The hypersensitive reaction, membrane damage and accumulation of autofluorescent phenolics in lettuce cells challenged by Bremia lactucae

Abstract

Summary The expression of resistance to Bremia lactucae determined by the resistance genes Dm5/8 and Dm7 in lettuce was examined; incompatibility involved the hypersensitive reaction (HR) which occurred only within penetrated cells at early and late stages of fungal development, respectively. 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 localized around penetration points and occurred during incompatible and compatible interactions. The second and major phase was only activated after the occurrence of irreversible membrane damage in the penetrated cell and was reduced by inhibitors of mRNA synthesis. Fungal structures, primary and secondary vesicles, intercellular hyphae and haustoria also 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 irreversible during the HR. In addition to localized deposition of phenolics, increases in the concentrations of the major free phenolic esters identified as dicaffeoyl tartaric and chlorogenic acids also occurred during 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 membrane damage in lettuce as a key signalling event leading to widespread activation of defence responses in surrounding cells.

Arabidopsis thaliana defense-related protein ELI3 is an aromatic alcohol:NADP + oxidoreductase

“…Although, to our knowledge the accumulation of wall-bound benzyl alcohols has not been described in relation to pathogen defense, two benzaldehydes, 4-hydroxybenzaldehyde and vanillin (Fig. 2), along with unidentified wall components, have been isolated from fungal elicitortreated, cultured P. crispum (22) and S. tuberosum cells (23), as well as from fungus-infected S. tuberosum leaves (23), and syringaldehyde was found to accumulate in fungus-infected lettuce leaves (24). Further analysis in this direction may reveal additional, structurally related compounds, possibly including the products of BAD activity.…”

Universally occurring phenylpropanoid and species-specific indolic metabolites in infected and uninfected Arabidopsis thaliana roots and leaves

“…This strong decline, together with the lack of an infection-induced increase of its wall-bound counterpart (9)-similar to the other two wall-bound cinnamate derivatives (7 and 8), but in contrast to most of the benzoate/benzaldehyde derivatives (1, 2, 4 and 6)-suggests a metabolic involvement of the three cinnamate residues (7-9) outside the presently investigated realms. The striking identity of their substitution patterns with those of the three characteristic lignin building blocks seems to indicate their direct or indirect involvement in the formation of lignin or of the less clearly defined 'lignin-like material', both of which have frequently been postulated to be re-enforced or newly formed as physical barriers around pathogen infection sites (Tiburzy and Reisener, 1990;Kovats et al, 1991;Bennett et al, 1996). Such a role of compounds 7-9 in lignin-related metabolism would readily explain not only their apparent lack of accumulation upon infection, but also the absence of detectable amounts of the corresponding aldehydes.…”

Benzothiadiazole-Mediated Induced Resistance toFusarium oxysporum f. sp.radicis-lycopersici in Tomato

“…In a recent study, Bennett et al (1996) suggested that accumulation of bright autofluorescing material within the fungus cell wall in the lettuce-Bremia lactucae interaction correlated with strong leakage of phenolics from the host vacuole, leading to changes in ionic balance and formation of compounds with fungitoxic activity. The authors concluded that irreversible membrane damage in lettuce was a key signaling event leading to widespread activation of defense responses in surrounding cells.…”