In the model angiosperm Arabidopsis thaliana, SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASES (SERKs), which are subfamily II of leucine-rich repeat receptor-like kinases (LRR-RLKs), play diverse roles in development and immunity as co-receptors for a multitude of LRR-RLKs including BRASSINOSTEROID INSENSITIVE 1 (BRI1) and FLAGELLIN SENSITIVE 2 (FLS2). The conserved tyrosine (Y) residue in AtSERK3 plays a crucial role in signaling specificity for differentiating AtBRI1- and AtFLS2-mediated pathways. BRI ASSOCIATED RECEPTOR KINASE 1 (BAK1)-INTERACTING RECEPTOR-LIKE KINASES (BIRs) interact with SERKs under resting conditions, negatively regulating SERK-mediated pathways. SERK and BIR are highly conserved in land plants, whereas BRI1 and FLS2 homologs are absent or poorly conserved in bryophyte lineages. Very little is known about biological functions of SERK homologs in non-flowering plants. The genome of the model liverwort Marchantia polymorpha encodes the single SERK and BIR homolog, namely MpSERK and MpBIR. We here show that Mpserk disruptants display growth and developmental defects with no observable sexual and vegetative reproduction. Complementation analysis revealed a contribution of the conserved Y residue of MpSERK in growth. Proximity labelling-based interactomics of MpSERK identified MpBIR as an interactor. Mpbir disruptants displayed defects in reproductive organ development. Transcriptomes of Mpserk and Mpbir showed antagonistic patterns, suggesting MpBIR functions as a MpSERK repressor. We revealed that the pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 could hardly grow on Mpbir, indicating a significant role of the MpSERK‒MpBIR module in immunity. Taken together, we propose that the SERK-BIR functional module was already established in the last common ancestor of land plants for regulating both development and immunity.