Plant cells employ the actin cytoskeleton to stably position organelles, as tracks for long distance transport, and to reorganize the cytoplasm in response to developmental and environmental cues. While diverse classes of actin binding proteins have been implicated in growth control, the mechanisms of cytoskeletal reorganization and the cellular functions of specific actin filament arrays are unclear. Arabidopsis trichome morphogenesis includes distinct requirements for the microtubule and actin filament cytoskeletons. It also is a genetically tractable process that is providing new knowledge about cytoskeleton function in plants. The "distorted group" of mutants defines a class of at least eight genes that are required during the actin-dependent phase of trichome growth. Using map-based cloning and a candidate gene approach, we identified mutations in ARP3 (ATARP3) and ARP2 (ATARP2) genes as the cause of the distorted1 (dis1) and wurm (wrm) phenotypes, respectively. ARP2 and ARP3 are components of the evolutionarily conserved ARP2/3 complex that nucleates actin filament polymerization [3]. Mutations in DIS1 and WRM caused severe trichome growth defects but had relatively mild effects on shoot development. DIS1 rescued the phenotype of Deltaarp3 when overexpressed in S. cerevisiae. Developing dis1 trichomes had defects in cytoplasmic actin bundle organization and reduced relative amounts of cytoplasmic actin filaments in developing branches.

Regulated growth and cell shape control are fundamentally important to the function of plant cells, tissues, and organs. The signal transduction cascades that control localized growth and cell shape, however, are not known. To better understand the relationship between cytoskeletal organization, organelle positioning, and regulated vesicle transport, we conducted a forward genetic screen to identify genes that regulate cytoskeletal organization in plants. Because of the distinct requirements for microtubules and actin filaments during leaf trichome development, a trichome-based morphology screen is an efficient approach to identify genes that affect cytoplasmic organization. The seedling lethal spike1 mutant was identified based on trichome, cotyledon, and leaf-shape defects. The predicted SPIKE1 protein shares amino acid identity with a large family of adapter proteins present in humans, flies, and worms that integrate extracellular signals with cytoskeletal reorganization. Both the trichome phenotype and immunolocalization data suggest that SPIKE1 also is involved in cytoskeletal reorganization. The assembly of laterally clustered foci of microtubules and polarized growth are early events in cotyledon development, and both processes are misregulated in spike1 epidermal cells. INTRODUCTIONIn multicellular organisms, specialized cytoplasmic organization and cell shape underlie the unique functions of cells, tissues, and organs. It is reasonable to propose that the dynamic properties of the microtubule and actin cytoskeletons and the proteins that bind them underlie much of the observed asymmetry in plant cells. During leaf and root trichoblast morphogenesis, organized actin filaments and microtubules are required (Bibikova et al., 1999;Mathur et al., 1999;Szymanski et al., 1999;Baluska et al., 2000). We are using the diagnostic shape defects of cytoskeleton-disrupted leaf trichomes to guide mutant screens for essential genes that participate in cytoskeletal reorganization and morphogenesis.Decades of cytological and biochemical research have demonstrated the importance of the actin filament and microtubule cytoskeletons during plant morphogenesis (for reviews of the interphase microtubule and actin cytoskeletons in plant cells, see Giddings and Staehelin, 1991;Cyr, 1994;Staiger, 2000). In most plant cell types, the interphase microtubule array is cortical. The most commonly cited function of the cortical microtubule array is the regulation of the alignment of newly synthesized cellulose microfibrils. However, there are cases in which microtubules and cellulose microfibrils are not coaligned (Baskin et al., 1999; Wasteneys, 2000). The interphase actin filament cytoskeleton is composed of bundled transvacuolar filaments, nucleus-associated filaments, and cortical actin filaments (Traas et al., 1987). Actin filaments provide both a scaffolding for the relatively immobile network of the endoplasmic reticulum and tracks for the rapid intracellular transport of Golgi stacks (Satiat-Jeunemaitre and Hawes, 1996;Boevink...

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Little is known about how cell shape is controlled. We are using the morphogenesis of trichomes (plant hairs) on the plant Arabidopsis thaliana as a model to study how cell shape is controlled. Wild-type Arabidopsis trichomes are large, single epidermal cells with a stalk and three or four branches, whereas in zwichel (zwi) mutants the trichomes have a shortened stalk and only two branches. To further understand the role of the ZWI gene in trichome morphogenesis we have cloned the wild-type ZWICHEL (ZWI) gene by T-DNA tagging, and report here that it encodes a member of the kinesin superfamily of microtubule motor proteins. Kinesin proteins transport diverse cellular materials in a directional manner along microtubules. Kinesin-like proteins are characterized by a highly conserved ''head'' region that comprises the motor domain, and a nonconserved ''tail'' region that is thought to participate in recognition and binding of the appropriate cargo.Plant cell expansion is central to plant development. During plant cell differentiation, the relative rates of expansion of different regions of the cell wall determine the final shape of the cell. For most cell types the functional significance of cell shape is evident. For example, the special shape of stomatal guard cells permits the opening and closing of stomata, the shape of spongy mesophyll cells allows for efficient gas exchange, and the broadly expanded but thin shape of leaf epidermal cells increases the surface area of a leaf and allows penetration of light to the underlying photosynthetic mesophyll cells.Plant cell expansion results from a complex interplay of diverse processes. These processes involve the loosening of the cell wall, the maintenance of turgor pressure which is needed to drive cell expansion, and the deposition of new cell wall material (reviewed in refs.