Effects of Cytokinins on the Nucleic Acids of Tobacco Pith

Abstract

Cylinders of pith of Nicotiana tabacum var. Wis. 38 were aseptically isolated and grown on a mineral-sucrose medium in the presence and absence of either kinetin or zeatin. On medium containing kinetin (5 x 10-6 M) the increase in the residual dry weight of cylinders was greater than that on control medium after 2 d culture and this was maintained at 5 d. At 2, 5, and 7 d there were increments in the DNA and RNA levels of cylinders due to kinetin treatment and these increased progressively. A double-labelling method coupled with acrylamide gel electrophoresis was used to study the effects of kinetin at 5 X 10 M and zeatin at 10~6 M on the incorporation of radioactive pre cursors into the RNA of cylinders given a 4-h label pulse. No effects of zeatin were observed after 1 d nor of kinetin at 2 d but after 4 d zeatin stimulated incorporation into ribosomal, transfer and poly-disperse RNA and kinetin did likewise after 5 d. Incorporation measured at 7 d was reduced in cylinders from which kinetin had been removed at 5 d compared to those to which it had been supplied continuously. Scans at 260 nm of the electrophoretic fractionations of RNA after 2,5,7, and 9 d of culture showed that both in the absence and in the presence of kinetin the proportion of 4S RNA increased from about 20 per cent to over 50 per cent of the total between 2 and 5 d of culture and after 5 d the ribosomal RNA gave aberrant scans apparently indicative of degradation. These aberrant scans persisted in RNA extracted after 27 d culture in both the presence and absence of kinetin.