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 ethyl ene 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 m orphologies. The minor NORs were expressed only in the ethylene-treate d tissue. These data confirm and extend our previous findings. In the quiescent basal region, major nucleoli increased in size and changed m orphology from round to oval, elongated-oval and dumbbell shapes in bo th control and ethylene-treated tissue. However, the number of elongat ed-oval and dumbbell types were one-third that observed in equatorial and near-apical tissue. Minor NORs were not expressed. We infer that t he responses of the major nucleoli and the inability to activate minor NORs were due to hormone interactions that affect the expression of r RNA cistrons (methylation-demethylation of CpG sites in the external s pacers) in this basal (meristematic) region. In the senescing apical r egion, major nucleoli in control and ethylene-treated tissue did not c hange 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 n o return in the senescence pathway. We propose that the inability to a ctivate major and minor NORs in senescing tissue is one of the earlies t symptoms of cellular senescence and is associated with karyoskeleton degeneration.