Dissection of Cell Death Induction by Wheat Stem Rust Resistance Protein Sr35 and Its Matching Effector AvrSr35

A Puccinia striiformis f. sp. tritici Effector with DPBB Domain Suppresses Wheat Defense

Wheat (Triticum aestivum L.) is a primary crop globally. Among the numerous pathogens affecting wheat production, Puccinia striiformis f. sp. tritici (Pst) is a significant biotic stress agent and poses a major threat to world food security by causing stripe rust or yellow rust disease. Understanding the molecular basis of plant-pathogen interactions is crucial for developing new means of disease management. It is well established that the effector proteins play a pivotal role in pathogenesis. Therefore, studying effector proteins has become an important area of research in plant biology. Our previous work identified differentially expressed candidate secretory effector proteins of stripe rust based on transcriptome sequencing data from susceptible wheat (Avocet S) and resistant wheat (Avocet YR10) infected with Pst. Among the secreted effector proteins, PSTG_14090 contained an ancient double-psi beta-barrel (DPBB) fold, which is conserved in the rare lipoprotein A (RlpA) superfamily. This study investigated the role of PSTG_14090 in plant immune responses, which encodes a protein, here referred to as Pst-DPBB, having 131 amino acids with a predicted signal peptide (SP) of 19 amino acids at the N-terminal end, and the DNA sequence of this effector is highly conserved among different stripe rust races. qRT-PCR analysis indicated that expression levels are upregulated during the early stages of infection. Subcellular localization studies in Nicotiana benthamiana leaves and wheat protoplasts revealed that it is distributed in the cytoplasm, nucleus, and apoplast. We demonstrated that Pst-DPBB negatively regulates the immune response by functioning in various compartments of the plant cells. Based on Co-IP and structural predictions and putative interaction analyses by AlphaFold 3, we propose the probable biological function(s). Pst-DPBB behaves as a papain inhibitor of wheat cysteine protease; Pst-DPBB has high structural homology to kiwellin, which is known to interact with chorismate mutase, suggesting that Pst-DPBB inhibits the native function of the host chorismate mutase involved in salicylic acid synthesis. The DPBB fold is also known to interact with DNA and RNA, which may suggest its possible role in regulating the host gene expression.

Assessment of Self-Activation and Inhibition of Wheat Coiled-Coil Domain Containing NLR Immune Receptor Yr10CG

Plant immunity is largely governed by nucleotide-binding leucine-rich repeat receptor (NLR). Here, we examine the molecular activation and inhibition mechanisms of the wheat CC-type NLR Yr10CG, a previously proposed candidate for the Yr10 resistance gene. Though recent studies have identified YrNAM as the true Yr10 gene, Yr10CG remains an important NLR in understanding NLR-mediated immunity in wheat. In this study, we found that the overexpression of either the full-length Yr10CG or its CC domain in Nicotiana benthamiana did not trigger cell death, suggesting a robust autoinhibitory mechanism within Yr10CG. However, we observed that mutations in the conserved MHD motif, specifically D502G, activated Yr10CG and induced cell death. Structural modeling indicated that this mutation disrupted key interactions within the MHD motif, promoting local flexibility and activation. We further explored the effector recognition potential of Yr10CG by creating chimeric proteins with Sr50 domains, revealing that both the NB-ARC and LRR domains are necessary for effector recognition, while the CC domain likely functions in downstream immune signaling. Additionally, disrupting membrane localization through an L11E mutation abolished Yr10CG self-activation, suggesting a requirement for membrane association in immune activation. Our findings contribute to the understanding of CC-NLR activation and autoinhibition mechanisms, highlighting the potential of Yr10CG in NLR engineering for crop resistance improvement.

