Spatial organization of NLR signaling complexes in plant immunity

This commentary examines recent work that explores the spatial organization of the plant internal sensors that detect infection and trigger immune responses, showing form small units that assemble into larger clusters at the cell surface. This spatial organization may strengthen defense signals and lead to controlled cell death. Plants are constantly exposed to diverse microbial pathogens and rely on efficient immune systems to detect infection and activate defense responses. Central to intracellular immunity are nucleotide-binding leucine-rich repeat receptors (NLRs), which recognize pathogen effectors or sense perturbations in host proteins and subsequently trigger defense signaling pathways (Cui et al., 2015; Jones et al., 2016). Plant NLRs typically contain an N-terminal signaling domain, a central nucleotide-binding NB–ARC domain, and C-terminal leucine-rich repeats. Based on their N-terminal domains, plant NLRs are broadly divided into coiled-coil NLRs (CC-NLRs) and Toll/interleukin-1 receptor NLRs (TIR-NLRs) (Cui et al., 2015). A major advance in plant immunity is that NLR activation is coupled to oligomerization into resistosomes. Structural studies have shown that the Arabidopsis CC-NLR ZAR1 assembles into a pentameric resistosome that inserts into the plasma membrane and functions as a Ca2+-permeable channel (Wang et al., 2019; Bi et al., 2021). In contrast, TIR–NLR receptors form tetrameric enzymatic complexes with NAD+-cleaving activity, generating nucleotide-derived signaling molecules (Horsefield et al., 2019; Wan et al., 2019). These second messengers activate EDS1 family proteins and helper NLRs (ADR1 and NRG1), leading to Ca2+ influx and defense activation (Sun et al., 2021). However, how these immune complexes are spatially organized in plant cells remains unclear. The study by Ge and colleagues focuses on the Arabidopsis CC-NLR SUMM2, a member of the relatively understudied CCG10-NLR clade (Ge et al., 2026). SUMM2 is anchored to the plasma membrane through N-myristoylation, which mediates stable membrane association without membrane penetration. In contrast, the canonical CC-NLRs ZAR1 forms a membrane-penetrating helix via its MADA motif (Wang et al., 2019; Bi et al., 2021). This structural distinction likely explains why SUMM2 does not function as a terminal pore-forming complex and instead requires downstream helper NLRs to execute cell death. Under resting conditions, SUMM2 activity is restrained by a MAP kinase module consisting of MEKK1, MKK1/2, and MPK4. Perturbation of this module by the bacterial effector HopAI1, which inactivates MPK4, triggers SUMM2-dependent immune activation (Figure 1). Ge and colleagues demonstrate that inactive SUMM2 associates with the immune regulators EDS1 and PAD4 at the plasma membrane. Upon activation, the EDS1–PAD4 complex is released and promotes activation of the helper NLR ADR1-L1. The nucleotide-derived second messengers facilitate the dissociation of the EDS1–PAD4 heterodimer from SUMM2 and enable ADR1-L1 activation (Horsefield et al., 2019; Sun et al., 2021). ADR1-L1 then assembles into pentameric oligomers, similar to other helper NLR complexes that can form Ca2+-permeable channels. Spatial organization of SUMM2–ADR1 immune signaling In the resting state, SUMM2 is anchored to the plasma membrane via N-myristoylation and is restrained by the MEKK1-MKK1/2-MPK4 MAP kinase module. EDS1-PAD4 and ADR1-L1 remain inactive under these conditions. Upon pathogen attack, the effector HopAI1 disrupts the MAP kinase module, leading to SUMM2 activation. Activated SUMM2 promotes the release of the EDS1–PAD4 complex, a process facilitated by TIR–NLR-derived nucleotide signals, enabling its interaction with ADR1-L1 at the plasma membrane. ADR1-L1 subsequently oligomerizes and assembles into higher-order ring-like clusters. These assemblies are proposed to facilitate cellular content release and amplify immune signaling, thereby leading to cell death. Created with BioRender.com. A notable observation in this study is that ADR1-L1 oligomers do not remain isolated after activation. Using high-resolution time-lapse microscopy, Ge and colleagues show that ADR1-L1 oligomers progressively assemble into ring-like clusters at the plasma membrane several hours after SUMM2 activation (Ge et al., 2026). Each cluster contains approximately two to six ADR1-L1 oligomers and also includes EDS1 and