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 importan t insights about pigment localization, ionic currents, and signal tran sduction. The first part of this article characterizes how light affec ts various aspects of gametophyte growth, such as spore germination, f ilamentous growth, and prothallial growth, in the three principal fern species used for photomorphogenetic research, namely, Adiantum capill us-veneris, Ceratopteris richardii, and Onoclea sensibilis. Although e ach species offers particular advantages for investigating certain pho toresponses, we conclude that the process of spore germination in Cera topteris is especially conducive to the selection of photomorphogeneti c mutants. The second part describes the available Ceratopteris mutant s isolated from three different selection schemes. Dark-germinating I (dkg1) exhibits the unique phenotype of reversed photoregulation of sp ore germination. Four other mutants, which are provisionally assigned the names of germ 1-4, are characterized in terms of germination perce ntages and prothallial growth in darkness, red light, and blue light. Germ 1 and 2 are impaired in their ability to respond to blue light, w ith 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 d arkness resemble broader prothalli exposed to continuous light. Thus, the Ceratopteris mutants isolated to date complement, and perhaps exte nd, the range of comparable mutants in Arabidopsis seedling growth.