SummaryThe contamination of agricultural products with Fusarium mycotoxins is a problem of world-wide importance. Fusarium graminearum and related species , which are important pathogens of small grain cereals and maize, produce an economically important and structurally diverse class of toxins designated trichothecenes. Trichothecenes inhibit eukaryotic protein synthesis. Therefore, a proposed role for these fungal toxins in plant disease development is to block or delay the expression of defence-related proteins induced by the plant. Using yeast as a model system, we have identified several mutations in the gene encoding ribosomal protein L3 (Rpl3), which confer semi-dominant resistance to trichothecenes. Expression of an engineered tomato RPL3 ( LeRPL3 ) cDNA, into which one of the amino acid changes identified in yeast was introduced, improved the ability of transgenic tobacco plants to adapt to the trichothecene deoxynivalenol (DON), but did not result in constitutive resistance. We show here that, in the presence of wild-type Rpl3 protein, the engineered Rpl3 protein is not utilized, unless yeast transformants or the transgenic plants are challenged with sublethal amounts of toxin. Our data from yeast twohybrid experiments suggest that affinity for the ribosome assembly factor Rrb1p could be altered by the toxin resistance-conferring mutation. This toxin-dependent utilization of the resistance-conferring Rpl3 protein could seriously limit efforts to utilize the identified target alterations in transgenic crops to increase trichothecene tolerance and Fusarium resistance.

The production of trichothecene toxins is a suspected virulence mechanism of several plant pathogenic fungi. This hypothesis has been confirmed forGibberella zeae (Fusarium graminearum) by gene disruption experiments, suggesting in turn, that resistance against the fungal toxin is a relevant component ofFusarium resistance of the host plant. Our goal is therefore to identify molecular mechanisms of trichothecene resistance. Using yeast as a model system we have found the following resistance mechanisms and genes: a) reduced uptake of deoxynivalenol (PDR5), b) toxin modification and reduction of toxicity (AYT1), and c) formation of a resistant toxin target (RPL3). Homologous plant genes exist and are attractive candidates forFusarium resistance genes.

No abstract