Effect of poly-L-ornithine on isolated tobacco mesophyll protoplasts: Evidence against stimulated pinocytosis.

The effects of poly-L-ornithine on the surface membrane of isolated tobacco protoplasts have been examined in the electron microscope using a colloidal metal oxide and a spherical virus as marker substances. No evidence was found to suggest that isolated protoplasts take up either of these markers by a pinocytotic process. Poly-L-ornithine increased the degree of damage observed in fixed preparations, and specifically caused lesions of the plasmalemma which were favoured sites for the binding of both external marker substances. It is suggested that the function of poly-L-ornithine and other treatments used to obtain virus infection of protoplasts is to stress the cell membrane to allow a non-physiological entry of high molecular weight materials. Pinocytosis appears not to occur nor to be necessary for uptake of these materials under conditions of membrane stress.