Importance of RNA synthesis whthin the lag phase preceding benzyladenine-induced growth of cucumber cotyledons in the dark.

Abstract

Changes in the rate of synthesis of cellular constituents were studied in detached cucumber cotyledons (Cucumis sativus L. cv. Ohio) after treatment with benzyladenine (BA). BA treatment stimulated RNA and DNA syntheses. The stimulation of RNA synthesis occurred within the lag phase preceding BA-induced growth, while that of DNA synthesis came during the growth. On the other hand, the stimulation by BA was not detected for protein and lipid syntheses. 5-Fluorouracil (FU, 10(-3) m) plus thymidine (T, 10(-4) m) did not inhibit the fresh weight increase in both water- and BA-treated cotyledons, while α-amanitin (AM, 10 µg/ml) inhibited it in both. AM added during the first 4 hr effectively inhibited the growth stimulation by BA, but AM added 6 hr or more after BA had almost no effect. FU+T strongly inhibited the incorporation of (3)H-uridine into cytoplasmic ribosomal RNA (rRNA) and brought about labelling in regions other than those of the regular rRNA species on gel electrophoresis. On the other hand, AM had less influence on the synthesis of cytoplasmic rRNA than FU+T. These results indicate that BA stimulates cotyledon growth in the dark through its effect on messenger RNA synthesis within the lag phase of growth and the synthesis of cytoplasmic rRNA is not always necessary for cotyledon growth.