ACTIN FILAMENT-INDEPENDENT FORMATION OF TUBULAR ENDOPLASMIC-RETICULUMIN ONION EPIDERMIS CELLS

Several earlier studies in vivo and in vitro suggest that the occurren ce of tubular ER depends on an intact actomyosin system in plant cells . Any interference with the actomyosin system (disintegration of the a ctin filaments by cytochalasin D; the inactivation of myosin by we of the ionophore calcimycin to elevate or lower the calcium concentration to an unfavourable level; artificially crosslinking myosin with actin filaments by NEM) results in the loss of tubular elements and the for mation of large flat sheet-like ER sacs. previously-formed flat sheet- like ER sacs, however, split into ER tubules after lowering the cytoso lic pH by loading with a weak acid or simply by chilling the epidermis cells, although the actomyosin system is still inactivated by either cytochalasin D, calcimycin, or NEM. Our results provide strong evidenc e that the actomyosin system is not necessarily involved in the format ion of tubular ER which, most likely, is an intrinsic property of the ER membrane. A possible role for protons and/or calcium is discussed.