Plant Actin Biology

Abstract

Actin is a major component of the plant cytoskeleton. Filamentous actin (F‐actin) contributes to the maintenance of the internal architecture of the cell, drives cytoplasmic streaming (the movement of components around the cytosol) and contributes to the process of cell division. The biochemistry of plant actin has many similarities to that of yeast and animals, but has developed its own strategies and molecular machinery to organise actin filaments. F‐actin provides tracks for myosin molecular motors to transport organelles at high velocities, and actin is required for specific stages of vesicle trafficking that underpin cell growth and development. The means by which plant cells control the nucleation of new actin filaments from G‐actin have diverged considerably from yeast and metazoans. The unique aspects of the plant actin cytoskeleton are likely to have arisen from a combination of the environmental challenges to a sessile multicellular existence and an ancient diversification from the ancestors of the Opisthokonta and Amoebozoa model organisms from which the majority of eukaryote cytoskeletal paradigms have been established. Key Concepts The basic principles of actin biochemistry applicable to animals, fungi and protists are conserved in plants but there are a number of specific modifications as plants do not possess the full complement of animal actin‐binding proteins. Actin filaments in plants have to fulfil unique functions. Actin focuses on the expansion of tip growing cells, maintains cytoarchitecture and responds to the environment (e.g. polarising in response to pathogen stimulation). During cell division actin filaments participate in all major plant‐specific arrays. A stationary cell is NOT a physically inactive cell. Actin cables are exploited for organelle transport. Actin‐based transport tends to dominate over microtubule‐based movement. Plants utilise the fastest known type V myosin (known in plants as type XI) within the eukaryote kingdom. Polymerisation dynamics are thought to add to trafficking capabilities as low levels of antiactin drugs disrupt vesicle delivery and fusion during cell growth yet preserve myosin‐mediated transport. Plant‐specific proteins and novel domain combinations have become established in plants to regulate the actin cytoskeleton in accordance with the demands of sessile multicellular photosynthetic autotrophy. These cytoskeletal adaptations to sedentary life within a cell wall are distinct from fungi, possibly reflecting the ancient diversification between the opisthokonts and plants.