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Plants are considered to have three major forms of resistance against pathogen infection. The first is ‘non host resistance’, which operates broadly at the species level, meaning all members of a plant species generally resist all members of the would-be pathogen species. The other two systems operate at the ‘host’ level. ‘Qualitative host resistance’ is seen when an individual plant cultivar (or genotype)completely resists attempted attack by an individual pathogen strain, whilst ‘partial resistance’ is when a plant cannot completely defend itself. Each of these systems have some underlying molecular principles which will be outlined later. In this issue of New Phytologist, Meile et al.(2023; pp. 1562–1577) describe a novel mechanism by which both partial and non host resistance can be achieved in wheat facing the fungal pathogen Zymoseptoria tritici and its closely related species.

Cell death is an important physiological phenomenon in life. It can be programmed or unprogrammed. Unprogrammed cell death is usually induced by abiotic or biotic stress. Recent studies have shown that many proteins regulate both cell death and immunity in plants. Here, we provide a review on the advances in plant immunity with cell death, especially the molecular regulation and underlying mechanisms of those proteins involved in both cell death and plant immunity. In addition, we discuss potential approaches toward improving plant immunity without compromising plant growth.

Alternative splicing of pre-mRNAs is an important gene regulatory mechanism shaping the transcriptome. AtMC1 is an Arabidopsis thaliana type I metacaspase that positively regulates the hypersensitive response. Here, we found that AtMC1 is involved in the regulation of plant immunity to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 and is physically associated with Sm-like4 (LSM4), which is involved in pre-mRNA splicing. AtMC1 and LSM4 protein levels both increased with their coexpression as compared with their separate expression in vivo. Like AtMC1, LSM4 negatively regulates plant immunity to P. syringae pv. tomato DC3000 infection. By RNA sequencing, AtMC1 was shown to modulate the splicing of many pre-mRNAs, including 4CL3, which is a negative regulator of plant immunity. Thus, AtMC1 plays a regulatory role in pre-mRNA splicing, which might contribute to AtMC1-mediated plant immunity. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

The hypersensitive response (HR) is a form of programmed cell death of plant cells occurring in the local region surrounding pathogen infection site to prevent the spread of infection by pathogens. Bax, a mammalian pro-apoptotic member of Bcl-2 family, triggers HR-like cell death when expressed in plants. However, constitutive expression of the Bax gene negatively affects plant growth and development. The Xa10 gene in rice (Oryza sativa) is an executor resistance (R) gene that confers race-specific disease resistance to Xanthomonas oryzae pv. oryzae strains harboring TAL effector gene AvrXa10. In this study, the Xa10 promoter was used to regulate heterologous expression of the Bax gene from mouse (Mus musculus) in Nicotiana benthamiana and rice. Cell death was induced in N. benthamiana after co-infiltration with the PXa10:Bax:TXa10 gene and the PPR1:AvrXa10:TNos gene. Transgenic rice plants carrying the PXa10:Bax:TXa10 gene conferred specific disease resistance to Xa10-incompatible X. oryzae pv. oryzae strain PXO99A(pHM1AvrXa10), but not to the Xa10-compatible strain PXO99A(pHM1). The resistance specificity was confirmed by the AvrXa10-dependent induction of the PXa10:Bax:TXa10 gene in transgenic rice. Our results demonstrated that the inducible expression of the Bax gene in transgenic rice was achieved through the control of the executor R gene promoter and the heterologous expression of the pro-apoptosis regulator gene in rice conferred disease resistance to X. oryzae pv. oryzae.

In plants, the reactive oxygen species (ROS) formed during normal conditions are essential in regulating several processes, like stomatal physiology, pathogen immunity and developmental signaling. However, biotic and abiotic stresses can cause ROS over-accumulation leading to oxidative stress. Therefore, a suitable equilibrium is vital for redox homeostasis in plants, and there have been major advances in this research arena. Salicylic acid (SA) is known as a chief regulator of ROS; however, the underlying mechanisms remain largely unexplored. SA plays an important role in establishing the hypersensitive response (HR) and systemic acquired resistance (SAR). This is underpinned by a robust and complex network of SA with Non-Expressor of Pathogenesis Related protein-1 (NPR1), ROS, calcium ions (Ca2+), nitric oxide (NO) and mitogen-activated protein kinase (MAPK) cascades. In this review, we summarize the recent advances in the regulation of ROS and antioxidant defense system signalling by SA at the physiological and molecular levels. Understanding the molecular mechanisms of how SA controls redox homeostasis would provide a fundamental framework to develop approaches that will improve plant growth and fitness, in order to meet the increasing global demand for food and bioenergy.

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. CCA 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 CCA 309 in Nicotiana benthamiana. CCA 309-induced extracellular alkalization causes plasma membrane depolarization, followed by cell death. Coimmunoprecipitation analyses suggest that CCA 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.

