Cytoplasmic and Nuclear Localizations Are Important for the Hypersensitive Response Conferred by Maize Autoactive Rp1-D21 Protein

A KNOX Ⅱ transcription factor suppresses the NLR immune receptor BRG8-mediated immunity in rice

Nucleotide-binding site and leucine-rich repeat (NLR) proteins are activated by detecting pathogen effectors, which in turn trigger host defenses and cell death. Although many NLRs have been identified, the mechanisms responsible for NLR-triggered defense responses are still poorly understood. In this study, through a genome-wide association study approach, we identified a novel NLR gene, Blast Resistance Gene 8 (BRG8), which confers resistance to rice blast and bacterial blight diseases. BRG8 overexpression and complementation lines exhibit enhanced resistance to both pathogens. Subcellular localization assays showed that BRG8 is localized in both the cytoplasm and the nucleus. Additional evidence revealed that nuclear-localized BRG8 can enhance rice immunity without a hypersensitive response (HR)-like phenotype. We also demonstrated that the coiled-coil domain of BRG8 not only physically interacts with itself but also interacts with the KNOX Ⅱ protein HOMEOBOX ORYZA SATIVA59 (HOS59). Knockout mutants of HOS59 in the BRG8 background show enhanced resistance to Magnaporthe oryzae strain CH171 and Xoo strain CR4, similar to that of the BRG8 background. By contrast, overexpression of HOS59 in the BRG8 background will compromise the HR-like phenotype and resistance response. Further analysis revealed that HOS59 promotes the degradation of BRG8 via the 26S proteasome pathway. Collectively, our study highlights HOS59 as an NLR immune regulator that fine-tunes BRG8-mediated immune responses against pathogens, providing new insights into NLR associations and functions in plant immunity.

OsLRR-RLP2 Gene Regulates Immunity to Magnaporthe oryzae in Japonica Rice

Rice is an important cereal crop worldwide, the growth of which is affected by rice blast disease, caused by the fungal pathogen Magnaporthe oryzae. As climate change increases the diversity of pathogens, the disease resistance genes (R genes) in plants must be identified. The major blast-resistance genes have been identified in indica rice varieties; therefore, japonica rice varieties with R genes now need to be identified. Because leucine-rich repeat (LRR) domain proteins possess R-gene properties, we used bioinformatics analysis to identify the rice candidate LRR domain receptor-like proteins (OsLRR-RLPs). OsLRR-RLP2, which contains six LRR domains, showed differences in the DNA sequence, containing 43 single-nucleotide polymorphisms (SNPs) in indica and japonica subpopulations. The results of the M. oryzae inoculation analysis indicated that indica varieties with partial deletion of OsLRR-RLP2 showed susceptibility, whereas japonica varieties with intact OsLRR-RLP2 showed resistance. The oslrr-rlp2 mutant, generated using clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), showed increased pathogen susceptibility, whereas plants overexpressing this gene showed pathogen resistance. These results indicate that OsLRR-RLP2 confers resistance to rice, and OsLRR-RLP2 may be useful for breeding resistant cultivars.

The maize ZmVPS23-like protein relocates the nucleotide-binding leucine-rich repeat protein Rp1-D21 to endosomes and suppresses the defense response

Plants often utilize nucleotide-binding leucine-rich repeat (NLR) proteins to perceive pathogen infections and trigger a hypersensitive response (HR). The endosomal sorting complex required for transport (ESCRT) machinery is a conserved multisubunit complex that is essential for the biogenesis of multivesicular bodies and cargo protein sorting. VPS23 is a key component of ESCRT-I and plays important roles in plant development and abiotic stresses. ZmVPS23L, a homolog of VPS23-like in maize (Zea mays), was previously identified as a candidate gene in modulating HR mediated by the autoactive NLR protein Rp1-D21 in different maize populations. Here, we demonstrate that ZmVPS23L suppresses Rp1-D21-mediated HR in maize and Nicotiana benthamiana. Variation in the suppressive effect of HR by different ZmVPS23L alleles was correlated with variation in their expression levels. ZmVPS23 also suppressed Rp1-D21-mediated HR. ZmVPS23L and ZmVPS23 predominantly localized to endosomes, and they physically interacted with the coiled-coil domain of Rp1-D21 and mediated the relocation of Rp1-D21 from the nucleo-cytoplasm to endosomes. In summary, we demonstrate that ZmVPS23L and ZmVPS23 are negative regulators of Rp1-D21-mediated HR, likely by sequestrating Rp1-D21 in endosomes via physical interaction. Our findings reveal the role of ESCRT components in controlling plant NLR-mediated defense responses.