Activation of an atypical plant NLR with an N-terminal deletion initiates cell death at the vacuole

Plants evolve nucleotide-binding leucine-rich repeat receptors (NLRs) to induce immunity. Activated coiled-coil (CC) domain containing NLRs (CNLs) oligomerize and form apparent cation channels promoting calcium influx and cell death, with the alpha-1 helix of the individual CC domains penetrating the plasma membranes. Some CNLs are characterized by putative N-myristoylation and S-acylation sites in their CC domain, potentially mediating permanent membrane association. Whether activated Potentially Membrane Localized NLRs (PMLs) mediate cell death and calcium influx in a similar way is unknown. We uncovered the cell-death function at the vacuole of an atypical but conserved Arabidopsis PML, PML5, which has a significant deletion in its CCG10/GA domain. Active PML5 oligomers localize in Golgi membranes and the tonoplast, alter vacuolar morphology, and induce cell death, with the short N-terminus being sufficient. Mutant analysis supports a potential role of PMLs in plant immunity. PML5-like deletions are found in several Brassicales paralogs, pointing to the evolutionary importance of this innovation. PML5, with its minimal CC domain, represents the first identified CNL utilizing vacuolar-stored calcium for cell death induction.

Uncovering the Mechanisms: The Role of Biotrophic Fungi in Activating or Suppressing Plant Defense Responses

This paper discusses the mechanisms by which fungi manipulate plant physiology and suppress plant defense responses by producing effectors that can target various host proteins. Effector-triggered immunity and effector-triggered susceptibility are pivotal elements in the complex molecular dialogue underlying plant-pathogen interactions. Pathogen-produced effector molecules possess the ability to mimic pathogen-associated molecular patterns or hinder the binding of pattern recognition receptors. Effectors can directly target nucleotide-binding domain, leucine-rich repeat receptors, or manipulate downstream signaling components to suppress plant defense. Interactions between these effectors and receptor-like kinases in host plants are critical in this process. Biotrophic fungi adeptly exploit the signaling networks of key plant hormones, including salicylic acid, jasmonic acid, abscisic acid, and ethylene, to establish a compatible interaction with their plant hosts. Overall, the paper highlights the importance of understanding the complex interplay between plant defense mechanisms and fungal effectors to develop effective strategies for plant disease management.

Activation and Autoinhibition Mechanisms of NLR Immune Receptor Pi36 in Rice

Nucleotide-binding and leucine-rich repeat receptors (NLRs) are the most important and largest class of immune receptors in plants. The Pi36 gene encodes a canonical CC-NBS-LRR protein that confers resistance to rice blast fungal infections. Here, we show that the CC domain of Pi36 plays a role in cell death induction. Furthermore, self-association is required for the CC domain-mediated cell death, and the self-association ability is correlated with the cell death level. In addition, the NB-ARC domain may suppress the activity of the CC domain through intramolecular interaction. The mutations D440G next to the RNBS-D motif and D503V in the MHD motif autoactivated Pi36, but the mutation K212 in the P-loop motif inhibited this autoactivation, indicating that nucleotide binding of the NB-ARC domain is essential for Pi36 activation. We also found that the LRR domain is required for D503V- and D440G-mediated Pi36 autoactivation. Interestingly, several mutations in the CC domain compromised the CC domain-mediated cell death without affecting the D440G- or D503V-mediated Pi36 autoactivation. The autoactivate Pi36 variants exhibited stronger self-associations than the inactive variants. Taken together, we speculated that the CC domain of Pi36 executes cell death activities, whereas the NB-ARC domain suppressed CC-mediated cell death via intermolecular interaction. The NB-ARC domain releases its suppression of the CC domain and strengthens the self-association of Pi36 to support the CC domain, possibly through nucleotide exchange.

Ectopic expression of the Arabidopsis mutant L3 NB-LRR receptor gene in Nicotiana benthamiana cells leads to cell death

Nucleotide-binding sites and leucine-rich repeat proteins (NLRs) act as critical intracellular immune receptors. Previous studies reported an Arabidopsis-resistant gene L3 (AT1G15890), which encoded a coiled-coil (CC) NLR that conferred cell death in bacteria; however, its function in planta remains unclear. This study describes a comprehensive structure-function analysis of L3 in Nicotiana benthamiana. The results of the transient assay showed that the L3 CC domain is sufficient for cell-death induction. The first 140 amino acid segment constituted the minimal function region that could cause cell death. The YFP-labeled L3 CC domain was localized to the plasma membrane, which was considered crucial for the function and self-interaction of the L3 CC domain. The results of point mutations analysis showed that L3 CC domain function is affected by mutations in some specific residues, and loss-of-function mutations in the CC domain affected the function of full-length L3. These study results offered considerable evidence to understand the activation mechanism of L3.

