Immunodiversity of the Arabidopsis ZAR1 NLR Is Conveyed by Receptor-Like Cytoplasmic Kinase Sensors

Receptor-like cytoplasmic kinases: orchestrating plant cellular communication

The receptor-like kinase (RLK) family of receptors and the associated receptor-like cytoplasmic kinases (RLCKs) have expanded in plants because of selective pressure from environmental stress and evolving pathogens. RLCKs link pathogen perception to activation of coping mechanisms. RLK-RLCK modules regulate hormone synthesis and responses, reactive oxygen species (ROS) production, Ca2+ signaling, activation of mitogen-activated protein kinase (MAPK), and immune gene expression, all of which contribute to immunity. Some RLCKs integrate responses from multiple receptors recognizing distinct ligands. RLKs/RLCKs and nucleotide-binding domain, leucine-rich repeats (NLRs) were found to synergize, demonstrating the intertwined genetic network in plant immunity. Studies in arabidopsis (Arabidopsis thaliana) have provided paradigms about RLCK functions, but a lack of understanding of crop RLCKs undermines their application. In this review, we summarize current understanding of the diverse functions of RLCKs, based on model systems and observations in crop species, and the emerging role of RLCKs in pathogen and abiotic stress response signaling.

Interplay between cell-surface receptor and intracellular NLR-mediated immune responses

The functional link between cell-surface receptors and intracellular NLR immune receptors is a critical aspect of plant immunity. To establish disease, successful pathogens have evolved mechanisms to suppress cell-surface immune signalling. In response, plants have adapted by evolving NLRs that recognize pathogen effectors involved in this suppression, thereby counteracting their immune-suppressing function. This ongoing co-evolutionary struggle has seemingly resulted in intertwined signalling pathways in some plant species, where NLRs form a separate signalling branch downstream of activated cell-surface receptor complexes essential for full immunity. Understanding these interconnected receptor networks could lead to novel strategies for developing durable disease resistance.

Ptr1 and ZAR1 immune receptors confer overlapping and distinct bacterial pathogen effector specificities

Some nucleotide-binding and leucine-rich repeat receptors (NLRs) indirectly detect pathogen effectors by monitoring their host targets. In Arabidopsis thaliana, RIN4 is targeted by multiple sequence-unrelated effectors and activates immune responses mediated by RPM1 and RPS2. These effectors trigger cell death in Nicotiana benthamiana, but the corresponding NLRs have yet not been identified. To identify N. benthamiana NLRs (NbNLRs) that recognize Arabidopsis RIN4-targeting effectors, we conducted a rapid reverse genetic screen using an NbNLR VIGS library. We identified that the N. benthamiana homolog of Ptr1 (Pseudomonas tomato race 1) recognizes the Pseudomonas effectors AvrRpt2, AvrRpm1, and AvrB. We demonstrated that recognition of the Xanthomonas effector AvrBsT and the Pseudomonas effector HopZ5 is conferred independently by the N. benthamiana homolog of Ptr1 and ZAR1. Interestingly, the recognition of HopZ5 and AvrBsT is contributed unequally by Ptr1 and ZAR1 in N. benthamiana and Capsicum annuum. In addition, we showed that the RLCK XII family protein JIM2 is required for the NbZAR1-dependent recognition of AvrBsT and HopZ5. The recognition of sequence-unrelated effectors by NbPtr1 and NbZAR1 provides an additional example of convergently evolved effector recognition. Identification of key components involved in Ptr1 and ZAR1-mediated immunity could reveal unique mechanisms of expanded effector recognition.

