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

In Allium cepa L. (onion) the number, size, and morphology of visible nucleoli per nucleus varies during cell division, growth, differentiation, storage, activation of quiescent tissue, senescence, wounding, and disease (host-pathogen interactions). Since there are two types (differentiated visually by size) of ribosomal cistrons in onion (major and minor nucleoli differ in the external spacer nucleotide sequences), we inferred that they function separately. In controls, major nucleolar organizer regions (NORs) were visible in epidermal cell nuclei as nucleoli. After exposure of the tissue to ambient conditions, these nucleoli enlarged and changed morphology. Minor NORs (when in the genome) required more than 6 h to become developed as visible nucleoli. In the ethylene treatments, the major NORs (visible nucleoli in quiescent epidermal cells) increased in size a (activation of a greater number of tandem rRNA genes) as in the controls. The minor, quiescent NORs became active and visible (activation of quiescent, inactive NORs) within 3 h. Actinomycin D, and cycloheximide, with or without ethylene, inhibited the increase in size of the major NORs, and prevented activation of the minor NORs (i.e. they did not become visible). Silver nitrate and cobalt chloride had no effect on major NORs but inhibited the appearance of minor nucleoli when combined with ethylene treatments. We infer: ethylene acts to regulate the expression of the minor NORs (selective ribosomal cistron regulation); and, other hormone(s) are involved in the regulation of the major NORs. Yellow and white sweet Spanish onions we studied had two major NORs and two minor NORs. Red sweet Spanish onions we studied had only the two major NORs.