SWI3A/B regulates the transition from vegetative to reproductive phase in the liverwort Marchantia polymorpha

Land plants alternate between multicellular haploid and diploid phases, requiring a tight coordination between vegetative growth and sexual reproduction. We investigated SWI3A/B, an ancient core subunit of the SWI/SNF complex, of the non-vascular liverwort Marchantia polymorpha. A mutation in the promoter of MpSWI3A/B affected gametangiophore development and spermiogenesis in males, revealing its male-specific role in reproductive development. The MpSWIA/B mutant line amplifies vegetative propagation in males under conditions that would normally induce reproductive growth. The phenotype was underpinned by transcriptomic changes, showing that MpSWI3A/B modulates key regulators of gametangiophore initiation (e.g. BONOBO, GLID), sperm development and motility (e.g. DUO1, PKAR), and asexual reproduction (e.g. KAI2). We propose that, as a chromatin-level regulator, MpSWI3A/B may contribute to balancing vegetative and reproductive phases and highlight the protein's potential when exploring ancient epigenetic functions that coordinate developmental phase transitions in land plants.