Coleoptile senescence in rice (Oryza sativa L.)

We investigated the cellular events associated with cell death in the coleoptile of rice plants (Oryza saliva L.). Seeds germinated under submergence produced coleoptiles that were more elongated than those grown under aerobic conditions. Transfer of seedlings to aerobic conditions was associated with coleoptile opening (i.e. splitting) due to death of specific cells in the side of the organ. Another type of cell death occurred in the formation of lysigenous aerenchyma. Senescence of the coleoptile was also noted, during which discolouration of the chlorophyll and tissue browning were apparent. DNA fragmentation was observed by deoxynucleotidyltransferase-mediated dUTP nick end labelling (TUNEL) assay, and further confirmed by the appearance of oligonucleosomal DNA ladders in senescent coleoptile cells. Two nucleases (Nuc-a and Nuc-b) were detected by in-gel-assay from proteins isolated from coleoptiles. Nuc-a, commonly observed in three cell death phases required either Ca2+ or Mg2+, whereas Nuc-b which appeared during senescence required both Ca2+ and Mg2+. Both nucleases were strongly inhibited by Zn2+. (C) 2000 Annals of Botany Company.