Probable involvement of cytoskeleton in stomatal-pore formation in Asplenium nidus L.

Summary

Stomatal-pore formation in the fern Asplenium nidus L. commences in postcytokinetic guard cells at the mid-region of the ventral wall, before the deposition of any cellulosic wall material on it, by the local movement of the adjacent plasmalemmata apart from each other. In this way a rudimentary “internal stomatal pore” is formed. At this stage the ventral wall exhibits an undulated appearance and gives a positive reaction to aniline blue. Detailed study of postcytokinetic guard cells by electron microscopy, as well as after tubulin immunolabeling and actin staining, shows that stomatal pore initiation coincides with the initiation of the organization of the anticlinal microtubule bundles along the middle of the ventral wall and the colocalization of actin filaments at the same sites. Afterwards, the stomatal pore broadens towards the periclinal walls, a phenomenon keeping pace with the further bundling of the cytoskeletal elements beneath the plasmalemmata lining the middle of the ventral wall. At this stage the anticlinal microtubule bundles lining the stomatal pore are very prominent. The above findings, as well as the fact that treatments with antimicrotubule drugs inhibit the “internal” stomatal-pore formation, denote that the cortical cytoskeleton lining the ventral wall and particularly the microtubules are involved in this process. Afterwards, distinct local wall thickenings are deposited at the sites of junction of the mid-region of the ventral wall with the periclinal walls as well as at the junctions of the polar ventral-wall ends with the external periclinal wall. Along the middle-lamella region of the former wall thickenings the fore- and rear-chambers of the stomatal pore are formed. The final stomatal-pore opening is achieved by disruption of the expanded thin median periclinal wall region inherited from the guard cell mother cell and of the overlying cuticle, which covers the stomatal pore externally and internally. At the same time the fore-chamber of the stomatal pore broadens by a schizogenous opening towards the polar ventral-wall ends. The observations show that the stomatal-pore formation in A. nidus is a unique process, which is probably restricted to ferns.