Production And Roles Of Endogenous Hydrogen Peroxide In Phytopathogenic Bacteria And Its Roles In The Interaction With Plant

Production and localization of endogenous hydrogen peroxide (H₂O₂) were investigated in strains of Xanthomonas using HRP-red phenol assay or by histochemical analysis under electron microscopy. Experiments identified the presence of endogenous H₂O₂ production in nearly all Xanthomonas strains tested. Furthermore, the levels of endogenous H₂O₂ production were different among Xanthomonas strains tested. Results showed that the accumulated H₂O₂ was mainly located in the cell wall; the H₂O₂ accumulation was undetectable in plasma membrane or cytoplasm of Xanthomonas cells. Further results showed that the endogenous H₂O₂ plays crucial roles in physiological or pathological process of bacterial cells. Steady-state level of H₂O₂ was localized in cell wall throughout the cell division cycle of bacteria. There is a close correlation between the endogenous H₂O₂ accumulation and the growth of bacteria. It was first reported by us that the H₂O₂ production and the localizations of the accumulated H₂O₂ changed during the process of cell division cycle. In addition to the cell wall localization of the accumulated H₂O₂, two other locations of H₂O₂ accumulation, nucleoids and the mesosome-like structures, were observed during the cell division cycle of Xanthomonas. It indicated that the abundant H₂O₂ accumulation was closely associated with the process of cellular division. The overall results intensely suggested that endogenous H₂O₂ production should play positive roles by integrating with chromosomes in cell division cycle of Xanthomonas. On the other hand, excess endogenous H₂O₂ accumulation was observed in cells under cellular injury. Intriguingly, mesosomes presented as an additional location of H₂O₂ accumulation within the injured cells. There was an association between the frequency and size of mesosomes and the quantity of excess H₂O₂ accumulation and the degree of cellular injury caused by cellular injury. Results indicated that it should be voluntary regulatory mechanisms of H₂O₂ accumulation in mesosomes for cells under cellular injury. The loss of the function of avirulence gene avrXa7 led to decrease in the virulence of strain and resulted in reduction of endogenous H₂O₂ accumulation in avrXa7 mutant Xanthomonas oryzae pv. oryzae strain. During the interaction with plant, the change of endogenous H₂O₂ accumulation caused by avrXa7 mutation in Xanthomonas oryzae pv. oryzae impacted on the responses of rice plants and also influence on the mean levels of H₂O₂ accumulation in the interaction systems. Our experiments suggested that bacterial pathogen is a potential source of the H₂O₂ accumulated in the interaction between rice and Xanthomonas oryzae pv. oryzae. Moreover, the change of endogenous H₂O₂ level caused by ahpC mutation of Xanthomonas campestris pv. phaseoli has also impact on the mean level of H₂O₂ accumulation during the interaction with plant. Our overall results suggested that bacterial pathogen is also a potential source of the H₂O₂ accumulated in the interaction between rice and Xanthomonas campestris pv. phaseoli. The H₂O₂ produced by bacteria may be a potent cause of membrane damage in plant cells undergoing the HR during non-host resistance. Either avrXa7 or ahpC mutation induced the decrease of H₂O₂ accumulation during the interaction between plant and bacterial pathogen. The ahpC mutant induced the hypersensitive response of plant, while the avrXa7 mutant did not. Author inferred that H₂O₂ is a downstream mediator of AvrXa7-dependent interaction between plant and bacterial pathogen. It was hypothesized that the H₂O₂ production is down-regulated by avrXa7 or other genes to act in the recognition with plant.