PAD4, suggesting that the EDS1–PAD4 complex may help stabilize or organize these higher-order immune assemblies. These supramolecular structures likely recruit additional components, explaining their larger size in live-cell imaging (approximately 300–450 nm) compared with cryo-EM (~25 nm) (Bi et al., 2021). These observations support a stepwise model for immune activation and cell death. Individual ADR1-L1 resistosomes likely function as initial Ca2+-permeable channels in early immune signaling. At a later stage, EDS1–PAD4–ADR1-L1 complexes progressively assemble into higher-order clusters, observed as ring-like structures at the plasma membrane (Figure 1). These higher-order assemblies are distinct from individual resistosomes and are associated with localized membrane disruption, ultimately leading to terminal cell death. Similar principles of regulated pore assembly and clustering have been described in animal innate immunity, where membrane-associated complexes control inflammatory signaling and cell death (Ding et al., 2016). More broadly, the spatial organization of signaling components into clusters is a recurring feature of Ca2+ signaling. In animal cells, inositol trisphosphate receptors (IP3Rs) dynamically assemble into small clusters that modulate their sensitivity to IP3 and Ca2+, enabling coordinated Ca2+ release events (Rahman et al., 2009). By analogy, clustering of ADR1-L1 resistosomes may similarly regulate Ca2+ influx and downstream signaling in plant immunity, suggesting a conserved role for spatial organization in shaping cellular responses. The work by Ge and colleagues also highlights connections between different NLR signaling pathways (Ge et al., 2026). Although SUMM2 is a CC-NLR receptor, its activation recruits components typically associated with TIR–NLR pathways, including EDS1–PAD4 and ADR1 helper NLRs. This supports the view that plant immune signaling is organized around shared downstream modules rather than strictly separated receptor classes. TIR–NLR receptors generate nucleotide-derived second messengers through NAD+ cleavage, which activate EDS1 complexes and promote helper NLR oligomerization (Sun et al., 2021). The findings from Ge and colleagues suggest that these downstream modules can also be engaged by the CC-NLR receptor SUMM2. In parallel, structural studies have revealed considerable diversity in the oligomeric architectures of NLR signaling complexes. Different NLRs assemble into distinct resistosome configurations with varying stoichiometries and biochemical activities (Horsefield et al., 2019; Wang et al., 2019; Bi et al., 2021). Collectively, these findings indicate that plants can deploy NLR complexes with distinct oligomeric architectures, which may help tune immune signaling by influencing the strength, timing, and spatial distribution of defense responses. Taken together, this work expands the current framework of plant immunity by showing that NLR signaling involves hierarchical assembly of membrane-associated complexes, from individual helper NLR oligomers to higher-order clusters. An important next step is to define the precise function of ADR1-L1 ring-like clusters, including their roles in regulating Ca2+ influx, altering membrane integrity, or serving as platforms for signal amplification. It also remains to be determined whether such higher-order clustering is a general feature of other non-MADA CC-NLRs or helper NLRs, such as RPS5 and NRG1. Addressing these questions will require combining structural approaches, such as cryo-electron tomography, with functional analysis including electrophysiology and live-cell imaging to directly link the architecture of NLR assemblies to their activities. A clearer understanding of how the spatial organization of NLRs shapes defense response will be essential for defining the principles of plant immune signaling and finally inform strategies to improve disease resistance while maintaining crop performance. We thank Dr. Dongdong Ge for the helpful suggestions to improve the manuscript. Chen Kang gratefully acknowledges Zhejiang University for funding the visiting scholarship at University of Oxford, and Prof. Chongde Sun for his support and encouragement. The authors declare no conflicts of interest. C.K. conceived the concept and structure of the commentary. C.K. wrote the draft of the manuscript. Z.L. prepared the schematic figure and revised the manuscript. All authors read and approved of its content.