Evidence that the Toxic Effect of Rishitin may be due to Membrane Damage

Potato leaf epidermal strips incubated with a high concentration of rishitin (300 μg ml−1) in 1% Evans blue rapidly accumulated the stain within the cells. The nuclei stained particularly strongly and frequently increased in diameter, and many chloroplasts lysed, suggesting rapid effects on several cell membranes. Approximately 30% of isolated chloroplasts suspended in 0.15 M phosphate buffer, pH 6.5, were lysed by the addition of 300 μg ml−1 rishitin. Rishitin only slightly affected the rate of respiration of isolated tobacco protoplasts whereas 100 μg ml−1 phaseollin caused a rapid increase prior to a marked decrease in respiration rate. Rishitin increased the permeability of liposomes to a range of low molecular weight non-electrolytes particularly when mixed with the lecithin prior to liposome formation. Rishitin affected negatively charged and positively charged liposomes equally. Liposome permeability was affected more by rishitin than by phytuberin thereby correlating with rishitin's higher toxicity to plants, fungi, and bacteria. Rishitin affected the transition temperature of liposomes prepared from dimyristoyl-L-3-lecithin. These results suggest that the effect of rishitin on liposomes may be by increasing membrane fluidity and that, in vivo, membranes may be its primary site of action.