Coding of plant immune signals by surface receptors

Immune signaling: receptor-like proteins make the difference

To resist biotic attacks, plants have evolved a sophisticated, receptor-based immune system. Cell-surface immune receptors, which are either receptor-like kinases (RLKs) or receptor-like proteins (RLPs), form the front line of the plant defense machinery. RLPs lack a cytoplasmic kinase domain for downstream immune signaling, and leucine-rich repeat (LRR)-containing RLPs constitutively associate with the RLK SOBIR1. The RLP/SOBIR1 complex was proposed to be the bimolecular equivalent of genuine RLKs. However, it appears that the molecular mechanisms by which RLP/SOBIR1 complexes and RLKs mount immunity show some striking differences. Here, we summarize the differences between RLP/SOBIR1 and RLK signaling, focusing on the way these receptors recruit the BAK1 co-receptor and elaborating on the negative crosstalk taking place between the two signaling networks.

Differential Transcriptome Reprogramming Induced by the Soybean Cyst Nematode Type 0 and Type 1.2.5.7 During Resistant and Susceptible Interactions

Soybean cyst nematode (SCN, Heterodera glycines [Hg]) is a serious root parasite of soybean (Glycine max) that induces extensive gene expression changes associated with pleiotropic biological activities in infected cells. However, the impacts of various SCN Hg types on host transcriptome reprogramming remain largely unknown. Here, we developed and used two recombinant inbred lines (RIL; RIL-72 and RIL-137) to profile transcriptome reprogramming in the infection sites during the resistant and susceptible interactions with SCN Hg Type 1.2.5.7 and Type 0. SCN bioassays indicated that RIL-72 was susceptible to Type 1.2.5.7 but resistant to Type 0, whereas RIL-137 was resistant to both types. Comparative analysis of gene expression changes induced by Type 1.2.5.7 in the resistant and susceptible lines revealed distinct transcriptome regulation with a number of similarly and oppositely regulated genes. The expression levels of similarly regulated genes in the susceptible line appeared to be insufficient to mount an effective defense against SCN. The functional importance of oppositely regulated genes was confirmed using virus-induced gene silencing (VIGS) and overexpression approaches. Further transcriptome comparisons revealed shared as well as Hg type- and genotype-specific transcriptome reprogramming. Shared transcriptome responses were mediated through common SCN-responsive genes and conserved immune signaling, whereas genotype-specific responses were derived from genetic variability, metabolic and hormonal differences, and varied regulation of protein phosphorylation and ubiquitination. The conserved defense mechanisms together with genotype-specific responses would enable plants to trigger effective and tailored immune responses to various Hg types and adapt the defense response to their genetic backgrounds. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Reprogramming of flagellin receptor responses with surrogate ligands

Receptor kinase (RK) families process information from small molecules, short peptides, or glycan ligands to regulate core cellular pathways in plants. To date, whether individual plant RKs are capable of processing signals from distinct types of ligands remains largely unexplored. Addressing this requires the discovery of structurally unrelated ligands that engage the same receptor. Here, we focus on FLAGELLIN-SENSING 2 (FLS2), an RK that senses a peptide of bacterial flagellin to activate antibacterial immunity in Arabidopsis. We interrogate >20,000 potential interactions between small molecules and the sensory domain of FLS2 using a large-scale reverse chemical screen. We discover two small molecules that interact with FLS2 in atypical ways. The surrogate ligands weakly activate the receptor to drive a functional antibacterial response channeled via unusual gene expression programs. Thus, chemical probes acting as biased ligands can be exploited to discover unexpected levels of output flexibility in RKs signal transduction.

Tomato roots exhibit distinct, development-specific responses to bacterial-derived peptides

Plants possess cell-surface recognition receptors that detect molecular patterns from microbial invaders and initiate an immune response. Understanding the conservation of pattern-triggered immunity within different plant organs and across species is crucial to its sustainable and effective use in plant disease management but is currently unclear. We examined the activation and immune response patterns of three pattern recognition receptors (PRRs: Sl FLS2, Sl FLS3, and Sl CORE) in different developmental regions of roots and in leaves of multiple accessions of domesticated and wild tomato ( Solanum lycopersicum and S. pimpinellifolium ) using biochemical and genetic assays. Roots from different tomato accessions differed in the amplitude and dynamics of their immune response, but all exhibited developmental-specific PTI responses in which the root early differentiation zone was the most sensitive to molecular patterns. PRR signaling pathways also showed distinct but occasionally overlapping responses downstream of each immune receptor in tomato roots.These results reveal that each PRR initiates a unique PTI pathway and suggest that the specificity and complexity of tomato root immunity are tightly linked to the developmental stage, emphasizing the importance of spatial and temporal regulation in PTI.

