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Book Summary:
Fusarium head blight or scab, a destructive disease of small grains is caused by members of the Fusarium graminearum species complex, which is comprised of at least nine distinct species. They produce trichothecene mycotoxins, including deoxynivalenol along with its acetylated derivatives and nivalenol. Thirty-one different strains, representing eight species of this complex and originating from diverse hosts or substrates were studied. Aggressiveness and the ability to produce trichothecenes on a susceptible wheat cultivar varied significantly among strains, while the amount of mycotoxin produced by each strain and its level of aggressiveness was correlated. Greenhouse studies with some of these strains showed that they are capable of infecting rice and causing a significant amount of disease; however, no trichothecenes could be detected from infected rice florets. Genomic studies were conducted to elucidate gene function related to pathogenicity. Three cDNA libraries were created by suppression subtractive hybridization using wheat heads inoculated with a highly aggressive strain and either water or a less aggressive strain, 48 hours after inoculation. Twenty-eight percent of sequenced ESTs were identified to be of fungal origin by matches to the F. graminearum whole genome sequence. Based on BLASTX comparisons with sequences from publicly available databases, large percentages of the fungal ESTs were found to correspond to mitochondrial and ribosomal genes. In total, 84 genes were identified that were expressed during initial disease development. The probable functions of 49 of these genes could be inferred by bioinformatic analysis. Thirty-five ESTs with no known homologs in current databases were considered new F. graminearum open reading frames that are being used for manual annotation of the genome sequence. Mitochondrial sequences from this library are being utilized for assembling the mitochondrial genome. An ABC transporter (FgAbc), a predicted lysine permease (FgLyp), a putative two-component response regulator receiver (FgRrr1), and a likely fungal transcriptional regulator gene (FgZbc1) were targeted for gene replacement. Mutants for all the four genes were successfully obtained, and the results suggest that FgRrr1 gene may be involved in pathogenicity of the fungus. Mutants for this gene showed reduced sporulation and were delayed in spread on the wheat head. |
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