THE HYPERSENSITIVE REACTION, MEMBRANE DAMAGE AND ACCUMULATION OF AUTOFLUORESCENT PHENOLICS IN LETTUCE CELLS CHALLENGED BY BREMIA-LACTUCAE

The expression of resistance to Bremia lactucae determined by the resi stance genes Dm5/8 and Dm7 in lettuce was examined; incompatibility in volved the hypersensitive reaction (HR) which occurred only within pen etrated cells at early and late stages of fungal development, respecti vely. Autofluorescence observed under UV and blue light excitation in cells undergoing the HR was associated with the accumulation of ester- linked syringaldehyde and caffeic acid on plant cell walls. Two phases of phenolic deposition were identified. The first was highly localize d around penetration points and occurred during incompatible and compa tible interactions. The second and major phase was only activated afte r the occurrence of irreversible membrane damage in the penetrated cel l and was reduced by inhibitors of mRNA synthesis. Fungal structures, primary and secondary vesicles, intercellular hyphae and haustoria als o became autofluorescent during incompatible interactions. Changes in the fluorescence due to preformed phenolics located in the plant cell vacuole were found just before plasma membrane damage became irreversi ble during the HR. In addition to localized deposition of phenolics, i ncreases in the concentrations of the major free phenolic esters ident ified as dicaffeoyl tartaric and chlorogenic acids also occurred durin g incompatible interactions. The results suggest that membrane damage in penetrated cells occurs at different rates in resistance controlled by Dm5/8 and Dm7 and indicate an important role for irreversible memb rane damage in lettuce as a key signalling event leading to widespread activation of defence responses in surrounding cells.