Calcium-mediated apoptosis in a plant hypersensitive disease resistance response

Background: Avirulent pathogens elicit a battery of plant defenses, often accompanied by collapse of the challenged cells. In soybean cells, sustained accumulation of H2O2 from an oxidative burst cues localized host cell death. Such hypersensitive cell death appears to be an active process, but little is known about the mechanisms underlying cellular collapse. Results: We show that H2O2 stimulates a rapid influx of Ca2+ into soybean cells, which activates a physiological cell death program resulting in the generation of large (similar to 50 kb) DNA fragments and cell corpse morphology - including cell shrinkage, plasma membrane blebbing and nuclear condensation characteristic of apoptosis. In contrast, H2O2 induction of the cellular protectant gene glutathione S-transferase is Ca2+-independent. Apoptosis in soybean cells and leaf tissue was induced by avirulent Pseudomonas syringae pv. glycinea but was not observed at comparable stages of the compatible interaction with the isogenic virulent strain, which fails to elicit a hypersensitive response. Apoptosis was also observed at the onset of the hypersensitive response in Arabidopsis leaves inoculated with avirulent P. syringae pv. tomato and in tobacco cells treated with the fungal peptide cryptogein, which is involved in the induction of non-host resistance to Phytophthora cryptogea. Conclusions: These observations establish a signal function for Ca2+ downstream of the oxidative burst in the activation of a physiological cell death program in soybean cells that is similar to apoptosis in animals. That the characteristic cell corpse morphology is also induced in Arabidopsis and tobacco by different avirulence signals suggests that apoptosis may prove to be a common, but not necessarily ubiquitous, feature of incompatible plant-pathogen interactions. Emerging similarities between facets of hypersensitive disease resistance and the mammalian native immune system indicate that apoptosis is a widespread defence mechanism in eukaryotes.