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.

Genetic Network between Leaf Senescence and Plant Immunity: Crucial Regulatory Nodes and New Insights

“…SAG12, which is a marker gene of leaf senescence, is considerably reduced in npr1 mutants compared to Col-0 plants. In addition, EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1) and PAD4 (PHYTOALEXIN DEFICIENT4) form a heterodimer to regulate plant immunity and are indispensable for the accumulation of SA induced by virulent pathogens [23]. EDS1 and PAD4 positively regulate disease resistance against Pst pathogens [23], but virus-induced gene silencing of EDS1 enhances the tobacco resistance to B. cinerea [24].…”

“…In addition, EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1) and PAD4 (PHYTOALEXIN DEFICIENT4) form a heterodimer to regulate plant immunity and are indispensable for the accumulation of SA induced by virulent pathogens [23]. EDS1 and PAD4 positively regulate disease resistance against Pst pathogens [23], but virus-induced gene silencing of EDS1 enhances the tobacco resistance to B. cinerea [24]. Similar to npr1, the pad4 mutant exhibits delayed senescence phenotypes when compared with Col-0 plants [22], and PAD4 positively modulates green peach aphid feeding-induced leaf senescence [25].…”

“…SAG101, a gene encoding an acyl hydrolase, plays a positive role in leaf senescence [80]. SAG101 has been demonstrated to function as an important component in plant immunity by forming a heterodimer with EDS1, thus mediating TNL-mediated PCD [23]. Arabidopsis HLS1 (HOOKLESS1) encodes a putative histone acetyltransferase and negatively regulates dark-, pathogen-, and ABA-induced leaf senescence [81].…”

The Evolutionary Origins of Programmed Cell Death Signaling

“…The factors required for cell death execution are not known and depend on the R-protein, which might belong to one of several subtypes. The TNL proteins carry an amino-terminal Toll and IL-1 receptor (TIR) domain and signal cell death via the EDS1-SAG101-NRG1 pathway (Lapin et al 2019) although the details of that mechanism are not known. A second class of R-proteins are the "CC-NB-LRR" (CNL) proteins, which carry an amino-terminal domain that was initially considered to form a coiled-coil (CC) structure, although this is not necessarily true.…”

“…In contrast to animals, plants do not use TIR domains in their surface receptors, but rather as the amino-terminal effector domains of one class of intracellular R-proteins (TIR-NB-LRR or TNL type). Surprisingly, plants lack TIR-containing adaptor proteins and the TNL proteins signal via a complex of EDS1 (enhanced disease susceptibility 1) and SAG101 (senescence-associated gene 101) through an as-yet uncharacterized mechanism (Lapin et al 2019). However, plant TIR domains share with their metazoan counterparts the capacity to mediate dimerization between different R-proteins (Bernoux et al 2011;Williams et al 2014;Zhang et al 2017).…”