14-3-3 proteins as a major hub for plant immunity

14-3-3 proteins, ubiquitously present in eukaryotic cells, are regulatory proteins involved in a plethora of cellular processes. In plants, they have been studied in the context of metabolism, development, and stress responses. Recent studies have highlighted the pivotal role of 14-3-3 proteins in regulating plant immunity. The ability of 14-3-3 proteins to modulate immune responses is primarily attributed to their function as interaction hubs, mediating protein-protein interactions and thereby regulating the activity and overall function of their binding partners. Here, we shed light on how 14-3-3 proteins contribute to plant defense mechanisms, the implications of their interactions with components of plant immunity cascades, and the potential for leveraging this knowledge for crop improvement strategies.

Plasmopara viticola effector PvCRN20 represses the import of VvDEG5 into chloroplasts to suppress immunity in grapevine

Chloroplasts play a crucial role in plant defense against pathogens, making them primary targets for pathogen effectors that suppress host immunity. This study characterizes the Plasmopara viticola CRN-like effector, PvCRN20, which interacts with DEG5 in the cytoplasm but not with its interacting protein, DEG8, which is located in the chloroplast. By transiently overexpressing in tobacco leaves, we show that PvCRN20 could inhibit INF1- and Bax-triggered cell death. Constitutive expression of PvCRN20 suppresses the accumulation of reactive oxygen species (ROS) and promotes pathogen colonization. PvCRN20 reduces DEG5 entry into chloroplasts, thereby disrupting DEG5 and DEG8 interactions in chloroplasts. Overexpression of VvDEG5 and VvDEG8 induces ROS accumulation and enhances grapevine resistance to P. viticola, whereas knockout of VvDEG8 represses ROS production and promotes P. viticola colonization. Consistently, ectopic expression of VvDEG5 and VvDEG8 in tobacco promotes chloroplast-derived ROS accumulation, whereas co-expression of PvCRN20 counteracted this promotion by VvDEG5. Therefore, DEG5 is essential for the virulence function of PvCRN20. Although PvCRN20 is located in both the nucleus and cytoplasm, only cytoplasmic PvCRN20 suppresses plant immunity and promotes pathogen infection. Our results reveal that PvCRN20 dampens plant defenses by repressing the chloroplast import of DEG5, thus reducing host ROS accumulation and facilitating pathogen colonization.

A pathogenesis-related protein, PRP1, negatively regulates root nodule symbiosis in Lotus japonicus

The legume-rhizobium symbiosis represents a unique model within the realm of plant-microbe interactions. Unlike typical cases of pathogenic invasion, the infection of rhizobia and their residence within symbiotic cells do not elicit a noticeable immune response in plants. Nevertheless, there is still much to uncover regarding the mechanisms through which plant immunity influences rhizobial symbiosis. In this study, we identify an important player in this intricate interplay: Lotus japonicus PRP1, which serves as a positive regulator of plant immunity but also exhibits the capacity to decrease rhizobial colonization and nitrogen fixation within nodules. The PRP1 gene encodes an uncharacterized protein and is named Pathogenesis-Related Protein1, owing to its orthologue in Arabidopsis thaliana, a pathogenesis-related family protein (At1g78780). The PRP1 gene displays high expression levels in nodules compared to other tissues. We observed an increase in rhizobium infection in the L. japonicus prp1 mutants, whereas PRP1-overexpressing plants exhibited a reduction in rhizobium infection compared to control plants. Intriguingly, L. japonicus prp1 mutants produced nodules with a pinker colour compared to wild-type controls, accompanied by elevated levels of leghaemoglobin and an increased proportion of infected cells within the prp1 nodules. The transcription factor Nodule Inception (NIN) can directly bind to the PRP1 promoter, activating PRP1 gene expression. Furthermore, we found that PRP1 is a positive mediator of innate immunity in plants. In summary, our study provides clear evidence of the intricate relationship between plant immunity and symbiosis. PRP1, acting as a positive regulator of plant immunity, simultaneously exerts suppressive effects on rhizobial infection and colonization within nodules.