Comparative Analysis of the Avirulence Effectors Produced by the Fungal Stem Rust Pathogen of Wheat

Crops are constantly exposed to pathogenic microbes. Rust fungi are examples of these harmful microorganisms, which have a major economic impact on wheat production. To protect themselves from pathogens like rust fungi, plants employ a multilayered immune system that includes immunoreceptors encoded by resistance genes. Significant efforts have led to the isolation of numerous resistance genes against rust fungi in cereals, especially in wheat. However, the evolution of virulence of rust fungi hinders the durability of resistance genes as a strategy for crop protection. Rust fungi, like other biotrophic pathogens, secrete an arsenal of effectors to facilitate infection, and these are the molecules that plant immunoreceptors target for pathogen recognition and mounting defense responses. When recognized, these effector proteins are referred to as avirulence (Avr) effectors. Despite the many predicted effectors in wheat rust fungi, only five Avr genes have been identified, all from wheat stem rust. Knowledge of the Avr genes and their variation in the fungal population will inform deployment of the most appropriate wheat disease-resistance genes for breeding and farming. The review provides an overview of methodologies as well as the validation techniques that have been used to characterize Avr effectors from wheat stem rust. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Rice NLR protein XinN1, induced by a pattern recognition receptor XA21, confers enhanced resistance to bacterial blight

KEY MESSAGE

Rice CC-type NLR XinN1, specifically induced by a PRR XA21, activates defense pathways against Xoo. Plants have evolved two layers of immune systems regulated by two different types of immune receptors, cell surface located pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat-containing receptors (NLRs). Plant PRRs recognize conserved molecular patterns from diverse pathogens, resulting in pattern-triggered immunity (PTI), whereas NLRs are activated by effectors secreted by pathogens into plant cells, inducing effector-triggered immunity (ETI). Rice PRR, XA21, recognizes a tyrosine-sulfated RaxX peptide (required for activation of XA21-mediated immunity X) as a molecular pattern secreted by Xanthomonas oryzae pv. oryzae (Xoo). Here, we identified a rice NLR gene, XinN1, that is specifically induced during the XA21-mediated immune response against Xoo. Transgenic rice plants overexpressing XinN1 displayed increased resistance to infection by Xoo with reduced lesion length and bacterial growth. Overexpression of autoactive mutant of XinN1 (XinN1D543V) also displayed increased resistance to Xoo, accompanied with severe growth retardation and cell death. In rice protoplast system, overexpression of XinN1 or XinN1D543V significantly elevated reactive oxygen species (ROS) production and cytosolic-free calcium (Ca2+) accumulations. In addition, XinN1 overexpression additionally elevated the ROS burst caused by the interaction between XA21 and RaxX-sY and induced the transcription of PTI signaling components, including somatic embryogenesis receptor kinases (OsSERKs) and receptor-like cytoplasmic kinases (OsRLCKs). Our results suggest that XinN1 induced by the PRR XA21 activates defense pathways and provides enhanced resistance to Xoo in rice.

Two functional CC-NBS-LRR proteins from rye chromosome 6RS confer differential age-related powdery mildew resistance to wheat

Rye (Secale cereale), a valuable relative of wheat, contains abundant powdery mildew resistance (Pm) genes. Using physical mapping, transcriptome sequencing, barley stripe mosaic virus-induced gene silencing, ethyl methane sulfonate mutagenesis, and stable transformation, we isolated and validated two coiled-coil, nucleotide-binding site and leucine-rich repeat (CC-NBS-LRR) alleles, PmTR1 and PmTR3, located on rye chromosome 6RS from different triticale lines. PmTR1 confers age-related resistance starting from the three-leaf stage, whereas its allele, PmTR3, confers typical all-stage resistance, which may be associated with their differential gene expression patterns. Overexpression in Nicotiana benthamiana showed that the CC, CC-NBS, and CC-LRR fragments of PMTR1 induce cell death, whereas in PMTR3 the CC and full-length fragments perform this function. Luciferase complementation imaging and pull-down assays revealed distinct interaction activities between the CC and NBS fragments. Our study elucidates two novel rye-derived Pm genes and their derivative germplasm resources and provides novel insights into the mechanism of age-related resistance, which can aid the improvement of resistance against wheat powdery mildew.

