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The Colletotrichum acutatum Gene Encoding a Putative pH-Responsive Transcription Regulator Is a Key Virulence Determinant During Fungal Pathogenesis on Citrus

September 2007 , Volume 20 , Number  9
Pages  1,149 - 1,160

Bang-Jau You, Mathias Choquer, and Kuang-Ren Chung

Citrus Research and Education Center and Department of Plant Pathology, Institute of Food and Agricultural Sciences (IFAS), University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850, U.S.A.

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Accepted 18 April 2007.

Postbloom fruit drop of citrus and Key lime anthracnose (KLA) are caused by different pathotypes of Colletotrichum acutatum. Both pathotypes are pathogenic to citrus flowers, resulting in blossom blight and induction of young fruit abscission. Two fungal mutants defective in pathogenicity were recovered from a KLA pathotype after Agrobacterium-mediated mutagenesis. A PacCKLAP2 gene encoding a polypeptide that resembles many pH-responsive PacC/Rim101 transcription regulators in fungi was identified from one of the mutants, and functionally characterized to play a crucial role in pathogenesis to both Key lime leaves and citrus flowers. Gene disruption at the PacCKLAP2 locus created fungal mutants that were hypersensitive to alkaline pH, altered in conidium and appressorium production and germination, and concomitant with reduced virulence to both tissues. The pacCklap2 null mutants had lower alkaline phosphatase and protease activities, but increased pectolytic and lipolytic activities. The mutants initiated penetration and incited lesion formation on Key lime, indistinguishable from the wild type, when a functional copy of PacCKLAP2 was reintroduced or the leaves were wounded prior to inoculation. The null mutants were blocked at the penetration stage and, thus, failed to initiate the necrotrophic phase. The PacCKLAP2 transcript was barely detectable when the fungus was grown on medium buffered to pH 3 or 4, yet accumulated to high levels at a pH between 5 and 7. The PacCKLAP2 transcript was detected 2 days postinoculation on Key lime leaves, correlating with the time of lesion formation. We conclude that PacCKLAP2 is essential for C. acutatum pathogenesis by regulating multiple physiological and developmental processes.

Additional keywords: Agrobacterium-mediated transformation, sweet orange, targeted gene disruption, transcription factors, zinc finger.

© 2007 The American Phytopathological Society