Actin-based domains of the "cell periphery complex" and their associationswith polarized "cell bodies" in higher plants

Nascent cellulosic cell wall microfibrils and transverse (with respect of c ell growth axis) arrays of cortical microtubules (MTs) beneath the plasma m embrane (PM) are two well established features of the periphery of higher p lant cells. Together with transmembrane synthase complexes, they represent the most characteristic form of a "cell periphery complex" of higher plant cells which determines the orientation of the diffuse (intercalary) type of their cell growth. However, there are some plant cell types having distinc t cell cortex domains which are depleted of cortical MTs. These particular cell cortex domains are, instead, typically enriched with components of the actin-based cytoskeleton. In higher plants, this feature is prominent at e xtending apices of two cell types displaying tip growth - pollen tubes and root hairs. In the latter cell type, highly dynamic F-actin meshworks accum ulate at extending tips, and they appear to be critical for the apparently motile character of these subcellular domains. Importantly, tip growth of b oth root hairs and pollen tubes is immediately stopped when the most dynami c F-actin population is depolymerized with low levels of anti-F-actin drugs . Intriguingly, MTs of tip-growing plant cells are organized in the form of longitudinal arrays, throughout the cytoplasm, which interconnect the exte nding tips with the subapical nuclei. This suggests that actin-rich cell co rtex domains polarize plant "cell bodies" represented by nucleus-MTs comple xes. A similar polarization of "cell bodies" is typical of mitotic and cyto kinetic plant cells. A further type of MT-depleted and actomyosin-enriched plant cell cortex domain comprises the plasmodesmata. Primary plasmodesmata are formed during cytokinesis as part of the myosin VIII-enriched callosic cell plates, representing "juvenile" forms of the plant "cell periphery co mplex". In phylogenetic terms the association between F-actin and the PM ma y be considered for a more "primitive" form of cellular organization than d oes the association of cortical MTs with the PM. We hypothesize that the ac tin cytoskeleton is a natural partner of the PM in all eukaryotic cells. In most plant cells, however, it was replaced by a tubulin-based "cell periph ery apparatus" which regulates, via still unknown mechanisms, the spatial d eposition of nascent cellulosic microfibrils synthesized by PM-associated s ynthase complexes.