THE FERN CERATOPTERIS-RICHARDII AS A LOWER PLANT-MODEL SYSTEM FOR STUDYING THE GENETIC-REGULATION OF PLANT PHOTOMORPHOGENESIS

Photomorphogenetic research on fern gametophytes has provided important insights about pigment localization, ionic currents, and signal transduction. The first part of this article characterizes how light affects various aspects of gametophyte growth, such as spore germination, filamentous growth, and prothallial growth, in the three principal fern species used for photomorphogenetic research, namely, Adiantum capillus-veneris, Ceratopteris richardii, and Onoclea sensibilis. Although each species offers particular advantages for investigating certain photoresponses, we conclude that the process of spore germination in Ceratopteris is especially conducive to the selection of photomorphogenetic mutants. The second part describes the available Ceratopteris mutants isolated from three different selection schemes. Dark-germinating I (dkg1) exhibits the unique phenotype of reversed photoregulation of spore germination. Four other mutants, which are provisionally assigned the names of germ 1-4, are characterized in terms of germination percentages and prothallial growth in darkness, red light, and blue light. Germ 1 and 2 are impaired in their ability to respond to blue light, with the mutated genes apparently encoding signal transduction proteins that act close to the blue light photoreceptor. Germ 3 and 4 exhibit a de-etiolated phenotype in which the gametophytes grown in complete darkness resemble broader prothalli exposed to continuous light. Thus, the Ceratopteris mutants isolated to date complement, and perhaps extend, the range of comparable mutants in Arabidopsis seedling growth.