The plant cytoskeleton

The plant cytoskeleton is a dynamic, three dimensional array of filamentous protein polymers, consisting of microtubules (MTs) and microfilaments (MFs). These cytoskeletal components play active roles in most, if not all aspects of plant cell growth, development and interaction with the environment. Recent advances in molecular genetics, cell imaging and the exploitation of plant systems, such as Arabidopsis thaliana have resulted in substantial advances in our understanding of the protein composition, regulation, dynamics of the cytoskeleton and interactions between MTs and MFs. MTs consist of a variety of tubulin isotypes (coded for by an array of tubulin genes and subsequently altered through post-translational modifications) and a large number of MT-associated proteins (MAPs). The complexity of MT chemistry (tubulins and MAPs) has helped explain how very similarly looking MTs can accomplish the wide array of observed stabilities, dynamics and functions. Although plants lack the recognizable MT organizing centers (MTOCs) found in other eukaryotes, MT assembly and reorganization is accomplished through complexes of gamma tubulin (γ-tubulin) and gamma tubulin complex proteins (GCPs) which regulate when, where, and in which pattern MTs are formed. Once formed, MT dynamics and function is controlled by a wide-array of MAPs, many of which have comparable homologs in other eukaryotic systems while other MAPs are plant-specific. Similar to MTs, MFs in plants exhibit a variety of actin isotypes (coded by an array of actin genes) and a large number of actin-binding proteins (ABPs). Actin proteins are divided into those involved in vegetative processes and those involved in reproductive processes. The specific functions of MFs are controlled by the activities of various ABPs. Many of the process originally thought to involve only MTs, have now been shown to require the proper functioning of both MTs and MFs. All aspects of cell reproduction (establishment of division plane, mitosis and cytokinesis) involve both MTs and MFs. Cell growth and shape is also controlled by the cytoskeleton. While MTs play active roles in regulating wall patterning and subsequent shape of cells growing via a diffuse growth mechanism, MFs are the key cytoskeletal elements important in defining and controlling cell expansion and shape in tip-growing systems. While great strides have been made in unraveling the complexities of cytoskeletal function in plants, much more needs doing. MTs and MFs appear to play a much more integrative role in plant development than previously believed. It is predicted that many more MAPs and ABPs await discovery, resulting more clarity in our understanding of the role of MTs and MFs in plant growth, development and interaction with the environment.