The Mutualistic Fungus Piriformospora indica Protects Barley Roots from a Loss of Antioxidant Capacity Caused by the Necrotrophic Pathogen Fusarium culmorum

Fusarium culmorum causes root rot in barley (Hordeum vulgare), resulting in severely reduced plant growth and yield. Pretreatment of roots with chlamydospores of the mutualistic root-colonizing basidiomycete Piriformospora indica (subdivision Agaricomycotina) prevented necrotization of root tissues and plant growth retardation commonly associated with Fusarium root rot. Quantification of Fusarium infections with a real-time polymerase chain reaction assay revealed a correlation between root rot symptoms and the relative amount of fungal DNA. Fusarium-infected roots showed reduced levels of ascorbate and glutathione (GSH), along with reduced activities of antioxidant enzymes such as superoxide dismutase, ascorbate peroxidase, GSH reductase, dehydroascorbate reductase, and monodehydroascorbate reductase. Consistent with this, Fusarium-infected roots showed elevated levels of lipid hydroperoxides and decreased ratios of reduced to oxidized forms of ascorbate and GSH. In clear contrast, roots treated with P. indica prior to inoculation with F. culmorum showed levels of ascorbate and GSH that were similar to controls. Likewise, lipid peroxidation and the overall reduction in antioxidant enzyme activities were largely attenuated by P. indica in roots challenged by F. culmorum. These results suggest that P. indica protects roots from necrotrophic pathogens, at least partly, through activating the plant's antioxidant capacity.