Utilising natural diversity of kinases to rationally engineer interactions with the angiosperm immune receptor ZAR1

The highly conserved angiosperm immune receptor HOPZ-ACTIVATED RESISTANCE1 (ZAR1) recognises the activity of diverse pathogen effector proteins by monitoring the ZED1-related kinase (ZRK) family. Understanding how ZAR1 achieves interaction specificity for ZRKs may allow for the expansion of the ZAR1-kinase recognition repertoire to achieve novel pathogen recognition outside of model species. We took advantage of the natural diversity of Arabidopsis thaliana kinases to probe the ZAR1-kinase interaction interface and found that A. thaliana ZAR1 (AtZAR1) can interact with most ZRKs, except ZRK7. We found evidence of alternative splicing of ZRK7, resulting in a protein that can interact with AtZAR1. Despite high sequence conservation of ZAR1, interspecific ZAR1-ZRK pairings resulted in the autoactivation of cell death. We showed that ZAR1 interacts with a greater diversity of kinases than previously thought, while still possessing the capacity for specificity in kinase interactions. Finally, using AtZAR1-ZRK interaction data, we rationally increased ZRK10 interaction strength with AtZAR1, demonstrating the feasibility of the rational design of a ZAR1-interacting kinase. Overall, our findings advance our understanding of the rules governing ZAR1 interaction specificity, with promising future directions for expanding ZAR1 immunodiversity.

ZED1-related kinase 13 is required for resistance against Pseudoidium neolycopersici in Arabidopsis accession Bla-6

To explore specific components of resistance against the tomato-adapted powdery mildew pathogen Pseudoidium neolycopersici (On) in the model plant Arabidopsis, we performed a disease assay in 123 accessions. When testing the resistance in the F1 from crossings between resistant accessions with susceptible Col-0 or Sha, only the progeny of the cross between accession Bla-6 and Col-0 displayed a completely resistant phenotype. The resistance in Bla-6 is known to be specific for Pseudoidium neolycopersici. QTL analysis and fine-mapping through several rounds of recombinant screenings allowed us to locate a major resistance QTL in an interval on chromosome 1, containing two candidate genes and an intergenic insertion. Via CRISPR/Cas9 targeted mutagenesis, we could show that knocking out the ZED-1 RELATED KINASE 13 (ZRK13) gene compromised the On resistance in Bla-6. Several polymorphisms are observed in the ZRK13 allelic variant of Bla-6 when compared to the Col-0 protein.

The Arabidopsis WRR4A and WRR4B paralogous NLR proteins both confer recognition of multiple Albugo candida effectors

The oomycete Albugo candida causes white blister rust, an important disease of Brassica crops. Distinct races of A. candida are defined by their capacity to infect different host plant species. Each A. candida race encodes secreted proteins with a CX₂ CX₅ G ('CCG') motif that are polymorphic and show presence/absence variation, and are therefore candidate effectors. The White Rust Resistance 4 (WRR4) locus in Arabidopsis thaliana accession Col-0 contains three genes that encode intracellular nucleotide-binding domain leucine-rich repeat immune receptors. The Col-0 alleles of WRR4A and WRR4B confer resistance to multiple A. candida races, although both WRR4A and WRR4B can be overcome by the Col-0-virulent race 4 isolate AcEx1. Comparison of CCG candidate effectors in avirulent and virulent races, and transient co-expression of CCG effectors from four A. candida races in Nicotiana sp. or A. thaliana, revealed CCG effectors that trigger WRR4A- or WRR4B-dependent hypersensitive responses. We found eight WRR4A-recognised CCGs and four WRR4B-recognised CCGs, the first recognised proteins from A. candida for which the cognate immune receptors in A. thaliana are known. This multiple recognition capacity potentially explains the broad-spectrum resistance to several A. candida races conferred by WRR4 paralogues. We further show that of five tested CCGs, three confer enhanced disease susceptibility when expressed in planta, consistent with A. candida CCG proteins being effectors.