Receptor-like cytoplasmic kinases of different subfamilies differentially regulate SOBIR1/BAK1-mediated immune responses in Nicotiana benthamiana

Cell-surface receptors form the front line of plant immunity. The leucine-rich repeat (LRR)-receptor-like kinases SOBIR1 and BAK1 are required for the functionality of the tomato LRR-receptor-like protein Cf-4, which detects the secreted effector Avr4 of the pathogenic fungus Fulvia fulva. Here, we show that the kinase domains of SOBIR1 and BAK1 directly phosphorylate each other and that residues Thr522 and Tyr469 of the kinase domain of Nicotiana benthamiana SOBIR1 are required for its kinase activity and for interacting with signalling partners, respectively. By knocking out multiple genes belonging to different receptor-like cytoplasmic kinase (RLCK)-VII subfamilies in N. benthamiana:Cf-4, we show that members of RLCK-VII-6, -7, and -8 differentially regulate the Avr4/Cf-4-triggered biphasic burst of reactive oxygen species. In addition, members of RLCK-VII-7 play an essential role in resistance against the oomycete pathogen Phytophthora palmivora. Our study provides molecular evidence for the specific roles of RLCKs downstream of SOBIR1/BAK1-containing immune complexes.

Plant immune receptors interact with hemibiotrophic pathogens to activate plant immunity

Phytopathogens pose a devastating threat to the productivity and yield of crops by causing destructive plant diseases in natural and agricultural environments. Hemibiotrophic pathogens have a variable-length biotrophic phase before turning to necrosis and are among the most invasive plant pathogens. Plant resistance to hemibiotrophic pathogens relies mainly on the activation of innate immune responses. These responses are typically initiated after the plant plasma membrane and various plant immune receptors detect immunogenic signals associated with pathogen infection. Hemibiotrophic pathogens evade pathogen-triggered immunity by masking themselves in an arms race while also enhancing or manipulating other receptors to promote virulence. However, our understanding of plant immune defenses against hemibiotrophic pathogens is highly limited due to the intricate infection mechanisms. In this review, we summarize the strategies that different hemibiotrophic pathogens interact with host immune receptors to activate plant immunity. We also discuss the significant role of the plasma membrane in plant immune responses, as well as the current obstacles and potential future research directions in this field. This will enable a more comprehensive understanding of the pathogenicity of hemibiotrophic pathogens and how distinct plant immune receptors oppose them, delivering valuable data for the prevention and management of plant diseases.

Phytophthora RxLR effector PcSnel4B promotes degradation of resistance protein AtRPS2

Phytophthora capsici deploys effector proteins to manipulate host immunity and facilitate its colonization. However, the underlying mechanisms remain largely unclear. In this study, we demonstrated that a Sne-like (Snel) RxLR effector gene PcSnel4 is highly expressed at the early stages of P. capsici infection in Nicotiana benthamiana. Knocking out both alleles of PcSnel4 attenuated the virulence of P. capsici, while expression of PcSnel4 promoted its colonization in N. benthamiana. PcSnel4B could suppress the hypersensitive reaction (HR) induced by Avr3a-R3a and RESISTANCE TO PSEUDOMONAS SYRINGAE 2 (AtRPS2), but it did not suppress cell death elicited by Phytophthora infestin 1 (INF1) and Crinkler 4 (CRN4). COP9 signalosome 5 (CSN5) in N. benthamiana was identified as a host target of PcSnel4. Silencing NbCSN5 compromised the cell death induced by AtRPS2. PcSnel4B impaired the interaction and colocalization of Cullin1 (CUL1) and CSN5 in vivo. Expression of AtCUL1 promoted the degradation of AtRPS2 and disrupted HR, while AtCSN5a stabilized AtRPS2 and promoted HR, regardless of the expression of AtCUL1. PcSnel4 counteracted the effect of AtCSN5 and enhanced the degradation of AtRPS2, resulting in HR suppression. This study deciphered the underlying mechanism of PcSnel4-mediated suppression of HR induced by AtRPS2.

