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 cha llenge 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-h ydroxyagmatine (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 resistan t to saponification at sites of attempted fungal ingress (papilla), in dicating a change in, or the addition of, different chemical bonding t ypes. The switch in saponification sensitivity occurred at least 2 h e arlier in the mlo-incompatible than in the Mlo-compatible interaction. Our results suggest that p-CHA and the speed of papillae compaction p lay important roles in non-race-specific powdery mildew defense.