Polyribosomes from Peas: II. Polyribosome Metabolism during Normal and Hormone-induced Growth

Polyribosomes as large as 10-mers (strands of messenger RNA bearing 10 ribosomes) were isolated from etiolated pea (Pisum sativum L. var. Alaska) stem tissue during all stages of development when methods were used which essentially eliminated ribonuclease activity during extraction. Actively growing tissue, harvested from the apical 10 mm, yielded many large polyribosomes and a low (<20%) proportion of monosomes. Similar tissue, allowed to age by applying lanolin to decapitated apices, showed a progressive decrease in number of larger polyribosomes and an increase in the proportion of monosomes. Hormone treatments, which prolonged growth and delayed aging, delayed the loss in large polyribosomes and the increase in proportion of monosomes. Growth-stimulating hormones, added to previously aged tissue, stimulated the production of many large polyribosomes in pre-existing cells.It is suggested that (a) large polyribosomes occur in all regions of the pea stem, (b) changes in polyribosome distribution appear to precede changes in growth rate, (c) loss of larger polyribosomes is closely related to a decrease in mRNA templates followed more gradually by loss of ribosomes, (d) hormone-stimulated continuation of growth is accomplished through maintenance of available mRNA.Methods are described, involving detailed analysis of polysome distribution, which, although they cannot be used to measure changes in initiation of ribosomes on to mRNA, do permit measurement of the amount of polysomal-associated mRNA present in tissues at different stages of growth. These analyses lead to the further suggestion that hormone stimulation of growth of previously nongrowing tissue is accomplished primarily through an increase in available mRNA prior to synthesis of ribosomes.