Lesion mimic mutants resembling the hypersensitive response without pathogen attack are an ideal material to understand programmed cell death, the defence response, and the cross‐talk between defence response and development in plants. In this study, mic, a lesion mimic mutant from cultivar Yunyin treated with ethyl methanesulphonate (EMS), was screened. By map‐based cloning, a short‐chain alcohol dehydrogenase/reductase with an atypical active site HxxxK was isolated and designated as SDR7‐6. It functions as a homomultimer in rice and is localized at the endoplasmic reticulum. The lesion mimic phenotype of the mutant is light‐dependent. The mutant displayed an increased resistance response to bacterial blight, but reduced resistance to rice blast disease. The mutant and knockout lines showed increased reactive oxygen species, jasmonic acid content, antioxidant enzyme activity, and expression of pathogenicity‐related genes, while chlorophyll content was significantly reduced. The knockout lines showed significant reduction in grain size, seed setting rate, 1000‐grain weight, grain weight per plant, panicle length, and plant height. SDR7‐6 is a new lesion mimic gene that encodes a short‐chain alcohol dehydrogenase with atypical catalytic site. Disruption of SDR7‐6 led to cell death and had adverse effects on multiple agricultural characters. SDR7‐6 may act at the interface of the two defence pathways of bacterial blight and rice blast disease in rice., SDR7‐6, a short‐chain alcohol dehydrogenase/reductase family protein, negatively regulates cell death and the defence response to rice bacterial blight, but positively regulates the defence response to Magnaporthe oryzae.

We reported an NBS-LRR gene, PsoRPM3, is highly expressed following RKN infection, initiating an HR response that promotes plant resistance. Meloidogyne spp. are root-knot nematodes (RKNs) that cause substantial economic losses worldwide. Screening for resistant tree resources and identifying plant resistance genes is currently the most effective way to prevent RKN infestations. Here, we cloned a novel TIR-NB-LRR-type resistance gene, PsoRPM3, from Xinjiang wild myrobalan plum (Prunus sogdiana Vassilcz.) and demonstrated that its protein product localized to the nucleus. In response to Meloidogyne incognita infection, PsoRPM3 gene expression levels were significantly higher in resistant myrobalan plum plants compared to susceptible plants. We investigated this difference, discovering that the − 309 to − 19 bp region of the susceptible PsoRPM3 promoter was highly methylated. Indeed, heterologous expression of PsoRPM3 significantly enhanced the resistance of susceptible tobacco plants to M. incognita. Moreover, transient expression of PsoRPM3 induced a hypersensitive response in tobacco, whereas RNAi-mediated silencing of PsoRPM3 in transgenic tobacco reduced this hypersensitive response. Several hypersensitive response marker genes were considerably up-regulated in resistant myrobalan plum plants when compared with susceptible counterparts inoculated with M. incognita. PsoPR1a (a SA marker gene), PsoPR2 (a JA marker gene), and PsoACS6 (an ET signaling marker gene) were all more highly expressed in resistant than in susceptible plants. Together, these results support a model in which PsoRPM3 is highly expressed following RKN infection, initiating an HR response that promotes plant resistance through activated salicylic acid, jasmonic acid, and ethylene signaling pathways.

In this study, we identified a new citrus vein enation virus (CVEV) isolate (named CVEV-DT1) through sRNA high-throughput sequencing and traditional sequencing. Phylogenetic analysis based on whole genome sequences of all known CVEV isolates revealed that CVEV-DT1 was in an evolutionary branch with other isolates from China. Molecular variation analysis showed that the single nucleotide variability along CVEV full-length sequences was less than 8%, with more transitions (60.55%) than transversions (39.43%), indicating a genetically homogeneous CVEV population. In addition, non-synonymous nucleotide mutations mainly occurred in ORF1 and ORF2. Based on disorder analysis of all encoded ORF by CVEV-DT1, we identified that the CVEV-DT1 coat protein (CP) formed spherical granules, mainly in the cell nucleus and partly throughout the cytoplasm, with liquid properties through subcellular localization and photobleaching assay. Furthermore, we also confirmed that the CVEV P0 protein has weak post-transcriptional RNA-silencing suppressor activity and could elicit a strong hypersensitive response (HR) in tobacco plants. Collectively, to the best of our knowledge, our study was the first to profile the genomic variation in all the reported CVEV isolates and reveal the functions of CVEV-DT1-encoded proteins.

Stripe (yellow) rust, caused by the biotrophic fungal pathogen Puccinia striiformis f. sp. tritici (Pst), is emerging as a serious threat to wheat (Triticum aestivum L.) production in many regions ...