Cloning of the wheat leaf rust resistance gene Lr47 introgressed from Aegilops speltoides

Leaf rust, caused by Puccinia triticina Eriksson (Pt), is one of the most severe foliar diseases of wheat. Breeding for leaf rust resistance is a practical and sustainable method to control this devastating disease. Here, we report the identification of Lr47, a broadly effective leaf rust resistance gene introgressed into wheat from Aegilops speltoides. Lr47 encodes a coiled-coil nucleotide-binding leucine-rich repeat protein that is both necessary and sufficient to confer Pt resistance, as demonstrated by loss-of-function mutations and transgenic complementation. Lr47 introgression lines with no or reduced linkage drag are generated using the Pairing homoeologous1 mutation, and a diagnostic molecular marker for Lr47 is developed. The coiled-coil domain of the Lr47 protein is unable to induce cell death, nor does it have self-protein interaction. The cloning of Lr47 expands the number of leaf rust resistance genes that can be incorporated into multigene transgenic cassettes to control this devastating disease.

Rapid evolution of an RNA virus to escape recognition by a rice nucleotide-binding and leucine-rich repeat domain immune receptor

Viral diseases are a major limitation for crop production, and their control is crucial for sustainable food supply. We investigated by a combination of functional genetics and experimental evolution the resistance of rice to the rice yellow mottle virus (RYMV), which is among the most devastating rice pathogens in Africa, and the mechanisms underlying the extremely fast adaptation of the virus to its host. We found that the RYMV3 gene that protects rice against the virus codes for a nucleotide-binding and leucine-rich repeat domain immune receptor (NLRs) from the Mla-like clade of NLRs. RYMV3 detects the virus by forming a recognition complex with the viral coat protein (CP). The virus escapes efficiently from detection by mutations in its CP, some of which interfere with the formation of the recognition complex. This study establishes that NLRs also confer in monocotyledonous plants immunity to viruses, and reveals an unexpected functional diversity for NLRs of the Mla clade that were previously only known as fungal disease resistance proteins. In addition, it provides precise insight into the mechanisms by which viruses adapt to plant immunity and gives important knowledge for the development of sustainable resistance against viral diseases of cereals.

Caveats of Using Bacterial Type Three Secretion Assays for Validating Fungal Avirulence Effectors in Wheat

Functional characterization of effector proteins of fungal obligate biotrophic pathogens, especially confirmation of avirulence (Avr) properties, has been notoriously difficult, due to the experimental intractability of many of these organisms. Previous studies in wheat have shown promising data suggesting the type III secretion system (T3SS) of bacteria may be a suitable surrogate for delivery and detection of Avr properties of fungal effectors. However, these delivery systems were tested in the absence of confirmed Avr effectors. Here, we tested two previously described T3SS-mediated delivery systems for their suitability when delivering two confirmed Avr effectors from two fungal pathogens of wheat, Puccinia graminis f. sp. tritici and Magnaporthe oryzae pathotype tritici. We showed that both effectors (AvrSr50 and AvrRmg8) were unable to elicit a hypersensitive response on wheat seedlings with the corresponding resistance gene when expressed by the Pseudomonas fluorescens "Effector to Host Analyser" (EtHAn) system. Furthermore, we found the utility of Burkholderia glumae for screening Avr phenotypes is severely limited, as the wild-type strain elicits nonhost cell death in multiple wheat accessions. These results provide valuable insight into the suitability of these systems for screening fungal effectors for Avr properties that may help guide further development of surrogate bacterial delivery systems in wheat. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Effectors from a Bacterial Vector-Borne Pathogen Exhibit Diverse Subcellular Localization, Expression Profiles, and Manipulation of Plant Defense

