NO provides mainly avr-dependent inputs into cell death mechanisms associated with the hypersensitive response in tobacco

The hypersensitive response (HR) is a localised programmed cell death that can aid in neutralising infections by pathogens. H2O2 and NO are proposed elicitors of the HR and we have employed in planta assays to measure the production of both (Mur et al., 2005 Plant Cell and Environment. 6: 65–78; Mur et al., 2005; Plant Physiology 138: 1247–1258) during a HR elicited in tobacco by Pseudomonas syringae pathovar phaseolicola (Psph). As in other studies, the simultaneous generation of NO and H2O2 was noted. H2O2 generation adopted a biphasic pattern reflecting two elicitation events; the first dependent on host recognition of pathogen-associated molecular patterns (PAMPs) and the second on cell death eliciting avirulence (avr) gene products also encoded by the pathogen. In contrast, NO generation was monophasic and, by 1 h following inoculation, continued at a constant level for at last 12 h. Inoculation of tobacco with a Psph hrp mutant where avr (but not PAMP) elicitation is abolished, resulted in the production of the first rise in H2O2 but negligible levels of NO. Reducing NO levels using inhibitors of nitric oxide generation affected only the avr-dependent H2O2 generation. Suppressing the oxidative burst had no observable effect on NO generation. These data suggest that NO provided primarily avr-dependent inputs into Psph-elicited HR. Evidence will also be presented that NO–ROS interactions influence cell death and certain defence gene expression via peroxynitrite and OH generation.