Grapevine VaRPP13 protein enhances oomycetes resistance by activating SA signal pathway

Expression of the VaRPP13 in Arabidopsis and tobacco enhanced resistance to oomycete pathogens, and this enhancement is closely related to the activation of salicylic acid (SA) signaling pathway. Resistance (R) genes, which usually contain a nucleotide-binding site and a leucine-rich repeat (NBS-LRR) domain, play crucial roles in disease resistance. In this study, we cloned a CC-NBS-LRR gene VaRPP13 from Vitis amurensis 'Shuang Hong' grapevine, and investigated its function on disease resistance. VaRPP13 expression was induced by Plasmopara viticola, an oomycetes pathogen causing downy mildew disease in grapevine. Heterologous expression VaRPP13 could also enhance resistance to Hyaloperonospora arabidopsidis in Arabidopsis thaliana and Phytophthora capsici in Nicotiana benthamiana, both oomycete pathogens. Further study indicated that VaRPP13 could enhance the expression of genes in SA signal pathway, while exogenous SA could also induce the expression of VaRPP13. In conclusion, our studies demonstrated that VaRPP13 contributes to a broad-spectrum resistance to oomycetes via activating SA signaling pathway.

NLR we there yet? Nucleocytoplasmic coordination of NLR-mediated immunity

Plant intracellular nucleotide-binding leucine-rich repeat immune receptors (NLRs) perceive the activity of pathogen-secreted effector molecules that, when undetected, promote colonisation of hosts. Signalling from activated NLRs converges with and potentiates downstream responses from activated pattern recognition receptors (PRRs) that sense microbial signatures at the cell surface. Efficient signalling of both receptor branches relies on the host cell nucleus as an integration point for transcriptional reprogramming, and on the macromolecular transport processes that mediate the communication between cytoplasm and nucleoplasm. Studies on nuclear pore complexes (NPCs), the nucleoporin proteins (NUPs) that compose NPCs, and nuclear transport machinery constituents that control nucleocytoplasmic transport, have revealed that they play important roles in regulating plant immune responses. Here, we discuss the contributions of nucleoporins and nuclear transport receptor (NTR)-mediated signal transduction in plant immunity with an emphasis on NLR immune signalling across the nuclear compartment boundary and within the nucleus. We also highlight and discuss cytoplasmic and nuclear functions of NLRs and their signalling partners and further consider the potential implications of NLR activation and resistosome formation in both cellular compartments for mediating plant pathogen resistance and programmed host cell death.

Cloning southern corn rust resistant gene RppK and its cognate gene AvrRppK from Puccinia polysora

Broad-spectrum resistance has great values for crop breeding. However, its mechanisms are largely unknown. Here, we report the cloning of a maize NLR gene, RppK, for resistance against southern corn rust (SCR) and its cognate Avr gene, AvrRppK, from Puccinia polysora (the causal pathogen of SCR). The AvrRppK gene has no sequence variation in all examined isolates. It has high expression level during infection and can suppress pattern-triggered immunity (PTI). Further, the introgression of RppK into maize inbred lines and hybrids enhances resistance against multiple isolates of P. polysora, thereby increasing yield in the presence of SCR. Together, we show that RppK is involved in resistance against multiple P. polysora isolates and it can recognize AvrRppK, which is broadly distributed and conserved in P. polysora isolates.

Southern corn rust (SCR) caused by Puccinia polysora is a major maize disease that can result in major yield loss. Here, the authors report the expression of a CC-NB-LRR type of R gene RppK results in SCR resistance in susceptible maize lines and it can recognize the effector AvrRppK produced by P. polysora.