Climate change is predicted to increase the prevalence of vector-borne disease due to expansion of insect populations. 'Candidatus Liberibacter solanacearum' is a phloem-limited pathogen associated with multiple economically important diseases in solanaceous crops. Little is known about the strategies and pathogenicity factors 'Ca. L. solanacearum' uses to colonize its vector and host. We determined the 'Ca. L. solanacearum' effector repertoire by predicting proteins secreted by the general secretory pathway across four different 'Ca. L. solanacearum' haplotypes, investigated effector localization in planta, and profiled effector expression in the vector and host. The localization of 'Ca. L. solanacearum' effectors in Nicotiana spp. revealed diverse eukaryotic subcellular targets. The majority of tested effectors were unable to suppress plant immune responses, indicating they possess unique activities. Expression profiling in tomato and the psyllid Bactericera cockerelli indicated 'Ca. L. solanacearum' differentially interacts with its host and vector and can switch effector expression in response to these environments. This study reveals 'Ca. L. solanacearum' effectors possess complex expression patterns, target diverse host organelles and the majority are unable to suppress host immune responses. A mechanistic understanding of 'Ca. L. solanacearum' effector function will reveal novel targets and provide insight into phloem biology. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A wheat resistosome defines common principles of immune receptor channels

Plant intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) detect pathogen effectors to trigger immune responses1. Indirect recognition of a pathogen effector by the dicotyledonous Arabidopsis thaliana coiled-coil domain containing NLR (CNL) ZAR1 induces the formation of a large hetero-oligomeric protein complex, termed the ZAR1 resistosome, which functions as a calcium channel required for ZAR1-mediated immunity2-4. Whether the resistosome and channel activities are conserved among plant CNLs remains unknown. Here we report the cryo-electron microscopy structure of the wheat CNL Sr355 in complex with the effector AvrSr356 of the wheat stem rust pathogen. Direct effector binding to the leucine-rich repeats of Sr35 results in the formation of a pentameric Sr35-AvrSr35 complex, which we term the Sr35 resistosome. Wheat Sr35 and Arabidopsis ZAR1 resistosomes bear striking structural similarities, including an arginine cluster in the leucine-rich repeats domain not previously recognized as conserved, which co-occurs and forms intramolecular interactions with the 'EDVID' motif in the coiled-coil domain. Electrophysiological measurements show that the Sr35 resistosome exhibits non-selective cation channel activity. These structural insights allowed us to generate new variants of closely related wheat and barley orphan NLRs that recognize AvrSr35. Our data support the evolutionary conservation of CNL resistosomes in plants and demonstrate proof of principle for structure-based engineering of NLRs for crop improvement.

Pathogen effector AvrSr35 triggers Sr35 resistosome assembly via a direct recognition mechanism

Nucleotide-binding, leucine-rich repeat receptors (NLRs) perceive pathogen effectors to trigger plant immunity. The direct recognition mechanism of pathogen effectors by coiled-coil NLRs (CNLs) remains unclear. We demonstrate that the Triticum monococcum CNL Sr35 directly recognizes the pathogen effector AvrSr35 from Puccinia graminis f. sp. tritici and report a cryo-electron microscopy structure of Sr35 resistosome and a crystal structure of AvrSr35. We show that AvrSr35 forms homodimers that are disassociated into monomers upon direct recognition by the leucine-rich repeat domain of Sr35, which induces Sr35 resistosome assembly and the subsequent immune response. The first 20 amino-terminal residues of Sr35 are indispensable for immune signaling but not for plasma membrane association. Our findings reveal the direct recognition and activation mechanism of a plant CNL and provide insights into biochemical function of Sr35 resistosome.

Three highly conserved hydrophobic residues in the predicted α2-helix of rice NLR protein Pit contribute to its localization and immune induction

Nucleotide-binding leucine-rich repeat (NLR) proteins work as crucial intracellular immune receptors. N-terminal domains of NLRs fall into two groups, coiled-coil (CC) and Toll-interleukin 1 receptor domains, which play critical roles in signal transduction and disease resistance. However, the activation mechanisms of NLRs, and how their N-termini function in immune induction, remain largely unknown. Here, we revealed that the CC domain of a rice NLR Pit contributes to self-association. The Pit CC domain possesses three conserved hydrophobic residues that are known to be involved in oligomer formation in two NLRs, barley MLA10 and Arabidopsis RPM1. Interestingly, the function of these residues in Pit differs from that in MLA10 and RPM1. Although three hydrophobic residues are important for Pit-induced disease resistance against rice blast fungus, they do not participate in self-association or binding to downstream signalling molecules. By homology modelling of Pit using the Arabidopsis ZAR1 structure, we tried to clarify the role of three conserved hydrophobic residues and found that they are located in the predicted α2-helix of the Pit CC domain and involved in the plasma membrane localization. Our findings provide novel insights for understanding the mechanisms of NLR activation as well as the relationship between subcellular localization and immune induction.

