A compromised Mlo pathway affects the response of barley to the necrotrophic fungus Bipolaris sorokiniana (Teleomorph : Cochliobolus sativus) and itstoxins

In search of new durable disease resistance traits in barley to control lea f spot blotch disease caused by the necrotrophic fungus Bipolaris sorokinia na (teleomorph: Cochliobolus sativus), we developed macroscopic and microsc opic scales to judge spot blotch disease development on barley. Infection o f barley was associated with cell wall penetration and accumulation of hydr ogen peroxide. The latter appeared to take place in cell wall swellings und er fungal penetration attempts as well as during cell death provoked by the necrotrophic pathogen. Additionally, we tested the influence of a compromi sed Mlo pathway that confers broad resistance against powdery mildew fungus (Blumeria graminis f. sp. hordei). Powdery mildew-resistant genotypes with mutations at the Mlo locus (mlo genotypes) showed a higher sensitivity to infiltration of toxic culture filtrate of Bipolaris sorokiniana as compared with wild-type barley. Mutants defective in Ror, a gene required for mlo-s pecified powdery mildew resistance, were also more sensitive to Bipolaris s orokiniana toxins than wild-type barley but showed less symptoms than mlo5 parents. Fungal culture filtrates induced an H2O2 burst in all mutants, whe reas wild-type (Mlo) barley was less sensitive. The results support the hyp othesis that the barley Mlo gene product functions as a suppresser of cell death. Therefore, a compromised Mlo pathway is effective for control of bio trophic powdery mildew fungus but not for necrotrophic Bipolaris sorokinian a. We discuss the problem of finding resistance traits that are effective a gainst both biotrophic and necrotrophic pathogens with emphasis on the role of the anti-oxidative system of plant cells.