Motile apparatus in Vallisneria leaf cells. I. organization of microfilaments

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Abstract
The organization of the microfilaments in epidermal cells of Vallisneria leaves was investigated with respect to the induction of cytoplasmic streaming (secondary streaming). In many of the epidermal cells, cytoplasm exhibited rotational streaming along the anticlinal wall of the cell after exposure to intense light. In these cells, many bundles of 6–7 nm microfilaments were observed in the cortex around the anticlinal wall. The bundles were arrayed in parallel to the streaming direction. They were recognized usually as 10–40 closely packed dense dots in cross-section. The spacing between bundles was not even. Bundles tended to form groups of 4 to 5 in which the spacing between bundles was usually 0·3 and 0·5 μm. The microfilaments were identified as F-actin. Together with the fact that rotational streaming in Vallisneria cells was inhibited by cytochalasin B, the motile mechanism of secondary streaming was concluded to be similar in its essential features to the cytoplasmic streaming seen in Characean cells (primary streaming). In epidermal cells that had been kept under low-intensity light the cytoplasm and the chloroplasts were mostly localized along the wall facing the exterior (the periclinal wall). No cytoplasmic streaming occurred in these cells. The bundles of microfilaments remained in the very thin layer of cytoplasm lining the anticlinal wall, although they were fewer and somewhat loosely packed. EGTA at appropriate concentration could induce cytoplasmic streaming in these cells. The mechanism of the induction is discussed on the basis of the effectiveness of EGTA and the requirement of a low concentration of free Ca2+ for cytoplasmic streaming in Characean cells.