Genomic sequencing of Thinopyrum elongatum chromosome arm 7EL, carrying fusarium head blight resistance, and characterization of its impact on the transcriptome of the introgressed line CS-7EL

Background

The tall wheatgrass species Thinopyrum elongatum carries a strong fusarium head blight (FHB) resistance locus located on the long arm of chromosome 7 (7EL) as well as resistance to leaf and stem rusts, all diseases with a significant impact on wheat production. Towards understanding the contribution of Th. elongatum 7EL to improvement of disease resistance in wheat, the genomic sequence of the 7EL fragment present in the wheat Chinese Spring (CS) telosomic addition line CS-7EL was determined and the contribution and impact of 7EL on the rachis transcriptome during FHB infection was compared between CS and CS-7EL.

Results

We assembled the Th. elongatum 7EL chromosome arm using a reference-guided approach. Combining this assembly with the available reference sequence for CS hexaploid wheat provided a reliable reference for interrogating the transcriptomic differences in response to infection conferred by the 7EL fragment. Comparison of the transcriptomes of rachis tissues from CS and CS-7EL showed expression of Th. elongatum transcripts as well as modulation of wheat transcript expression profiles in the CS-7EL line. Expression profiles at 4 days after infection with Fusarium graminearum, the causal agent of FHB, showed an increased in expression of genes associated with an effective defense response, in particular glucan endo-1,3-beta-glucosidases and chitinases, in the FHB-resistant line CS-7EL while there was a larger increase in differential expression for genes associated with the level of fungal infection in the FHB-susceptible line CS. One hundred and seven 7EL transcripts were expressed in the smallest 7EL region defined to carry FHB resistance.

Conclusion

7EL contributed to CS-7EL transcriptome by direct expression and through alteration of wheat transcript profiles. FHB resistance in CS-7EL was associated with transcriptome changes suggesting a more effective defense response. A list of candidate genes for the FHB resistance locus on 7EL has been established.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08433-8.

Advances in Understanding Defense Mechanisms in Persea americana Against Phytophthora cinnamomi

Avocado (Persea americana) is an economically important fruit crop world-wide, the production of which is challenged by notable root pathogens such as Phytophthora cinnamomi and Rosellinia necatrix. Arguably the most prevalent, P. cinnamomi, is a hemibiotrophic oomycete which causes Phytophthora root rot, leading to reduced yields and eventual tree death. Despite its' importance, the development of molecular tools and resources have been historically limited, prohibiting significant progress toward understanding this important host-pathogen interaction. The development of a nested qPCR assay capable of quantifying P. cinnamomi during avocado infection has enabled us to distinguish avocado rootstocks as either resistant or tolerant - an important distinction when unraveling the defense response. This review will provide an overview of our current knowledge on the molecular defense pathways utilized in resistant avocado rootstock against P. cinnamomi. Notably, avocado demonstrates a biphasic phytohormone profile in response to P. cinnamomi infection which allows for the timely expression of pathogenesis-related genes via the NPR1 defense response pathway. Cell wall modification via callose deposition and lignification have also been implicated in the resistant response. Recent advances such as composite plant transformation, single nucleotide polymorphism (SNP) analyses as well as genomics and transcriptomics will complement existing molecular, histological, and biochemical assay studies and further elucidate avocado defense mechanisms.

Diversity, structure and function of the coiled-coil domains of plant NLR immune receptors

Plant nucleotide-binding, leucine-rich repeat receptors (NLRs) perceive pathogen avirulence effectors and activate defense responses. Nucleotide-binding, leucine-rich repeat receptors are classified into coiled-coil (CC)-containing and Toll/interleukin-1 receptor (TIR)-containing NLRs. Recent advances suggest that NLR CC domains often function in signaling activation, especially for induction of cell death. In this review, we outline our current understanding of NLR CC domains, including their diversity/classification and structure, their roles in cell death induction, disease resistance, and interaction with other proteins. Furthermore, we provide possible directions for future work.

