The initiation and development of a radial array of microtubules (MTs) in guard cells of A. cepa was studied using immunofluorescence microscopy of tubulin in isolated epidermal layers. Soon after the completion of cytokinesis, MTs originate in the cortex adjacent to a central strip of the new, anticlinally oriented ventral wall separating the two guard cells.
Cortical MTs extend from the mid-region of the central strip toward the cell edge where the ventral wall joins the inner periclinal wall. They then spread in a fan-like formation along the periclinal wall and gradually extend along the lateral and end walls as well. Many MTs criss-cross at various angles as they arc past the edge formed by the junction of the ventral and periclinal walls, but they do not terminate there, indicating that, contrary to previous report, the edge is not involved in MT initiation.
Instead, the mid-region of the central strip appears to function as a planar MT-organizing zone. Initially, MTs radiate from this zone through the inner cytoplasm as well as the cortex.
During cell expansion, however, the cortical MTs increasingly predominate and consolidate into relatively thick, long bundles, while the frequency of non-cortical MTs diminishes. The apparent density of MTs per unit surface area is maintained as the cells expand and gradually flex into an elliptical shape. The guard cells eventually separate completely at the pore site. The entire process is accomplished within about 12 h.