Transcription Factor Pso9TF Assists Xinjiang Wild Myrobalan Plum (Prunus sogdiana) PsoRPM3 Disease Resistance Protein to Resist Meloidogyne incognita

The root-knot nematode (Meloidogyne incognita) causes huge economic losses in the agricultural industry throughout the world. Control methods against these polyphagous plant endoparasites are sparse, the preferred one being the deployment of plant cultivars or rootstocks bearing resistance genes against Meloidogyne species. Our previous study has cloned one resistance gene, PsoRPM3, from Xinjiang wild myrobalan plum (Prunus sogdiana). However, the function of PsoRPM3 remains elusive. In the present study, we have investigated the regulatory mechanism of PsoRPM3 in plant defense responses to M. incognita. Our results indicate that fewer giant cells were detected in the roots of the PsoRPM3 transgenic tobacco than wild tobacco lines after incubation with M. incognita. Transient transformations of full-length and TN structural domains of PsoRPM3 have induced significant hypersensitive responses (HR), suggesting that TIR domain might be the one which caused HR. Further, yeast two-hybrid results revealed that the full-length and LRR domain of PsoRPM3 could interact with the transcription factor Pso9TF. The addition of Pso9TF increased the ROS levels and induced HR. Thus, our data revealed that the LRR structural domain of PsoRPM3 may be associated with signal transduction. Moreover, we did not find any relative inductions of defense-related genes PsoEDS1, PsoPAD4 and PsoSAG101 in P. sogdiana, which has been incubated with M. incognita. In summary, our work has shown the key functional domain of PsoRPM3 in the regulation of defense responses to M. incognita in P. sogdiana.

Maize Plants Chimeric for an Autoactive Resistance Gene Display a Cell-Autonomous Hypersensitive Response but Non-Cell Autonomous Defense Signaling

The maize gene Rp1-D21 is a mutant form of the gene Rp1-D that confers resistance to common rust. Rp1-D21 triggers a spontaneous defense response that occurs in the absence of the pathogen and includes a programed cell death called the hypersensitive response (HR). Eleven plants heterozygous for Rp1-D21, in four different genetic backgrounds, were identified that had chimeric leaves with lesioned sectors showing HR abutting green nonlesioned sectors lacking HR. The Rp1-D21 sequence derived from each of the lesioned portions of leaves was unaltered from the expected sequence whereas the Rp1-D21 sequences from nine of the nonlesioned sectors displayed various mutations, and we were unable to amplify Rp1-D21 from the other two nonlesioned sectors. In every case, the borders between the sectors were sharp, with no transition zone, suggesting that HR and chlorosis associated with Rp1-D21 activity was cell autonomous. Expression of defense response marker genes was assessed in the lesioned and nonlesioned sectors as well as in near-isogenic plants lacking and carrying Rp1-D21. Defense gene expression was somewhat elevated in nonlesioned sectors abutting sectors carrying Rp1-D21 compared with near-isogenic plants lacking Rp1-D21. This suggests that, whereas the HR itself was cell autonomous, other aspects of the defense response initiated by Rp1-D21 were not.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Maize nicotinate N-methyltransferase interacts with the NLR protein Rp1-D21 and modulates the hypersensitive response

Most plant intracellular immune receptors belong to nucleotide-binding, leucine-rich repeat (NLR) proteins. The recognition between NLRs and their corresponding pathogen effectors often triggers a hypersensitive response (HR) at the pathogen infection sites. The nicotinate N-methyltransferase (NANMT) is responsible for the conversion of nicotinate to trigonelline in plants. However, the role of NANMT in plant defence response is unknown. In this study, we demonstrated that the maize ZmNANMT, but not its close homolog ZmCOMT, an enzyme in the lignin biosynthesis pathway, suppresses the HR mediated by the autoactive NLR protein Rp1-D21 and its N-terminal coiled-coil signalling domain (CC_D21 ). ZmNANMT, but not ZmCOMT, interacts with CC_D21 , and they form a complex with HCT1806 and CCoAOMT2, two key enzymes in lignin biosynthesis, which can also suppress the autoactive HR mediated by Rp1-D21. ZmNANMT is mainly localized in the cytoplasm and nucleus, and either localization is important for suppressing the HR phenotype. These results lay the foundation for further elucidating the molecular mechanism of NANMTs in plant disease resistance.

