Fig. 2. Schematic diagram of cytokinin (CK) regulation of leaf primordium initiation.

Cells in the shoot apical meristem (SAM) are arranged into the L1, L2, and L3 layers and four distinct zones: the central zone (CZ), peripheral zone (PZ), organizing center (OC), and rib zone (RZ). KNOX is expressed in almost the entire SAM. KNOX positively regulates the synthesis of cytokinins and keeps their levels high. Cytokinins promote the expression of WUS through signal transduction and transcription factors, which maintain a high cell division rate in the OC. ERECTA blocks the effect of cytokinins and promotes the transport of auxin. In areas with higher auxin concentrations, the leaf primordium begins to form. In the early stage of leaf development, KNOX, which is highly expressed in the marginal blastozone, changes the leaf morphology by promoting cytokinin synthesis to form compound leaves. Solid lines indicate direct relationships that have been confirmed; dashed lines represent potential mechanisms. The abbreviations are as defined in the text