Synthesis of Plastid and Cytoplasmic Ribosomal RNAs in Isolated Pumpkin Cotyledons: II. Effect of Cytokinin and Light

Light and 6-benzylaminopurine (BAP) increase the amount of plastid and cytoplasmic ribosomal RNA in isolated pumpkin cotyledons during four days of growth. Continuous light promotes plastid rRNA accumulation to a greater extent than that of cytoplasmic rRNA showing a maximum value at 48h. BAP more than light stimulates the increase of rRNA content. The BAP effect is higher in dark than in light. During the first 24h of cotyledon development after BAP treatment the accumulation of cytoplasmic rRNA is more pronounced but within the 48 to 96h period the accumulation of chloroplast RNA is higher than that of cytoplasmic rRNA. Autoradiographs show that the incorporation of 3H-uridine into the RNA per nucleus and plastid increases in the light in the presence of BAP in comparison to water control. It is suggested that BAP stimulation of plastid rRNA accumulation is due to both increase in number of plastids and increased rRNA content per plastid.