A genetically controlled process of cellular suicide previously known to occur only in animals had been identified in plants by a team of scientists at the University of California, Davis. Researchers hope this finding will open the door to new ways of genetically engineering disease-resistance in plants and will help identify key genes involved in normal plant development. UC Davis plant pathologist David Gilchrist and colleagues report their findings in the March 25 issue of the journal Plant Cell. The process, in which the cell's genes orchestrate the death of the cell, is known as "apoptosis," a term derived from Greek roots to signify an orderly process of cell death. Apoptosis is characterized by a series of abnormal changes in the DNA of the cell nucleus, leading up to the cell's destruction. The researchers observed these trademark events in plant cells during normal development and in response to toxins from a fungal disease. "The results from this study suggest that disease-causing agents such as viruses, bacteria and fungi are able to hijack this normally occurring process of programmed cell death in the plant," said Gilchrist, who is associate director of the UC Davis Center for Engineering Plants for Resistance Against Pathogens. "Knowing that apoptosis occurs in plants and can be triggered during plant infection, we can direct future research toward identifying the genes involved in apoptosis and developing new targets for genetically engineering disease resistance into plants." Apoptosis has been identified in animals -- including humans and insects -- as a process by which chemical signals from inside or outside the cell trigger certain genes in the cell to initiate the programmed destruction of that cell. Unlike degenerative cell death, which usually is caused by environmental injury, apoptosis is part of the animal's orderly life process. It occurs during development of new tissue as it becomes necessary for the organism not only to produce new cells, but also to get rid of old cells. Apoptosis also can be activated defensively against disease or suppressed by an attacking disease agent, such as when cells proliferate wildly in cancerous tumors. Although apoptosis in recent years has been well-characterized in the animal kingdom, existence of such a process in plants has been an unresolved question among plant biologists. Gilchrist and colleagues explored this area of research in an attempt to find out how genes normally involved with the developmental processes of plants might be involved in the plant's susceptibility to disease. As a research model, they were studying a fungal disease of tomato plants known as Alternaria stem canker disease, which appears as a blackened area of dead cells at the site of infection. Previous research had shown that the Alternaria fungus produces a toxin that, alone or in the presence of the fungus, causes cell death. That toxin and other chemically similar toxins are known to cause cell death in plants, as well as cancer and degenerative diseases in animals. The UC Davis researchers had observed that the Alternaria toxin produced the DNA abnormalities characteristic of apoptosis when tested on animal cells. First, the DNA in the cell's nucleus broke up into specific fragments. Those fragments, along with other cellular components, were then organized into smaller units called apoptotic bodies. Eventually, the apoptotic bodies were taken up by nearby cells and degraded. In short, the cells self-destructed. Curious to see whether the Alternaria toxin would cause a similar reaction in plant cells, Gilchrist and colleagues applied the toxin to tissue from tomato leaflets and to tomato protoplasts, which are plant cells that have been stripped of their cell walls. In both cases, the nuclear DNA fragmented, then formed apoptotic-like bodies that migrated to the edge of the cell. Finally, the treated cells disintegrated in only a few hours. The researchers then confirmed that the process was dependent on expression of plant genes and that the same events were observed in plant infection by the fungus. The researchers also examined cells from the tips of plant roots and conductive cells of the tomato leaf, both areas intricately involved in the plant's developmental process and where cell death is a preordained, necessary event. The characteristic apoptotic processes were observed in these plant cells as part of the normal development and growth of the plant, and in the absence of the Alternaria toxin. "The results of this study were strikingly consistent with the characteristics of apoptosis in animal cells," said Gilchrist. "However, as in animals, the confirmation and manipulation of such a gene-controlled process will require identification of the genes or sets of genes that respond to specific chemical signals and trigger apoptosis." Collaborating with Gilchrist on this study were Hong Wang and Juan Li of the UC Davis Center for Engineering Plants for Resistance Against Pathogens and Richard Bostock, a UC Davis professor of plant pathology. Support for this study was provided by the National Science Foundation, which funds the Center for Engineering Plants for Resistance Against Pathogens. The UC Davis center focuses on the tomato as a research model to study fundamental mechanisms regulating disease in plants and to develop new technologies for engineering disease resistance in crop plants.