Available online at www.sciencedirect.com Actin dynamics in plant cells: a team effort from multiple proteins orchestrates this very fast-paced game Laurent Blanchoin 1 , Rajaa Boujemaa-Paterski 1 , Jessica L Henty 2 , Parul Khurana 2 and Christopher J Staiger 2,3 Gazing at a giant redwood tree in the Pacific Northwest, that has grown to enormous heights over centuries, does little to convince one that plants are built for speed and versatility. Even at the cellular level, a system of polymers — the cell skeleton or cytoskeleton — integrates signals and generates subcellular structures spanning scales of a few nanometers to hundreds of micrometers that coordinate cell growth. The term cytoskeleton itself connotes a stable structure. Clearly, this is not the case. Recent studies using advanced imaging modalities reveal the plant actin cytoskeleton to be a highly dynamic, ever changing assemblage of polymers. These insights along with growing evidence about the biochemical/ biophysical properties of plant cytoskeletal polymers, especially those obtained by single filament imaging and reconstituted systems of purified proteins analyzed by total internal reflection fluorescence microscopy, allow the generation of a unique model for the dynamic turnover of actin filaments, termed stochastic dynamics. Here, we review several significant advances and highlight opportunities that will position plants at the vanguard of research on actin organization and turnover. A challenge for the future will be to apply the power of reverse-genetics in several model organisms to test the molecular details of this new model. Addresses 1 Institut de Recherches en Technologies et Sciences pour le Vivant – iRTSV, Laboratoire de Phyiologie Cellulaire et Ve ´ ge ´ tale, Commissariat a ` l’Energie Atomique /Centre National de la Recherche Scientifique/ Institut National de la Recherche Agronomique/Universite ´ Joseph Fourier, CEA Grenoble, F38054, Grenoble, France 2 Department of Biological Sciences, Hansen Life Sciences Research Building, Purdue University, West Lafayette, IN 47907-2064, USA 3 The Bindley Bioscience Center, Purdue University, West Lafayette, IN 47907, USA Corresponding authors: Blanchoin, Laurent (laurent.blanchoin@cea.fr) and Staiger, Christopher J (staiger@purdue.edu) Current Opinion in Plant Biology 2010, 13:714–723 This review comes from a themed issue on Cell biology Edited by Christian Luschnig and Claire Grierson Available online 21st October 2010 1369-5266/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2010.09.013 Actin dynamics are well understood from in vitro analyses In eukaryotes, the actin cytoskeleton is an essential molecular machine that creates structures and generates forces that support a diverse array of cellular functions, including morphogenesis, establishment of polarity and motility [1,2]. Powering these cellular functions often depends on the ability of the cytoskeleton to auto- organize and modulate its dynamics to create novel and often transient higher-order structures. In plant cells, responses to hormones or to attack by micro-organisms, along with cell morphogenesis pathways, induce signaling cascades that correlate with the rearrangement or turn- over of actin-based structures [3,4]. Actin dynamics consist of the assembly and disassembly of a helical polymer, or ‘actin filament’, with mechanical properties that are tuned for its specific cellular functions [5]. Assembly of actin filaments occurs through two funda- mental steps, called nucleation and elongation (Figure 1, green box; [6]). Nucleation is a set of reactions involving the association of actin monomers into dimers, followed by trimer formation (Figure 1, Step 1). These trimers are thermodynamically unstable, but provide the ‘seeds’ or nuclei necessary for actin filament assembly [7]. Certain accessory proteins, like profilin, potently inhibit spon- taneous nucleation of actin ([3,8]; Figure 1, Step 1). Fol- lowing this nucleation step, the elongation of actin trimers occurs rapidly at a rate that depends on the actin monomer concentration and the particular end of the actin filament at which these monomers are added. Indeed, the two ends of actin filaments are not equivalent; for historical reasons, they are referred to as ‘barbed’ and ‘pointed’. In the absence of regulatory proteins, growth at the barbed ends of actin filaments is up to 10 times faster than at the pointed ends [9]. However, this difference can climb up to 100 times in the presence of proteins that accelerate actin assembly, such as formins (Figure 1, Step 3; [10,11,12  ]). Following elongation, aging of actin filaments is controlled by the hydrolysis and phosphate release of the nucleotide bound to actin subunits [7]. At steady state in vitro, the concentration of actin monomers reaches a critical concen- tration that varies from 0.1 mM if barbed ends are free, to 0.7 mM if only pointed ends are available [7]. Under these conditions, the turnover of actin filaments utilizes a mech- anism called ‘treadmilling’ (Figure 1, grey box), where the rate of assembly at the barbed ends is balanced by the rate of depolymerization at the pointed ends with a rate of about Current Opinion in Plant Biology 2010, 13:714–723 www.sciencedirect.com