The Coiled-Coil and Leucine-Rich Repeat Domain of the Potyvirus Resistance Protein Pvr4 Has a Distinct Role in Signaling and Pathogen Recognition

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.

The dynamic TaRACK1B-TaSGT1-TaHSP90 complex modulates NLR-protein-mediated antiviral immunity in wheat

Nucleotide-binding leucine-rich repeat (NLR) proteins contribute widely to plant immunity by regulating defense mechanisms through the elicitation of a hypersensitive response (HR). Here, we find that TaRACK1B (the receptor for activated C-kinase 1B) regulates wheat immune response against Chinese wheat mosaic virus (CWMV) infection. TaRACK1B recruits TaSGT1 and TaHSP90 to form the TaRACK1B-TaSGT1-TaHSP90 complex. This complex is essential for maintaining NLR proteins' stability (TaRGA5-like and TaRGH1A-like) in order to control HR activation and inhibit viral infection. However, the cysteine-rich protein encoded by CWMV can disrupt TaRACK1B-TaSGT1-TaHSP90 complex formation, leading to the reduction of NLR-protein stability and suppression of HR activation, thus promoting CWMV infection. Interestingly, the 7K protein of wheat yellow mosaic virus also interferes with this antiviral immunity. Our findings show a shared viral counter-defense strategy whereby two soil-borne viruses may disrupt the TaRACK1B-TaSGT1-TaHSP90 complex, suppressing NLR-protein-mediated broad-spectrum antiviral immunity and promoting viral infection in wheat.

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.

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.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

The melon Fom-1-Prv resistance gene pair: Correlated spatial expression and interaction with a viral protein

The head-to-head oriented pair of melon resistance genes, Fom-1 and Prv, control resistance to Fusarium oxysporum races 0 and 2 and papaya ringspot virus (PRSV), respectively. They encode, via several RNA splice variants, TIR-NBS-LRR proteins, and Prv has a C-terminal extra domain with a second NBS homologous sequence. In other systems, paired R-proteins were shown to operate by "labor division," with one protein having an extra integrated domain that directly binds the pathogen's Avr factor, and the second protein executing the defense response. We report that the expression of the two genes in two pairs of near-isogenic lines was higher in the resistant isoline and inducible by F. oxysporum race 2 but not by PRSV. The intergenic DNA region separating the coding sequences of the two genes acted as a bi-directional promoter and drove GUS expression in transgenic melon roots and transgenic tobacco plants. Expression of both genes was strong in melon root tips, around the root vascular cylinder, and the phloem and xylem parenchyma of tobacco stems and petioles. The pattern of GUS expression suggests coordinated expression of the two genes. In agreement with the above model, Prv's extra domain was shown to interact with the cylindrical inclusion protein of PRSV both in yeast cells and in planta.

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.

Combination of transcriptomic, proteomic, and degradomic profiling reveals common and distinct patterns of pathogen-induced cell death in maize

Regulated cell death (RCD) is crucial for plant development, as well as in decision-making in plant-microbe interactions. Previous studies revealed components of the molecular network controlling RCD, including different proteases. However, the identity, the proteolytic network as well as molecular components involved in the initiation and execution of distinct plant RCD processes, still remain largely elusive. In this study, we analyzed the transcriptome, proteome, and N-terminome of Zea mays leaves treated with the Xanthomonas effector avrRxo1, the mycotoxin Fumonisin B1 (FB1), or the phytohormone salicylic acid (SA) to dissect plant cellular processes related to cell death and plant immunity. We found highly distinct and time-dependent biological processes being activated on transcriptional and proteome levels in response to avrRxo1, FB1, and SA. Correlation analysis of the transcriptome and proteome identified general, as well as trigger-specific markers for cell death in Zea mays. We found that proteases, particularly papain-like cysteine proteases, are specifically regulated during RCD. Collectively, this study characterizes distinct RCD responses in Z. mays and provides a framework for the mechanistic exploration of components involved in the initiation and execution of cell death.