In Rosaceae , tandem duplication caused the drastic expansion of CNGC gene family Group I. The members MdCN11 and MdCN19 negatively regulate Valsa canker resistance. Apple (Malus domestica) and pear (Pyrus bretschneideri and P. communis) are important fruit crops in Rosaceae family but are suffering from threats of Valsa canker. Cyclic nucleotide-gated ion channels (CNGCs) take crucial roles in plant immune responses. In the present study, a total of 355 CNGCs was identified from 8 Rosaceae plants. Based on phylogenetic analysis, 540 CNGCs from 18 plants (8 in Rosaceae and 10 others) could be divided into four groups. Group I was greatly expanded in Rosaceae resulted from tandem duplications. A large number of cis-acting regulatory elements (cis-elements) responsive to signals from multiple stresses and hormones were identified in the promoter regions of CNGCs in Malus spp. and Pyrus spp. Expressions of most Group I members were obviously up-regulated in Valsa canker susceptible varieties but not in the resistant ones. Furthermore, overexpression of the MdCN11 and MdCN19 in both apple fruits and ‘Duli’ (P. betulifolia) suspension cells compromised Valsa canker resistance. Overexpression of MdCN11 induced expression of hypersensitive response (HR)-related genes. In conclusion, tandem duplication resulted in a drastic expansion of CNGC Group I members in Rosaceae. Among these, MdCN11 and MdCN19 negatively regulate the Valsa canker resistance via inducting HR.

Agroinfiltration in Nicotiana benthamiana is widely used to transiently express heterologous proteins in plants. However, the state of Agrobacterium itself is not well studied in agroinfiltrated tissues, despite frequent studies of immunity genes conducted through agroinfiltration. Here, we generated a bioluminescent strain of Agrobacterium tumefaciens GV3101 to monitor the luminescence of Agrobacterium during agroinfiltration. By integrating a single copy of the lux operon into the genome, we generated a stable 'AgroLux' strain, which is bioluminescent without affecting Agrobacterium growth in vitro and in planta. To illustrate its versatility, we used AgroLux to demonstrate that high light intensity post infiltration suppresses both Agrobacterium luminescence and protein expression. We also discovered that AgroLux can detect Avr/Cf-induced immune responses before tissue collapse, establishing a robust and rapid quantitative assay for the hypersensitive response (HR). Thus, AgroLux provides a non-destructive, versatile and easy-to-use imaging tool to monitor both Agrobacterium and plant responses.

Chloroplasts are key players in plant immune signaling, contributing to not only de novo synthesis of defensive phytohormones but also the generation of reactive oxygen and nitrogen species following activation of pattern recognition receptors or resistance (R) proteins. The local hypersensitive response (HR) elicited by R proteins is underpinned by chloroplast-generated reactive oxygen species. HR-induced lipid peroxidation generates important chloroplast-derived signaling lipids essential to the establishment of systemic immunity. As a consequence of this pivotal role in immunity, pathogens deploy effector complements that directly or indirectly target chloroplasts to attenuate chloroplast immunity (CI). Our review summarizes the current knowledge of CI signaling and highlights common pathogen chloroplast targets and virulence strategies. We address emerging insights into chloroplast retrograde signaling in immune responses and gaps in our knowledge, including the importance of understanding chloroplast heterogeneity and chloroplast involvement in intraorganellular interactions in host immunity.

The infection of Pseudomonas syringae pv. actinidiae in kiwifruit is currently assessed by numerous methodologies, each with their own limitations. Most studies are based on either a laborious method of growth quantification of the pathogen or qualitative assessments by visual scoring following stem or cutting inoculation. Additionally, when assessing for resistance against specific pathogen effectors, confounding interactions between multiple genes in the pathogen can make mapping resistance phenotypes nearly impossible. Here, we present robust alternative methods to quantify pathogen load based on rapid bacterial DNA quantification by PCR, the use of Pseudomonas fluorescens, and a transient reporter eclipse assay for assessing resistance conferred by isolated bacterial avirulence genes. These assays compare well with bacterial plate counts to assess bacterial colonization as a result of plant resistance activation. The DNA-based quantification, when coupled with the P. fluorescens and reporter eclipse assays to independently identify bacterial avirulence genes, is rapid, highly reproducible, and scalable for high-throughput screens of multiple cultivars or genotypes. Application of these methodologies will allow rapid and high-throughput identification of resistant cultivars and the bacterial avirulence genes they recognize, facilitating resistance gene discovery for plant breeding programs.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

