Dynamic auxin maxima regulate male-to-hermaphrodite conversion and de novo meristem formation in the fern Ceratopteris gametophytes

Land plants alternate between generations of asexual sporophytes and sexual gametophytes. Unlike seed plants, ferns produce free-living gametophytes that grow independently from their sporophytes. Gametophytes of the model fern Ceratopteris exist in two sex types: hermaphrodites and males. Hermaphrodites maintain meristems and secrete the pheromone antheridiogen, inducing undecided gametophytes to become males. In the absence of antheridiogen, males exhibit developmental plasticity and dynamic cell fate specification by initiating de novo meristems to convert into hermaphrodites. Despite its essential role, the molecular signals governing this process remain unclear. Here, we show that local auxin biosynthesis, dynamically regulated during sex-type conversion, establishes new auxin maxima that are critical for specifying and promoting the proliferation of the meristem progenitor cell (MPC) lineage, ultimately enabling the de novo formation of a multicellular meristem from a single MPC. Time-lapse imaging revealed that upon antheridiogen removal, auxin signaling is specifically activated at the initial site of proliferation in Ceratopteris males, triggering new meristem formation. This auxin signaling subsequently becomes concentrated at the center of the proliferating meristem, aligning with localized auxin biosynthesis and the emergence of the meristem notch. Computationally reconstrued lineage maps further showed that chemical inhibition of CrTAA1 abolishes these dynamic auxin patterns, blocking MPC lineage initiation and its subsequent proliferation. Furthermore, genetic knockout of CrTAA1 via CRISPR-Cas9 phenocopies the effects of chemical inhibition, preventing new meristem formation and disrupting male-to-hermaphrodite conversion. Together, these findings uncover a molecular mechanism underlying sex-type conversion in land plants and highlight the pivotal role of de novo auxin biosynthesis in orchestrating cell fate and proliferation during meristem formation.