Fungal Pathogens of Oat Roots and Tomato Leaves Employ Closely Related Enzymes to Detoxify Different Host Plant Saponins. Anne Osbourn . Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK . Paul Bowyer, Patricia Lunness, Belinda Clarke, and Michael Daniels. Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK. MPMI 8:971-978. Accepted 15 August 1995. Copyright 1995 The American Phytopathological Society.

Antifungal saponins arc produced by many plants and have been implicated as preformed determinants of resis tance to fungal attack. The importance of saponin detoxi fication in fungal palhogenesis has recently been demon strated for the fungus Gaeumannomyces graminis van avenae, which produces the enzyme avenacinase. Avena- cinase detoxifies the triterpenoid oat root saponin avenacin A-l, and is essential for pathogenicity of G. graminis van avenae to oats. Here we demonstrate an un expected relatedness between avenacinase and the lomat- inase enzyme produced by Seploria lycopersici (a tomato leaf-infecting fungus), which acts on the steroidal glycoal- kaloid ?tomatine. The two enzymes share common phys- icochemical properties and are immunologically cross- reactive; however, there are critical differences in their substrate specificities which reflect the host preferences of the fungi from which the enzymes were purified. The DNA encoding tomatinase was isolated from a 5. lycopersici cDNA library using avenacinase DNA as a probe. Com parison of the predicted amino acid sequences of avenac inase and tomatinase revealed that the enzymes are clearly similar.