Plant NLRs: Evolving with pathogen effectors and engineerable to improve resistance

Pathogens are important threats to many plants throughout their lifetimes. Plants have developed different strategies to overcome them. In the plant immunity system, nucleotide-binding domain and leucine-rich repeat-containing proteins (NLRs) are the most common components. And recent studies have greatly expanded our understanding of how NLRs function in plants. In this review, we summarize the studies on the mechanism of NLRs in the processes of effector recognition, resistosome formation, and defense activation. Typical NLRs are divided into three groups according to the different domains at their N termini and function in interrelated ways in immunity. Atypical NLRs contain additional integrated domains (IDs), some of which directly interact with pathogen effectors. Plant NLRs evolve with pathogen effectors and exhibit specific recognition. Meanwhile, some NLRs have been successfully engineered to confer resistance to new pathogens based on accumulated studies. In summary, some pioneering processes have been obtained in NLR researches, though more questions arise as a result of the huge number of NLRs. However, with a broadened understanding of the mechanism, NLRs will be important components for engineering in plant resistance improvement.

ZAR1: Guardian of plant kinases

A key facet of innate immunity in plants entails the recognition of pathogen "effector" virulence proteins by host Nucleotide-Binding Leucine-Rich Repeat Receptors (NLRs). Among characterized NLRs, the broadly conserved ZAR1 NLR is particularly remarkable due to its capacity to recognize at least six distinct families of effectors from at least two bacterial genera. This expanded recognition spectrum is conferred through interactions between ZAR1 and a dynamic network of two families of Receptor-Like Cytoplasmic Kinases (RLCKs): ZED1-Related Kinases (ZRKs) and PBS1-Like Kinases (PBLs). In this review, we survey the history of functional studies on ZAR1, with an emphasis on how the ZAR1-RLCK network functions to trap diverse effectors. We discuss 1) the dynamics of the ZAR1-associated RLCK network; 2) the specificity between ZRKs and PBLs; and 3) the specificity between effectors and the RLCK network. We posit that the shared protein fold of kinases and the switch-like properties of their interactions make them ideal effector sensors, enabling ZAR1 to act as a broad spectrum guardian of host kinases.

Diversity, Evolution, and Function of Pseudomonas syringae Effectoromes

Pseudomonas syringae is an evolutionarily diverse bacterial species complex and a preeminent model for the study of plant-pathogen interactions due in part to its remarkably broad host range. A critical feature of P. syringae virulence is the employment of suites of type III secreted effector (T3SE) proteins, which vary widely in composition and function. These effectors act on a variety of plant intracellular targets to promote pathogenesis but can also be avirulence factors when detected by host immune complexes. In this review, we survey the phylogenetic diversity (PD) of the P. syringae effectorome, comprising 70 distinct T3SE families identified to date, and highlight how avoidance of host immune detection has shaped effectorome diversity through functional redundancy, diversification, and horizontal transfer. We present emerging avenues for research and novel insights that can be gained via future investigations of plant-pathogen interactions through the fusion of large-scale interaction screens and phylogenomic approaches.

Two plant NLR proteins confer strain-specific resistance conditioned by an effector from Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa) causes bacterial canker, a devastating disease threatening the Actinidia fruit industry. In a search for non-host resistance genes against Psa, we found that the nucleotide-binding leucine-rich repeat receptor (NLR) protein ZAR1 from both Arabidopsis and Nicotiana benthamiana (Nb) recognizes HopZ5 and triggers cell death. The recognition requires ZED1 in Arabidopsis and JIM2 in Nb plants, which are members of the ZRK pseudokinases and known components of the ZAR1 resistosome. Surprisingly, Arabidopsis ZAR1 and RPM1, another NLR known to recognize HopZ5, confer disease resistance to HopZ5 in a strain-specific manner. Thus, ZAR1, but not RPM1, is solely required for resistance to P. s. maculicola ES4326 (Psm) carrying hopZ5, whereas RPM1 is primarily required for resistance to P. s. tomato DC3000 (Pst) carrying hopZ5. Furthermore, the ZAR1-mediated resistance to Psm hopZ5 in Arabidopsis is insensitive to SOBER1, which encodes a deacetylase known to suppress the RPM1-mediated resistance to Pst hopZ5. In addition, hopZ5 enhances P. syringae virulence in the absence of ZAR1 or RPM1, and that SOBER1 abolishes such virulence function. Together the study suggests that ZAR1 may be used for improving Psa resistance in Actinidia and uncovers previously unknown complexity of effector-triggered immunity and effector-triggered virulence.

