Structural analyses and dynamics of soluble and cell wall-bound phenolics in a broad spectrum resistance to the powdery mildew fungus in barley

High pressure liquid chromatography profiles of barley leaf epidermal soluble and cell wall-bound phenolics were analyzed in response to challenge with the fungal pathogen Erysiphe graminis f. sp. hordei. Only one soluble phenolic was found to accumulate differentially in a broad spectrum resistance reaction controlled by mlo resistance alleles in comparison to susceptible near isogenic Mlo lines. Structural analysis identified this compound as a novel phenolic conjugate, p-coumaroyl-hydroxyagmatine (p-CHA). p-CHA but not the nonhydroxylated derivative p-coumaroylagmatine exhibited antifungal activity both in vitro and in vivo. The accumulation of p-CHA in epidermal tissue correlated tightly with fungal penetration attempts of attacked host cells. Furthermore, upon penetration, epidermal cell wall-bound phenolics became resistant to saponification at sites of attempted fungal ingress (papilla), indicating a change in, or the addition of, different chemical bonding types. The switch in saponification sensitivity occurred at least 2 h earlier in the mlo-incompatible than in the Mlo-compatible interaction. Our results suggest that p-CHA and the speed of papillae compaction play important roles in non-race-specific powdery mildew defense.