The formation of different tissue systems in multicellular organisms depends on the activity of groups of undifferentiated cells called stem cells. In vascular plants, the three principal tissue systems--dermal, ground, and vascular--are derived from specific groups of stem cells that are laid down during embryogenesis. We show here that SCHIZORIZA (SCZ) is necessary for the early establishment of the stem cells that produce the ground tissue in the embryonic root meristem. Our results show that SCZ expression in stem cells is sufficient to maintain these cells in an undifferentiated stem cell state. Furthermore, we show that embryos that lack SCZ and SCARECROW (SCR) functions do not form a ground tissue because they do not develop ground tissue stem cells. We demonstrate that the formation of a complex tissue system requires the interaction between specific stem cell fate regulatory genes and patterning genes. Our study reveals a new function for a member of the heat-shock transcription factor family in stem cell development and provides a molecular framework to understand stem cell formation during embryogenesis.
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