The Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, U.K.
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Accepted 5 July 2000.
The phytopathogenic fungus Stagonospora avenae is able to infect oat leaves despite the presence of avenacoside saponins in the leaf tissue. In response to pathogen attack, avenacosides are converted into 26-desglucoavenacosides (26-DGAs), which possess antifungal activity. These molecules are comprised of a steroidal backbone linked to a branched sugar chain consisting of one α-L-rhamnose and two (avenacoside A) or three (avenacoside B) β-D-glucose residues. Isolates of the fungus that are pathogenic to oats are capable of sequential hydrolysis of the sugar residues from the 26-DGAs. Degradation is initiated by removal of the L-rhamnose, which abolishes antifungal activity. The D-glucose residues are then hydrolyzed by β-glucosidase activity. A comprehensive analysis of saponin-hydrolyzing activities was undertaken, and it was established that S. avenae isolate WAC1293 secretes three enzymes, one α-rhamnosidase and two β-glucosidases, that carry out this hydrolysis. The major β-glucosidase was purified and the gene encoding the enzyme cloned. The protein is similar to saponin-hydrolyzing enzymes produced by three other phytopathogenic fungi, Gaeumannomyces graminis, Septoria lycopersici, and Botrytis cinerea, and is a family 3 β-glucosidase. The gene encoding the β-glucosidase is expressed during infection of oat leaves but is not essential for pathogenicity.
© 2000 The American Phytopathological Society