Protein stability governed by α1-2 helices in Pvr4 is essential for localization and cell death

Plant nucleotide-binding domain leucine-rich-repeat receptor (NLR) confers disease resistance to various pathogens by recognizing effectors derived from the pathogen. Previous studies have shown that overexpression of the CC domain in several NLRs triggers cell death, implying that the CC domain plays an important role as a signaling module. However, how CC domain transduces immune signals remains largely unknown. A Potyvirus-resistant NLR protein, Pvr4, possesses a CC domain (CCPvr4 ) that induces cell death upon transient overexpression in Nicotiana benthamiana. In this study, loss-of-function mutants were generated by error-prone PCR-based random mutagenesis to understand the molecular mechanisms underlying CCPvr4 -mediated cell death. Cell biology and biochemical studies revealed that M16 and Q52 in the α1 and α2 helices, respectively, are crucial for protein stability, and mutation of these residues disrupts localization to the plasma membrane and oligomerization activity. The increase of the protein stability of these mutants by tagging a green fluorescent protein (GFP) variant led to restoration of cell death-inducing activity and plasma membrane localization. Another mutant, I7E in the very N-terminal region, lost cell death-inducing activity by weakening the interaction with plasma membrane H+ -ATPase compared to CCPvr4 , although the protein remained in the plasma membrane. Moreover, most of the mutated residues are on the outer surface of the funnel shape in the predicted pentameric CCPvr4 , implying that the disordered N-terminal region plays a crucial role in association with PMA as well as targeting to the plasma membrane. This work could provide insights into the molecular mechanisms of cell death induced by NLR immune receptors.

The Hypersensitive Response to Plant Viruses

Plant proteins with domains rich in leucine repeats play important roles in detecting pathogens and triggering defense reactions, both at the cellular surface for pattern-triggered immunity and in the cell to ensure effector-triggered immunity. As intracellular parasites, viruses are mostly detected intracellularly by proteins with a nucleotide binding site and leucine-rich repeats but receptor-like kinases with leucine-rich repeats, known to localize at the cell surface, have also been involved in response to viruses. In the present review we report on the progress that has been achieved in the last decade on the role of these leucine-rich proteins in antiviral immunity, with a special focus on our current understanding of the hypersensitive response.

Wheat yellow mosaic virus NIb targets TaVTC2 to elicit broad-spectrum pathogen resistance in wheat

GDP-L-galactose phosphorylase (VTC2) catalyses the conversion of GDP-L-galactose to L-galactose-1-P, a vital step of ascorbic acid (AsA) biosynthesis in plants. AsA is well known for its function in the amelioration of oxidative stress caused by most pathogen infection, but its function against viral infection remains unclear. Here, we have identified a VTC2 gene in wheat named as TaVTC2 and investigated its function in association with the wheat yellow mosaic virus (WYMV) infection. Our results showed that overexpression of TaVTC2 significantly increased viral accumulation, whereas knocking down TaVTC2 inhibited the viral infection in wheat, suggesting a positive regulation on viral infection by TaVTC2. Moreover, less AsA was produced in TaVTC2 knocking down plants (TaVTC2-RNAi) which due to the reduction in TaVTC2 expression and subsequently in TaVTC2 activity, resulting in a reactive oxygen species (ROS) burst in leaves. Furthermore, the enhanced WYMV resistance in TaVTC2-RNAi plants was diminished by exogenously applied AsA. We further demonstrated that WYMV NIb directly bound to TaVTC2 and inhibited TaVTC2 enzymatic activity in vitro. The effect of TaVTC2 on ROS scavenge was suppressed by NIb in a dosage-dependent manner, indicating the ROS scavenging was highly regulated by the interaction of TaVTC2 with NIb. Furthermore, TaVTC2 RNAi plants conferred broad-spectrum disease resistance. Therefore, the data indicate that TaVTC2 recruits WYMV NIb to down-regulate its own enzymatic activity, reducing AsA accumulation to elicit a burst of ROS which confers the resistance to WYMV infection. Thus, a new mechanism of the formation of plant innate immunity was proposed.

