Embryogenesis

After fertilization, synergid, antipodal, endosperm, coleorhiza and nucellar pillar cells die (Barlow, 1982; Murgia et al., 1993). These events are all assumed to represent PCD. The suspensor, which develops at the base of the embryo in angiosperms, is another example of developmental PCD (Barlow, 1982; Raghavan, 1986). The cells of the suspensor are metabolically active, they promote embryo survival in vitro, and they synthesize GAs and other hormones; nevertheless, they degenerate in the later stages of embryo development (Yeung andMeinke, 1993; Schwartz etal., 1997).

During seed maturation, cell death occurs in the endosperm (DeMason, 1997). Endosperm cell death in maize seeds begins at about 16 days after pollination (DAP) near the center of the endosperm, progresses centrifugally toward the top of the kernel and is complete at 40 DAP (Young et al., 1997). DNA fragmentation can be observed in extracts beginning around day 28 and expression intensifies until day 44. DNA fragmentation is not seen in the embryos from the same seed. The endosperm mutation called shrunken 2 (sh2), which results in 2-fold elevation of kernel sugar levels, also results in more extensive and rapid DNA fragmentation. Ethylene production rates per kernel exhibited two peaks, one around 16 to 20 DAP and the other around 32 to 40 DAP. Levels of the ethylene precursor ACC generally followed ethylene synthesis patterns. Ethylene treatments accelerated DNA fragmentation and death in the endosperm, and they reduced germination of seed. When ethylene synthesis was reduced to 20% of the control rate with AVG, DNA fragmentation was reduced but cell death was not prevented in sh2 kernels. The data suggest that ethylene may act as a signal in PCD in maize endosperm cells, and more recent evidence indicates that ABA and reactive oxygen species may also be involved (Young and Gallie, 2000). PCD in wheat endosperm cells exhibits common and unique features with the similar program in maize (Young and Gallie, 1999).

Endosperm PCD in castor bean occurs during germination and is associated with DNA fragmentation and accumulation of small organelles termed ricinosomes (Schmid et al, 1999). The ricinosomes contain a cysteine aminopeptidase which is released into the cytoplasm and then activated by cleavage. Homologous proteases occur in other species and tissues.

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