ACTTS3 encoding a polyketide synthase is essential for the biosynthesis of ACT-Toxin and pathogenicity in the tangerine pathotype of alternaria alternata

Abstract

The tangerine pathotype of Alternaria alternala produces host-selective ACT-toxin and causes Alternaria brown spot disease of tangerine and tangerine hybrids. Sequence analy-sis of a genomic BAC clone identified part of the ACT-toxin TOX (ACTT) gene cluster, and knockout experiments have implicated several open reading frames (ORF) contained within the cluster in the biosynthesis of ACT-toxin. One of the ORF, designated ACTTS3, encoding a putative poly-ketide synthase, was isolated by rapid amplification of cDNA ends and genomic/reverse transcription-polymerase chain reactions using the specific primers designed from the BAC sequences. The 7,374-bp ORF encodes a polyketide synthase with putative ß-ketoacyl synthase, acyltransferase, methyltransferase, ß-ketoacyl reductase, and phosphopante-theine attachment site domains. Genomic Southern blots demonstrated that ACTTS3 is present on the smallest chro-mosome in the tangerine pathotype of A. alternata, and the presence of ACTTS3 is highly correlated with ACT-toxin production and pathogenicity. Targeted gene disruption of two copies of ACTTS3 led to a complete loss of ACT-toxin production and pathogenicity. These results indicate that ACTTS3 is an essential gene for ACT-toxin biosynthesis in the tangerine pathotype of A. alternata and is required for pathogenicity of this fungus. © 2010 The American Phytopathological Society.