38 The Plant Cytoskeleton

Abstract

The cytoskeleton is a system of fibrous polymers (F-actin, microtubules, and intermediate filaments) that provide spatial organization to many crucial subcellular processes. The plant polypeptides related to those forming intermediate filaments in animal cells are not discussed here, since their in vivo aggregation state and biological functions are not yet known (Shaw et al. 1991; McNulty and Saunders 1992). In contrast, it is known that F-actin and microtubules participate in mitosis, cytokinesis, cellulose microfibril alignment, and organelle movements. Although the cytoskeleton is an entirely cytoplasmic system, many cytoskeletal components lie near the plasma membrane, where they are well placed to respond to chemical and physical stimuli transmitted across the plasma membrane (Luna 1991). Importantly, in plants, they influence wall placement at cytokinesis and wall architecture during cell expansion. The subcellular organization of the cytoskeleton during cell division and cell expansion and the characteristic planes of cell division seen in various organs are reflected in cell and organ shape. Thus, the effects on cell-wall structure amplify and propagate the cytoskeleton’s influence on development.

The subunits of the cytoskeleton’s major fibrous polymers (actins and tubulins) are quite highly conserved among all eukaryotes, even though the roles they perform vary enormously through the eukaryotic kingdoms and even within single cells during ontogeny. Such versatility probably requires the participation of many less highly conserved proteins that interact with these primary cytoskeletal polymers, such as microtubule-associated proteins (MAPs) and actin-binding proteins (ABPs), together with motor proteins specific for both actin and microtubules. In addition, many...