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TUNEL of death
It is very well known that in cereal seed development, the cells of the endosperm undergo programmed cell death (PCD). Indeed, the roles of the embryo and aleurone in mobilizing the stored reserves of the dead endosperm during germination have been the subject of extensive research in relation to the action of gibberellins. But what of other parts of the seed, including the embryo itself? Giuliani et al. (pp. 287-292) from the University of Milan have used the TUNEL assay to study DNA fragmentation in the suspensor and embryo of maize. This assay detects strand breaks in DNA which are taken as symptomatic of an ordered degradation of chromatin. As expected, TUNEL-positive nuclei were readily detected in the suspensor - it has only a limited lifetime during embryogenesis - and their presence preceded the breakdown and collapse of the suspensor cells. More surprising, however, was the transient presence at particular times during embryogenesis of TUNEL-positive nuclei in specific parts of the embryo, particularly the scutellum (the single cotyledon of the cereal seed) and the coleoptile. However, we need to be cautious in interpreting a positive TUNEL assay as being diagnostic for PCD. For example, TUNEL-positive nuclei are detectable in ungerminated seeds. The nicks in DNA in mature seeds are due to DNA damage sustained during desiccation which are repaired during germination. However, the TUNEL-positive nuclei that occur at specific phases of embryogenesis are unlikely to result from DNA damage that is subsequently repaired. First, the authors find some evidence for breakdown and loss of nuclei, and secondly they detect much more extensive endonucleolytic hydrolysis of DNA as evidenced by the presence of a 'ladder' when extracted DNA is fractionated in agarose gels. At present, we do not know the significance of this localized PCD in cereal embryogenesis, but the data provide an excellent impetus for further research.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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