TIR domains of plant immune receptors are 2',3'-cAMP/cGMP synthetases mediating cell death

2',3'-cAMP is a positional isomer of the well-established second messenger 3',5'-cAMP, but little is known about the biology of this noncanonical cyclic nucleotide monophosphate (cNMP). Toll/interleukin-1 receptor (TIR) domains of nucleotide-binding leucine-rich repeat (NLR) immune receptors have the NADase function necessary but insufficient to activate plant immune responses. Here, we show that plant TIR proteins, besides being NADases, act as 2',3'-cAMP/cGMP synthetases by hydrolyzing RNA/DNA. Structural data show that a TIR domain adopts distinct oligomers with mutually exclusive NADase and synthetase activity. Mutations specifically disrupting the synthetase activity abrogate TIR-mediated cell death in Nicotiana benthamiana (Nb), supporting an important role for these cNMPs in TIR signaling. Furthermore, the Arabidopsis negative regulator of TIR-NLR signaling, NUDT7, displays 2',3'-cAMP/cGMP but not 3',5'-cAMP/cGMP phosphodiesterase activity and suppresses cell death activity of TIRs in Nb. Our study identifies a family of 2',3'-cAMP/cGMP synthetases and establishes a critical role for them in plant immune responses.

From plant immunity to crop disease resistance

Plant diseases caused by diverse pathogens lead to serious reduction in crop yield and threaten food security worldwide. Genetic improvement of plant immunity is considered as the most effective and sustainable approach to control crop diseases. In the last decade, our understanding of plant immunity at both molecular and genomic levels has improved greatly. Combined with advances in biotechnologies, particularly CRISPR/Cas9-based genome editing, we can now rapidly identify new resistance genes and engineer disease resistance crop plants like never before. In this review, we summarize the current knowledge of plant immunity and outline existing and new strategies for disease resistance improvement in crop plants. We also discuss existing challenges in this field and suggest directions for future studies.

Seeing is believing: Exploiting advances in structural biology to understand and engineer plant immunity

Filamentous plant pathogens cause disease in numerous economically important crops. These pathogens secrete virulence proteins, termed effectors, that modulate host cellular processes and promote infection. Plants have evolved immunity receptors that detect effectors and activate defence pathways, resulting in resistance to the invading pathogen. This leads to an evolutionary arms race between pathogen and host that is characterised by highly diverse effector repertoires in plant pathogens. Here, we review the recent advances in understanding host-pathogen co-evolution provided by the structural determination of effectors alone, and in complex with immunity receptors. We highlight the use of recent advances in structural prediction within this field and its role for future development of designer resistance proteins.

Metaeffector interactions modulate the type III effector-triggered immunity load of Pseudomonas syringae

The bacterial plant pathogen Pseudomonas syringae requires type III secreted effectors (T3SEs) for pathogenesis. However, a major facet of plant immunity entails the recognition of a subset of P. syringae’s T3SEs by intracellular host receptors in a process called Effector-Triggered Immunity (ETI). Prior work has shown that ETI-eliciting T3SEs are pervasive in the P. syringae species complex raising the question of how P. syringae mitigates its ETI load to become a successful pathogen. While pathogens can evade ETI by T3SE mutation, recombination, or loss, there is increasing evidence that effector-effector (a.k.a., metaeffector) interactions can suppress ETI. To study the ETI-suppression potential of P. syringae T3SE repertoires, we compared the ETI-elicitation profiles of two genetically divergent strains: P. syringae pv. tomato DC3000 (PtoDC3000) and P. syringae pv. maculicola ES4326 (PmaES4326), which are both virulent on Arabidopsis thaliana but harbour largely distinct effector repertoires. Of the 529 T3SE alleles screened on A. thaliana Col-0 from the P. syringae T3SE compendium (PsyTEC), 69 alleles from 21 T3SE families elicited ETI in at least one of the two strain backgrounds, while 50 elicited ETI in both backgrounds, resulting in 19 differential ETI responses including two novel ETI-eliciting families: AvrPto1 and HopT1. Although most of these differences were quantitative, three ETI responses were completely absent in one of the pathogenic backgrounds. We performed ETI suppression screens to test if metaeffector interactions contributed to these ETI differences, and found that HopQ1a suppressed AvrPto1m-mediated ETI, while HopG1c and HopF1g suppressed HopT1b-mediated ETI. Overall, these results show that P. syringae strains leverage metaeffector interactions and ETI suppression to overcome the ETI load associated with their native T3SE repertoires.