The wheat Sr22, Sr33, Sr35 and Sr45 genes confer resistance against stem rust in barley

In the last 20 years, stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt), has re-emerged as a major threat to wheat and barley production in Africa and Europe. In contrast to wheat with 60 designated stem rust (Sr) resistance genes, barley's genetic variation for stem rust resistance is very narrow with only ten resistance genes genetically identified. Of these, only one complex locus consisting of three genes is effective against TTKSK, a widely virulent Pgt race of the Ug99 tribe which emerged in Uganda in 1999 and has since spread to much of East Africa and parts of the Middle East. The objective of this study was to assess the functionality, in barley, of cloned wheat Sr genes effective against race TTKSK. Sr22, Sr33, Sr35 and Sr45 were transformed into barley cv. Golden Promise using Agrobacterium-mediated transformation. All four genes were found to confer effective stem rust resistance. The barley transgenics remained susceptible to the barley leaf rust pathogen Puccinia hordei, indicating that the resistance conferred by these wheat Sr genes was specific for Pgt. Furthermore, these transgenic plants did not display significant adverse agronomic effects in the absence of disease. Cloned Sr genes from wheat are therefore a potential source of resistance against wheat stem rust in barley.

Recognition and defence of plant-infecting fungal pathogens

Attempted infections of plants with fungi result in diverse outcomes ranging from symptom-less resistance to severe disease and even death of infected plants. The deleterious effect on crop yield have led to intense focus on the cellular and molecular mechanisms that explain the difference between resistance and susceptibility. This research has uncovered plant resistance or susceptibility genes that explain either dominant or recessive inheritance of plant resistance with many of them coding for receptors that recognize pathogen invasion. Approaches based on cell biology and phytochemistry have contributed to identifying factors that halt an invading fungal pathogen from further invasion into or between plant cells. Plant chemical defence compounds, antifungal proteins and structural reinforcement of cell walls appear to slow down fungal growth or even prevent fungal penetration in resistant plants. Additionally, the hypersensitive response, in which a few cells undergo a strong local immune reaction, including programmed cell death at the site of infection, stops in particular biotrophic fungi from spreading into surrounding tissue. In this review, we give a general overview of plant recognition and defence of fungal parasites tracing back to the early 20th century with a special focus on Triticeae and on the progress that was made in the last 30 years.

Pm21 CC domain activity modulated by intramolecular interactions is implicated in cell death and disease resistance

Nucleotide-binding (NB) leucine-rich repeat (LRR) receptors (NLRs) provide resistance against several plant pathogens. We previously cloned the wheat powdery mildew resistance gene Pm21, which encodes a coiled-coil (CC) NLR that confers broad-spectrum resistance against Blumeria graminis f. sp. tritici. Here, we report comprehensive biochemical and functional analyses of Pm21 CC domain in Nicotiana benthamiana. Transient overexpression assay suggested that only the extended CC (eCC, amino acid residues 1-159) domain has cell-death-inducing activity, whereas the CC-containing truncations, including CC-NB and CC-NB-LRR, do not induce cell-death responses. Coimmunoprecipitation (Co-IP) assay showed that the eCC domain self-associates and interacts with the NB and LRR domains in planta. These results imply that the activity of the eCC domain is inhibited by the intramolecular interactions of different domains in the absence of pathogens. We found that the LRR domain plays a crucial role in D491V-mediated full-length (FL) Pm21 autoactivation. Some mutations in the CC domain leading to the loss of Pm21 resistance to powdery mildew impaired the CC activity of cell-death induction. Two mutations (R73Q and E80K) interfered with D491V-mediated Pm21 autoactivation without affecting the cell-death-inducing activity of the eCC domain. Notably, some susceptible mutants harbouring mutations in the CC domain still exhibited cell-death-inducing activity. Taken together, these results implicate the CC domain of Pm21 in cell-death signalling and disease-resistance signalling, which are potentially independent of each other.

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.