In maize, like all monocots, the endosperm is the tissue where food reserves are stored. Developmentally, it is fated or programmed to be cannabilized by the seedling during the first 2-3 weeks of growth following germination. Thus, the death of endosperm is a desirable event, but exactly when this death initiates and how it progresses is not known. In animals, one of the hallmarks of cells or tissues that undergo such programmed cell death, often termed apoptosis, is the degradation of chromatin into nucleosome-sized fragments. These degradation products can be visualized as a DNA ladder of ~180 bp by electrophoresis on agarose gels.
In a preliminary experiment designed to study the fate of DNA of maize endosperm, we found that most of the DNA is degraded in a mature kernel. In a B73 inbred, this degradation first becomes obvious during the 5th week after pollination, and by the time endosperm is seven weeks old, most of the DNA is degraded. However, we have been unable to detect intranucleosomal fragmentation of chromatin so far. Although in our experiment degradation was not seen till the 5th week after pollination, an allele-specific loss of some RFLP markers, which may represent chromosomal foci where DNA degradation first initiates, was observed as early as 18 days after pollination by Yerk et al. (MNL 67:103, 1993).
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