Regulators of Cell Division in Plant Tissues. XXII* Physiological Aspects of Cytokinin-induced Radish Cotyledon Growth

Abstract

The cytokinin 6-benzylaminopurine (BAP) markedly stimulated the lateral expansion of excised immature radish cotyledons after a lag period of about 10 h. This growth occurred principally by cell enlargement, especially in the light which enhanced the response. However, a marked response 'to cytokinin occurred in the complete absence of red light during germination, cotyledon excision and incubation. Contact with BAP for 5 h significantly stimulated growth, but a maximum response required more than 24 h of contact; potassium chloride also promoted cotyledon expansion and acted synergistically with cytokinin. The response to cytokinin did not appear to be mediated by ethylene, gibberellins, polyamines or cyclic nucleotides. Growth induction did not alter the respiration rate and appeared to be inde- pendent of chloroplast function. Inhibitors of DNA and RNA synthesis and of protein synthesis on cytoplasmic ribosomes almost completely abolished BAP-induced growth, control growth being less markedly affected. There were, however, no significant BAP-induced increases in total DNA or RNA levels or specific activity before the initiation of growth stimulation. Similarly, BAP had no effect on any individual RNA species until after the lag period, when there was a small enhancement of uridine incorporation into RNA species with similar electrophoretic mobility to rRNA. Although total protein levels were not affected by BAP, the cytokinin enhanced amino acid incorporation into protein within the lag period, an effect which persisted when transcription was strongly inhibited by actinomycin D. Phosphorylation of total protein was stimulated by BAP only well after the onset of cytokinin-induced growth. Protein methylation, however, was stimulated by BAP during the lag period, and the effect was at least as early as the BAP-enhanced incorporation of methionine into protein. The possible role of translational control in the mechanism of cytokinin action is discussed.