The endophytic bacterium Pantoea agglomerans DAPP‐PG 734 was previously isolated from olive knots caused by infection with Pseudomonas savastanoi pv. savastanoi DAPP‐PG 722. Whole‐genome analysis of this P. agglomerans strain revealed the presence of a Hypersensitive response and pathogenicity (Hrp) type III secretion system (T3SS). To assess the role of the P. agglomerans T3SS in the interaction with P. savastanoi pv. savastanoi, we generated independent knockout mutants in three Hrp genes of the P. agglomerans DAPP‐PG 734 T3SS (hrpJ, hrpN, and hrpY). In contrast to the wildtype control, all three mutants failed to cause a hypersensitive response when infiltrated in tobacco leaves, suggesting that P. agglomerans T3SS is functional and injects effector proteins in plant cells. In contrast to P. savastanoi pv. savastanoi DAPP‐PG 722, the wildtype strain P. agglomerans DAPP‐PG 734 and its Hrp T3SS mutants did not cause olive knot disease in 1‐year‐old olive plants. Coinoculation of P. savastanoi pv. savastanoi with P. agglomerans wildtype strains did not significantly change the knot size, while the DAPP‐PG 734 hrpY mutant induced a significant decrease in knot size, which could be complemented by providing hrpY on a plasmid. By epifluorescence microscopy and confocal laser scanning microscopy, we found that the localization patterns in knots were nonoverlapping for P. savastanoi pv. savastanoi and P. agglomerans when coinoculated. Our results suggest that suppression of olive plant defences mediated by the Hrp T3SS of P. agglomerans DAPP‐PG 734 positively impacts the virulence of P. savastanoi pv. savastanoi DAPP‐PG 722., The presence of an active Hrp‐1 T3SS in Pantoea agglomerans (Pag) DAPP‐PG 734 can suppress plant defences and therefore Pag synergistically interacts with Pseudomonas savastanoi pv. savastanoi DAPP‐PG 722 inside the olive knot.

Whitefly-transmitted begomoviruses infect and damage a wide range of food, feed, and fiber crops worldwide. Some of these viruses are associated with betasatellite molecules that are known to enhance viral pathogenesis. In this study, we investigated the function of a novel βV1 protein encoded by radish leaf curl betasatellite (RaLCB) by overexpressing the protein using potato virus X (PVX)-based virus vector in Nicotiana benthamiana. βV1 protein induced lesions on leaves, suggestive of hypersensitive response (HR), indicating cell death. The HR reaction induced by βV1 protein was accompanied by an increased accumulation of reactive oxygen species (ROS), free radicals, and HR-related transcripts. Subcellular localization through confocal microscopy revealed that βV1 protein localizes to the cellular periphery. βV1 was also found to interact with replication enhancer protein (AC3) of helper virus in the nucleus. The current findings suggest that βV1 functions as a protein elicitor and a pathogenicity determinant.

Lipids are major and essential constituents of plant cells and provide energy for various metabolic processes. However, the function of the lipid signal in defence against Verticillium dahliae, a hemibiotrophic pathogen, remains unknown. Here, we characterized 19 conserved stearoyl‐ACP desaturase family proteins from upland cotton (Gossypium hirsutum). We further confirmed that GhSSI2 isoforms, including GhSSI2‐A, GhSSI2‐B, and GhSSI2‐C located on chromosomes A10, D10, and A12, respectively, played a dominant role to the cotton 18:1 (oleic acid) pool. Suppressing the expression of GhSSI2s reduced the 18:1 level, which autoactivated the hypersensitive response (HR) and enhanced cotton Verticillium wilt and Fusarium wilt resistance. We found that low 18:1 levels induced phenylalanine ammonia‐lyase‐mediated salicylic acid (SA) accumulation and activated a SA‐independent defence response in GhSSI2s‐silenced cotton, whereas suppressing expression of GhSSI2s affected PDF1.2‐dependent jasmonic acid (JA) perception but not the biosynthesis and signalling cascade of JA. Further investigation showed that structurally divergent resistance‐related genes and nitric oxide (NO) signal were activated in GhSSI2s‐silenced cotton. Taken together, these results indicate that SA‐independent defence response, multiple resistance‐related proteins, and elevated NO level play an important role in GhSSI2s‐regulated Verticillium wilt resistance. These findings broaden our knowledge regarding the lipid signal in disease resistance and provide novel insights into the molecular mechanism of cotton fungal disease resistance., GhSACPD gene family isoforms GhSSI2‐A, GhSSI2‐B, and GhSSI2‐C displayed an important role in regulating 18:1 fatty acid level and negatively regulated cotton disease resistance to Verticillium wilt.

