The effect of the purified host-selective toxin victorin C, a cyclized penta peptide, was compared to that of CCCP and vanadate on membrane functions of susceptible leaves, roots, and single root cap cells of Avena sativa with conventional electrophysiology. The plasmalemma depolarized irreversibly by about 80 millivolts and to below the diffusion potential within 1 hour. Concentrations as low as 12.5 picomolar were effective in the susceptible but not the resistant cultivar. Electrical membrane potential difference changes were independent of pH and could not be prevented by fusicoccin or Ca2 . Membranes began to depolarize after a lag phase that never was shorter than 6.5 minutes, even with concentrations as high as 1.25 micromolar. Membrane depolarization was accompanied by a distinct decrease in specific membrane resistance from 4.5 to 1.0 ohm times square meter on average. These changes were followed by K+ and Clefflux and extracellular alkalinization. ATP level and 02 uptake did not decrease within 2 hours. It is concluded that the victorin-induced deleterious membrane alterations are not caused by direct interaction with the plasmalemma H+-ATPase, K+ channels, lipid structure, nor energy metabolism, but they seem to be triggered by a cascade of events leading to an unspecific increase in membrane permeability. Victorin C is a host-selective toxin, a cyclized penta peptide (32), produced by the phytopathogenic fungus Cochliobolus victoriae Nelson. It is known to increase the cell membrane permeability of susceptible oat plants (Avena sativa), containing the Victoria (Vb) gene, which also encodes for resistance (Pc-2) against Puccinia coronata (16, 31). Changes in membrane permeability were most typically expressed in irreversible K+ efflux from leaves, roots, and leaf protoplasts (2, 12, 18, 24, 30). K+ efflux might be triggered by several mechanisms: (a) a direct or indirect (by stalling the energy supply) inhibition of the plasmalemma H+-ATPase and thus depolarization of the membrane and opening of the K+ channels; (b) an interaction with K+ channels; (c) a direct change in channel-unrelated unspecific membrane permeability by alteration 'Supported by the Deutsche Forschungsgemeinschaft (C. U.), by an Alexander von Humboldt Senior U.S. Scientist award, and by the National Science Foundation, DMB-8516038 (A. N.) 675 of the lipid structure or an indirect one via an intracellular sequence of events, starting with binding of the toxin to a specific membrane-located binding protein (31). The molecular mechanism, however, is still unknown. With K+ selective electrodes only flux changes beyond the nmol range can be detected. However, for a measurable change in the electrical membrane potential of plant cells, a charge transfer ofless than one pmol * cm-2 is required. Hence, measurements of membrane potential changes with conventional electrophysiological methods are at least 1000 times more sensitive in indicating changes in membrane properties than continuous K+ concentration measurements. To detect victorin-induced membrane changes as early as possible, this method was introduced in 1974 (3, 17). At that time, the toxin was available only as a mixture of several related compounds and only partly purified from the fungal culture filtrate. It is known, however, that a variety of ions, sugars, and amino acids induce large transient membrane potential changes upon addition. Therefore, the design of the present experiments was (a) to investigate if victorin C, which has been purified and characterized as the most toxic component, induces the same specific toxin-related membrane changes, (b) to determine the minimum toxic concentration, and (c) to determine the minimum period between application and the first effect in cells of different oat organs. With outside-out plasmalemma patches prepared from isolated protoplasts from susceptible oat leaves, no direct victorin interaction with single-ion channels could be detected by patch clamp techniques ( 15). Isolated protoplasts were found to respond less (9) or more slowly (6) to the toxin than intact tissue. This might be related to the isolation procedure during which generation of 0Oand H202 was found even under the most careful protocol (1 1). The consequences are an increase in plasma membrane permeability and decreased membrane potential difference (23). To avoid these problems with isolated protoplasts, in the present investigation large leaf sections and intact single root cap cells were used. Root cap cells from susceptible plants were found to be as sensitive to victorin as leaf cells (9, 10). Viable root cap cells (4, 5) are easily available from any higher plant species and from particular plant cultivars of interest. They are not coupled with each other by plasmodesmata like most suspension culture cells, which grow in small clusters (21). Due to the natural www.plantphysiol.org on September 22, 2017 Published by Downloaded from Copyright © 1991 American Society of Plant Biologists. All rights reserved.