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Physiology and Biochemistry

Selective Toxin of Helminthosporium victoriae: Thermal Relationships in Effects on Oat Tissues and Protoplasts. Steven P. Briggs, Graduate research assistant, Department of Botany and Plant Pathology, Pesticide Research Center, Michigan State University, East Lansing 48824, Present address of senior author: Department of Biotechnology Research, Pioneer Hi-Bred International, Inc., Box 38, Johnston, IA 50131.; Robert P. Scheffer(2), and Alfred R. Haug(3). (2)Professor, Department of Botany and Plant Pathology, Pesticide Research Center, Michigan State University, East Lansing 48824; (3)Professor, Department of Microbiology, Pesticide Research Center, Michigan State University, East Lansing 48824. Phytopathology 74:768-773. Accepted for publication 14 January 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-768.

Electrolyte leakage induced in oat leaves susceptible to toxin from Helminthosporium victoriae was minimal at temperatures below 12 C; above 12 C, the rate of leakage increased with increases in temperature. Protoplasts from leaves were held at temperatures from 0 to 40 C, and the fluidity of membranes was measured by electron spin resonance with 5-doxylstearic acid as a spin label. A membrane phase transformation was observed at 12 ± 1 C. The temperature correlation between toxin-induced leakage and membrane fluidity suggests that loss of electrolytes occurs through the membrane matrix, possibly via a shuttle-type carrier. Toxin rapidly killed oat mesophyll protoplasts as shown by the use of fluorescein diacetate, a vital stain. Collapse of protoplasts followed death at 35 C but not at 23 C. The vital stain showed that protoplasts are as sensitive to toxin as are leaf tissues and intact roots. Tissues treated with toxin at 0 C and held at that temperature had no toxin-induced leakage. Tissues exposed to toxin at 0 C, washed, and then warmed to 23 C lost as much as did tissues held at 23 C throughout. Presumably, the initial step in toxic activity can occur at 0 C, but processes leading to electrolyte leakage occur only at higher temperatures.