Homologous and heterologous desensitization and synergy in pathways leading to the soybean oxidative burst

Because the H₂O₂ and O₂/^- generated during a pathogen-triggered oxidative burst could either protect or destroy a besieged plant cell, their synthesis might be expected to be tightly regulated. We have examined the nature of this regulation as it is communicated between homologous and heterologous oxidative-burst pathways, using both chemical (oligogalacturonic acid, harpin, fensulfothion) and mechanical (osmotic stress) stimuli to induce the burst. We report here that the above three chemical elicitors attenuate a subsequent oxidative burst induced in cultured soybean (Glycine max L.) cells by either the same (homologous desensitization) or a different chemical elicitor (heterologous desensitization). Further, when the magnitude of the initial oxidative burst is maximal, the cells remain refractory to subsequent elicitation for at least 10 min and then revive their sensitivities to re-stimulation with a half-time of > 20 min. Mechanical stimulation of the oxidative burst appears to be regulated by a different set of constraints. Although initiation of a mechanically induced burst leads to attenuation of a subsequent mechanically induced burst, the same mechanical stimulus is peculiarly unable to reduce a subsequent chemically induced burst. The converse is also true, suggesting that heterologous desensitization of the oxidative burst does not extend to mixed chemical and mechanical/osmotic stimuli. However, communication between these disparate forms of elicitation is still demonstrated to occur, since low-level chemical stimuli strongly synergize concurrent low-level osmotic stimuli and vice versa. Furthermore, the pattern of synergy changes dramatically if one stimulus is administered immediately prior to the other. Taken together, these data demonstrate that significant cross-talk occurs among the different signaling pathways of the oxidative burst and that the overall process is tightly regulated.