BACKGROUND Tobacco mosaic virus (TMV) is one of destructive plant viruses, causing serious economic losses in the world. Using antiviral proteins or elicitors to inhibit viral infection or promote plant immunity is one of the efficient strategies against TMV. Our previous study identified that the fermentation broth of Brevibacillus laterosporus strain B8 showed strong antiviral activity against TMV. However, the active antiviral ingredient is still unclear. RESULTS Here, BLB8 (B. laterosporus strain B8 protein, BLB8), an antiviral protein from B. laterosporus strain B8 was isolated and characterized. BLB8 showed protective, inactive and curative effects against TMV, and the inhibition rate reached up to 63%, 83% and 55%, respectively. BLB8 infiltrated around the infection site of the recombinant virus TMV-GFP inhibited the systemic extend and movement of TMV. Pretreatment of the bottom leaves with BLB8 inhibited the spread and accumulation of TMV in upper systemic leaves. Furthermore, BLB8 caused hypersensitive response (HR) in a dose-dependent way, promoted H2 O2 accumulation, and induced the expression of defense-relative genes in Nicotiana benthamiana. CONCLUSION The antiviral protein BLB8 from B. laterosporus strain B8 effectively inhibits TMV infection in inactivation, protective and curative effects through triggering plant immunity in tobacco. Therefore, the present study provides a new antiviral agent for prevention and control of viral disease. © 2021 Society of Chemical Industry.

12 Pág. Centro de Biotecnología y Genómica de Plantas, Mutations in the eukaryotic translation initiation factors eIF4E and eIF(iso)4E confer potyvirus resistance in a range of plant hosts. This supports the notion that, in addition to their role in translation of cellular mRNAs, eIF4E isoforms are also essential for the potyvirus cycle. CERES is a plant eIF4E- and eIF(iso)4E-binding protein that, through its binding to the eIF4Es, modulates translation initiation; however, its possible role in potyvirus resistance is unknown. In this article, we analyse if the ectopic expression of AtCERES is able to interfere with turnip mosaic virus replication in plants. Our results demonstrate that, during infection, the ectopic expression of CERES in Nicotiana benthamiana promotes the development of a mosaic phenotype when it is accumulated to moderate levels, but induces veinal necrosis when it is accumulated to higher levels. This necrotic process resembles a hypersensitive response (HR)-like response that occurs with different HR hallmarks. Remarkably, Arabidopsis plants inoculated with a virus clone that promotes high expression of CERES do not show signs of infection. These final phenotypical outcomes are independent of the capacity of CERES to bind to eIF4E. All these data suggest that CERES, most likely due to its leucine-rich repeat nature, could act as a resistance protein, able to promote a range of different defence responses when it is highly overexpressed from viral constructs., This research has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 260468 to MMC and from the grant S2013-ABI2734 from CAM. In addition, this work has been financially supported by RTI2018-095946-B100 from MICIU and by the ‘Severo Ochoa Programme for Centres of Excellence in R&D’ from the Agencia Estatal de Investigación of Spain (grant SEV-2016-0672 [2017–2021]) to the CBGP. In the frame of this latter program, RT was supported with a postdoctoral contract.

The bacterial leaf blight in rice caused by Xanthomonas oryzae pv. oryzae (Xoo) affects crop losses worldwide. In spite of developing resistant varieties by introgressing different Xa genes, the occurrence of diseases is evident. Here we report identification of several genes that are associated with improved plant immunity against Xoo in a resistant genotype BPT-5204 in comparison with susceptible genotype TN-1. The RNA sequencing information was developed to identify the genes that could provide durable resistance in rice. Xoo-resistant rice genotype BPT-5204 with Xa 5, 13 and 21 genes is compared with sensitive Taichung Native 1 (TN-1) to identify the genetic pathways and gene networks involved in resistance mechanisms. The higher levels of salicylic acid resulted in upregulation of many pathogenesis-related (PR) and redox protein encoding transcripts which resulted in higher hypersensitive response in BPT-5204. Many Serine/threonine protein kinase, leucine-rich repeat (LRR) transmembrane protein kinase, protein kinase family genes, Wall-associated kinase (WAK) were upregulated that resulted in activation of bZIP, WRKY, MYB, DOF and HSFs transcription factors that are associated with improved plant immunity. The study provided roles of many genes and their associated plant immunity pathways that can be used for developing resistant rice cultivars.

The content of reactive oxygen species, the activity of key antioxidant enzymes ‒ ascorbate peroxidase and glutathione reductase, the level of expression of the hypersensitive response marker gene (HSR) as well as potato virus X particles content in cv. Uladar seedlings grown on an ion-exchange substrate in the presence of a complex preparation containing chitosan, Bacillus subtilis bacteria-based preparation and salicylic acid, when infected potato virus X. Accumulation of the reactive oxygen species, increase in ascorbate peroxidase activity, a lower level of HSR gene expression and a lower content of virus X particles in potato plants under such conditions are shown. An increase in both the number of potato minitubers and dry matter content in them was also registered when plants were grown on an ion-exchange substrate in the presence of a complex preparation.

