The arrangement of actin bundles and chloroplasts in the nodal regions of Characean internodal cells

Abstract

We have examined, with scanning electron microscopy, the actin bundle-stationary chloroplast association which is involved with cytoplasmic streaming in the Characean intermodal cell. Actin bundles are known to be aligned on rows of stationary chloroplasts along the cylindrical internodal cell. The nodal ends of the cells, however, are free of chloroplasts and here the actin bundles pass over a zone of tapered chloroplasts, towards the plasma membrane. Actin bundles then continue across the transverse walls of the internodal cell and down the opposite side of the cell, facilitating a continuous, uninterrupted flow of cytoplasm around the entire cell. We found the internal organisation and the development of the streaming apparatus basically to be consistent in the several species examined, although the extent to which the ends of the intemodal cells were free of chloroplasts varied considerably and was determined by the presence and distribution of nodal cells. Since plasmodesmata occur between nodal and internodal cells, the absence of chloroplasts in the endoplasm in this region may facilitate cytoplasmic continuity along the whole Characean frond. © 1993 The British Phycological Society.