| Plant diseases cause billions of dollars in damage to crops annually, making dissection of the molecular mechanisms of plant disease resistance an economically important task. Gene-for-gene resistance mediated by plant R genes and pathogen avr genes has long been associated with the hypersensitive response (HR). Previously, Arabidopsis thaliana “defense, no death” (dnd) mutants were identified based on their failure to produce the HR when inoculated with P. syringae carrying the avirulence gene avrRpt2. The first objective of this study was to characterize the dnd2 mutants in relation to defense. Like the previously characterized dnd1 mutant plants, dnd2 mutants responded to avirulent pathogens by activating avr-dependent defenses despite the absence of the HR. In addition, dnd2 plants exhibited a constitutive systemic resistance associated with elevated levels of salicylic acid (SA). SA was found to be required for the systemic resistance of dnd2 plants, but was not necessary for the HR− phenotype displayed by the mutant plants. SA-independent pathways not normally activated by P. syringae infection were activated in dnd plants, as indicated by the marker for SA-independent signaling, PDF 1.2. A map-based cloning project was initiated which identified a precise genetic interval spanning DND2. Focused shotgun complementation experiments were initiated using subclones from within this region. The roles of reactive oxygen species (ROS) and the enzymes that scavenge these signaling molecules were examined in relation to the HR− phenotype of dnd plants. Northern analysis of ROS scavenger mRNA revealed no significant differences in gene expression between wild type and dnd plants, suggesting that ROS scavengers are not blocking ROS signaling for the HR. Finally, the dwarfed stature of dnd plants was could be partially corrected by altered growth conditions, specifically the soil mix. Chemical analysis of soil mixes after 6 weeks of use suggested that cations and pH might be involved in the growth phenotype. The effect of K+ , Ca2+, Na+, and pH on dnd plant growth was then tested in agar-based media. The dnd plants did not respond to increasing concentrations of Ca2+ in parallel with wild-type plants. K+, Na+ and pH impacted dnd1, dnd2, and wild-type plants similarly. |