Analysis of the transcriptomic, metabolomic, and gene regulatory responses to Puccinia sorghi in maize

Common rust, caused by Puccinia sorghi, is a widespread and destructive disease of maize. The Rp1-D gene confers resistance to the P. sorghi IN2 isolate, mediating a hypersensitive cell death response (HR). To identify differentially expressed genes (DEGs) and metabolites associated with the compatible (susceptible) interaction and with Rp1-D-mediated resistance in maize, we performed transcriptomics and targeted metabolome analyses of P. sorghi IN2-infected leaves from the near-isogenic lines H95 and H95:Rp1-D, which differed for the presence of Rp1-D. We observed up-regulation of genes involved in the defence response and secondary metabolism, including the phenylpropanoid, flavonoid, and terpenoid pathways. Metabolome analyses confirmed that intermediates from several transcriptionally up-regulated pathways accumulated during the defence response. We identified a common response in H95:Rp1-D and H95 with an additional H95:Rp1-D-specific resistance response observed at early time points at both transcriptional and metabolic levels. To better understand the mechanisms underlying Rp1-D-mediated resistance, we inferred gene regulatory networks occurring in response to P. sorghi infection. A number of transcription factors including WRKY53, BHLH124, NKD1, BZIP84, and MYB100 were identified as potentially important signalling hubs in the resistance-specific response. Overall, this study provides a novel and multifaceted understanding of the maize susceptible and resistance-specific responses to P. sorghi.

Maize metacaspases modulate the defense response mediated by the NLR protein Rp1-D21 likely by affecting its subcellular localization

Plants usually employ resistance (R) genes to defend against the infection of pathogens, and most R genes encode intracellular nucleotide-binding, leucine-rich repeat (NLR) proteins. The recognition between R proteins and their cognate pathogens often triggers a rapid localized cell death at the pathogen infection sites, termed the hypersensitive response (HR). Metacaspases (MCs) belong to a cysteine protease family, structurally related to metazoan caspases. MCs play crucial roles in plant immunity. However, the underlying molecular mechanism and the link between MCs and NLR-mediated HR are not clear. In this study, we systematically investigated the MC gene family in maize and identified 11 ZmMCs belonging to two types. Further functional analysis showed that the type I ZmMC1 and ZmMC2, but not the type II ZmMC9, suppress the HR-inducing activity of the autoactive NLR protein Rp1-D21 and of its N-terminal coiled-coil (CCD21 ) signaling domain when transiently expressed in Nicotiana benthamiana. ZmMC1 and ZmMC2 physically associate with CCD21 in vivo. We further showed that ZmMC1 and ZmMC2, but not ZmMC9, are predominantly localized in a punctate distribution in both N. benthamiana and maize (Zea mays) protoplasts. Furthermore, the co-expression of ZmMC1 and ZmMC2 with Rp1-D21 and CCD21 causes their re-distribution from being uniformly distributed in the nucleocytoplasm to a punctate distribution co-localizing with ZmMC1 and ZmMC2. We reveal a novel role of plant MCs in modulating the NLR-mediated defense response and derive a model to explain it.

What are the Top 10 Unanswered Questions in Molecular Plant-Microbe Interactions?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.The past few decades have seen major discoveries in the field of molecular plant-microbe interactions. As the result of technological and intellectual advances, we are now able to answer questions at a level of mechanistic detail that we could not have imagined possible 20 years ago. The MPMI Editorial Board felt it was time to take stock and reassess. What big questions remain unanswered? We knew that to identify the fundamental, overarching questions that drive our research, we needed to do this as a community. To reach a diverse audience of people with different backgrounds and perspectives, working in different areas of plant-microbe interactions, we queried the more than 1,400 participants at the 2019 International Congress on Molecular Plant-Microbe Interactions meeting in Glasgow. This group effort resulted in a list of ten, broad-reaching, fundamental questions that influence and inform our research. Here, we introduce these Top 10 unanswered questions, giving context and a brief description of the issues. Each of these questions will be the subject of a detailed review in the coming months. We hope that this process of reflecting on what is known and unknown and identifying the themes that underlie our research will provide a framework to use going forward, giving newcomers a sense of the mystery of the big questions and inspiring new avenues and novel insights.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Maize ZmFNSI Homologs Interact with an NLR Protein to Modulate Hypersensitive Response

