Merocyanine 540 as an optical probe to monitor the effects of culture filtrates of Phytophthora cactorum on apple cell membranes

Abstract The response of apple ( Malus domestica ) cells to valinomycin, gramicidin and culture filtrates (CF) from an avirulent and from virulent strains of Phytophthora cactorum was monitored by measuring changes in fluorescence of cells stained with Merocyanine 540, an optical probe for changes in transmembrane electrical potential (PD). Cells of MM. 106 rootstock (susceptible to P. cactorum ) exposed to either valinomycin or to CF from virulent strains of P. cactorum showed a decrease in fluorescence, whereas cells exposed to gramicidin showed an increase in fluorescence. The response of cells to fungal culture medium (FCM) was similar to the response to CF from an avirulent strain of the fungus. These results support the feasibility of using Merocyanine 540 as a probe of PD changes in apple cells and suggest a role for P. cactorum toxin in disease development. This research opens up the possibility of using fluorescence measurements as a screening system for identifying resistance in apple germplasm to P. cactorum and for using the toxin as an in vitro selective agent.