Precise control of cell division is essential for proper patterning and growth during the development of multicellular organisms. Coordination of formative divisions that generate new tissue patterns with proliferative divisions that promote growth is poorly understood. SHORTROOT (SHR) and SCARECROW (SCR) are transcription factors (TFs) required for formative divisions in the stem cell niche of Arabidopsis roots. Here we show that levels of SHR and SCR early in the cell cycle determine the orientation of the division plane resulting in either formative or proliferative cell division. We employed 4D quantitative long-term and frequent (every 15 minutes for up to 48 hours) light sheet and confocal microscopy to probe the dynamics of SHR and SCR in tandem within single cells of living roots. Directly controlling their dynamics with a SHR induction system enabled us to challenge an existing bistable model of the SHR/SCR gene regulatory network and to identify key features that are essential for rescue of formative divisions in shr mutants. SHR and SCR kinetics do not align with the expected behavior of a bistable system and only low transient levels present early in the cell cycle are required for formative divisions. These results reveal an uncharacterized mechanism by which developmental regulators directly coordinate patterning and growth.
