Novel mutant phenotypes of a dark-germinating mutant dkg1 in the fern Ceratopteris richardii.

The mutant dark-germinating 1 ( dkg1) of the fern Ceratopteris richardii was originally characterized by two phenotypes, germination in the dark and inhibition of germination by light. In this work, we examined whether other phenotypes are present in the gametophytic generation of the dkg1 mutant. Although dkg1 prothalli grown in darkness were elongated as in the case of the wild type, some developmental processes were found to proceed even in complete darkness: (1) the apical and subapical zones developed largely by forming a lateral meristem; (2) asymmetric cell division for rhizoid differentiation occurred in the subapical elongation zone; (3) an archegonium was formed in the proximity of the meristem; and (4) chloroplast relocation could occur without de novo protein synthesis. Furthermore, these processes were shown to be under the control of phytochrome in the wild-type gametophytes on the basis of red/far-red reversibility. These results indicate that the DKG1 gene is pleiotropic and is involved in several phytochrome-mediated responses in the gametophyte development of C. richardii.