Although plants are sessile organisms with unmoving cells, plant cell organelles are in constant motion. Cytoplasmic streaming is a phenomenon exclusive to plant cells wherein organelles will rapidly migrate around the cell. Streaming is controlled by myosin motors anchored to an actin filament network. This phenomenon necessitates only the actin cytoskeleton as a series of experiments showed that streaming doesn’t require the cooperation of microtubules. Additionally, these experiments demonstrated that actin filaments can maintain cytoplasmic streaming by simply disassembling and reassembling recycled actin monomers. In light of these discoveries, there are three non-mutually exclusive theories regarding the mechanism of cytoplasmic streaming. Active streaming theory asserts that myosin motors riding along actin filaments walk organelles around the plant cell. Passive streaming theory posits that the bulk of organelle movement around the plant cell is driven by a cytosolic stream produced by the active transport of only a few organelles. Endoplasmic reticulum (ER) anchor theory states that the majority of organelles are bound to the ER and that myosin motors move the ER, 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 one model predominates.
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