Sugar beet cultivation is dependent on an effective control of beet necrotic yellow vein virus (BNYVV, family Benyviridae), which causes tremendous economic losses in sugar production. As the virus is transmitted by a soilborne protist, the use of resistant cultivars is currently the only way to control the disease. The Rz2 gene product belongs to a family of proteins conferring resistance towards diverse pathogens in plants. These proteins contain coiled-coil and leucine-rich repeat domains. After artificial inoculation of homozygous Rz2 resistant sugar beet lines, BNYVV and beet soilborne mosaic virus (BSBMV, family Benyviridae) were not detected. Analysis of the expression of Rz2 in naturally infected plants indicated constitutive expression in the root system. In a transient assay, coexpression of Rz2 and the individual BNYVV-encoded proteins revealed that only the combination of Rz2 and triple gene block protein 1 (TGB1) resulted in a hypersensitive reaction (HR)-like response. Furthermore, HR was also triggered by the TGB1 homologues from BSBMV as well as from the more distantly related beet soilborne virus (family Virgaviridae). This is the first report of an R gene providing resistance across different plant virus families.

In the present study, genes encoding for six major classes of enzymatic antioxidants, namely superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), Peroxidase (Prx) and glutathione S-transferase (GST) are identified in tomato. Their expression was studied in tomato cultivars contrastingly tolerant to ToLCNDV during virus infection and different hormone treatments. Significant upregulation of SlGR3, SlPrx25, SlPrx75, SlPrx95, SlGST44, and SlGST96 was observed in the tolerant cultivar during disease infection. Virus-induced gene silencing of SlGR3 in the tolerant cultivar conferred disease susceptibility to the knock-down line, and higher accumulation (~80%) of viral DNA was observed in the tolerant cultivar. Further, subcellular localization of SlGR3 showed its presence in cytoplasm, and its enzymatic activity was found to be increased (~65%) during ToLCNDV infection. Knock-down lines showed ~3- and 3.5-fold reduction in GR activity, which altogether underlines that SlGR3 is vital component of the defense mechanism against ToLCNDV infection.

The plant immune system involves cell-surface receptors that detect intercellular pathogen-derived molecules, and intracellular receptors that activate immunity upon detection of pathogen-secreted effector proteins that act inside the plant cell. Immunity mediated by surface receptors has been extensively studied1, but that mediated by intracellular receptors has rarely been investigated in the absence of surface-receptor-mediated immunity. Furthermore, interactions between these two immune pathways are poorly understood. Here, by activating intracellular receptors without inducing surface-receptor-mediated immunity, we analyse interactions between these two distinct immune systems in Arabidopsis. Pathogen recognition by surface receptors activates multiple protein kinases and NADPH oxidases, and we find that intracellular receptors primarily potentiate the activation of these proteins by increasing their abundance through several mechanisms. Likewise, the hypersensitive response that depends on intracellular receptors is strongly enhanced by the activation of surface receptors. Activation of either immune system alone is insufficient to provide effective resistance against the bacterial pathogen Pseudomonas syringae. Thus, immune pathways activated by cell-surface and intracellular receptors in plants mutually potentiate to activate strong defences against pathogens. These findings reshape our understanding of plant immunity and have broad implications for crop improvement. In Arabidopsis, two distinct types of immunity—that mediated by cell-surface receptors and that mediated by intracellular receptors—interact with and mutually enhance each other to provide effective defence against pathogens.

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 (CCD21). ZmNANMT, but not ZmCOMT, interacts with CCD21, 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., Maize ZmNANMT, but not its close homolog ZmCOMT, interacts with the NLR protein Rp1‐D21 and modulates the hypersensitive response when transiently expressed in Nicotiana benthamiana.

The innate immune system of plants is crucial in defining the fate of a plant cell during plant-pathogen interactions. This response is often accompanied by a hypersensitive reaction leading to the death of a plant cell and restricted pathogen growth. Plant mitochondria, in this case, play a key role by maintaining a balance between cell respiration and reactive oxygen species formation. One of the key features of the hypersensitive response is the shift of the normal plant respiratory pathway to a special 'alternative' pathway. Plants contain an enzyme, alternative oxidase, for maintaining metabolic homeostasis of the cell. This energy dissipating respiration provides a branch in normal respiration by using ubiquinone to form water and heat, thus maintaining the energy status of the cell. Alternative oxidase is thought to minimize production of reactive oxygen species and can also function in 'anti-apoptotic' machinery in plant cells. In this mini review, we briefly describe the alternative respiratory pathway and explain the role of alternative oxidase in important cellular processes, such as programmed cell death and the hypersensitive response.

