Broader functions of TIR domains in Arabidopsis immunity
TIR domains are NAD-degrading enzymes that function during immune signaling in prokaryotes, plants, and animals. In plants, most TIR domains are incorporated into intracellular immune receptors. In Arabidopsis, TIR-derived small molecules bind and activate EDS1 heterodimers, which in turn activate RNLs, a class of cation channel-forming immune receptors. RNL activation drives cytoplasmic Ca2+ influx, transcriptional reprogramming, pathogen resistance and host cell death. We screened for mutants that suppress an RNL activation mimic allele and identified a TIR-containing immune receptor, SADR1. Despite functioning downstream of an auto-activated RNL, SADR1 is not required for defense signaling triggered by other tested TIR-containing immune receptors. SADR1 is required for defense signaling initiated by some trans-membrane pattern recognition receptors and contributes to the unbridled spread of cell death in lesion simulating disease 1. Together with RNLs, SADR1 regulates defense gene expression at infection site borders, likely in a non-autonomous manner. RNL mutants that cannot sustain this pattern of gene expression are unable to prevent disease spread beyond localized infection sites, suggesting that this pattern corresponds to a pathogen containment mechanism. SADR1 potentiates RNL-driven immune signaling partially through the activation of EDS1, but also partially independently of EDS1. We studied EDS1-independent TIR function using nicotinamide, an NADase inhibitor. We observed decreased defense induction from trans-membrane pattern recognition receptors and decreased calcium influx, pathogen growth restriction and host cell death following intracellular immune receptor activation. We demonstrate that TIR domains can potentiate calcium influx and defense and are thus broadly required for Arabidopsis immunity.