N-hydroxypipecolic acid primes plants for enhanced microbial pattern-induced responses

The bacterial elicitor flagellin induces a battery of immune responses in plants. However, the rates and intensities by which metabolically-related defenses develop upon flagellin-sensing are comparatively moderate. We report here that the systemic acquired resistance (SAR) inducer N-hydroxypipecolic acid (NHP) primes Arabidopsis thaliana plants for strongly enhanced metabolic and transcriptional responses to treatment by flg22, an elicitor-active peptide fragment of flagellin. While NHP powerfully activated priming of the flg22-induced accumulation of the phytoalexin camalexin, biosynthesis of the stress hormone salicylic acid (SA), generation of the NHP biosynthetic precursor pipecolic acid (Pip), and accumulation of the stress-inducible lipids γ-tocopherol and stigmasterol, it more modestly primed for the flg22-triggered generation of aromatic and branched-chain amino acids, and expression of FLG22-INDUCED RECEPTOR-KINASE1. The characterization of the biochemical and immune phenotypes of a set of different Arabidopsis single and double mutants impaired in NHP and/or SA biosynthesis indicates that, during earlier phases of the basal immune response of naïve plants to Pseudomonas syringae infection, NHP and SA mutually promote their biosynthesis and additively enhance camalexin formation, while SA prevents extraordinarily high NHP levels in later interaction periods. Moreover, SA and NHP additively contribute to Arabidopsis basal immunity to bacterial and oomycete infection, as well as to the flagellin-induced acquired resistance response that is locally observed in plant tissue exposed to exogenous flg22. Our data reveal mechanistic similarities and differences between the activation modes of flagellin-triggered acquired resistance in local tissue and the SAR state that is systemically induced in plants upon pathogen attack. They also corroborate that the NHP precursor Pip has no independent immune-related activity.

Plant immunity in soybean: progress, strategies, and perspectives

Soybean (Glycine max) is one of the most important commercial crops worldwide. Soybean hosts diverse microbes, including pathogens that may cause diseases and symbionts that contribute to nitrogen fixation. Study on soybean-microbe interactions to understand pathogenesis, immunity, and symbiosis represents an important research direction toward plant protection in soybean. In terms of immune mechanisms, current research in soybean lags far behind that in the model plants Arabidopsis and rice. In this review, we summarized the shared and unique mechanisms involved in the two-tiered plant immunity and the virulence function of pathogen effectors between soybean and Arabidopsis, providing a molecular roadmap for future research on soybean immunity. We also discussed disease resistance engineering and future perspectives in soybean.

SHOU4/4L link cell wall cellulose synthesis to pattern-triggered immunity

Pattern recognition receptors (PRRs) are plasma membrane-localised proteins that sense molecular patterns to initiate pattern-triggered immunity (PTI). Receptor-like cytoplasmic kinases (RLCKs) function downstream of PRRs to propagate signal transduction via the phosphorylation of substrate proteins. The identification and characterisation of RLCK-regulated substrate proteins are critical for our understanding of plant immunity. We showed that SHOU4 and SHOU4L are rapidly phosphorylated upon various patterns elicitation and are indispensable for plant resistance to bacterial and fungal pathogens. Protein-protein interaction and phosphoproteomic analysis revealed that BOTRYTIS-INDUCED KINASE 1, a prominent protein kinase of RLCK subfamily VII (RLCK-VII), interacted with SHOU4/4L and phosphorylated multiple serine residues on SHOU4L N-terminus upon pattern flg22 treatment. Neither phospho-dead nor phospho-mimic SHOU4L variants complemented pathogen resistance and plant development defect of the loss-of-function mutant, suggesting that reversible phosphorylation of SHOU4L is critical to plant immunity and plant development. Co-immunoprecipitation data revealed that flg22 induced SHOU4L dissociation from cellulose synthase 1 (CESA1) and that a phospho-mimic SHOU4L variant inhibited the interaction between SHOU4L and CESA1, indicating the link between SHOU4L-mediated cellulose synthesis and plant immunity. This study thus identified SHOU4/4L as new components of PTI and preliminarily revealed the mechanism governing SHOU4L regulation by RLCKs.

