Apikalmeristem

Apikalmeristem, typically written as apical meristem, is a region of undifferentiated, actively dividing plant cells located at the tips of shoots and roots. It drives primary growth by producing new tissues, enabling lengthening of stems and roots.

In the shoot apical meristem (SAM), the tissue is located at the tip of the shoot and is often dome-shaped. It maintains a small pool of stem cells in an organizing center. Peripheral cells give rise to leaf primordia and initiate new internodes, while central cells contribute to the ground and vascular tissues during development.

In the root apical meristem (RAM), the tissue sits at the root apex and comprises a quiescent center and surrounding initial cells that generate the root cap and the tissues of the root axis (epidermis, cortex, endodermis, vascular tissue). RAM activity involves a balance of cell division and differentiation, creating distinct zones: the meristematic zone, elongation zone, and differentiation zone as cells move away from the tip.

Regulation of apical meristems involves hormonal signals, especially auxin and cytokinin, and gene regulatory networks that govern stem cell maintenance and patterning. In the SAM, regulatory pathways such as WUS-CLAVATA control stem cell numbers and organ initiation, while in the RAM, PLT, SHR, and SCR genes contribute to maintenance and radial patterning. Environmental factors like light, nutrients, and stress also influence meristem activity.

Apical meristems are essential for plant growth and development, shaping plant architecture and enabling continuous organ formation throughout the life of many species.