Figure 1.

Diagram of SAM and RAM in Arabidopsis thaliana. In the SAM, a stem cell pool is located in the Central Zone (CZ, purple) above the organizing center (OC, yellow), which expresses the WUSCHEL transcription factor (TF) (yellow). Cells that pass the boundary defined by CLAVATA (CLV) function start differentiation, establishing the organ founder cell population. The RAM consists of a pool of quiescent cells (QC, cyan) surrounded by a pool of initial cells (the consideration of initial cells as stem cells depends on the author). This regulatory model highlights the complex interplay of phytohormones and TFs in the WOX domain. Arrows indicate activation and barred lines indicate inhibition. The dashed arrows beside auxin (AUX) and cytokinin (CK) indicate the hormone flow direction. AUX produced in the SAM and young leaves are basipetally transported through the stem by the polar auxin transport (PAT) stream toward the RAM. CK biosynthesis genes are expressed in the RAM differentiation zone and acropetally transported. CZ, central zone; PZ, peripheral zone; LP, lateral primordium [33,34].