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

## TL;DR

This study reveals how auxin signaling enables male fern gametophytes to convert into hermaphrodites by forming new meristems.

## Contribution

The study identifies de novo auxin biosynthesis as a key driver of cell fate changes during male-to-hermaphrodite conversion in ferns.

## Key findings

- Auxin signaling is activated at the site of proliferation in male gametophytes, triggering new meristem formation.
- Genetic and chemical inhibition of CrTAA1 blocks auxin patterns and prevents meristem development.
- Dynamic auxin maxima are critical for meristem progenitor cell lineage and sex-type conversion.

## Abstract

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.

Ferns alternate between generations of asexual sporophytes and sexual free-living gametophytes. This study uncovers a molecular mechanism underlying male-to-hermaphrodite conversion in a model fern species showing de novo auxin biosynthesis role in orchestrating cell fate and proliferation during meristem formation.