The SHORT-ROOT protein acts as a mobile, dose-dependent signal in patterning the ground tissue.

A key question in developmental biology is how cellular patterns are created and maintained. During the formation of the Arabidopsis root, the endodermis, middle cortex (MC), and cortex are produced by periclinal cell divisions that occur at different positions and at different times in root development. The endodermis and cortex arise continuously from the periclinal divisions of cells that surround the quiescent center (QC) at the tip of the root. The MC arises between days 7 and 14 from periclinal divisions of the endodermis. The divisions that produce the middle cortex begin in the basal region of the root meristem away from the QC and then spread apically and circumferentially around the root. Although the transcription factor SHORT-ROOT (SHR) is required for both of these divisions, the mechanism that determines where and when SHR acts to promote cell division along the longitudinal axis of the root is unknown; SHR is present along the entire length of the root tip, but only promotes periclinal divisions at specific sites. Here we show that the abundance of the SHR protein changes dynamically as the root develops, and that the pattern of cell division within the endodermis is sensitive to the dose of this protein: high levels of SHR prevent the formation of the MC, whereas intermediate levels of SHR promote MC formation. These results provide a mechanism for the longitudinal patterning of the endodermis, and represent the first example in plants of a mobile transcription factor whose function (activator or repressor) depends upon concentration.