Fig. 7

HS-induced plant cell death is necrotic rather than apoptotic process. Plasma membrane (PM) integrity is maintained during apoptosis to prevent spillage of toxic dying cell contents. However, plant cell exposed to HS and undergoing the so-called AL-PCD exhibit early irreversible permeabilization of PM, a feature typical for necrotic cell death. Apoptosis is associated with step-wise massive re-organization of the cellular morphology (nuclear segmentation, PM blebbing, fragmentation into apoptotic bodies) and thus is an energy consuming process that relies on controlled release of cytochrome c, functional mitochondria and intracellular ATP. On the contrary, accidental necrotic death is a passive lysis of the cellular contents associated with low intracellular ATP and decoupled mitochondria. Similarly to animal cells undergoing necrosis, plant cells subjected to HS have a low intracellular ATP content and uncoupled mitochondria. The PM of apoptotic cell forms outward protrusions, called blebs, that will eventually undergo scission and become apoptotic bodies. Vesicles that are observed in the proximity of plant PM during HS-induced cell death are on the inner side of the PM and thus cannot be considered as blebs. While apoptotic cells condense and disassemble into apoptotic bodies that are engulfed by phagocytes, a necrotic cell typically undergoes swelling that is followed by a decrease of the cell volume and leakage of the dead cell contents. Presence of the rigid cell wall most probably does not permit necrosis-like swelling of plant cells during HS-induced cell death, while rupture of the vacuole coinciding with permeabilization of the PM would lead to rapid leakage of the cellular contents and thus decrease in volume. Such shrinkage has very little in common with AVD, as it is a passive process not associated with the formation of apoptotic bodies