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

Plants possess a remarkable capacity for regeneration, which involves the redeployment of developmental programs and diverse regulatory mechanisms. However, how related regulators with overlapping functions are differentially deployed during regeneration remains poorly understood. The model liverwort Marchantia polymorpha provides a powerful experimental system for studying regeneration because it readily regenerates apical meristems from basal thallus fragments after removal of the original meristem, even without exogenous plant hormones. Here, we identify the R2R3-MYB transcription factor GEMMA CUP-ASSOCIATED MYB1-LIKE (MpGC1L), the closest paralog of the clonal propagation regulator MpGCAM1, as a positive regulator of regeneration. MpGC1L was rapidly induced at the cut site following meristem removal. Ectopic overexpression of MpGC1L caused the proliferation of undifferentiated cells, whereas Mpgc1l mutants showed delayed regeneration and reduced S-phase entry. Loss of MpGCAM1 alone had little effect on regeneration but markedly enhanced the Mpgc1l phenotype, indicating partially redundant functions. Transcriptome analysis of the double mutant revealed reduced induction of genes associated with ribosome biogenesis and the cell cycle. We next examined the relationship between MpGC1L and the known jasmonate- and auxin- related regeneration regulators, MpERF15 and MpLAXR. MpGC1L induction was retained in Mperf15 and Mplaxr mutants and was unaffected by OPDA or auxin treatment, whereas MpERF15 and MpLAXR were still induced in Mpgc1l Mpgcam1 double mutants. Thus, these regulators are not arranged in a simple linear transcriptional pathway. Our findings reveal that the paralogous MYB transcription factors MpGC1L and MpGCAM1 promote cell proliferation in distinct developmental contexts, thereby linking clonal propagation and wound-induced regeneration.