Microtubule Components of the Plant Cell Cytoskeleton

Like the skeleton of any vertebrate animal, the cytoskeleton plays a major role in detennining the three-dimensional form of a cell. However, in plants the cytoskeleton is probably more analogous to a scaffold, being directly involved in the establishment of cell morphology and less involved in the maintenance of cell shape once the relatively rigid cell walls are in place. The term "skeleton" unfortunately tends to connote a rigid, somewhat permanent structure, but it is clear that much of the cytoskeleton is dynamic and polymorphous. Microtubules in particular are continually being disassembled, assembled, and rearranged into new configurations or arrays as a cell progresses through division and differentiation. The cellulosic wall of a typical plant cell largely precludes cell migration, which plays a central role in animal development. Thus, morphogenesis in plants is govemed primarily by the processes that establish the planes of cell divisions and the axes of cell expansions, and components of the cytoskeleton play important roles in both processes. In addition, the plant cytoskeleton is directly involved in the intracellular transport of a wide variety of macromolecules, vesicles, organelles, and other macromolecular structures essentia1 to plant growth and development. A11 three major classes of cytoskeletal filaments-microtubules, intermediate filaments, and microfilaments-are present in plant cells. However, our discussion will be restricted to the microtubule-based components of the plant cytoskeleton. First, we will describe the major microtubule arrays present in plant cells and discuss their known and/or predicted functions. Then we will focus on (a) recent information about the large tubulin gene families in plant cells and their possible significance, (b) questions about MTOCs in plants and the probable nucleation of microtubule assembly by y-tubulin, and (c) the roles of MAPs in plant cells. Finally, we emphasize that a clear picture of plant cell growth and differentiation will become available only when we understand the coordinated interactions between the cytoskeleton, the cell nucleus, the nuclear envelope, the ER, the plasma membrane, and the cell wall. At present there is too little definitive information about most of these important interactions to warrant further discussion here.