Nuclear Activities of DNA-Associated Immune Receptors
Plant Biochemistry and Biophysics
Plant Physiology
Plant Cell and Developmental Biology
Final Report 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 resistance. In Arabidopsis TNL-mediated immunity, EDS1 heterodimers with PHYTOALEXIN DEFICIENT4 (PAD4) transcriptionally induce defenses. EDS1 uses the same surface to interact with PAD4-related SENESCENCE-ASSOCIATED GENE101 (SAG101), but the role of EDS1-SAG101 heterodimers remained unclear. The focus of this project was to structurally dissect activities of these two EDS1 dimers in Arabidopsis. By interrogating an Arabidopsis EDS1 C-terminal ɑ-helical EP-domain we identified positively charged residues lining a cavity that are essential for TNL immunity, beyond heterodimer formation. We established the EDS1 EP-domain is necessary for resistance conferred by different NLR receptor types. These data provide a unique structural insight to early signalling in NLR receptor immunity. We further demonstrated that EDS1-SAG101 functions together with N REQUIRED GENE1 (NRG1) coiled-coil domain helper NLRs as a coevolved TNL cell death-signaling module. EDS1- SAG101-NRG1 cell death activity is transferable to the Solanaceous species Nicotiana benthamiana and cannot be substituted by EDS1-PAD4 with NRG1 or EDS1-SAG101 with endogenous tobacco NRG1. Analysis of EDS1-family evolutionary rate variation and heterodimer structure-guided phenotyping of EDS1 variants and PAD4-SAG101 chimeras identify closely aligned ɑ-helical coil surfaces in the EDS1-SAG101 partner C-terminal domains that are necessary for reconstituted TNL 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. Another part of the project interrogated the role of PAD4 as a multitasking protein. In Arabidopsis, PAD4 functions with EDS1 to limit pathogen growth. Independently of EDS1, PAD4 reduces infestation by green peach aphid (GPA). How PAD4 regulates these defense outputs was unclear. We showed that transgenic expression of the N-terminal PAD4 domain in Arabidopsis is sufficient for limiting GPA infestation but not for conferring pathogen immunity. Our data suggest that PAD4 has domain-specific functions. Furthermore, in this project we resolved solution and crystal structures of unbound Arabidopsis EDS1 using nanobodies for crystallization. These structures, together with gel filtration and immunoprecipitation data, show that PAD4/SAG101-unbound EDS1 is stable as a monomer and does not form the homodimers recorded in public databases. A PAD4/SAG101 anchoring helix in EDS1 is disordered unless engaged in protein/protein interactions. As in complex with SAG101, monomeric AtEDS1 has a substrate-inaccessible esterase triad with a blocked oxyanion hole and without space for a covalent acyl intermediate. The new structures suggest that the AtEDS1 monomer represents an inactive or pre-activated ground state. Together, these studies have considerably advanced understanding of an important plant immune regulating hub.
Publications
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(2019). A Coevolved EDS1-SAG101-NRG1 Module Mediates Cell Death Signaling by TIR-Domain Immune Receptors. Plant Cell 31, 2430-2455
Lapin, D., Kovacova, V., Sun, X., Dongus, J.A., Bhandari, D., von Born, P., Bautor, J., Guarneri, N., Rzemieniewski, J., Stuttmann, J., Beyer, A., and Parker, J.E.
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(2019). An EDS1 heterodimer signaling surface enforces timely reprogramming of immunity genes in Arabidopsis. Nat Commun 10, 772
Bhandari, D.D., Lapin, D., Kracher, B., von Born, P., Bautor, J., Niefind, K., and Parker, J.E.
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(2019). Arabidopsis immunity regulator EDS1 in a PAD4/SAG101-unbound form is a monomer with an inherently inactive conformation. J Struct Biol 208, 107390
Voss, M., Toelzer, C., Bhandari, D.D., Parker, J.E., and Niefind, K.
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(2020). The Arabidopsis PAD4 lipase-like domain is sufficient for resistance to Green Peach Aphid. Mol Plant Microbe Interact 33, 328-33
Dongus, J.A., Bhandari, D.D., Patel, M., Archer, L., Dijkgraaf, L., Deslandes, L., Shah, J., and Parker, J.E.
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2021. EDS1 signaling: At the nexus of intracellular and surface receptor immunity. Curr Opin Plant Biol 66:102039
Dongus JA and Parker JE
