|Tai, Yin Shan|
Submitted to: Research Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 26, 2006
Publication Date: March 31, 2007
Citation: Tai, Y. 2007. Detection of reactive oxygen species can be used to distinguish ToxA-induced cell death from the hypersensitive response. Research Journal of Botany 2(1):1-12. Interpretive Summary: Pathogens deploy a variety of effector proteins into host cells. In the arms race between pathogen and host, one of the effector proteins may be recognized by a cognate resistance gene product in the host. This recognition usually triggers cell death in a very fast manner and is called the hypersensitive response (HR). Accumulating evidence indicates that the early events in initiation of HR include the production of reactive oxidative species (ROS), such as hydrogen peroxide. The interaction between tomato plants and the bacterium Pseudomonas syringae pv. tomato provides a model system for the study of HR. Meanwhile, the fungal pathogen Pyrenophora tritici-repentis also causes cell death in wheat, because of the action of a secreted proteinaceous toxin known as ToxA. The mechanism for the cell death induced by ToxA is not well-studied. In this report, the time course for the production of ROS and cell death was measured. Results indicate that the production of hydrogen peroxide occurs prior to HR. In contrast, the ToxA-induced cell death in wheat was observed prior to the production of hydrogen peroxide. Therefore, these results infer that ToxA-caused cell death is different from HR.
Technical Abstract: Unlike the situation of resistance (R) gene mediated cell death, the biochemical mechanism for the fungal toxin ToxA-caused cell death is not well-defined. Resistance associated with the gene-for-gene interaction is usually manifested in a form of programmed cell death termed the hypersensitive response (HR). Nitric oxide (NO) and various reactive oxidative species (ROS) are key components required for the HR in several species. The interaction between tomato plants and Pseudomonas syringae pv. tomato (Pst) is a well-studied gene-for-gene model of plant disease resistance. Two tomato genes, Pto and Prf, are required for the signaling pathway leading to resistance against strains of Pst that express the avirulence gene AvrPto. A transgenic line of Nicotiana benthamiana carrying constitutive expression of Pto and chemical-inducible AvrPto was used to study the involvement of NO and ROS. After the application of a chemical called dexamethasone (DEX), the production of nitric oxide, superoxide anion, and hydrogen peroxide could be detected by staining with 4,5-diaminofluorescein diacetate (DAF), nitroblue tetrazolium (NBT), and 3,3-diaminobenzidine (DAB), respectively. Results of genetic and histochemical studies indicated that Prf is acting upstream to the production of nitric oxide, superoxide anion, and hydrogen peroxide induced by the Pto-AvrPto interaction. Furthermore, the production of hydrogen peroxide occurs prior to the HR. In contrast, the ToxA-induced cell death in wheat was observed prior to the production of hydrogen peroxide. Therefore, the detection of ROS can be applied to indicate that the ToxA-caused cell death is different from the model of HR.