Cell cycle duration and time of DNA synthesis in binucleate cells induced inV. faba meristems by caffeine or isobutyl-methylxanthine

Summary Lateral roots ofVicia faba were treated with a solution of 5-aminouracil (3.93×10^–3M) for 6 hours. After 15 hours roots were recovering from the temporary inhibition of mitosis induced by 5-AU and were approaching peak mitotic indices; they were then treated with 0.1% caffeine or 0.1% isobutylmethylxanthine (IBMX) for 1 hour. Treatment with methylxanthines when the mitotic index was high gave relatively high yields of binucleate cells, 3.8 to 7.5%. DNA synthesis, cell cycle duration and nuclear growth were determined for binucleate cells. Caffeine induced binucleate cells underwent a marked reduction in nuclear volume, from 1,074 m³ at 1+1 hours to 534 m³ at 1+14 hours. Only 15% of these binucleates entered S phase; those that did so were in mitosis or had divided by 1+14 hours. We conclude that 85% of the binucleate cells are so inhibited by caffeine that their G₁ is extended to>14 hours or that they are no longer proliferating cells. IBMX-induced binucleate cells, by contrast, did enter S phase and many of them also divided. Though in IBMX-induced binucleate cells there was also a decrease in nuclear volume up to 1+10 hours, subsequently mean nuclear volume increased e.g. at 1+16 and 1+18 hours. Both caffeine and IBMX treatments resulted in decreases in mean volume of prophase nuclei of mononucleate cells; this is further evidence that both methylxanthines inhibit the macromolecular synthesis required to sustain nuclear growth. It also suggests that nuclear division can be initiated at considerably lower nuclear volumes than those of untreated cells. We suggest that caffeine may act as a mimic of the normal mechanism that regulates the switch from a proliferating to a non-proliferative condition.