LOCAL MECHANICAL STIMULATION INDUCES COMPONENTS OF THE PATHOGEN DEFENSE RESPONSE IN PARSLEY

Cell suspension cultures of parsley (Petroselinum crispum) have previo usly been used as a suitable system for studies of the nonhost resista nce response to Phytophthora sojae, In this study, we replaced the pen etrating fungus by local mechanical stimulation by using a needle of t he same diameter as a fungal hypha, by local application of a structur ally defined fungus-derived elicitor, or by a combination of the two s timuli. Similar to the fungal infection hypha, the local mechanical st imulus alone induced the translocation of cytoplasm and nucleus to the site of stimulation, the generation of intracellular reactive oxygen intermediates (ROI), and the expression of some, but not all, elicitor -responsive genes. When the elicitor was applied locally to the cell s urface without mechanical stimulation, intracellular ROI also accumula ted rapidly, but morphological changes were not detected. A combinatio n of the mechanical stimulus with simultaneous application of low dose s of elicitor closely simulated early reactions to fungal infection, i ncluding cytoplasmic aggregation, nuclear migration, and ROI accumulat ion. By contrast, cytoplasmic rearrangements were impaired at high eli citor concentrations. Neither papilla formation nor hypersensitive cel l death occurred under the conditions tested. These results suggest th at mechanical stimulation by the invading fungus is responsible for th e observed intracellular rearrangements and may trigger some of the pr eviously demonstrated changes in the activity of elicitor-responsive g enes, whereas chemical stimulation is required for additional biochemi cal processes. As yet unidentified signals may be involved in papilla formation and hypersensitive cell death.