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Role of the Pathotype-Specific ACRTS1 Gene Encoding a Hydroxylase Involved in the Biosynthesis of Host-Selective ACR-Toxin in the Rough Lemon Pathotype of Alternaria alternata

August 2012 , Volume 102 , Number  8
Pages  741 - 748

Yuriko Izumi, Eri Kamei, Yoko Miyamoto, Kouhei Ohtani, Akira Masunaka, Takeshi Fukumoto, Kenji Gomi, Yasuomi Tada, Kazuya Ichimura, Tobin L. Peever, and Kazuya Akimitsu

First, second, third, fourth, fifth, sixth, seventh, eighth, ninth, and eleventh authors: Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795 Japan; and tenth author: Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430.

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Accepted for publication 16 April 2012.

The rough lemon pathotype of Alternaria alternata produces host-selective ACR-toxin and causes Alternaria leaf spot disease of the rootstock species rough lemon (Citrus jambhiri) and Rangpur lime (C. limonia). Genes controlling toxin production were localized to a 1.5-Mb chromosome carrying the ACR-toxin biosynthesis gene cluster (ACRT) in the genome of the rough lemon pathotype. A genomic BAC clone containing a portion of the ACRT cluster was sequenced which allowed identification of three open reading frames present only in the genomes of ACR-toxin producing isolates. We studied the functional role of one of these open reading frames, ACRTS1 encoding a putative hydroxylase, in ACR-toxin production by homologous recombination-mediated gene disruption. There are at least three copies of ACRTS1 gene in the genome and disruption of two copies of this gene significantly reduced ACR-toxin production as well as pathogenicity; however, transcription of ACRTS1 and production of ACR-toxin were not completely eliminated due to remaining functional copies of the gene. RNA-silencing was used to knock down the remaining ACRTS1 transcripts to levels undetectable by reverse transcription-polymerase chain reaction. The silenced transformants did not produce detectable ACR-toxin and were not pathogenic. These results indicate that ACRTS1 is an essential gene in ACR-toxin biosynthesis in the rough lemon pathotype of A. alternata and is required for full virulence of this fungus.

© 2012 The American Phytopathological Society