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.