Nucleotide binding, leucine-rich-repeat (NLR) proteins are the major class of resistance (R) proteins used by plants to defend against pathogen infection. The recognition between NLRs and their cognate pathogen effectors usually triggers a rapid localized cell death, termed the hypersensitive response (HR). Flavone synthase I (FNSI) is one of the key enzymes in the flavone biosynthesis pathway. It also displays salicylic acid (SA) 5-hydroxylase (S5H) activity. A close homolog of FNSI/S5H displays SA 3-hydroxylase (S3H) activity. Both FNSI/S5H and S3H play important roles in plant innate immunity. However, the underlying molecular mechanisms and the relationship between S5H and S3H with the NLR-mediated HR are not known in any plant species. In this study, we identified three genes encoding ZmFNSI-1, ZmFNSI-2 and ZmS3H that are significantly upregulated in a maize line carrying an autoactive NLR Rp1-D21 mutant. Functional analysis showed that ZmFNSI-1 and ZmFNSI-2, but not ZmS3H, suppressed HR conferred by Rp1-D21 and its signaling domain CCD21 when transiently expressed in N. benthamiana. ZmFNSI-1 and ZmFNSI-2 physically interacted with CCD21. Furthermore, ZmFNSI-1 and ZmFNSI-2 interacted with HCT, a key enzyme in lignin biosynthesis pathway, which can also suppress Rp1-D21-mediated HR. These results lay the foundation for the further functional analysis of the roles of FNSI in plant innate immunity.

Blast resistance gene Pi54 over-expressed in rice to understand its cellular and sub-cellular localization and response to different pathogens

Rice blast resistance gene, Pi54 provides broad-spectrum resistance against different strains of Magnaporthe oryzae. Understanding the cellular localization of Pi54 protein is an essential step towards deciphering its place of interaction with the cognate Avr-gene. In this study, we investigated the sub-cellular localization of Pi54 with Green Fluorescent Protein (GFP) as a molecular tag through transient and stable expression in onion epidermal cells (Allium cepa) and susceptible japonica cultivar rice Taipei 309 (TP309), respectively. Confocal microscopy based observations of the onion epidermal cells revealed nucleus and cytoplasm specific GFP signals. In the stable transformed rice plants, GFP signal was recorded in the stomata, upper epidermal cells, mesophyll cells, vascular bundle, and walls of bundle sheath and bulliform cells of leaf tissues. These observations were further confirmed by Immunocytochemical studies. Using GFP specific antibodies, it was found that there was sufficient aggregation of GFP::Pi54protein in the cytoplasm of the leaf mesophyll cells and periphery of the epidermal cells. Interestingly, the transgenic lines developed in this study could show a moderate level of resistance to Xanthomonas oryzae and Rhizoctonia solani, the causal agents of the rice bacterial blight and sheath blight diseases, respectively. This study is a first detailed report, which emphasizes the cellular and subcellular distribution of the broad spectrum blast resistance gene Pi54 in rice and the impact of its constitutive expression towards resistance against other fungal and bacterial pathogens of rice.

Plant and Animal Innate Immunity Complexes: Fighting Different Enemies with Similar Weapons

Both animals and plants express intracellular innate immunity receptors known as NLR (NOD-like receptors or nucleotide-binding domain and leucine-rich repeat receptors, respectively). For various mammalian systems, the specific formation of macromolecular structures, such as inflammasomes by activated NLR receptors, has been extensively reported. However, for plant organisms, the formation of such structures was an open scientific question for many years. This year, the first plant 'resistosome' structure was reported, revealing significant structural similarities to mammalian apoptosome and inflammasome structures. In this review, we summarize the key components comprising the mammalian apoptosome/inflammasome structures and the newly discovered plant resistosome, highlighting their commonalities and differences.

The plant hypersensitive response: concepts, control and consequences

The hypersensitive defence response is found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance, though, in specific situations, it may have other consequences such as pathogen susceptibility, growth retardation and, over evolutionary timescales, speciation. Due to the potentially severe costs of inappropriate activation, plants employ multiple mechanisms to suppress inappropriate activation of HR and to constrain it after activation. The ubiquity of this response among higher plants despite its costs suggests that it is an extremely effective component of the plant immune system.

