Conserved role of the SERK-BIR module in development and immunity across land plants

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).^1,2,3,4 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.^6,7 SERK and BIR are highly conserved in land plants, whereas BRI1 and FLS2 homologs are absent or poorly conserved in bryophyte lineages.^8,9 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.⁹ 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-labeling-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 an 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.