Overexpression of the MADS-box gene CRM3 in the fern Ceratopteris richardii affects sporophyte development

MADS-box genes encode MADS-domain transcription factors that play important roles in the development and evolution of land plants. Especially well-characterized is their role as organ identity genes during flower development of angiosperms. Even though the first MADS-box genes have been molecularly cloned from ferns almost 30 years ago, little is known about their function, mainly due to a lack of mutants. Also their quite broad expression domain, often comprising both the gametophytic and the sporophytic phase of the life cycle of ferns, provide few specific clues concerning gene function. However, as sister group of seed plants monilophytes (ferns and their allies such as horsetails) are of great interest in many attempts to understand both MADS-box gene and land plant evolution. Here we report, to the best of our knowledge, the first mutant phenotype based on transgenic overexpression of a fern MADS-box gene. Transgenic plants of the fern model system Ceratopteris richardii overexpressing the MIKCC-type MADS-box gene CRM3 under control of the Cauliflower Mosaic Virus (CaMV) 35S promoter show a significant reduction of the length of sporophylls. This finding reveals that overexpression of CRM3 affects sporophyll development in C. richardii, possibly by inhibiting cell proliferation or elongation.