Programmed Cell Death in Plants.

Programmed cell death (PCD) is a physiological cell death process involved in the selective elimination of unwanted cells (Ellis et al., 1991). In animals, these unwanted cells include those that have served temporary functions, such as tadpole tail cells at metamorphosis; cells that are overproduced, such as vertebrate neurons; cells that are unwanted or present in inappropriate positions, such as cells between the developing digits and Müllerian duct cells required in females but not males; and cells that die during the process of cell specialization, such as keratinocytes at the surface of the skin (Jacobsen et al., 1997). PCD in specific cell types can also give rise to disease. These cell types include helper T cells, which undergo PCD in AIDS, and selected brain neurons, which die during Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and Lou Gehrig’s disease (Duke et al., 1996). Thus, PCD plays an important role in cell and tissue homeostasis and specialization, tissue sculpting, and disease. PCD in Caenorhabditis elegans and other animals depends on the induction and action of specific genes that bring about the controlled disassembly of a cell (Wadewitz and Lockshin, 1988; Ellis et al., 1991). This disassembly involves the condensation, shrinkage, and fragmentation of the cytoplasm and nucleus and the fragmentation of the nuclear DNA into -50-kb and, in some cases, -0.14-kb pieces that represent oligonucleosomes (Cohen, 1993; Eastman et al., 1994). These DNA fragments can be detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) of DNA 3’-OH groups in sections of cells (Gavrieli et al., 1992). When PCD features marginalization of the chromatin in the nucleus, breakage of the cytoplasm and nucleus into small, sealed packets, and processing and fragmentation of the DNA at nucleosome linker sites, the process is termed apoptosis (Cohen, 1993). However, not all PCD involves all of these changes (Schwartz et al., 1993). Many of the proteins responsible for PCD in animal cells have now been identified. Proteases of the interleukin-1 pconverting enzyme (ICE) family bring about cell condensation and shrinkage (Martin and Green, 1995), and endonucleases cause the fragmentation of nuclear DNA (Eastman et al., 1994). The basic leucine zipper transcription factor ces-2 (for