APOPTOSIS - A FUNCTIONAL PARADIGM FOR PROGRAMMED PLANT-CELL DEATH INDUCED BY A HOST-SELECTIVE PHYTOTOXIN AND INVOKED DURING DEVELOPMENT

The host-selective AAL toxins secreted by Alternaria alternata f sp ly copersici are primary chemical determinants in the Alternaria stem can ker disease of tomato. The AAL toxins are members of a new class of sp hinganine analog mycotoxins that cause cell death in both animals and plants, Here, we report detection of stereotypic hallmarks of apoptosi s during cell death induced by these toxins in tomato. DNA ladders wer e observed during cell death in toxin-treated tomato protoplasts and l eaflets. The intensity of the DNA ladders was enhanced by Ca2+ and inh ibited by Zn2+. The progressive delineation of fragmented DNA into dis tinct bodies, coincident with the appearance of DNA ladders, also was observed during death of toxin-treated tomato protoplasts. In situ ana lysis of cells dying during development in both onion root caps and to mato leaf tracheary elements revealed DNA fragmentation localized to t he dying cells as well as the additional formation of apoptotic-like b odies in sloughing root cap cells. We conclude that the fundamental el ements of apoptosis, as characterized in animals, are conserved in pla nts. The apoptotic process may be expressed during some developmental transitions and is the functional process by which symptomatic lesions are formed in the Alternaria stem canker disease of tomato. Sphingani ne analog mycotoxins may be used to characterize further signaling pat hways leading to apoptosis in plants.