ACTOMYOSIN-BASED MOTILITY OF ENDOPLASMIC-RETICULUM AND CHLOROPLASTS IN VALLISNERIA MESOPHYLL-CELLS

Intracellular localization and motile behaviour of the endoplasmic ret iculum (ER), plastids and mitochondria were studied in living mesophyl l cells of Vallisneria using the vital fluorochrome 3,3'-dihexyloxacar bocyanine iodide (DIOC6(3)). In quiescent cells, the ER was composed o f a three-dimensional network of tubular and lamellar elements. Chloro plasts were distributed evenly throughout the cell periphery and appea red embedded within the ER network. The ER network was relatively stat ionary, with the exception of rare motile episodes occurring as moveme nt of tubular ER strands and adjacent areas of the polygonal network i n localized areas of the cell. During experimental induction of stream ing, most of the Lamellar ER elements transformed into tubules and tog ether with the chloroplasts they began to translocate to the anticlina l walls to establish the circular streaming around the circumference o f the cell. Microwave-accelerated fixation followed by immunofluoresce nce revealed an hitherto unknown phase of actin reorganization occurri ng within the cells and most interestingly at the surface of the chlor oplasts during streaming induction. Myosin was localized in an ER-like pattern in quiescent as well as in streaming cells, with bright fluor escent label localized on mitochondria and proplastids. In addition, m yosin label appeared on the surface of the chloroplasts, preferentiall y in streaming mesophyll cells. Motile activities were impeded by the actin-depolymerizing drug cytochalasin D (CD), the thioreagent N-ethyl maleimide (NEM), and thapsigargin, an inhibitor of the ER-Ca2+-ATPase. These inhibitors also interfered with the integrity of actin filament s, the intracellular distribution of myosin and calcium-homeostasis, r espectively. These effects suggested an obligate association of at lea st one type of myosin with the membranes of ER and smaller organelles and are consistent with the appearance of another type of myosin on th e chloroplast surface upon streaming induction.