Basic layout of the shoot apical meristem and our model.

<p>A) Schematic representation of the SAM. The central zone (CZ) is located in the center of the shoot apex and contains the stem cells (SCs). <i>CLV3</i> expression marks the SC domain. Directly below the SC domain is the organizing center (OC), which is defined by the expression of <i>WUS</i>. Lateral to the CZ is the peripheral zone (PZ), which consists of rapidly proliferating cells. <i>STM</i> expression in the three outer cell layers partially correlates with the PZ. The surface width of the <i>STM</i> expression domain (dashed line) as well as the surface distance between opposing primordia (solid line) is indicated. B) Schematic representation of the stem cell pool model. Cells of the SC pool (<i>S</i>) and the OC (<i>O</i>) proliferate with rate α₁. SCs differentiate into cells of the OC with rate <i>λ</i>₁ and into cells of the proliferation zone P with rate <i>λ</i>₂. Cells of the proliferation zone can re-specify into SCs with a rate <i>ρ</i>₁. This rate depends on signals from the OC as indicated by the dotted line. Cells of the OC terminally differentiate with a rate <i>λ</i>₃, which depends on the SC pool.