A paralog of a clonal propagation regulator promotes cell-cycle re-entry during thallus regeneration in Marchantia polymorpha

Cells have the ability to regenerate, which entails reactivating developmental processes and various regulatory systems. Nonetheless, the manner in which related regulators with similar functions are selectively utilized during regeneration is not fully grasped. The liverwort Marchantia polymorpha offers a valuable experimental model for studying regeneration, as it can regenerate apical meristems from basal thallus fragments after the original meristem is removed, without the need for external plant hormones.

Our research identifies the R2R3-MYB transcription factor GEMMA CUP-ASSOCIATED MYB1-LIKE (MpGC1L), a close relative of the clonal propagation regulator MpGCAM1, as a positive factor in regeneration. MpGC1L was rapidly activated at the cut site after the meristem was removed. When MpGC1L was overexpressed, undifferentiated cells proliferated, while Mpgc1l mutants demonstrated delayed regeneration and reduced entry into the S-phase.

The absence of MpGCAM1 alone had minimal impact on regeneration but exacerbated the Mpgc1l phenotype, suggesting partially overlapping roles. Transcriptomic analysis of the double mutant showed a diminished induction of genes related to ribosome biogenesis and the cell cycle. We then investigated the connection between MpGC1L and known jasmonate- and auxin-related regeneration regulators, MpERF15 and MpLAXR.

MpGC1L induction persisted in Mperf15 and Mplaxr mutants and was unaffected by OPDA or auxin treatment, whereas MpERF15 and MpLAXR were still activated in the Mpgc1l Mpgcam1 double mutants. Consequently, these regulators do not follow a straightforward linear transcriptional pathway. Our results indicate that the paralogous MYB transcription factors MpGC1L and MpGCAM1 stimulate cell proliferation in different developmental settings, thus establishing a connection between clonal propagation and wound-induced regeneration.