The regulation of meristem development in Ceratopteris gametophytes and sporophytes

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Plant shoot apical meristems, which contain stem cells and are found at the growing tips of plants, are essential for plants to grow tall and make leaves, flowers, fruits and seeds. Over three hundred million years, shoot meristems have evolved from a simple structure in non-seed plants, including ferns, to a complex structure in flowering plants. Using Ceratopteris (known as C-fern) as a model, the work interrogates the evolution of the meristem in land plants. In its life cycle, C-fern alternates between a haploid gametophyte and a diploid sporophyte stage, both having very distinct meristems. This research addresses how meristem development is regulated during these two generations. The research takes advantage of Ceratopteris as an effective and popular educational tool to engage K-12 science teachers, high school students, and undergraduates. Through summer workshops, high school science teachers from Indiana and Illinois, including those with limited resources for biology education, gain hands-on training to develop interactive lab exercises for their biology classes. High school students from underrepresented groups, recruited through established summer programs at Purdue, directly participate in the research. Aspects of the project also are integrated into undergraduate courses that the investigators teach, including plant anatomy, development and systematics. Lab exercises in plant anatomy, for example, include microscopic observations and quantification of fern gametophytes from wild type and transgenic plants, providing unique opportunities for undergraduates to get hands-on research experiences while benefiting the research project.Shoot apical meristems (SAMs) of seed plants are typically composed of multiple cell layers with distinct cell types, for example, the tunica-corpus organization in most angiosperms. In contrast, SAMs of non-seed vascular plants, including ferns, are simple, composed of a single large apical initial cell and its derivatives. Furthermore, non-seed vascular plants develop meristems in their independent, haploid and free-living gametophytes; whereas seed plant gametophytes do not have a meristem and are dependent on surrounding diploid sporophytic tissues. These differences raise two important but unresolved questions: are gametophyte and sporophyte meristems regulated by common pathways in ferns, and are the complex SAMs in seed plants and the simple SAMs in non-seed vascular plants controlled by similar mechanisms? Using the fern Ceratopteris richardii as a model system, the research tackles these questions employing multiple approaches, including transcriptome and phylogenetic analyses, gene-editing and gene silencing, time-lapse live imaging of fluorescence reporters for genes, proteins and distinct cell types, and computational image quantification. The research has great potential to reveal the ancestral function of meristem regulators in non-seed vascular plants, and uncover the molecular mechanisms underlying the initiation and maintenance of haploid gametophyte meristems and diploid sporophyte meristems of a model fern species. Given the importance of stem cells and the conserved function of the meristems in land plants, this work lays the groundwork for understanding how complex meristems observed in seed plants evolved from the simple meristems typical of ferns.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.