Microtubule organization and cell morphogenesis in two semi-lobed cell types of Adiantum capillus-veneris L. leaflets.

Protodermal cells of Adiantum capillus-veneris leaflets are polyhedral, displaying regularly arranged cortical microtubules transverse to the main cell axis. The nascent epidermal cells partly detach from the underlying mesophyll cells by formation of intercellular spaces, containing PAS-positive material. In early differentiating upper epidermal cells discrete U-like bundles of cortical microtubules form on the internal periclinal and the anticlinal walls. In contrast, microtubules are randomly scattered along the external periclinal wall. Microtubule bundles of neighbouring anticlinal walls of epidermal cells exhibit an alternating disposition but are directly opposite to those of underlying mesophyll cells. Epidermal cell wall is locally reinforced by thickenings arising under the microtubule bundles and including parallel cellulose microfibrils. The pattern of wall thickenings reflects that of the microtubule bundles. The internal periclinal epidermal cell region expands at the sectors free of wall thickenings, forming several lobes. Simultaneously, intercellular spaces open at the thickened regions of anticlinal walls, which finally become wavy. In contrast, the external periclinal wall does not form any lobes but remains smooth. As a result, epidermal cells become 'semi-lobed'. The lobes of lower epidermal cells are less prominent. Mesophyll cells surrounding the endodermis are also 'semi-lobed'. Their morphogenesis is achieved by the same mechanism. Colchicine treatment inhibits the 'semi-lobed' morphogenesis of epidermal cells and mesophyll cells surrounding the endodermis and the concomitant intercellular space opening. These observations reveal that the primary event of 'semi-lobed' cell morphogenesis is the organization of two different patterns of the cortical microtubule cytoskeleton in the same cell.