Cytoplasmic Streaming

Though plants are sessile organisms with unmoving cells, the inside of plant cells is in constant motion. Cytoplasmic streaming is a phenomenon exclusive to plant cells wherein organelles will migrate around the cell at speeds that exceed most intracellular motion. This motion is controlled by myosin motors that travel along an actin filament network. Though microtubules are the main scaffold for protein motors in animal cells, plant cells favour their actin network. Additionally, through a series of experiments, it was shown that streaming didn’t require the cooperation of the microtubule cytoskeleton nor did it need continuous production of actin monomers. Thus, actin filaments can maintain cytoplasmic streaming simply through disassembly and reassembly of recycled monomers. There are three non-mutually exclusive theories regarding the mechanism of cytoplasmic streaming. Active streaming theory suggests that myosin motors on actin filaments move the organelles around the plant cell. Passive streaming theory posits that the bulk of organelle movement around the plant cell is driven by a cytosol stream produced by the movement of only a few organelles. Endoplasmic reticulum anchor theory states that organelles are bound to the endoplasmic reticulum and that myosin motors move the endoplasmic reticulum, thus dragging the organelles anchored to it. Research has provided evidence for all three of these theories, and it has yet to be shown if they contribute to streaming in equal part or if there one model predominates.