Plant subtilases (SBTs) or subtilisin-like proteases comprise a very diverse family of serine peptidases that participates in a broad spectrum of biological functions. Despite increasing evidence for roles of SBTs in plant immunity in recent years, little is known about wheat (Triticum aestivum) SBTs (TaSBTs). Here, we identified 255 TaSBT genes from bread wheat using the latest version 2.0 of the reference genome sequence. The SBT family can be grouped into five clades, from TaSBT1 to TaSBT5, based on a phylogenetic tree constructed with deduced protein sequences. In silico protein-domain analysis revealed the existence of considerable sequence diversification of the TaSBT family which, together with the local clustered gene distribution, suggests that TaSBT genes have undergone extensive functional diversification. Among those TaSBT genes whose expression was altered by biotic factors, TaSBT1.7 was found to be induced in wheat leaves by chitin and flg22 elicitors, as well as six examined pathogens, implying a role for TaSBT1.7 in plant defense. Transient overexpression of TaSBT1.7 in Nicotiana benthamiana leaves resulted in necrotic cell death. Moreover, knocking down TaSBT1.7 in wheat using barley stripe mosaic virus-induced gene silencing compromised the hypersensitive response and resistance against Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust. Taken together, this study defined the full complement of wheat SBT genes and provided evidence for a positive role of one particular member, TaSBT1.7, in the incompatible interaction between wheat and a stripe rust pathogen.

The intracellular plant resistance (R) proteins, nucleotide-binding and leucine-rich repeat (NLR) proteins, mediate resistance to pathogens by enabling recognition and rapid response. The response consists of the induction of a defensive suite that typically culminates in the hypersensitive response (HR), death of the plant cells at and around an infection site. The Arabidopsis intracellular innate immune receptor protein RESISTANCE TO PSEUDOMONAS MACULICOLA1 (RPM1) is a coiled-coil (CC) type of NLR protein that specifies resistance to strains of the bacterial pathogen Pseudomonas syringae expressing the type III effector proteins AvrRpm1 and AvrB. We previously demonstrated that RPM1-myc (an epitope-tagged version of RPM1) disappears coincident with the onset of HR induced by AvrRpm1. Infection with P. syringae expressing two other type III effector proteins, AvrRpt2 and AvrRps4, also initiated RPM1-myc disappearance at time points coincident with the HR they initiate through the NLR proteins RESISTANCE TO P. SYRINGAE2 (RPS2) and RESISTANCE TO P. SYRINGAE 4 (RPS4), respectively. Here, we use mutants impaired in NLR gene dependent signaling to demonstrate that disappearance of RPM1-myc requires normal NLR gene dependent signaling steps, but does not require HR. Inhibitors of the 26S proteasome block the disappearance of RPM1-myc and enhance RPM1-myc-dependent cell death. Our data are consistent with a model in which RPM1 is degraded by the 26S proteasome to limit the extent of RPM1-dependent signaling and/or cell death. Furthermore, AvrRpt2 induces disappearance of RPM1-myc in rps2 mutant plants without HR, suggesting that RPM1 is part of the host target of the virulence activity of AvrRpt2.

Bacterial spot, which is caused by several Xanthomonas species, is an economically important disease in tomato (Solanum lycopersicum). Great efforts have been made for the identification of resistant sources and the genetic analysis of resistance. However, the development of resistant commercial varieties is slow due to the existence of multiple species of the pathogen and a poor understanding of the resistance mechanism in tomato. The current study revealed that the Rx4 gene encodes a nucleotide-binding leucine-rich repeat protein in the wild tomato species Solanum pimpinellifolium and specifically recognizes and confers a hypersensitive response (HR) to Xanthomonas euvesicatoria pv. perforans race T3 expressing the AvrXv3 avirulence protein. Complementation of the Rx4 gene in the susceptible tomato line Ohio 88119 using a transgenic approach resulted in HR, whereas knockout of the gene through CRISPR/Cas9 editing in resistant lines Hawaii 7981 and PI 128216 led to non-HR to race T3. Transcription of Rx4 was not induced by the presence of race T3. Furthermore, the Rx4 protein did not show physical interaction with AvrXv3 but interacted with SGT1-1 and RAR1. Virus-induced gene silencing of SGT1-1 and RAR1 in the resistant line PI128216 suppressed the HR to race T3. Taken together, our study confirms Rx4 is the gene conferring the HR to bacterial spot race T3 and reveals the potential roles of SGT1-1 and RAR1 as signals in the Rx4-mediated HR. This discovery represents a step forward in our understanding of the mechanism of resistance to bacterial spot in tomato and may have important implications for understanding plant-bacterial interactions.

Expansins, cerato-platanins and swollenins (which we will henceforth refer to as expansin-related proteins) are a group of microbial proteins involved in microbe-plant interactions. Although they share very low sequence similarity, some of their composing domains are near-identical at the structural level. Expansin-related proteins have their target in the plant cell wall, in which they act through a non-enzymatic, but still uncharacterized, mechanism. In most cases, mutagenesis of expansin-related genes affects plant colonization or plant pathogenesis of different bacterial and fungal species, and thus, in many cases they are considered virulence factors. Additionally, plant treatment with expansin-related proteins activate several plant defenses resulting in the priming and protection towards subsequent pathogen encounters. Plant-defence responses induced by these proteins are reminiscent of pattern-triggered immunity or hypersensitive response in some cases. Plant immunity to expansin-related proteins could be caused by the following: (i) protein detection by specific host-cell receptors, (ii) alterations to the cell-wall-barrier properties sensed by the host, (iii) displacement of cell-wall polysaccharides detected by the host. Expansin-related proteins may also target polysaccharides on the wall of the microbes that produced them under certain physiological instances. Here, we review biochemical, evolutionary and biological aspects of these relatively understudied proteins and different immune responses they induce in plant hosts.

