H₂O₂ plays different roles in determining penetration failure in three diverse plant–fungal interactions

Fungal plant pathogens that attempt to penetrate and feed on living cells frequently trigger a localized plant defence response that results in fungal penetration failure. In the current study we demonstrate that breakdown products of the cell wall released by the localized application of hemicellulase elicit localized responses including, sequentially, extracellular H2O2 generation; accumulation of phenolic compounds; and cross-linking of proteins in the cell wall. In a detailed time-course study of three plant-fungus interactions that result in a high frequency of penetration failure, only one plant-fungus combination displayed a similar profile of responses to that induced by localized cell-wall degradation. The additional generation of extracellular O2- in one interaction, and the absence of phenolic compounds in the cell wall in another, demonstrate that plant responses to the penetration process may be influenced by activities of the penetrating fungus. Significantly, H2O2 generation was the only response detected in all three plant-fungal combinations at the correct time and place to account for penetration failure, and in all three combinations the enzymatic removal of H2O2 resulted in increased penetration success. Pharmacological studies suggest that in two of the three interactions, H2O2 generation required cytoskeletal involvement but was independent of transcription or translation, although inhibition of the latter processes increased fungal penetration. In at least one of these two interactions, the data suggest that H2O2 generation and new gene expression act within the same penetration-inhibiting pathway, possibly through the involvement of phenolic materials. However, enzymatic removal of H2O2 from the third interaction almost completely eliminated penetration failure, while interference with cytoplasmic processes had no effect, suggesting that H2O2 generation in this system did not require protoplast involvement and, alone, was necessary and sufficient to account for fungal penetration failure.