Nuclear degeneration in epidermal cells of drying onion bulb leaf bases

Abstract

We tested the null hypothesis ‘that activated nuclei and nucleoli in outer-epidermal cells of newly exposed equatorial tissue of the turgid leaf bases of white onions (exposed to the ambient atmosphere by removal of two dry and two turgid leaf bases) remained in that state as the tissue dried’ by following nuclear macromolecules (total nucleic acid, DNA, RNA, total protein, histone, and non-histone protein: compared with T 0 = 100%) and nucleolar morphologies over a 5-day period. The nuclei became activated within 6 h and remained in that state for 2–3 days [increases in RNA, non-histone protein, and volume of major nucleoli occurred by T 12 (about 191, 177, and 289%, respectively) and appearance of the minor nucleoli between T 12 and T 24 (activation of silent rRNA cistrons)]. Combined nucleolar (major and minor) volumes decreased to 228% by T 24 and to 150% by T 48. Minor nucleoli were visible at T 24 and T 48. DNA (DAPI) remained unchanged over that period of time. At the T 96 sampling, all nuclear indices had decreased to levels below those obtained at the time of exposure to the ambient atmosphere; minor rRNA cistrons had became silent genes; nuclear volume was about 89% of the original volume; and, nucleolar volume (major nucleoli) was about 93%. The percentages for nuclear indices at T 120 were DNA, 85% of T 0; RNA, 35%; histone, 87%; non-histone protein, 47%; nuclear volume, 81%; and nucleolar volume, 67%. Of interest is the lack of change in major nucleolar morphologies between T 96 and T 120 although they decreased in volume during that period. We infer that the karyoskeleton (nuclear matrix) had undergone irreversible degeneration after T 48 and that the cells had passed the point-of-no-return in the senescence pathway by T 120. We propose that this model for cell senescence and death (drying of turgid leaf bases to form the dry, dead outer covering of the bulbs) simulates post-harvest storage conditions and will prove helpful to those studying cellular senescence mechanisms and associated host-pathogen interactions in plants.