Characterization of the oligogalacturonide-induced oxidative burst in cultured soybean (Glycine max) cells

The rapid release of H₂O₂ by elicited plant cells, recently termed the oxidative burst, was investigated in suspension-cultured soybean (Glycine max Merr. cv Kent) cells stimulated with a purified polygalacturonic acid (PCA) elicitor. Examination of the elicited cells by fluorescence microscopy revealed that virtually every living cell participates in the elicitor-induced H₂O₂ burst. Measurement of the kinetics of the response using a macroscopic fluorescence-based assay indicated that approximately 100 molecules of H₂O₂ are generated per PCA molecule added, achieving a cumulative H₂O₂ concentration of approximately 1.2 mmol L^-1 of packed cells. At the height of the defense response, 3 × 10^-14 mol of H₂O₂ cell^-1 min^-1 are produced, a value comparable to the rate of H₂O₂ production by myeloid cells of mammals. Variables affecting the rate and magnitude of the soybean oxidative burst were found to be mechanical stress, extracellular pH, and cell age. The PGA-induced oxidative burst was shown to undergo both homologous and heterologous desensitization, a characteristic of signal transduction pathways in animals. Homologous desensitization was obtained with PCA, and heterologous desensitization was observed with the G protein activator mastoparan, consistent with earlier observations showing that G proteins perform a regulatory function in this pathway. Finally, a model describing the possible role of the PGA-induced oxidative burst in the overall scheme of plant defense is proposed.