Cytological studies on the inhibition by 5-fluorouracil of ribosome synthesis and growth in jerusalem artichoke tuber slices

Summary Rapid auxin-induced cell expansion in artichoke tuber slices is obtained by aerating the slices in water (aging) prior to auxin treatment. 5-fluorouracil (5-FU), an inhibitor of ribosomal RNA synthesis in plant cells, markedly inhibits auxin-induced growth only if present in the pre-growth aging period. Autoradiographic studies show that 5-FU given in the aging and/or growth periods reduces the incorporation of RNA precursors into the cytoplasm. Pulse-chase experiments suggest that the reduced cytoplasmic incorporation is in large part due to decreased stability of ribosomal rNA, as nucleolar and chromatin label are only slightly depressed at the end of the pulse. Though the nucleoli continue to incorporate RNA precursors following 5-FU treatment, they lack a distinct granular zone, and appear as homogeneous fibrillar structures under the electron microscope. 5-FU has a parallel inhibitory effect on growth and protein synthesis as shown by ³H-leucine studies during the growth period. Electron-microscope studies show that treatment with 5-FU causes decreased numbers of ribosomes and rough endoplasmic reticulum. The results suggest that the ribosomes and rough endoplasmic reticulum formed during aging are important in obtaining subsequent rapid auxin-induced expansion. The new ribosomes serve in part to replace pre-existing ribosomes present at the time of excision, which from electron microscopic evidence from 5-FU treated tissue, appear to slowly disappear.