A coherent feed-forward loop in the Arabidopsis root stem cell organizer regulates auxin biosynthesis and columella stem cell maintenance.

Stem cells in plant meristems are kept undifferentiated by signals from surrounding cells and provide the basis for continuous organ formation. In the stem cell organizer of the Arabidopsis thaliana root, the quiescent centre (QC), the WOX5 transcription factor, functions as a central hub in regulating columella stem cell (CSC) homoeostasis. However, the processes mediating WOX5 function are only poorly understood. Here we identify the transcription factor HAN as a central mediator of WOX5-regulated stem cell maintenance. HAN is required for mitotic quiescence of QC and CSC maintenance and is sufficient to induce ectopic stem cells. WOX5 and HAN repress transcription of the differentiation factor gene CDF4 in a coherent feed-forward loop (cFFL), one output of which is the expression of the auxin biosynthesis gene TAA1 and maintenance of auxin response maxima in the organizer. These findings and mathematical modelling provide a mechanistic framework for WOX5 function in the root stem cell niche.