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A Polyketide Synthase Gene, ACRTS2, Is Responsible for Biosynthesis of Host-Selective ACR-Toxin in the Rough Lemon Pathotype of Alternaria alternata

November 2012 , Volume 25 , Number  11
Pages  1,419 - 1,429

Y. Izumi,1 K. Ohtani,1 Y. Miyamoto,1 A. Masunaka,1 T. Fukumoto,1 K. Gomi,1 Y. Tada,1 K. Ichimura,1 T. L. Peever,2 and K. Akimitsu1

1Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795 Japan; 2Department of Plant Pathology, Washington State University, Pullman 99164-6430 U.S.A.

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Accepted 12 July 2012.

The rough lemon pathotype of Alternaria alternata produces host-selective ACR-toxin and causes Alternaria leaf spot disease of rough lemon (Citrus jambhiri). The structure of ACR-toxin I (MW = 496) consists of a polyketide with an α-dihydropyrone ring in a 19-carbon polyalcohol. Genes responsible for toxin production were localized to a 1.5-Mb chromosome in the genome of the rough lemon pathotype. Sequence analysis of this chromosome revealed an 8,338-bp open reading frame, ACRTS2, that was present only in the genomes of ACR-toxin-producing isolates. ACRTS2 is predicted to encode a putative polyketide synthase of 2,513 amino acids and belongs to the fungal reducing type I polyketide synthases. Typical polyketide functional domains were identified in the predicted amino acid sequence, including β-ketoacyl synthase, acyl transferase, methyl transferase, dehydratase, β-ketoreductase, and phosphopantetheine attachment site domains. Combined use of homologous recombination-mediated gene disruption and RNA silencing allowed examination of the functional role of multiple paralogs in ACR-toxin production. ACRTS2 was found to be essential for ACR-toxin production and pathogenicity of the rough lemon pathotype of A. alternata.

© 2012 The American Phytopathological Society