Effector-dependent activation and oligomerization of plant NRC class helper NLRs by sensor NLR immune receptors Rpi-amr3 and Rpi-amr1

Plant pathogens compromise crop yields. Plants have evolved robust innate immunity that depends in part on intracellular Nucleotide-binding, Leucine rich-Repeat (NLR) immune receptors that activate defense responses upon detection of pathogen-derived effectors. Most "sensor" NLRs that detect effectors require the activity of "helper" NLRs, but how helper NLRs support sensor NLR function is poorly understood. Many Solanaceae NLRs require NRC (NLR-Required for Cell death) class of helper NLRs. We show here that Rpi-amr3, a sensor NLR from Solanum americanum, detects AVRamr3 from the potato late blight pathogen, Phytophthora infestans, and activates oligomerization of helper NLRs NRC2 and NRC4 into high-molecular-weight resistosomes. In contrast, recognition of P. infestans effector AVRamr1 by another sensor NLR Rpi-amr1 induces formation of only the NRC2 resistosome. The activated NRC2 oligomer becomes enriched in membrane fractions. ATP-binding motifs of both Rpi-amr3 and NRC2 are required for NRC2 resistosome formation, but not for the interaction of Rpi-amr3 with its cognate effector. NRC2 resistosome can be activated by Rpi-amr3 upon detection of AVRamr3 homologs from other Phytophthora species. Mechanistic understanding of NRC resistosome formation will underpin engineering crops with durable disease resistance.

Structure, biochemical function, and signaling mechanism of plant NLRs

To counter pathogen invasion, plants have evolved a large number of immune receptors, including membrane-resident pattern recognition receptors (PRRs) and intracellular nucleotide-binding and leucine-rich repeat receptors (NLRs). Our knowledge about PRR and NLR signaling mechanisms has expanded significantly over the past few years. Plant NLRs form multi-protein complexes called resistosomes in response to pathogen effectors, and the signaling mediated by NLR resistosomes converges on Ca²⁺-permeable channels. Ca²⁺-permeable channels important for PRR signaling have also been identified. These findings highlight a crucial role of Ca²⁺ in triggering plant immune signaling. In this review, we first discuss the structural and biochemical mechanisms of non-canonical NLR Ca²⁺ channels and then summarize our knowledge about immune-related Ca²⁺-permeable channels and their roles in PRR and NLR signaling. We also discuss the potential role of Ca²⁺ in the intricate interaction between PRR and NLR signaling.

Development specifies, diversifies and empowers root immunity

Roots are a highly organised plant tissue consisting of different cell types with distinct developmental functions defined by cell identity networks. Roots are the target of some of the most devastating diseases and possess a highly effective immune system. The recognition of microbe- or plant-derived molecules released in response to microbial attack is highly important in the activation of complex immunity gene networks. Development and immunity are intertwined, and immunity activation can result in growth inhibition. In turn, by connecting immunity and cell identity regulators, cell types are able to launch a cell type-specific immunity based on the developmental function of each cell type. By this strategy, fundamental developmental processes of each cell type contribute their most basic functions to drive cost-effective but highly diverse and, thus, efficient immune responses. This review highlights the interdependence of root development and immunity and how the developmental age of root cells contributes to positive and negative outcomes of development-immunity cross-talk.

Advances in Fungal Elicitor-Triggered Plant Immunity

There is an array of pathogenic fungi in the natural environment of plants, which produce some molecules including pathogen-associated molecular patterns (PAMPs) and effectors during infection. These molecules, which can be recognized by plant specific receptors to activate plant immunity, including PTI (PAMP-triggered immunity) and ETI (effector-triggered immunity), are called elicitors. Undoubtedly, identification of novel fungal elicitors and their plant receptors and comprehensive understanding about fungal elicitor-triggered plant immunity will be of great significance to effectively control plant diseases. Great progress has occurred in fungal elicitor-triggered plant immunity, especially in the signaling pathways of PTI and ETI, in recent years. Here, recent advances in fungal elicitor-triggered plant immunity are summarized and their important contribution to the enlightenment of plant disease control is also discussed.

PERKing up our understanding of the proline-rich extensin-like receptor kinases, a forgotten plant receptor kinase family

Proline-rich extensin-like receptor kinases (PERKs) are an important class of receptor-like kinases (RLKs) containing an extracellular proline-rich domain. While they are thought to be putative sensors of the cell wall integrity, there are very few reports on their biological functions in the plant, as compared with other RLKs. Several studies support a role for PERKs in plant growth and development, but their effect on the cell wall composition to regulate cell expansion is still lacking. Gene expression data suggest that they may intervene in response to environmental changes, in agreement with their subcellular localization. And there is growing evidence for PERKs as novel sensors of environmental stresses such as insects and viruses. However, little is known about their precise role in plant immunity and in the extracellular network of RLKs, as no PERK-interacting RLK or any coreceptor has been identified as yet. Similarly, their signaling activities and downstream signaling components are just beginning to be deciphered, including Ca²⁺ fluxes, reactive oxygen species accumulation and phosphorylation events. Here we outline emerging roles for PERKs as novel sensors of environmental stresses, and we discuss how to better understand this overlooked class of receptor kinases via several avenues of research.

