Cell Morphogenesis and Macromolecule Synthesis during Phytochrome-controlled Germination of Spores of the Fern,Pteris vittata

Abstract

The object of this study was to characterize the pattern of cell morphogenesis and synthesis of nucleic acids and proteins during phytochrome-controlled germination of spores of the fern, Pteris vittata. Phytochrome activation and germination were initiated in fully imbibed spores by exposure to a saturating dose of red light. At timed intervals thereafter, spores were fixed in acrolein and embedded in glycol methacrylate for examination in the light microscope. The first sign of germination, visible in sections of the spore 12 h after irradiation, was the hydrolysis of storage protein granules. This was followed by a migration of the nucleus from its central location to one side of the spore. Subsequently, the protoplast enlarged at the site of the nucleus and appeared outside the exine as a papillate structure. An asymmetrical division of the protoplast gave rise to a small colourless rhizoid cell and a large, chloroplast-containing protonemal cell. During the early phase of germination, DNA was synthesized both in the nucleus and cytoplasm as judged by autoradiography of [3H]thymidine incorporation. [3H]Uridine, a precursor of RNA synthesis, was incorporated into the nucleolus and the rest of the nuclear material of germinating spores. Protein synthesis monitored by [3H]leucine incorporation occurred both in the nucleus and cytoplasm during the early stage of germination, although a strictly cytoplasmic protein synthesis was observed later. Addition of cycloheximide completely inhibited germination of photoinduced spores and incorporation of labelled precursors of macromolecule synthesis into cellular components. Actinomycin D was much less effective as an inhibitor of germination and, even in high concentrations of the drug which effectively inhibited DNA and RNA synthesis in spores, proteolysis and protein synthesis appeared normal. These findings are discussed with respect to the regulation of nucleic acid and protein synthesis during spore germination and the role of phytochrome in the process.