Fig. 8. Schematic of de novo shoot meristem organization from callus. Auxin-rich CIM (A) induces proliferation of multipotent cells in the root leading to callus formation (B). (C) Transfer to cytokinin-rich SIM induces partition of cell identity and behavior within callus marked by the CUC2 (yellow) and WUS (red) reporters. Arrowhead indicates shoot progenitors. (D) Clusters of CUC2-labeled shoot progenitors proliferate among neighboring WUS expressing (red lines) non-progenitor cells in areas of high cytokinin and low auxin response. (E) 24-48 hours later, PIN1 and ML1 reporters (both green) are upregulated within the superficial layer of the shoot promeristem while STM (blue) is upregulated in a ring of surrounding cells and within the promeristem. Within the membrane of shoot progenitors, PIN1 protein is directed towards the apex of the promeristem (arrows), and thus is predicted to transport auxin into the promeristem from surrounding cells. (F) 48-96 hours later, PIN1 becomes locally upregulated within the peripheral zone and marks sites of primordial initiation. PIN1 protein becomes locally polarized towards sites of primordia formation (arrows). FIL (magenta) is expressed in the abaxial sides of newly initiated primordia. CLV3 expression (teal) is initiated within the central zone after WUS expression (red) initiates within the center of the meristem. pSTM::STM-VENUS is expressed within the meristem.