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RNA SILENCING OF THE TRICHOTHECENE BIOSYNTHESIS GENE TRI6 IN FUSARIUM CULMORUM. B. Scherm1, M. Orr?1, V. Balmas1, T.M. Hammond2, N.P. Keller2, and Q. Migheli1.1Dipartimento di Protezione delle Piante, Universit? degli Studi di Sassari, Via E. De Nicola 9, I-07100 Sassari, Italy and 2Department of Plant Pathology, University of Wisconsin, Madison, USA; Fax: +++39079229316; E-mail:

Post-transcriptional regulation of eukaryotic genes through interception and degradation of mRNA is known as RNA silencing. This mechanism is activated by an RNAse III enzyme, which digests double-stranded RNA (dsRNA) molecules into 21- to 25-bp fragments. These fragments (siRNAs) are incorporated into a complex of proteins, the "RNA-induced silencing complex" (RISC), which uses the incorporated siRNAs to target and degrade mRNA with complementary sequences. It was recently demonstrated that inverted repeat transgenes (IRT) are efficient activators of RNA silencing in fungal species. The aim of this study was to evaluate whether RNA silencing could be applied to suppress mycotoxin production in the plant pathogen F. culmorum (W.G. Smith) Sacc., incitant of crown and foot rot on wheat. Transformation of a highly virulent strain of F. culmorum with IRT containing sequences corresponding to the trichothecene biosynthesis gene tri6 was achieved by using the hygromycin B resistance gene hph as selectable marker in PEG-mediated co-transformation of fungal protoplasts. The pattern of integration indicates that most transformants underwent homologous recombination events with partial deletion of the endogenous tri6 gene. A subset of transformants possessing both the endogenous gene and the tri6-specific IRT construct were selected for further studies. The tri6-specific IRT did not alter physiological characteristics, such as spore production, pigmentation, and growth rate on solid media. Pathogenicity assays are being carried to evaluate whether impairment in deoxynivalenol production in the tri6-IRT strains correlates with a loss of virulence.