CONTROL OF MITOSIS BY PHYTOCHROME AND A BLUE-LIGHT RECEPTOR IN FERN SPORES

The first mitosis in spores of the fern A. capillus-veneris was observ ed under a microscope equipped with Nomarski optics with irradiation f rom a safelight at 900 nm, and under a fluorescent microscope after st aining with 4',6-diamidino-2-phenylindole. During imbibition the nucle us remained near one corner of each tetrahedron-shaped dormant spore, and asymmetric cell division occurred upon brief irradiation with red light. This red light-induced mitosis was photoreversibly prevented by subsequent brief exposure to far-red light and was photo-irreversibly prevented by brief irradiation with blue light. However, neither farr ed nor blue light affected the germination rate when spores were irrad iated after the first mitosis. Therefore, the first mitosis in the spo res appears to be the crucial step for photoinduction of spore germina tion. Furthermore, experiments using a microbeam of red or blue light demonstrated that blue light was effective only when exposed to the nu cleus, and no specific intracellular photoreceptive site for red light was found in the spores. Therefore, phytochrome in the far-red absorb ing form induces the first mitosis in germinating spores but prevents the subsequent mitosis in protonemata, whereas a blue-light receptor p revents the former but induces the latter.