Stagonospora avenae secretes multiple enzymes that hydrolyze oat leaf saponins

The phytopathogenic fungus Stagonospora avenae is able to infect oat leaves despite the presence of avenacoside saponins in the leaf tissue. In respon se to pathogen attack, avenacosides are converted into 26-desglucoavenacosi des (26-DGAs), which possess antifungal activity, These molecules are compr ised of a steroidal backbone linked to a branched sugar chain consisting of one alpha-L-rhamnose and two (avenacoside A) or three (avenacoside B) beta -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, D egradation is initiated by removal of the L-rhamnose, which abolishes antif ungal activity, The D-glucose residues are then hydrolyzed by beta-glucosid ase activity. A comprehensive analysis of saponin-hydrolyzing activities wa s undertaken, and it was established that S, avenae isolate WAC1293 secrete s three enzymes, one alpha-rhamnosidase and two beta-glucosidases, that car ry out this hydrolysis, The major beta-glucosidase was purified and the gen e encoding the enzyme cloned. The protein is similar to saponin-hydrolyzing enzymes produced by three other phytopathogenic fungi, Gaercmannomyces gra minis, Septoria lycopersici, and Botrytis cinerea, and is a family 3 beta-g lucosidase, The gene encoding the beta-glucosidase is expressed during infe ction of oat leaves but is not essential for pathogenicity.