The origin and evolution of a plant resistosome

The nucleotide-binding, leucine-rich receptor (NLR) protein HOPZ-ACTIVATED RESISTANCE 1 (ZAR1), an immune receptor, interacts with HOPZ-ETI-DEFICIENT 1 (ZED1)-related kinases (ZRKs) and AVRPPHB SUSCEPTIBLE 1-like proteins to form a pentameric resistosome, triggering immune responses. Here, we show that ZAR1 emerged through gene duplication and that ZRKs were derived from the cell surface immune receptors wall-associated protein kinases (WAKs) through the loss of the extracellular domain before the split of eudicots and monocots during the Jurassic period. Many angiosperm ZAR1 orthologs, but not ZAR1 paralogs, are capable of oligomerization in the presence of AtZRKs and triggering cell death, suggesting that the functional ZAR1 resistosome might have originated during the early evolution of angiosperms. Surprisingly, inter-specific pairing of ZAR1 and AtZRKs sometimes results in the formation of a resistosome in the absence of pathogen stimulation, suggesting within-species compatibility between ZAR1 and ZRKs as a result of co-evolution. Numerous concerted losses of ZAR1 and ZRKs occurred in angiosperms, further supporting the ancient co-evolution between ZAR1 and ZRKs. Our findings provide insights into the origin of new plant immune surveillance networks.

The Arabidopsis effector-triggered immunity landscape is conserved in oilseed crops

The bacterial phytopathogen Pseudomonas syringae causes disease on a wide array of plants, including the model plant Arabidopsis thaliana and its agronomically important relatives in the Brassicaceae family. To cause disease, P. syringae delivers effector proteins into plant cells through a type III secretion system. In response, plant nucleotide-binding leucine-rich repeat proteins recognize specific effectors and mount effector-triggered immunity (ETI). While ETI is pervasive across A. thaliana, with at least 19 families of P. syringae effectors recognized in this model species, the ETI landscapes of crop species have yet to be systematically studied. Here, we investigated the conservation of the A. thaliana ETI landscape in two closely related oilseed crops, Brassica napus (canola) and Camelina sativa (false flax). We show that the level of immune conservation is inversely related to the degree of evolutionary divergence from A. thaliana, with the more closely related C. sativa losing ETI responses to only one of the 19 P. syringae effectors tested, while the more distantly related B. napus loses ETI responses to four effectors. In contrast to the qualitative conservation of immune response, the quantitative rank order is not as well-maintained across the three species and diverges increasingly with evolutionary distance from A. thaliana. Overall, our results indicate that the A. thaliana ETI profile is qualitatively conserved in oilseed crops, but quantitatively distinct.

The small molecule Zaractin activates ZAR1-mediated immunity in Arabidopsis

Pathogenic effector proteins use a variety of enzymatic activities to manipulate host cellular proteins and favor the infection process. However, these perturbations can be sensed by nucleotide-binding leucine-rich-repeat (NLR) proteins to activate effector-triggered immunity (ETI). Here we have identified a small molecule (Zaractin) that mimics the immune eliciting activity of the Pseudomonas syringae type III secreted effector (T3SE) HopF1r and show that both HopF1r and Zaractin activate the same NLR-mediated immune pathway in Arabidopsis Our results demonstrate that the ETI-inducing action of pathogenic effectors can be harnessed to identify synthetic activators of the eukaryotic immune system.