Leaf transcriptome analysis of Medicago ruthenica revealed its response and adaptive strategy to drought and drought recovery

BACKGROUND

Drought is one of the main causes of losses in forage crop yield and animal production. Medicago ruthenica (L.) cv. Zhilixing is a high-yielding alfalfa cultivar also known for its high tolerance to drought. We analyzed the transcriptome profile of this cultivar throughout drought stress and recovery and we were able to describe its phased response through the expression profiles of overlapping gene networks and drought-specific genes.

RESULTS

The ABA and auxin signal transduction pathways are overlapping pathways in response to drought and drought recovery in forage crops. Medicago ruthenica (L.) cv. Zhilixing adopts different strategies at different degrees of drought stress. On the 9th day of drought, transcriptional regulations related to osmoregulation are enhanced mainly through increased activities of carbohydrate and amino acid metabolism, while photosynthetic activities were reduced to slow down growth. With drought prolonging, on the 12th day of drought, the synthesis of proline and other stored organic substances was suppressed in general. After recovery, Medicago ruthenica synthesizes flavonoids through the flavonoid biosynthesis pathway to remove accumulated ROS and repair the oxidative damage from water stress. In addition, the regulation of circadian rhythm seems to accelerate the drought recovery process.

CONCLUSIONS

Medicago ruthenica adapts to drought by regulating the osmoregulatory system and photosynthesis, which appears to involve the ABA and auxin signaling pathways as key regulators. Furthermore, the synthesis of flavonoids and the regulation of the circadian rhythm can accelerate the recovery process. These results enriched our knowledge of molecular responses to drought and drought recovery in Medicago ruthenica and provide useful information for the development of new legume forage grass varieties with improved adaptability to drought stress.

Tsw - A case study on structure-function puzzles in plant NLRs with unusually large LRR domains

Plant disease immunity heavily depends on the recognition of plant pathogens and the subsequent activation of downstream immune pathways. Nod-like receptors are often crucial in this process. Tsw, a Nod-like resistance gene from Capsicum chinense conferring resistance against Tomato spotted wilt virus (TSWV), belongs to the small group of Nod-like receptors with unusually large LRR domains. While typical protein domain dimensions rarely exceed 500 amino acids due to stability constraints, the LRR of these unusual NLRs range from 1,000 to 3,400 amino acids and contain over 30 LRR repeats. The presence of such a multitude of repeats in one protein is also difficult to explain considering protein functionality. Interactions between the LRR and the other NLR domains (CC, TIR, NBS) take place within the first 10 LRR repeats, leaving the function of largest part of the LRR structure unexplained. Herein we discuss the structural modeling limits and various aspects of the structure-function relation conundrums of large LRRs focusing on Tsw, and raise questions regarding its recognition of its effector NSs and the possible inhibition on other domains as seen in other NLRs.

Plasma membrane-localized plant immune receptor targets H+ -ATPase for membrane depolarization to regulate cell death

The hypersensitive response (HR) is a robust immune response mediated by nucleotide-binding, leucine-rich repeat receptors (NLRs). However, the early molecular event that links activated NLRs to cell death is unclear. Here, we demonstrate that NLRs target plasma membrane H⁺ -ATPases (PMAs) that generate electrochemical potential, an essential component of living cells, across the plasma membrane. CC^A 309, an autoactive N-terminal domain of a coiled-coil NLR (CNL) in pepper, is associated with PMAs. Silencing or overexpression of PMAs reversibly affects cell death induced by CC^A 309 in Nicotiana benthamiana. CC^A 309-induced extracellular alkalization causes plasma membrane depolarization, followed by cell death. Coimmunoprecipitation analyses suggest that CC^A 309 inhibits PMA activation by preoccupying the dephosphorylated penultimate threonine residue of PMA. Moreover, pharmacological experiments using fusicoccin, an irreversible PMA activator, showed that inhibition of PMAs contributes to CNL-type (but not Toll interleukin-1 receptor NLR-type) resistance protein-induced cell death. We suggest PMAs as primary targets of plasma membrane-associated CNLs leading to HR-associated cell death by disturbing the electrochemical gradient across the membrane. These results provide new insight into NLR-mediated cell death in plants, as well as innate immunity in higher eukaryotes.

