Pathogen-induced programmed cell death in tobacco.

Sacrificing an infected cell or cells in order to prevent systemic spread of a pathogen appears to be a conserved strategy in both plants and animals. We studied some of the morphological and biochemical events that accompany programmed cell death during the hypersensitive response of tobacco plants infected with tobacco mosaic virus. Certain aspects of this cell death process appeared to be similar to those that take place during apoptosis in animal cells. These included condensation and vacuolization of the cytoplasm and cleavage of nuclear DNA to 50 kb fragments. In contrast, internucleosomal fragmentation, condensation of chromatin at the nuclear periphery and apoptotic bodies were not observed in tobacco plants during tobacco mosaic virus-induced hypersensitive response. A unique aspect of programmed cell death during the hypersensitive response of tobacco to tobacco mosaic virus involved an increase in the amount of monomeric chloroplast DNA. Morphological changes to the chloroplast and cytosol of tobacco cells and increase in monomeric chloroplast DNA occurred prior to gross changes in nuclear morphology and significant chromatin cleavage. Our findings suggest that certain aspects of programmed cell death may have been conserved during the evolution of plants and animals.