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GENE REGULATORY NETWORK CONTROLLING THE ROOT STEM CELL NICHE IN ARABIDOPSIS THALIANA

Final Report Summary - STEM CELL REGULATION (GENE REGULATORY NETWORK CONTROLLING THE ROOT STEM CELL NICHE IN ARABIDOPSIS THALIANA)

Plant stem cells are located in apical meristems at the growing tips of the shoot and the root. Within the root meristem, signals from the slowly dividing cells of the so-called quiescent center (QC) maintain the neighboring stem cells in an undifferentiated state. The homeobox gene WUSCHEL-RELATED HOMEOBOX 5 (WOX5) is QC-specifically expressed and plays a central role in the control of the distal stem cells as well as in QC specification. Lack of WOX5 activity results in loss of distal stem cells, QC marker expression and altered stem cell niche morphology.
To understand the molecular events triggered by WOX5 activity, we performed genome-wide binding and transcriptome profiling studies. First, we identified WOX5 primary target genes by chromatin immunoprecipitation with a WOX5 specific antibody followed by hybridization to a tiling array. The analysis in combination with QC-specific expression data sets allowed us to identify direct targets of WOX5. Candidate genes have been verified for direct binding by WOX5 with independent ChIP-qPCR experiments and by fluorescent reporters in the wild type and wox5 mutant background.
The majority of genes directly targeted by WOX5 were up-regulated. Thus, we conclude that WOX5 acts mainly as a repressor in the QC. To test the biological relevance of the repression of the candidate genes, we started a functional characterization by overexpressing selected candidate genes specifically in the QC in the wild type background. The first results suggest that several candidate genes of WOX5 have to be repressed in the QC to maintain a functional stem cell niche. Thus, the modulation of the transcriptional profile of the identified target genes in the QC might be crucial for QC as well as for stem cell maintenance.
The functional characterization of the identified direct targets of WOX5 will advance our understanding of the regulatory network required for stem cell maintenance and cell fate maintenance in general. These are crucial steps towards future agricultural applications such as the enhancement of the regeneration of economically important plants. Furthermore, studies of the stem cell niche in plants can be used as a complementary approach to gain insights into stem cell regulation in general. Animals and plants have evolved multi-cellularity separately and how the integrity of stem cells is maintained and how the transition from pluripotency to differentiation is balanced are key questions in biology.