Sprossmeristem

Sprossmeristem, also known as the shoot apical meristem (SAM), is a dome-shaped region of undifferentiated cells at the tip of the plant shoot. It remains meristematic throughout the plant’s life and continuously generates new leaves, stems, and floral organs, providing the basis for primary (axial) growth in the shoot.

Anatomy and organization: The SAM is organized into distinct functional zones. The central zone contains a pool of slowly dividing stem cells that maintain the meristem, with signals from the organizing center underneath helping to preserve their undifferentiated state. The peripheral zone surrounds the central region and consists of more rapidly dividing cells that form leaf primordia and initiate lateral organs. A rib meristem, located interior to the central zone, contributes to internal shoot tissues such as ground tissue and supports the growth of the stem. The organizing center, central to this architecture, interacts with the central zone to regulate stem cell number.

Regulation and development: Maintenance of the SAM involves genetic and hormonal networks, including signaling pathways that involve WUSCHEL and CLAVATA genes, as well as hormones like cytokinins and auxin. These cues balance stem cell renewal with the initiation of new organs, enabling the meristem to sustain indeterminate growth while producing a periodic sequence of leaves and other shoot structures.

Significance: The shoot apical meristem is essential for the architecture and continuous growth of most land plants. Its function and size influence plant form, organ patterning, and the timing of development, adapting to environmental conditions and developmental stage.