Plant NLRs: From discovery to application

Plants require a complex immune system to defend themselves against a wide range of pathogens which threaten their growth and development. The nucleotide-binding leucine-rich repeat proteins (NLRs) are immune sensors that recognize effectors delivered by pathogens. The first NLR was cloned more than twenty years ago. Since this initial discovery, NLRs have been described as key components of plant immunity responsible for pathogen recognition and triggering defense responses. They have now been described in most of the well-studied mulitcellular plant species, with most having large NLR repertoires. As research has progressed so has the understanding of how NLRs interact with their recognition substrates and how they in turn activate downstream signalling. It has also become apparent that NLR regulation occurs at the transcriptional, post-transcriptional, translational, and post-translational levels. Even before the first NLR was cloned, breeders were utilising such genes to increase crop performance. Increased understanding of the mechanistic details of the plant immune system enable the generation of plants resistant against devastating pathogens. This review aims to give an updated summary of the NLR field.

Physical interaction between nuclear accumulated CC-NB-ARC-LRR protein and WRKY64 promotes EDS1 dependent Fusarium wilt resistance in chickpea

Fusarium wilt is one of the most serious diseases affecting chickpea (Cicer arietinum L.). Here, we identified a putative Resistance Gene Analog (CaRGA) from chickpea, encoding a coiled-coil (CC) nucleotide-binding oligomerization domain (NB-ARC) containing leucine-rich repeat (LRR) protein (CC-NLR protein) that confers resistance against Fusarium oxysporum f. sp. ciceri race1 (Foc1). Over-expression and silencing of CaRGA in chickpea resulted in enhanced resistance and hyper-susceptibility, respectively against Foc1. Furthermore, defense response to Foc1 depends on CC-NLR interaction with WRKY64 transcription factor. CaRGA mediated wilt resistance largely compromised when WRKY64 was silenced. We also determined in planta intramolecular interactions and self-association of chickpea CC-NLR protein. The study shows CC domain suppressing auto-activation of the full-length CC-NLR protein in the absence of pathogen through self-inhibitory intramolecular interaction with NB-ARC domain, which is attenuated by self-interactions to LRR domain. Chickpea CC-NLR protein forms homocomplexes and then interacts with WRKY64. CC-NLR protein further phosphorylates WRKY64 thereby, ubiquitination and proteasome mediated degradation are protected. Phosphorylated WRKY64 with increased stability binds to EDS1 promoter and stimulates its transcription that induces in planta ectopic cell-death. The detailed analysis of CC-NLR and WRKY interactions provide a better understanding of the immune regulation by NLR proteins under biotic stresses.

What Do We Know About NOD-Like Receptors in Plant Immunity?

The first plant disease resistance (R) genes were identified and cloned more than two decades ago. Since then, many more R genes have been identified and characterized in numerous plant pathosystems. Most of these encode members of the large family of intracellular NLRs (NOD-like receptors), which also includes animal immune receptors. New discoveries in this expanding field of research provide new elements for our understanding of plant NLR function. But what do we know about plant NLR function today? Genetic, structural, and functional analyses have uncovered a number of commonalities and differences in pathogen recognition strategies as well as how NLRs are regulated and activate defense signaling, but many unknowns remain. This review gives an update on the latest discoveries and breakthroughs in this field, with an emphasis on structural findings and some comparison to animal NLRs, which can provide additional insights and paradigms in plant NLR function.

Cell death triggering and effector recognition by Sw-5 SD-CNL proteins from resistant and susceptible tomato isolines to Tomato spotted wilt virus

