CONTROL OF THE ENTRY INTO S PHASE BY PHYTOCHROME AND BLUE-LIGHT RECEPTOR IN THE FIRST CELL-CYCLE OF FERN SPORES

Phytochrome-and a blue light receptor-dependent pathway antagonistical ly regulate the first mitosis in spores of the fern Adiantum capillus- veneris L. This study focused on determining which phase(s) of the cel l cycle is positively regulated by phytochrome and negatively regulate d by a blue light receptor in germinating spores. Incorporation of the radioactivity of H-3-thymidine into the acid-insoluble material prepa red from the spores indicated that phytochrome in the P-FR form induce d the entry into S phase of the first cell cycle in the spores 20-28 h after irradiation with red light. Blue light treatment before or afte r red light treatment totally prevented the P-FR-induced DNA synthesis . Brief irradiation with red, far-red or blue light showed no effects on mitosis if the irradiation was given 28 h after the red light induc tion, during S and M phases. These results indicate that phytochrome a nd a blue light receptor regulate the entry into S phase during the fi rst cell cycle of fern spores.