Modulation of Expression of PVYNTN RNA-Dependent RNA Polymerase (NIb) and Heat Shock Cognate Host Protein HSC70 in Susceptible and Hypersensitive Potato Cultivars

Potato virus Y (PVY) belongs to the genus Potyvirus and is considered to be one of the most harmful and important plant pathogens. Its RNA-dependent RNA polymerase (RdRp) is known as nuclear inclusion protein b (NIb). The recent findings show that the genome of PVY replicates in the cytoplasm of the plant cell by binding the virus replication complex to the membranous structures of different organelles. In some potyviruses, NIb has been found to be localized in the nucleus and associated with the endoplasmic reticulum membranes. Moreover, NIb has been shown to interact with other host proteins that are particularly involved in promoting the virus infection cycle, such as the heat shock proteins (HSPs). HSP70 is the most conserved among the five major HSP families that are known to affect the plant-pathogen interactions. Some plant viruses can induce the production of HSP70 during the development of infection. To understand the molecular mechanisms underlying the interactive response to PVY^NTN (necrotic tuber necrosis strain of PVY), the present study focused on StHSC70-8 and PVY^NTN-NIb gene expression via localization of HSC70 and NIb proteins during compatible (susceptible) and incompatible (hypersensitive) potato-PVY^NTN interactions. Our results demonstrate that NIb and HSC70 are involved in the response to PVY^NTN infections and probably cooperate at some stages of the virus infection cycle. Enhanced deposition of HSC70 proteins during the infection cycle was associated with the dynamic induction of PVY^NTN-NIb gene expression and NIb localization during susceptible infections. In hypersensitive response (HR), a significant increase in HSC70 expression was observed up to 3 days post-inoculation (dpi) in the nucleus and chloroplasts. Thereafter, between 3 and 21 dpi, the deposition of NIb decreased, which can be attributed to a reduction in the levels of both virus accumulation and PVY^NTN-NIb gene expression. Therefore, we postulate that increase in the expression of both StHSC70-8 and PVY^NTN-NIb induces the PVY infection during susceptible infections. In contrast, during HRs, HSC70 cooperates with PVY^NTN only at the early stages of interaction and mediates the defense response signaling pathway at the later stages of infection.

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.

Genome-wide functional analysis of hot pepper immune receptors reveals an autonomous NLR clade in seed plants

Plants possess hundreds of intracellular immune receptors encoding nucleotide-binding domain leucine-rich repeat (NLR) proteins. Full-length NLRs or a specific domain of NLRs often induce plant cell death in the absence of pathogen infection. In this study we used genome-wide transient expression analysis to identify a group of NLRs (ANLs; ancient and autonomous NLRs) carrying autoactive coiled-coil (CC^A ) domains in pepper (Capsicum annuum). CC^A -mediated cell death mimics hypersensitive cell death triggered by the interaction between NLRs and pathogen effectors. Sequence alignment and mutagenesis analyses revealed that the intact α1 helix of CC^A s is critical for both CC^A - and ANL-mediated cell death. Cell death induced by CC^A s does not require NRG1/ADR1 or NRC type helper NLRs, suggesting ANLs may function as singleton NLRs. We also found that CC^A s localize to the plasma membrane, as demonstrated for Arabidopsis singleton NLR ZAR1. Extended studies revealed that autoactive CC^A s are well conserved in other Solanaceae plants as well as in rice, a monocot plant. Further phylogenetic analyses revealed that ANLs are present in all tested seed plants (spermatophytes). Our study not only uncovers the autonomous NLR clade in plants but also provides powerful resources for dissecting the underlying molecular mechanism of NLR-mediated cell death in plants.

