Diversity of Plant Actin–Myosin Systems

Interactions between the actin cytoskeleton and myosin motor proteins are crucial for force generation, intracellular transport, and morphogenesis in eukaryotic cells. In plant cells, the rapid intracellular transport system—cytoplasmic streaming—is generated by the interaction between actin and the plant-specific myosin XI. Genomic analyses have revealed numerous actin and myosin genes (paralogues) in angiosperms, suggesting that the plant actin–myosin XI system is more complex than expected. Recent molecular biological and biochemical approaches have revealed the functional diversity of actins and myosins in vascular plants. Actin isoforms show various biochemical properties in vitro and form distinct filamentous structures in cells. Myosin XIs exhibit various enzymatic properties and velocities, and their classification based on velocities crudely correlates with their expression pattern in tissues. Myosin XI isoform numbers increase with the evolution of plants from algae to angiosperms, suggesting that diversity of the actin–myosin system is essential for higher plant systems, such as development, morphogenesis, fertilisation, and environmental response. In this review, we summarise recent advances in research into the plant actin–myosin system and discuss the diversity entwined with plant evolution, and then propose a new model for intracellular transport regulated by multiple actin–myosin isoforms.