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 (GA 3), 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. GA 3 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, GA 3, 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.