Defining the Path from Stem Cells to Differentiated Tissue.

The molecular events that control the progression from stem cells to differentiated tissue are largely unknown. The root of Arabidopsis is an excellent model to study this process, as cells at every stage of development are present at all times. A network that involves the mobile transcription factor, SHORT-ROOT, controls the asymmetric division of the stem cell that gives rise to endodermis and cortex. After initially interacting with another transcription factor, SCARECROW (SCR), together they activate a D-type cyclin, which specifically regulates this asymmetric cell division. Another direct target of SCR is a MYB transcription factor that regulates formation of the Casparian strip, the signature differentiated feature of the endodermis. Thus, the outline of a regulatory network from stem cell to differentiated tissue has been traced, with much remaining to be filled in.