P-291 - Lineage-specific divergence of an immune chaperone complex important for NLR-based immunity in land plants

During 500 million years of evolution, land plants have developed a diverse range of mechanisms to neutralize non-adapted pathogens, including NLR immune receptors and the RAR1-SGT1-HSP90 chaperone complex required to sustain them. To unravel the evolutionary history of the immune chaperone complex, we interrogated the critical RAR1-SGT1 binding interfaces across land plants. Using yeast 2-hybrid assays, we determined that the RAR1-SGT1 interaction is so highly conserved that interactions form irrespective of plant lineage. Interestingly, we identified a single exception in the model fern Ceratopteris richardii, where the RAR1-SGT1 interaction was species-specific. Through subsequent phylogenetic analyses and protein-protein interaction assays, we discovered fern-specific mutations in the RAR1^CHORD2-SGT1^CS binding interface that were sequentially acquired in early leptosporangiate ferns and fully explain RAR1-SGT1 ortholog incompatibility. Ancestral reconstruction indicated that the change in the protein interaction dynamics occurred first in SGT1 in Hymenophyllales ferns, which preceeded further divergence in most extant fern species. Strikingly, introducing fern-specific mutations into Arabidopsis thaliana homologs could mimic the species-specific binding interface. Collectively, our findings demonstrate that the RAR1-SGT1 interaction is conserved in land plants, with ferns experiencing an atypical divergence at the RAR1^CHORD2-SGT1^CS interface.