is the formation of the root epidermis in Arabidopsis. This is a useful tissue because it contains a single cell layer with only two cell-types that are morphologically distinct (hair cells and non-hair cells), and it forms rapidly and continuously after seed germination. Furthermore, mutations affecting the formation of either epidermal cell type do not affect plant viability. The hair and non-hair cell types arise in a position-dependent pattern that implies cell-cell interactions are critical for cell fate specification. We have identified and analyzed several genes that influence the fate of developing epidermal cells. Some of these genes (e.g. GLABRA2, WEREWOLF, and GLABRA3) encode transcription factors important for non-hair cell specification, whereas others (e.g. CAPRICE) help define the hair cell type. By studying the expression and interactions between these genes, we have found that transcriptional feedback loops acting within and between adjacent cells are important in establishing the cell type pattern. Specifically, the WEREWOLF MYB-type protein and the GLABRA3 bHLH-type protein are positive regulators of GLABRA2 and CAPRICE, whereas the truncated MYB encoded by CAPRICE participates in a lateral inhibition mechanism to negatively regulate WEREWOLF, GLABRA2, and CAPRICE gene expression. Furthermore, GLABRA3 negatively autoregulates its own expression. During epidermis development, positional cues, acting through a leucine-rich-repeat receptor-like kinase (SCRAMBLED), apparently influence WEREWOLF gene expression and cause a relatively high level of GLABRA2 and CAPRICE expression in cells located in a particular position (the N cell position), which leads to the non-hair cell fate. Cells in the alternate position (the H cell position) are subjected to the CAPRICE-mediated lateral inhibition and adopt the hair fate. These findings provide a molecular genetic framework for understanding the determination of a cell type pattern in plants. We are currently using genomic and genetic approaches to expand our understanding of the regulatory network that governs the differentiation and patterning of these cell types.