Is Riboflavin the Photoreceptor in the Induction of Two-dimensional Growth in Fern Gametophytes?

Abstract

Blue light controls the development of 2-dimensional form in the gametophyte phase of numerous ferns (2 and literature therein). Although the photoreceptor for this system is still unknown, it has been hypothesized to be a flavoprotein (7). Yeoh and Raghavan (11) have presented data which they interpreted as evidence for the existence of riboflavin (or lumichrome) as the photoreceptor. This conclusion was based upon the observation that these compounds reverse the inhibitory effects of 2-thiouracil or 2-thiocytosine. Several problems are encountered with this interpretation: (1) riboflavin is very unstable in light (5, 10), and (2) riboflavin is a well known sensitizer for the photodestruction of various compounds (4,5). Because of the importance placed upon the knowledge of the receptor in any photophysiological reaction, the above experimental design was reinvestigated. Spores of Pteridium aquilinum (L.) Kuhn were collected in September, 1963, in Pulaski County, Indiana, and stored under refrigeration. The spores were filtered through 4 to 6 layers of lens paper to remove sporangia and leaf fragments before inoculation onto the surface of a modified Bold's algal medium (3), 25 ml per 10 cm petri dish. After 65 to 70 hours (approx 60% germination, cell number of 1.1, 0.1 % of the plants 2-dimensional), samples were transferred with a sterile bacteriological loop to 7.5 cm X 1.0 cm test tubes containing 0.5 ml of the basal medium plus supplements. Cultures were assayed 4 days after treatment, as compared to 28 days required for the gametophytes of Phymtatodes nigrescens used by Yeoh and Raghavan (11). All materials were incubated at 25 ± 1° over 2 G. E. Cool White fluorescent tubes, approximately 200 ft-c. Ten cultures were assayed for each treatment. At least 200 gametophytes from each culture were examined microscopically for the percentage of 2-dimensional plants (plants with at least 1 oblique cell wall, relative to the long axis of the organism); at least 50 gametophytes per culture were assayed for the cell number per filamentous