Etiology, Epidemiology and Disease Forecasting

DON, a trichothecene toxin produced by Fusarium graminearum, is postulated to have a role in pathogenesis in Fusarium head blight of barley and wheat. To understand possible roles of DON, we investigated effects of the toxin in healthy tissues. Portions of epidermis were removed from 1.1 cm barley leaf segments and the segments floated with exposed mesophyll cells in contact with DON solutions. Within 2-4 days, DON at 10-90 ppm had a bleaching effect on chloroplast pigments and damaged cell membranes, including the plasmalemma (Bushnell et al., 2002; 2005). Pending more complete publication of this work (Bushnell et al., 2007), the major findings and conclusions of this project are presented here. 1. The bleaching of leaf tissues was light dependent. Tissues turned white in light as they lost chlorophylls a and b, as well as carotenoids pigments. In the dark, tissues remained green but became flaccid and easily damaged when manipulated with forceps. 2. In both light and dark, DON damaged integrity of the plasmalemma as shown by electrolyte leakage and uptake of Evans blue. Damage was somewhat greater in dark than in light. We conclude that toxicity of DON is not light dependent and that photobleaching is a secondary consequence of damage to cell membranes and not a direct cause of cell degradation. 3. The first membrane to be damaged, as viewed by transmission electron microscopy, was the tonoplast (Bushnell et al., 2005). This allowed cytoplasm to disperse into the vacuole, an irreversible step toward cell death. This preceded dissolution of the plasmalemma, mitochondria, and chloroplasts, as well as the loss of chloroplast pigments. 4. Damage to membranes and chloroplasts apparently is related to the known ability of DON to inhibit cytoplasmic protein synthesis. Cycloheximide, an inhibitor of protein synthesis in cytoplasm of eukaryotes, caused photobleaching of barley leaf segments in our experiments much like the photobleaching caused by DON. Chloramphenicol, an inhibitor of protein synthesis in chloroplasts and prokaryotes, had little or no effect on leaf pigmentation. 5. Ca2+ added at 10 mM to test solutions greatly increased toxicity of DON. With Ca2+, DON at 10 ppm bleached leaf segments; without Ca2+, concentrations of 30 ppm or higher were required. The reasons for DON’s effectiveness need investigation. In any case, differences in sensitivity to DON among genotypes of barley and wheat or plants at different stages of development, may relate to variations in Ca2+ availability within tissues. 6. Fumonisin B1 (FB1), a non-trichothecene toxin produced by F. moniliforme, caused photobleaching and membrane damage in jimsonweed leaves as reported by Abbas et al. (1992). The results were remarkably