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A Quantitative Bioassay for Necrosis Toxin from Pyrenophora tritici-repentis Based on Electrolyte Leakage. Chil Y. Kwon, Dept. of Plant Pathology. J. B. Rasmussen, L. J. Francl, and S. W. Meinhardt. Second, and third authors: Dept, of Plant Pathology; and fourth author: Dept, of Biochemistry, North Dakota Slate University, Fargo. Phytopathology 86:1360-1363. Accepted for publication 17 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-1360.

Pyrenophora tritici-repentis, the causal agent of tan spot of wheat, produces a cultivar-specific toxin (Ptr necrosis toxin) closely associated with the disease symptomatology. We initiated studies aimed at (he mode-of-action of Ptr necrosis toxin and, as a first step, report here a bioassay for toxin based on electrolyte leakage. Toxin exposures of 4 h or more caused enhanced electrolyte leakage from wheat line ND495 (toxin sensitive), but not cv. Erik (toxin insensitive), relative to water-treated controls. Electrolyte leakage increased with exposure times up to 12 h, the longest time tested, when toxin concentration and leaching time were held constant. Electrolyte leakage increased with toxin concentration between 1 and 20 ?g ml-1, when exposure and leaching times were constant. All toxin-sensitive wheats tested (based on toxin-induced necrosis) showed enhanced electrolyte leakage, and all insensitive wheats did not. A toxin exposure of 4 h permitted completion of the assay in 1 day. We used this rapid assay to demonstrate that incubation of ND495 seedlings at 30C for 24 h prior to toxin exposure causes a loss of toxin sensitivity. Enhanced electrolyte leakage is the most rapid response to toxin observed to date. However, the toxin exposure times required for leakage to develop (4 h or longer) suggest that effects on the plasmalemma are secondary and occur relatively late in the development of necrosis.

Additional keywords: Triticum aestivum