A Coevolved EDS1-SAG101-NRG1 Module Mediates Cell Death Signaling by TIR-Domain Immune Receptors

Abstract

Plant nucleotide binding/leucine-rich repeat (NLR) immune receptors are activated by pathogen effectors to trigger host defenses and cell death. Toll-interleukin 1 receptor domain NLRs (TNLs) converge on the ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) family of lipase-like proteins for all resistance outputs. In Arabidopsis (Arabidopsis thaliana) TNL-mediated immunity, AtEDS1 heterodimers with PHYTOALEXIN DEFICIENT4 (AtPAD4) transcriptionally induced basal defenses. AtEDS1 uses the same surface to interact with PAD4-related SENESCENCE-ASSOCIATED GENE101 (AtSAG101), but the role of AtEDS1-AtSAG101 heterodimers remains unclear. We show that AtEDS1-AtSAG101 functions together with N REQUIRED GENE1 (AtNRG1) coiled-coil domain helper NLRs as a coevolved TNL cell death-signaling module. AtEDS1-AtSAG101-AtNRG1 cell death activity is transferable to the Solanaceous species Nicotiana benthamiana and cannot be substituted by AtEDS1-AtPAD4 with AtNRG1 or AtEDS1-AtSAG101 with endogenous NbNRG1. Analysis of EDS1-family evolutionary rate variation and heterodimer structure-guided phenotyping of AtEDS1 variants and AtPAD4-AtSAG101 chimeras identify closely aligned ɑ-helical coil surfaces in the AtEDS1-AtSAG101 partner C-terminal domains that are necessary for reconstituted TNL cell death signaling. Our data suggest that TNL-triggered cell death and pathogen growth restriction are determined by distinctive features of EDS1-SAG101 and EDS1-PAD4 complexes and that these signaling machineries coevolved with other components within plant species or clades to regulate downstream pathways in TNL immunity.