Use of virus-induced gene silencing to characterize genes involved in modulating hypersensitive cell death in maize

Plant disease resistance proteins (R-proteins) detect specific pathogen-derived molecules, triggering a defence response often including a rapid localized cell death at the point of pathogen penetration called the hypersensitive response (HR). The maize Rp1-D21 gene encodes a protein that triggers a spontaneous HR causing spots on leaves in the absence of any pathogen. Previously, we used fine mapping and functional analysis in a Nicotiana benthamiana transient expression system to identify and characterize a number of genes associated with variation in Rp1-D21-induced HR. Here we describe a system for characterizing genes mediating HR, using virus-induced gene silencing (VIGS) in a maize line carrying Rp1-D21. We assess the roles of 12 candidate genes. Three of these genes, SGT1, RAR1, and HSP90, are required for HR induced by a number of R-proteins across several plant-pathogen systems. We confirmed that maize HSP90 was required for full Rp1-D21-induced HR. However, suppression of SGT1 expression unexpectedly increased the severity of Rp1-D21-induced HR while suppression of RAR1 expression had no measurable effect. We confirmed the effects on HR of two genes we had previously validated in the N. benthamiana system, hydroxycinnamoyltransferase and caffeoyl CoA O-methyltransferase. We further showed the suppression the expression of two previously uncharacterized, candidate genes, IQ calmodulin binding protein (IQM3) and vacuolar protein sorting protein 37, suppressed Rp1-D21-induced HR. This approach is an efficient way to characterize the roles of genes modulating the hypersensitive defence response and other dominant lesion phenotypes in maize.

Molecular Characterization of a Pathogen-Inducible Bidirectional Promoter from Hot Pepper (Capsicum annuum)

In hot pepper, the sesquiterpene phytoalexin capsidiol is catalyzed by the two final-step enzymes, a sesquiterpene cyclase (EAS) and a hydroxylase (EAH), which are genetically linked and present as head-to-head orientation in the genome. Transcriptomic analysis revealed that a subset of EAS and EAH is highly induced following pathogen infection, suggesting the coregulation of EAS and EAH by a potential bidirectional activity of the promoter (pCaD). A series of the nested deletions of pCaD in both directions verified the bidirectional promoter activity of the pCaD. Promoter deletion analysis revealed that the 226 bp of the adjacent promoter region of EAS and GCC-box in EAH orientation were determined as critical regulatory elements for the induction of each gene. Based on promoter analyses, we generated a set of synthetic promoters to maximize reporter gene expression within the minimal length of the promoter in both directions. We found that the reporter gene expression was remarkably induced upon infection with Phytophthora capsici, Phytophthora infestans, and bacterial pathogen Pseudomonas syringae pv. tomato DC3000 but not with necrotrophic fungi Botrytis cinerea. Our results confirmed the bidirectional activity of the pCaD located between the head-to-head oriented phytoalexin biosynthetic genes in hot pepper. Furthermore, the synthetic promoter modified in pCaD could be a potential tool for pathogen-inducible expression of target genes for developing disease-resistant crops.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

Extreme resistance to Potato virus Y in potato carrying the Rysto gene is mediated by a TIR-NLR immune receptor

Potato virus Y (PVY) is a major potato (Solanum tuberosum L.) pathogen that causes severe annual crop losses worth billions of dollars worldwide. PVY is transmitted by aphids, and successful control of virus transmission requires the extensive use of environmentally damaging insecticides to reduce vector populations. Ry_sto , from the wild relative S. stoloniferum, confers extreme resistance (ER) to PVY and related viruses and is a valuable trait that is widely employed in potato resistance breeding programmes. Ry_sto was previously mapped to a region of potato chromosome XII, but the specific gene has not been identified to date. In this study, we isolated Ry_sto using resistance gene enrichment sequencing (RenSeq) and PacBio SMRT (Pacific Biosciences single-molecule real-time sequencing). Ry_sto was found to encode a nucleotide-binding leucine-rich repeat (NLR) protein with an N-terminal TIR domain and was sufficient for PVY perception and ER in transgenic potato plants. Ry_sto -dependent extreme resistance was temperature-independent and requires EDS1 and NRG1 proteins. Ry_sto may prove valuable for creating PVY-resistant cultivars of potato and other Solanaceae crops.

