First and second authors: USDA-ARS, Alternate Crops and Systems Laboratory, Bldg. 001, Rm. 342, BARC-West, Beltsville, MD 20705; and third author: USDA-ARS, Foreign Diseases-Weed Science Research Unit, Fort Detrick, MD 21702
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Accepted for publication 28 March 2002.
The necrosis inducing extracellular protein Nep1 is produced by Fusarium oxysporum f. sp. erythroxyli in liquid culture. NEP1, the Nep1 protein structural gene, was disrupted in F. oxysporum f. sp. erythroxyli isolate EN-4 by gene replacement using polyethylene glycol (PEG)-mediated transformation. NEP1 disruption was verified by polymerase chain reaction (PCR), Southern blot, and northern blot analysis. NEP1-disrupted transformants failed to produce Nep1 in liquid culture. NEP1 disruption did not affect the pathogenicity of isolate EN-4 toward Erythroxylum coca. Transformation of isolate EN-4 with construct pPB-FO11-45 carrying NEP1 between the trpC promoter and terminator resulted in increased production of Nep1 in potato dextrose broth plus 1% casamino acids or Czapek-Dox broth plus 1% casamino acids but not in potato dextrose broth alone. Transformation of EN-4 with construct pPB-FO11-45 was verified by PCR and Southern blot analysis. Overexpression of NEP1 was confirmed by northern blot and Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. NEP1-overexpressing transformant 15 produced 64 to 128 times as much Nep1 as EN-4 wild type when grown in shake cultures. Transformants overexpressing Nep1 in liquid culture were no more or less pathogenic toward E. coca than wild-type isolates. Nep1 was not detected in E. coca seedlings infected with NEP1-overexpressing transformants or with EN-4 wild type. In large-scale fermentations of NEP1-overexpressing transformant 15, the amount of secreted protein including Nep1 was 15.1 times that of the wild-type EN-4, providing a ready source of Nep1 for future study.
The American Phytopathological Society, 2002