Subcellular localization requirements and specificities for plant immune receptor Toll‐interleukin‐1 receptor signaling

Abstract

Recent work shed light on how plant intracellular immune receptors of the nucleotide-binding leucine-rich repeat (NLR) family are activated upon pathogen effector recognition to trigger immune responses. Activation of Toll-interleukin-1 receptor (TIR) domain-containing NLRs (TNLs) induces receptor oligomerization and close proximity of the TIR domain, which is required for TIR enzymatic activity. TIR-catalyzed small signaling molecules bind to EDS1 family heterodimers and subsequently activate downstream helper NLRs, which function as Ca 2+ permeable channel to activate immune responses eventually leading to cell death. Subcellular localization requirements of TNLs and signaling partners are not well understood, although they are required to understand fully the mechanisms underlying NLR early signaling. TNLs show diverse subcellular localization while EDS1 shows nucleocytosolic localization. Here, we studied the impact of TIR and EDS1 mislocalization on the signaling activation of different TNLs. In Nicotiana benthamiana, our results suggest that close proximity of TIR domains isolated from flax L6 and Arabidopsis RPS4 and SNC1 TNLs drives signaling activation from different cell compartments. Nevertheless, both Golgi-membrane anchored L6 and nucleocytosolic RPS4 have the same requirements for EDS1 subcellular localization in Arabidopsis thaliana. By using mislocalized variants of EDS1, we found that autoimmune L6 and RPS4 TIR domain can induce seedling cell death when EDS1 is present in the cytosol. However, when EDS1 is restricted to the nucleus, both induce a stunting phenotype but no cell death. Our data point out the importance of thoroughly investigating the dynamics of TNLs and signaling partners subcellular localization to understand TNL signaling fully.

Plasma membrane association and resistosome formation of plant helper immune receptors

“…S6 ). These data suggest that the cytoplasmic pools of AtEDS1 and AtSAG101 activate AtNRG1.1 and are critical determinants of XopQ-activated cell death responses, which is consistent with a recent study showing that different TIR domains preferentially signal cell death via the cytosolic pool of EDS1 in Arabidopsis ( 38 ). Given that TIR-dependent signals induce the formation of AtEDS1/AtSAG101/AtNRG1 heterotrimers ( 20 ), we performed BN-PAGE to further investigate whether AtEDS1 and AtSAG101 are subsequently retained in the oligomerized AtNRG1.1 resistosome.…”

Plant Toll/interleukin-1 receptor/resistance protein domains physically associate with enhanced disease susceptibility1 family proteins in immune signaling

“…Neither L6TIR BD nor L6TIR BD E135A showed association with VP16 Nb EDS1, VP16 Nb SAG101b or VP16 Nb PAD4 in this assay, which is likely because L6TIR is excluded from the nucleus due to its N-terminal Golgi membrane anchor. 19 Nuclear localization of the fusion proteins is required for the activation of Ruby expression in this assay. 43 Overall, both the split luciferase and plant two-hybrid assays supported the physical association of plant TIRs with individual EDS1 family members.…”

“… 14 , 15 TIR signaling can be activated by forced oligomerisation through fusion to the tandem SAM domains of the human SARM1 protein, which form an octameric ring assembly, 15 or to the mammalian NLRC4 immune receptor which forms an oligomeric inflammasome in cooperation with NAIP NLRs and a corresponding ligand, 16 , 17 , 18 or to the flax rust effector AvrM, which forms a stable dimer in planta . 19 Activated full-length TIR-NLRs RPP1 and Roq1 form tetramers where the TIR domains self-associate resulting in conformational changes that expose the NADase catalytic sites in two of the four TIR subunits, 20 , 21 providing an explanation for how effector-driven TIR-NLR oligomerization leads to signaling activation via its catalytic function.…”

Pathogen perception and signaling in plant immunity

“…This has also been reported for numerous other TIR and CC domains ( Williams et al 2014 ; Casey et al 2016 ; Cesari et al 2016 ; Zhang et al 2017 ). Furthermore, induced proximity of TIR domains by fusion to constitutively or inducibly oligomeric proteins was sufficient to activate their signaling function ( Horsefield et al 2019 ; Duxbury et al 2020 ; Bernoux et al 2023 ). TIR domains were also shown to possess an NADase activity that can cleave NAD + or NADP + in vitro and is dependent on self-association, suggesting activation of the enzymatic function by oligomerization ( Horsefield et al 2019 ; Wan et al 2019 ).…”