Differential Accumulation of Innate- and Adaptive-Immune-Response-Derived Transcripts during Antagonism between Papaya Ringspot Virus and Papaya Mosaic Virus

Papaya ringspot virus (PRSV), a common potyvirus infecting papaya plants worldwide, can lead to either antagonism or synergism in mixed infections with Papaya mosaic virus (PapMV), a potexvirus. These two unrelated viruses produce antagonism or synergism depending on their order of infection in the plant. When PRSV is inoculated first or at the same time as PapMV, the viral interaction is synergistic. However, an antagonistic response is observed when PapMV is inoculated before PRSV. In the antagonistic condition, PRSV is deterred from the plant and its drastic effects are overcome. Here, we examine differences in gene expression by high-throughput RNA sequencing, focused on immune system pathways. We present the transcriptomic expression of single and mixed inoculations of PRSV and PapMV leading to synergism and antagonism. Upregulation of dominant and hormone-mediated resistance transcripts suggests that the innate immune system participates in synergism. In antagonism, in addition to innate immunity, upregulation of RNA interference-mediated resistance transcripts suggests that adaptive immunity is involved.

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

Nucleotide-binding leucine-rich repeat receptors (NLRs) are the most abundant type of immune receptors in plants and can trigger a rapid cell-death (hypersensitive) response upon sensing pathogens. We previously cloned the wheat NLR Sr35, which encodes a coiled-coil (CC) NLR that confers resistance to the virulent wheat stem rust race Ug99. Here, we investigated Sr35 signaling after Agrobacterium-mediated transient expression in Nicotiana benthamiana. Expression of Sr35 in N. benthamiana leaves triggered a mild cell-death response, which is enhanced in the autoactive mutant Sr35 D503V. The N-terminal tagging of Sr35 with green fluorescent protein (GFP) blocked the induction of cell death, whereas a C-terminal GFP tag did not. No domain truncations of Sr35 generated cell-death responses as strong as the wild type, but a truncation including the NB-ARC (nucleotide binding adaptor) shared by APAF-1, R proteins, and CED-4 domains in combination with the D503V autoactive mutation triggered cell death. In addition, coexpression of Sr35 with the matching pathogen effector protein AvrSr35 resulted in robust cell death and electrolyte leakage levels that were similar to autoactive Sr35 and significantly higher than Sr35 alone. Coexpression of Sr35-CC-NB-ARC and AvrSr35 did not induce cell death, confirming the importance of the leucine-rich repeat (LRR) domain for AvrSr35 recognition. These findings were confirmed through Agrobacterium-mediated transient expression in barley. Taken together, these results implicate the CC-NB-ARC domains of Sr35 in inducing cell death and the LRR domain in AvrSr35 recognition.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

The RNA-Dependent RNA Polymerase NIb of Potyviruses Plays Multifunctional, Contrasting Roles during Viral Infection

Potyviruses represent the largest group of known plant RNA viruses and include many agriculturally important viruses, such as Plum pox virus, Soybean mosaic virus, Turnip mosaic virus, and Potato virus Y. Potyviruses adopt polyprotein processing as their genome expression strategy. Among the 11 known viral proteins, the nuclear inclusion protein b (NIb) is the RNA-dependent RNA polymerase responsible for viral genome replication. Beyond its principal role as an RNA replicase, NIb has been shown to play key roles in diverse virus–host interactions. NIb recruits several host proteins into the viral replication complexes (VRCs), which are essential for the formation of functional VRCs for virus multiplication, and interacts with the sumoylation pathway proteins to suppress NPR1-mediated immunity response. On the other hand, NIb serves as a target of selective autophagy as well as an elicitor of effector-triggered immunity, resulting in attenuated virus infection. These contrasting roles of NIb provide an excellent example of the complex co-evolutionary arms race between plant hosts and potyviruses. This review highlights the current knowledge about the multifunctional roles of NIb in potyvirus infection, and discusses future research directions.

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.