Marchantia stem cell maintenance and re-establishment are controlled by MpPIN1-mediated auxin transport and ARF signalling

Meristem organization in land plants depends on coordinated auxin distribution, transport and response, yet how these processes are integrated during vegetative growth of the gametophyte of non-seed plants remains poorly understood. Here, we describe dissection of the auxin network in the liverwort Marchantia polymorpha using a suite of endogenous and synthetic reporters. Quantitative imaging with the R2DII auxin response sensor revealed that the apical meristem is positioned at a stable auxin minimum, with auxin levels increasing toward differentiated tissues. This spatial configuration correlates with polar localization of the auxin efflux carrier MpPIN1 along the direction of auxin flux. Disruption of Mp PIN1 alters auxin distribution and compromises tissue regeneration, but does not prevent initial meristem formation, indicating that PIN-mediated transport reinforces rather than defines stem-cell niche identity. Imaging of auxin response reporters and Mp ARF1 and Mp ARF2 knock-in reporters further revealed a constrained auxin response within the meristem, characterized by low auxin signalling, dependent on the inhibitory activity of Mp ARF2 in the stem cell zone. During regeneration, transient, oscillatory ARF dynamics were observed before stabilization of a new auxin minimum and re-establishment of normal meristem architecture. Together, our results are consistent with auxin responses in the Marchantia meristem being shaped by an incoherent feed-forward network topology that couples auxin flux, transcriptional constraint and niche permissiveness. This architecture provides a robust framework for stem-cell maintenance and re-establishment, revealing ancestral principles of meristem regulation in land plants.