The shoot apical meristem (SAM) is responsible for formation of all bove-ground structures, such as leaves, flowers and fruits. It is subdivided into a central zone (Cz) where stem cells are located and a peripheral zone (Pz) where fast dividing progenitor cells are located. Precursors of plant organs, so-called primordia, originate in the peripheral zone and detach from the meristem at its outer boundary.
Plant cells are encased in cell walls and cannot migrate. The stem cell fate is induced by local signals. The signal that induces stemness in the SAM is the transcription factor WUSCHEL (WUS). In response to WUS the stem cells produce CLAVATA3 (CLV3) which negatively acts on WUS production. This feedback loop regulates the number of stem cells. HECATE (HEC) is a family of factors that fine-tune the WUS-CLV3 feedback loop.
Ordinary differential equation models of the different SAM cell populations help to interpret experiments and to understand mutant phenotypes. The hec triple mutant hec1,2,3 which possesses no functional HEC is characterized by smaller meristems compared to the wildtype. Counterintuitively, the organ production per time unit is higher in the triple mutant although its meristem has a reduced number of stem and progenitor cells. The mathematical models suggest that this can be explained by a higher transition from the central to the peripheral zone and a higher rate of primordia formation.