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 (PGA) 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 PGA 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 x 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 PGA, 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.