Detoxification of oat leaf saponins by Septoria avenae

Isolates of Septoria avenae and related fungi were analyzed for pathogenicity to oats and wheat and found to be either pathogenic on oat or wheat. Two compounds were identified in methanolic extracts of oat leaves that inhibited the growth of wheat-attacking isolates. Analysis of these compounds by thin-layer chromatography and fast atom bombardment-mass spectrometry supported their identification as the steroidal saponins 26-desglucoavenacosides A and B. Oat-attacking (but not wheat-attacking) isolates of Septoria were able to detoxify these saponins by enzymatic hydrolysis of the sugar chain attached at carbon 3. An enzyme that carried out this hydrolysis was purified from the culture filtrate of S. avenae f. sp. avenae. This enzyme (avenacosidase) was capable of removing both L-rhamnose and D-glucose molecules from the C-3 sugar chain of the saponins. The enzyme had a molecular mass of 110 kDa, an isoelectric point between pH 3.8 and 4.1, and optimal beta-D-glucosidase activity at pH 5.4.