Background: Avirulent pathogens elicit a battery of plant defenses, of
ten 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 acti
ve process, but little is known about the mechanisms underlying cellul
ar collapse. Results: We show that H2O2 stimulates a rapid influx of C
a2+ into soybean cells, which activates a physiological cell death pro
gram resulting in the generation of large (similar to 50 kb) DNA fragm
ents and cell corpse morphology - including cell shrinkage, plasma mem
brane blebbing and nuclear condensation characteristic of apoptosis. I
n contrast, H2O2 induction of the cellular protectant gene glutathione
S-transferase is Ca2+-independent. Apoptosis in soybean cells and lea
f tissue was induced by avirulent Pseudomonas syringae pv. glycinea bu
t was not observed at comparable stages of the compatible interaction
with the isogenic virulent strain, which fails to elicit a hypersensit
ive response. Apoptosis was also observed at the onset of the hypersen
sitive response in Arabidopsis leaves inoculated with avirulent P. syr
ingae 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 act
ivation 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 avi
rulence signals suggests that apoptosis may prove to be a common, but
not necessarily ubiquitous, feature of incompatible plant-pathogen int
eractions. Emerging similarities between facets of hypersensitive dise
ase resistance and the mammalian native immune system indicate that ap
optosis is a widespread defence mechanism in eukaryotes.