Synchronous Cell Division in Cultured Explants of Jerusalem Artichoke Tubers: the Effects of 5-Fluorouracil on Messenger RNA Synthesis and the Induction of Cell Division

Abstract

Explants of secondary xylern parenchyma tissue from Jerusalem artichoke tubers were induced to undergo cell division and de-differentiate by culture in nutrient medium. The first division was inherently synchronous. The system was used to study the involvement of messenger RNA synthesis in the induction and continuance of cell division in previously non-dividing cells. The base analogue 5-fluorouracil (5-FU) inhibited ribosomal RNA synthesis and the processing of ribosomal RNA precursor to mature 25 S and 18 S RNAs. The synthesis of messenger-like RNAs (heterogeneous in size, labelled to a high specific activity in a pulse incubation, and containing a polyadenylic acid sequence) was less inhibited by 5-FU. Explants grown in 5-FU did not synthesize DNA and did not divide. A direct inhibition of DNA synthesis by 5-FU added late in culture was reversed by thymidine. An indirect inhibition of DNA synthesis occurred when 5-FU was present from the start of culture and was not reversed by thymidine. Because ribosomal RNA synthesis is not necessary for the induction of cell division (Fraser, 1975) and because 5-FU was incorporated into mENA, probably interfering with its function, these results suggest that 5-FU inhibited the metabolism of mRNA which was required for DNA synthesis and cell division. The timing of mRNA synthesis required for DNA synthesis and cell division was investigated by adding 5-FU plus thymidine to cultures at various times. By the beginning of DNA synthesis for the first division, explants were competent, in terms of mRNA synthesized, to complete the first division. Messenger RNA synthesis occurring before the end of the first division allowed explants to undergo at least three more divisions.