Only a limited number of dominant resistance genes acting against plant viruses have been cloned, and further functional studies of these have been almost entirely limited to the resistance genes Rx against Potato virus X (PVX) and N against Tobacco mosaic virus (TMV). Recently, the cell-to-cell movement protein (NSM ) of Tomato spotted wilt virus (TSWV) has been identified as the avirulence determinant (Avr) of Sw-5b-mediated resistance, a dominant resistance gene which belongs to the class of SD-CC-NB-LRR (Solanaceae domain-coiled coil-nucleotide-binding-leucine-rich repeat, SD-CNL) resistance genes. On transient expression of the NSM protein in tomato and transgenic Nicotiana benthamiana harbouring the Sw-5b gene, a hypersensitive cell death response (HR) is triggered. Here, it is shown that high accumulation of the Sw-5b protein in N. benthamiana leaves, achieved by co-expression of the Sw-5b protein with RNA silencing suppressors (RSSs), leads to auto-activity in the absence of NSM . In a similar approach, Sw-5a, the highest conserved paralogue of Sw-5b from Solanum peruvianum, also triggered HR by auto-activation, whereas the highest conserved orthologue from susceptible S. lycopersicum, named Sw-5aS , did not. However, neither of the last two homologues was able to trigger an NSM -dependent HR. Truncated and mutated versions of these Sw-5 proteins revealed that the NB-ARC [nucleotide-binding adaptor shared by Apaf-1 (from humans), R proteins and CED-4 (from nematodes)] domain is sufficient for the triggering of HR and seems to be suppressed by the SD-CC domain. Furthermore, a single mutation was sufficient to restore auto-activity within the NB-ARC domain of Sw-5aS . When the latter domain was fused to the Sw-5b LRR domain, NSM -dependent HR triggering was regained, but not in the presence of its own Sw-5aS LRR domain. Expression analysis in planta revealed a nucleocytoplasmic localization pattern of Sw-5b, in which the SD-CC domain seems to be required for nuclear translocation. Although the Sw-5 N-terminal CC domain, in contrast with Rx, contains an additional SD, most findings from this study support a conserved role of domains within NB-LRR (NLR) proteins against plant viruses.

The CC domain structure from the wheat stem rust resistance protein Sr33 challenges paradigms for dimerization in plant NLR proteins

Plants use intracellular immunity receptors, known as nucleotide-binding oligomerization domain-like receptors (NLRs), to recognize specific pathogen effector proteins and induce immune responses. These proteins provide resistance to many of the world's most destructive plant pathogens, yet we have a limited understanding of the molecular mechanisms that lead to defense signaling. We examined the wheat NLR protein, Sr33, which is responsible for strain-specific resistance to the wheat stem rust pathogen, Puccinia graminis f. sp. tritici We present the solution structure of a coiled-coil (CC) fragment from Sr33, which adopts a four-helix bundle conformation. Unexpectedly, this structure differs from the published dimeric crystal structure of the equivalent region from the orthologous barley powdery mildew resistance protein, MLA10, but is similar to the structure of the distantly related potato NLR protein, Rx. We demonstrate that these regions are, in fact, largely monomeric and adopt similar folds in solution in all three proteins, suggesting that the CC domains from plant NLRs adopt a conserved fold. However, larger C-terminal fragments of Sr33 and MLA10 can self-associate both in vitro and in planta, and this self-association correlates with their cell death signaling activity. The minimal region of the CC domain required for both cell death signaling and self-association extends to amino acid 142, thus including 22 residues absent from previous biochemical and structural protein studies. These data suggest that self-association of the minimal CC domain is necessary for signaling but is likely to involve a different structural basis than previously suggested by the MLA10 crystallographic dimer.

Cytosolic activation of cell death and stem rust resistance by cereal MLA-family CC-NLR proteins

Plants possess intracellular immune receptors designated "nucleotide-binding domain and leucine-rich repeat" (NLR) proteins that translate pathogen-specific recognition into disease-resistance signaling. The wheat immune receptors Sr33 and Sr50 belong to the class of coiled-coil (CC) NLRs. They confer resistance against a broad spectrum of field isolates of Puccinia graminis f. sp. tritici, including the Ug99 lineage, and are homologs of the barley powdery mildew-resistance protein MLA10. Here, we show that, similarly to MLA10, the Sr33 and Sr50 CC domains are sufficient to induce cell death in Nicotiana benthamiana Autoactive CC domains and full-length Sr33 and Sr50 proteins self-associate in planta In contrast, truncated CC domains equivalent in size to an MLA10 fragment for which a crystal structure was previously determined fail to induce cell death and do not self-associate. Mutations in the truncated region also abolish self-association and cell-death signaling. Analysis of Sr33 and Sr50 CC domains fused to YFP and either nuclear localization or nuclear export signals in N benthamiana showed that cell-death induction occurs in the cytosol. In stable transgenic wheat plants, full-length Sr33 proteins targeted to the cytosol provided rust resistance, whereas nuclear-targeted Sr33 was not functional. These data are consistent with CC-mediated induction of both cell-death signaling and stem rust resistance in the cytosolic compartment, whereas previous research had suggested that MLA10-mediated cell-death and disease resistance signaling occur independently, in the cytosol and nucleus, respectively.