MCB-9418377 Beers Naturally occurring cell death is an active process that serves a number of important functions. In animals, cell death affects morphogenesis, and removes a variety of cells, including those that have completed their function, cells that have developed improperly or those that exist in excess. In plants, programmed cell death occurs as a consequence of normal senescence, local lesion formation, and as the terminal developmental event in the formation of certain specialized tissues, such as the xylem. The ultimate fate of the cells programmed to die in animals is engulfment and degradation. Degradation during programmed cell death in plants appears to be mediated not by engulfment, but rather by intracellular hydroytic enzymes including nucleases, carbohydrases and proteases. While little is known concerning the exact role of proteolysis in cell death, regulation of the ubiquitin-dependent pathway of proteolysis appears to be important during programmed cell death in animals and plants. Using biochemical approaches, the role of ubiquitin-dependent proteolysis in programmed cell death will be investigated by characterizing changes in activity of the ubiquitin pathway and identifying substrates of this pathway unique to the autolyic phase of tracheary element differentiation in Zinnia elegans mesophyll cell culture system. The results of these studies are applicable to all higher plants and potentially to animals because of the high degree of conservation of the ubiquitin pathway and of mechanisms of programmed cell death among species. A more complete understanding of cell death in plants may lead to genetic engineering strategies for manipulating senescence and cell death for the purposes of increasing productivity, nutritional value, and disease resistance of food crops and may also enhance our understanding of situations where cell death goes wrong in mammalian systems, such as during abnormal tumor growth and neurodegenerative diseases. %%% Naturally occurring cell death is an active process that serves a number of important functions. In animals, cell death affects morphogenesis, and removes a variety of cells, including those that have completed their function, cells that have developed improperly or those that exist in excess. In plants, programmed cell death occurs as a consequence of normal senescence, local lesion formation, and as the terminal developmental event in the formation of certain specialized tissues, such as the xylem. The ultimate fate of the cells programmed to die in animals is engulfment and degradation. Degradation during programmed cell death in plants appears to be mediated not by engulfment, but rather by intracellular hydroytic enzymes including nucleases, carbohydrases and proteases. While little is known concerning the exact role of proteolysis in cell death, regulation of the ubiquitin-dependent pathway of proteolysis appears to be important during programmed cell death in animals and plants. Ubiquitin is a macromolecule that "tags" proteins for proteolysis. The results of these studies are applicable to all higher plants and potentially to animals because of the high degree of conservation of the ubiquitin pathway and of mechanisms of programmed cell death among species. A more complete understanding of cell death in plants may lead to genetic engineering strategies for manipulating senescence and cell death for the purposes of increasing productivity, nutritional value, and disease resistance of food crops and may also enhance our understanding of situations where cell death goes wrong in mammalian systems, such as during abnormal tumor growth and neurodegenerative diseases. ***