Characterisation of the saponin hydrolysing enzyme avenacoside-α-L- rhamnosidase from the fungal pathogen of cereals, Stagonospora avenae

The fungal pathogen Stagonospora avenae f. sp. avenaria infects oat leaves, which contain the saponins avenacoside A and B. The avenacosides are glycosylated steroidal saponins that occur within oat leaves in a non-fungitoxic form and are converted upon damage or pathogen invasion to their antifungal form by a plant enzyme. It has previously been shown that oat-attacking isolates of S. avenae are able to hydrolyse the sugar chain at the C3 position of the avenacosides. This carbohydrate moiety is a branched chain that consists of one α-L-rhamnose and two or three β-D-glucose residues in avenacosides A and B, respectively. Removal of the α-L-rhamnose residue is sufficient to detoxify the avenacosides. This work describes the purification of the avenacoside-degrading α-L-rhamnosidase-and determination of peptide sequence from the protein which represents the first α-L-rhamnosidase thus characterised in a fungal plant pathogen.