Ribonucleic Acid Metabolism and Cell Expansion in Oat Coleoptile

RNA metabolism in oat coleoptiles was studied using physiological responses to 5-FU and actinomycin D; autoradiographic detection of RNA and protein synthesis; and estimation of ribosomal concentration by analytical ultracentrifugation. 5-FU failed to inhibit growth of either intact coleoptiles or isolated coleoptile segments but completely blocked cell division in roots. Actinomycin D markedly inhibited auxin-induced expansion of coleoptile segments. When supplied to isolated segments from coleoptiles of various lengths the RNA precursors cytidine, adenine and adenosine all showed weak incorporation into RNA of nuclei and in some cases, to a lesser extent, RNA of cytoplasm. IAA did not affect this RNA synthesis but it was considerably reduced by actinomycin D. A proportion of the label incorporated from RNA precursors was not removable with either RNase, PCA or hot TCA but was extracted by trypsin. The amount of this spurious incorporation increased with coleoptile age, as did the ability to incorporate labelled amino acids. The concentration of both free and bound ribosomes does not increase in growing coleoptiles and may even decline. Free ribosomes decline markedly in fully grown coleoptiles while the proportion of bound ribosomes increases. It is concluded that young coleoptiles contain a full complement of ribosomes necessary for subsequent growth but normal growth is dependent on continued production of an actinomycin D-sensitive messenger-type RNA. No evidence for auxin mediation of RNA synthesis was found.