How activated NLRs induce anti-microbial defenses in plants

Plants utilize cell-surface localized and intracellular leucine-rich repeat (LRR) immune receptors to detect pathogens and to activate defense responses, including transcriptional reprogramming and the initiation of a form of programmed cell death of infected cells. Cell death initiation is mainly associated with the activation of nucleotide-binding LRR receptors (NLRs). NLRs recognize the presence or cellular activity of pathogen-derived virulence proteins, so-called effectors. Effector-dependent NLR activation leads to the formation of higher order oligomeric complexes, termed resistosomes. Resistosomes can either form potential calcium-permeable cation channels at cellular membranes and initiate calcium influxes resulting in activation of immunity and cell death or function as NADases whose activity is needed for the activation of downstream immune signaling components, depending on the N-terminal domain of the NLR protein. In this mini-review, the current knowledge on the mechanisms of NLR-mediated cell death and resistance pathways during plant immunity is discussed.

Pseudokinases repurpose flexibility signatures associated with the protein kinase fold for noncatalytic roles

The bilobal protein kinase-like fold in pseudokinases lack one or more catalytic residues, conserved in canonical protein kinases, and are considered enzymatically deficient. Tertiary structures of pseudokinases reveal that their loops topologically equivalent to activation segments of kinases adopt contracted configurations, which is typically extended in active conformation of kinases. Herein, anisotropic network model based normal mode analysis (NMA) was conducted on 51 active conformation structures of protein kinases and 26 crystal structures of pseudokinases. Our observations indicate that although backbone fluctuation profiles are similar for individual kinase-pseudokinase families, low intensity mean square fluctuations in pseudo-activation segment and other sub-structures impart rigidity to pseudokinases. Analyses of collective motions from functional modes reveal that pseudokinases, compared to active kinases, undergo distinct conformational transitions using the same structural fold. All-atom NMA of protein kinase-pseudokinase pairs from each family, sharing high amino acid sequence identities, yielded distinct community clusters, partitioned by residues exhibiting highly correlated fluctuations. It appears that atomic fluctuations from equivalent activation segments guide community membership and network topologies for respective kinase and pseudokinase. Our findings indicate that such adaptations in backbone and side-chain fluctuations render pseudokinases competent for catalysis-independent roles.

Regulation of Cell Death and Signaling by Pore-Forming Resistosomes

Nucleotide-binding leucine-rich repeat receptors (NLRs) are the largest class of immune receptors in plants. They play a key role in the plant surveillance system by monitoring pathogen effectors that are delivered into the plant cell. Recent structural biology and biochemical analyses have uncovered how NLRs are activated to form oligomeric resistosomes upon the recognition of pathogen effectors. In the resistosome, the signaling domain of the NLR is brought to the center of a ringed structure to initiate immune signaling and regulated cell death (RCD). The N terminus of the coiled-coil (CC) domain of the NLR protein HOPZ-ACTIVATED RESISTANCE 1 likely forms a pore in the plasma membrane to trigger RCD in a way analogous to animal pore-forming proteins that trigger necroptosis or pyroptosis. NLRs that carry TOLL-INTERLEUKIN1-RECEPTOR as a signaling domain may also employ pore-forming resistosomes for RCD execution. In addition, increasing evidence supports intimate connections between NLRs and surface receptors in immune signaling. These new findings are rapidly advancing our understanding of the plant immune system.

The ETS-ETI cycle: evolutionary processes and metapopulation dynamics driving the diversification of pathogen effectors and host immune factors

The natural diversity of pathogen effectors and host immune components represents a snapshot of the underlying evolutionary processes driving the host-pathogen arms race. In plants, this arms race is manifested by an ongoing cycle of disease and resistance driven by pathogenic effectors that promote disease (effector-triggered susceptibility; ETS) and plant resistance proteins that recognize effector activity to trigger immunity (effector-triggered immunity; ETI). Here we discuss how this ongoing ETS-ETI cycle has shaped the natural diversity of both plant resistance proteins and pathogen effectors. We focus on the evolutionary forces that drive the diversification of the molecules that determine the outcome of plant-pathogen interactions and introduce the concept of metapopulation dynamics (i.e., the introduction of genetic variation from conspecific organisms in different populations) as an alternative mechanism that can introduce and maintain diversity in both host and pathogen populations.

