Yan, Y.; Melluh, J.; Mecchia, M. A.; Jeon, H.-W.; Melkonian, K.; Holzberger, C.; Harzen, A.; Stolze, S. C.; Franzen, R.; Hirakawa, Y.; Delgado, A. I. C.; Nakagami, H.

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 in the angiosperm Arabidopsis thaliana. AtSERKs act as co-receptors for many LRR-RLKs, including BRASSINOSTEROID INSENSITIVE 1 (BRI1) and FLAGELLIN SENSITIVE 2 (FLS2). The conserved tyrosine (Y) residue in AtSERK3 is crucial for signaling specificity in differentiating BRI1- and FLS2-mediated pathways.5 BRI1-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. The biological functions of SERK homologs in non-flowering plants are largely unknown. The genome of the liverwort Marchantia polymorpha encodes single homologs for SERK and BIR, namely MpSERK and MpBIR. We here show that Mpserk disruptants display growth and developmental defects with no observable sexual or vegetative reproduction. Complementation analysis revealed a contribution of the conserved Y residue of MpSERK to growth. Proximity labelling-based interactomics identified MpBIR as a MpSERK interactor. Mpbir disruptants displayed defects in reproductive organ development. Patterns of development- and immunity-related gene expression in Mpserk and Mpbir were antagonistic, suggesting that MpBIR functions as a MpSERK repressor. The pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 grew poorly 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 regulating both development and immunity in the last common ancestor of land plants.