Relatively little is known about programmed cell death (PCD) in plants. It is 
nonetheless suggested here that tonoplast rupture and the subsequent rapid 
destruction of the cytoplasm can distinguish two large PCD classes. One class, 
which is here called 'autolytic', shows this feature, whilst the second class 
(called 'non-autolytic') can include tonoplast rupture but does not show the 
rapid cytoplasm clearance. Examples of the 'autolytic' PCD class mainly occur 
during normal plant development and after mild abiotic stress. The 
'non-autolytic' PCD class is mainly found during PCD that is due to 
plant-pathogen interactions. Three categories of PCD are currently recognized in 
animals: apoptosis, autophagy, and necrosis. An attempt is made to reconcile the 
recognized plant PCD classes with these groups. Apoptosis is apparently absent 
in plants. Autophagic PCD in animals is defined as being accompanied by an 
increase in the number of autophagosomes, autolysosomes, and small lytic 
vacuoles produced by autolysosomes. When very strictly adhering to this 
definition, there is no (proof for) autophagic PCD in plants. Upon a slightly 
more lenient definition, however, the 'autolytic' class of plant PCD can be 
merged with the autophagic PCD type in animal cells. The 'non-autolytic' class 
of plant PCD, as defined here, can be merged with necrotic PCD in animals.