The ZAR1 resistosome is a calcium-permeable channel triggering plant immune signaling

Nucleotide-binding, leucine-rich repeat receptors (NLRs) are major immune receptors in plants and animals. Upon activation, the Arabidopsis NLR protein ZAR1 forms a pentameric resistosome in vitro and triggers immune responses and cell death in plants. In this study, we employed single-molecule imaging to show that the activated ZAR1 protein can form pentameric complexes in the plasma membrane. The ZAR1 resistosome displayed ion channel activity in Xenopus oocytes in a manner dependent on a conserved acidic residue Glu11 situated in the channel pore. Pre-assembled ZAR1 resistosome was readily incorporated into planar lipid-bilayers and displayed calcium-permeable cation-selective channel activity. Furthermore, we show that activation of ZAR1 in the plant cell led to Glu11-dependent Ca²⁺ influx, perturbation of subcellular structures, production of reactive oxygen species, and cell death. The results thus support that the ZAR1 resistosome acts as a calcium-permeable cation channel to trigger immunity and cell death.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

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.

Cutting the line: manipulation of plant immunity by bacterial type III effector proteases

Pathogens and their hosts are engaged in an evolutionary arms race. Pathogen-derived effectors promote virulence by targeting components of a host's innate immune system, while hosts have evolved proteins that sense effectors and trigger a pathogen-specific immune response. Many bacterial effectors are translocated into host cells using type III secretion systems. Type III effector proteases irreversibly modify host proteins by cleavage of peptide bonds and are prevalent among both plant and animal bacterial pathogens. In plants, the study of model effector proteases has yielded important insights into the virulence mechanisms employed by pathogens to overcome their host's immune response, as well as into the mechanisms deployed by their hosts to detect these effector proteases and counteract their effects. In recent years, the study of a larger number of effector proteases, across a wider range of pathogens, has yielded novel insights into their functions and recognition. One key limitation that remains is the lack of methods to detect protease cleavage at the proteome-wide level. We review known substrates and mechanisms of plant pathogen type III effector proteases and compare their functions with those of known type III effector proteases of mammalian pathogens. Finally, we discuss approaches to uncover their function on a system-wide level.

A natural diversity screen in Arabidopsis thaliana reveals determinants for HopZ1a recognition in the ZAR1-ZED1 immune complex

Pathogen pressure on hosts can lead to the evolution of genes regulating the innate immune response. By characterizing naturally occurring polymorphisms in immune receptors, we can better understand the molecular determinants of pathogen recognition. ZAR1 is an ancient Arabidopsis thaliana NLR (Nucleotide-binding [NB] Leucine-rich-repeat [LRR] Receptor) that recognizes multiple secreted effector proteins from the pathogenic bacteria Pseudomonas syringae and Xanthomonas campestris through its interaction with receptor-like cytoplasmic kinases (RLCKs). ZAR1 was first identified for its role in recognizing P. syringae effector HopZ1a, through its interaction with the RLCK ZED1. To identify additional determinants of HopZ1a recognition, we performed a computational screen for ecotypes from the 1001 Genomes project that were likely to lack HopZ1a recognition, and tested ~300 ecotypes. We identified ecotypes containing polymorphisms in ZAR1 and ZED1. Using our previously established Nicotiana benthamiana transient assay and Arabidopsis ecotypes, we tested for the effect of naturally occurring polymorphisms on ZAR1 interactions and the immune response. We identified key residues in the NB or LRR domain of ZAR1 that impact the interaction with ZED1. We demonstrate that natural diversity combined with functional assays can help define the molecular determinants and interactions necessary to regulate immune induction in response to pathogens.