Ethylene is a selective ribosomal cistron regulator in Allium cepa epidermal cells

Nuclear degeneration in epidermal cells of drying onion bulb leaf bases

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 T0 = 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 T12 (about 191, 177, and 289%, respectively) and appearance of the minor nucleoli between T12 and T24 (activation of silent rRNA cistrons)]. Combined nucleolar (major and minor) volumes decreased to 228% by T24 and to 150% by T48. Minor nucleoli were visible at T24 and T48. DNA (DAPI) remained unchanged over that period of time. At the T96 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 T120 were DNA, 85% of T0; 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 T96 and T120 although they decreased in volume during that period. We infer that the karyoskeleton (nuclear matrix) had undergone irreversible degeneration after T48 and that the cells had passed the point-of-no-return in the senescence pathway by T120. 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.

Brassinolide is a selective ribosomal cistron regulator in onion leaf base tissue

Outer epidermal cells from the basal, equatorial, near-apical, and apical regions of the third turgid onion (Allium cepa L. var. yellow, sweet Spanish) leaf base were treated (3 and 6 h in the dark = T3 and T6, respectively) with brassinolide (Br, a brassinosteroid plant growth regulator; effects on excised pieces compared with those in water controls: there were no statistical differences between the T3 and T6 results). Br induced increases in the volume and changes in morphologies of the major nucleoli to a greater extent than observed for major nucleoli in basal through near-apical controls. No major nucleoli were activated in control or Br-treated apical tissue. Minor nucleolar organizer regions in control and Br-treated tissue remained inactive in all locations. We propose that Br is a major ribosomal cistron regulator.

Effect of abscisic acid, gibberellic acid, indoleacetic acid, and kinetin on selective ribosomal cistron regulation in quiescent and senescent onion leaf base tissue

Small pieces of tissue from the basal, equatorial, near-apical, and apical regions of the third turgid onion leaf base were treated (3 and 6 h in the dark) with abscisic acid (ABA), gibberellic acid (GA3), indoleacetic acid (IAA), and kinetin (K) and compared with responses in water controls. ABA inhibited the activation (increase in size and changes in morphologies from round or oval to elongated-oval and dumbbell) of major nucleolar organizer regions (NORs) in basal, equatorial, and near-apical tissue. GA3 and K activated the major NORs in the basal, equatorial, and near-apical tissue. IAA stimulated the activation of major NORs in basal tissue but inhibited their activation in equatorial and near-apical tissue. No major nucleoli were activated in control or plant growth regulator-treated apical tissue. Minor NORs were not expressed in the control and plant growth regulator-treated tissue in these four locations. Actinomycin D and cycloheximide inhibited major NOR activation in equatorial control and kinetin-treated tissue. We propose that ABA, GA3, IAA, and K are major NOR regulators. We infer that the basal through near-apical cells were quiescent during post-harvest storage and that the cells in the apical tissue had senesced beyond the point of no return (degeneration of the karyoskeleton) in the cellular senescence pathway.

Effect of ethylene on selective ribosomal cistron regulation in quiescent and senescent onion leaf base tissue

Excised pieces of tissue from four regions (base, equator, near-apex, and apex) of yellow, sweet Spanish onion bulbs were treated with ethylene in an attempt to activate minor nucleolar organizer regions (NORs). In the quiescent equatorial and near-apical regions, major nucleoli in control and ethylene-treated tissue increased in size and changed morphologies. The minor NORs were expressed only in the ethylene-treated tissue. These data confirm and extend our previous findings. In the quiescent basal region, major nucleoli increased in size and changed morphology from round to oval, elongated-oval and dumbbell shapes in both control and ethylene-treated tissue. However, the number of elongated-oval and dumbbell types were one-third that observed in equatorial and near-apical tissue. Minor NORs were not expressed. We infer that the responses of the major nucleoli and the inability to activate minor NORs were due to hormone interactions that affect the expression of rRNA cistrons (methylation-demethylation of CpG sites in the external spacers) in this basal (meristematic) region. In the senescing apical region, major nucleoli in control and ethylene-treated tissue did not change in size or morphology and ethylene had no effect on minor NORs. We infer that the cells in the apical region had passed the point of no return in the senescence pathway. We propose that the inability to activate major and minor NORs in senescing tissue is one of the earliest symptoms of cellular senescence and is associated with karyoskeleton degeneration.