Coordinated cell division and growth drive meristem notch formation in Ceratopteris gametophytes

Meristems drive organogenesis and shape plant body plans, yet their morphology and cellular organization vary widely across land plants. Fern gametophytes develop a multicellular meristem that initiates at one flat margin of the prothallus and progressively forms a distinctive concave notch, producing the characteristic cordate gametophyte. Despite its importance for gametophyte growth and sexual organ formation, the cellular basis of meristem notch morphogenesis remains unclear. Here, we address this question in the fern Ceratopteris richardii using long-term time-lapse confocal imaging, chemical perturbations, and quantitative analyses at single-cell resolution. We reconstructed complete cell lineages and generated dynamic maps of cell division, size, growth rate, and collective cell displacement throughout development. Our analyses reveal that position-specific cell proliferation and division patterns, coordinated with differential cell expansion and dynamic cell displacement between the notch-forming lineage and adjacent wing regions, drive notch morphogenesis. Chemical perturbations disrupting microtubule function or cell-cycle progression disrupted notch formation, demonstrating that coordinated cell division and growth are required. Together, these findings establish a quantitative cellular framework for understanding meristem morphogenesis across land plants.