Recognition of pathogen-derived sphingolipids in Arabidopsis

In plants, many invading microbial pathogens are recognized by cell-surface pattern recognition receptors, which induce defense responses. Here, we show that the ceramide Phytophthora infestans-ceramide D (Pi-Cer D) from the plant pathogenic oomycete P. infestans triggers defense responses in Arabidopsis. Pi-Cer D is cleaved by an Arabidopsis apoplastic ceramidase, NEUTRAL CERAMIDASE 2 (NCER2), and the resulting 9-methyl-branched sphingoid base is recognized by a plasma membrane lectin receptor-like kinase, RESISTANT TO DFPM-INHIBITION OF ABSCISIC ACID SIGNALING 2 (RDA2). 9-Methyl-branched sphingoid base is specific to microbes and induces plant immune responses by physically interacting with RDA2. Loss of RDA2 or NCER2 function compromised Arabidopsis resistance against an oomycete pathogen. Thus, we elucidated the recognition mechanisms of pathogen-derived lipid molecules in plants.

MAP kinase cascades in plant development and immune signaling

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules regulating diverse biological processes. During the past 20 years, much progress has been made on the functions of MAPK cascades in plants. This review summarizes the roles of MAPKs, known MAPK substrates, and our current understanding of MAPK cascades in plant development and innate immunity. In addition, recent findings on the molecular links connecting surface receptors to MAPK cascades and the mechanisms underlying MAPK signaling specificity are also discussed.

Silencing a Simple Extracellular Leucine-Rich Repeat Gene OsI-BAK1 Enhances the Resistance of Rice to Brown Planthopper Nilaparvata lugens

Many plant proteins with extracellular leucine-rich repeat (eLRR) domains play an important role in plant immunity. However, the role of one class of eLRR plant proteins—the simple eLRR proteins—in plant defenses against herbivores remains largely unknown. Here, we found that a simple eLRR protein OsI-BAK1 in rice localizes to the plasma membrane. Its expression was induced by mechanical wounding, the infestation of gravid females of brown planthopper (BPH) Nilaparvata lugens or white-backed planthopper Sogatella furcifera and treatment with methyl jasmonate or abscisic acid. Silencing OsI-BAK1 (ir-ibak1) in rice enhanced the BPH-induced transcript levels of three defense-related WRKY genes (OsWRKY24, OsWRKY53 and OsWRKY70) but decreased the induced levels of ethylene. Bioassays revealed that the hatching rate was significantly lower in BPH eggs laid on ir-ibak1 plants than wild-type (WT) plants; moreover, gravid BPH females preferred to oviposit on WT plants over ir-ibak1 plants. The exogenous application of ethephon on ir-ibak1 plants eliminated the BPH oviposition preference between WT and ir-ibak1 plants but had no effect on the hatching rate of BPH eggs. These findings suggest that OsI-BAK1 acts as a negative modulator of defense responses in rice to BPH and that BPH might exploit this modulator for its own benefit.

Molecular mechanisms of early plant pattern-triggered immune signaling

All eukaryotic organisms have evolved sophisticated immune systems to appropriately respond to biotic stresses. In plants and animals, a key part of this immune system is pattern recognition receptors (PRRs). Plant PRRs are cell-surface-localized receptor kinases (RKs) or receptor proteins (RPs) that sense microbe- or self-derived molecular patterns to regulate pattern-triggered immunity (PTI), a robust form of antimicrobial immunity. Remarkable progress has been made in understanding how PRRs perceive their ligands, form active protein complexes, initiate cell signaling, and ultimately coordinate the cellular reprogramming that leads to PTI. Here, we discuss the critical roles of PRR complex formation and phosphorylation in activating PTI signaling, as well as the emerging paradigm in which receptor-like cytoplasmic kinases (RLCKs) act as executors of signaling downstream of PRR activation.