Cellular basis of ABA-induced de novo meristem formation and sex-type conversion in Ceratopteris gametophytes.

Meristems house pluripotent stem cells and sustain continuous growth and organogenesis in land plants. Unlike seed plants, whose gametophytes lack meristems, fern gametophytes initiate and maintain meristems, enabling growth independently from sporophytes. In the model fern Ceratopteris richardii, gametophytes develop into either hermaphrodites or males. Hermaphrodites maintain multicellular meristems and secrete the pheromone antheridiogen, directing undetermined gametophytes to become male. Under constant antheridiogen exposure, males lack meristems and exclusively produce sperm-bearing antheridia. The phytohormone abscisic acid (ABA) antagonizes antheridiogen signaling during this process, suggesting a crucial, lineage-specific role of ABA in fern meristem specification. Nonetheless, the cellular dynamics underlying ABA-induced meristem formation and sex-type conversion remain unclear. Here, we used non-invasive, long-term confocal time-lapse imaging to capture the dynamic process of ABA-induced male-to-hermaphrodite conversion in Ceratopteris at single-cell resolution. Cell lineage analyses revealed that ABA triggers the formation of a de novo meristem, originating entirely from a single non-antheridium meristem progenitor cell (MPC), mirroring the meristem formation observed following antheridiogen removal. Importantly, ABA exhibited dual functions: promoting the cell fate re-specification essential for meristem initiation and concurrently suppressing cell division within the developing meristem lineage. Genetic analyses with different combinations of antheridiogen and ABA treatments demonstrated these dual roles could be uncoupled, yet both required functional ABA signaling. Our findings reveal both conserved and lineage-specific mechanisms for meristem initiation triggered by distinct environmental cues, providing insight into hormone-mediated cellular reprogramming and proliferation during sex-type specification in land plants.