CHANGES IN NUCLEAR MACROMOLECULES AND NUCLEOLAR VOLUME ASSOCIATED WITH SELECTIVE RIBOSOMAL CISTRON ACTIVATION BY ETHYLENE

Pieces of leaf base tissue from the equatorial region of the third tur gid onion (Allium cepa L.; yellow, sweet Spanish, quiescent tissue) le af base with two visible major nucleoli (volume = 0.51 mu m(3)) and tw o inactive minor nucleolar organizer regions (NORs) in the outer epide rmal cells were placed in shallow water (time = T-0 control). After 3 h (T-3 control), the two visible nucleoli enlarged (178% of T-0) and c hanged shape (from 88% round and 12% oval at T-0 to 5% round, 68% oval , 20% elongated-oval, and 7% dumbbell). The minor NORs remained inacti ve. Nuclear RNA and non-histone protein (nHP) increased (RNA = 128% an d nHP = 134%, compared with T-0 data, set at 100%). The content of DNA and histone protein (H) in nuclei remained unchanged. Major nucleoli in pieces of onion placed in 10(-5) M ethylene enlarged and minor nucl eoli became visible in 33% of the nuclei; i.e., ethylene acted as a se lective ribosomal cistron regulator. The nucleolar volume (212% of T-0 ) was statistically greater than those for T-0 and T-3. DNA and H rema ined unchanged (not statistically different) but RNA (159% of T-0) and nHP (169% of T-0) were greater (statistically significant) than those for T-0 and T-3 controls. Cobalt chloride (10(-5) M Co) alone or in 1 0(-5) M ethylene (Co + E) inhibited the enlargement of some of the maj or nucleoli and completely inhibited the appearance of minor nucleoli. The nucleolar volumes for the cobalt and for cobalt-ethylene treatmen ts were significantly less than those for T-3 controls and ethylene tr eatment at T-3. DNA and histone did not change. RNA and nHP were signi ficantly less than the amounts obtained in T-3 controls and ethylene t reatments suggesting inhibition or reversal of the ethylene-induced ac tivation. We infer that when ethylene activated minor NORs, it stimula ted rRNA, mRNA, and nHP synthesis. The amounts of rRNA that moved from the nucleus into the cytoplasm and of nHP that remained in the cytopl asm are not known.