The relationship of Resistance (R) gene-mediated defense to other forms of resistance in plants is considered, and the natures of the products of dominant and recessive R genes are reviewed. Various factors involved in expressing R gene-mediated resistance are described. These include phytohormones and plant effector molecules: the former regulating different pathways for disease resistance and the latter having direct effects on viral genomes or encoded proteins. Finally, the status of our knowledge concerning the cell-death hypersensitive response and its relationship to the actual resistance response involved in inhibiting virus infection is examined.

The content of reactive oxygen species, the activities of ascorbate-glutathione cycle enzymes, the content of low molecular weight antioxidants: ascorbate and glutathione, the activity of phenolic peroxidase, the content of water-soluble phenols and the expression level of the hypersensitive response marker gene (HSR) and PR protein genes (Chit, Glu and TLP) in potato seedlings grown on an ion-exchange substrate in the presence of jasmonic acid at a concentration of 10 -8 and 10 -6 M and infected with potato virus X were studied. Accumulation of reactive oxygen species, an increase in the content of reduced ascorbate and an intensive consumption of reduced glutathione upon action of jasmonic acid, as well as a sharp increase in the expression of HSR, Chit, Glu, and TLP genes in the leaves of potato seedlings grown on a substrate with the addition of 10 -6 M jasmonic acid before infection were shown. In control plants, the expression of the HSR and TLP genes increased only after infection with potato virus X.

Red rot caused by Colletotrichum falcatum poses a serious threat to sugarcane cultivation in many tropical and sub-tropical countries. Deciphering the molecular network of major defense-signaling pathways in sugarcane cultivars with varying red rot resistance is essential to elucidate the phenomenon of defense priming exerted by resistance inducers. Therefore, in this study, expression pattern of transcripts coding for major defense-signaling pathway regulatory genes was profiled during compatible and incompatible interactions and in response to defense priming using qRT-PCR. Candidate genes that were profiled are involved in or related to hypersensitive response and reactive oxygen species production (HR/ROS), salicylic acid (SA), and jasmonic acid/ethylene (JA/ET) pathways. For compatible and incompatible interactions, susceptible (CoC 671), field tolerant (Co 86032) and resistant (Co 93009) sugarcane cultivars were used, whereas for defense priming, benzothiadiazole (BTH) and the pathogen-associated molecular patterns (PAMPs) of C. falcatum viz., CfEPL1 (eliciting plant response-like) and CfPDIP1 (plant defense inducing protein) were used in CoC 671 cultivar. Results indicated that the master regulator of defense pathways, nonexpressor of pathogenesis-related genes 1 (NPR1) was highly upregulated in incompatible interactions (in both Co 86032 and Co 93009) than the compatible interaction along with SA pathway-associated genes. Similarly, in response to defense priming with BTH, CfEPL1 and CfPDIP1, only the SA pathway-associated genes showed considerable upregulation at 0 h post inoculation (hpi) and other intermittent time points. Overall, this study showed that SA-mediated defense pathway is the most predominant pathway reprogrammed during priming with BTH, CfEPL1 and CfPDIP1 and substantiated the earlier findings that these agents indeed induce systemic resistance against red rot of sugarcane.

Brassinosteroids (BRs) are plant steroidal phytohormones that play an important role in plant growth, stress responses, and adaptation. BIP130 is a BR-INSENSITIVE 1 (BRI1)-interacting protein, and BIP130 phosphorylation by OsMPK1 upregulates abscisic acid (ABA)-induced antioxidant defense in rice protoplasts. However, studies on BIP130-induced modulation of plant responses to salt stress are lacking. This study investigated the function of BIP130 in response to salt stress in transgenic rice (Oryza sativa L.). We assayed the relative BIP130 transcript levels in wild-type (WT) plants with NaCl treatment and observed the effects on scavenging reactive oxygen species (ROS) in transgenic plants. Changes in ABA content in transgenic plants were also analyzed. NaCl treatment upregulated relative transcript and protein levels of BIP130 in WT plants. The bip130 RNA interference (RNAi) rice mutants displayed a hypersensitive response to salt stress, while BIP130-overexpressing plants showed significant enhancement of salt tolerance, which was confirmed by changes in survival rates, antioxidant enzyme activities, H2O2 levels, and electrolyte leakage. RNA-sequencing analysis showed that BIP130 upregulates nine ABA-related genes (OsPSY2, OsNCED3, OsNCED4, OsCIPK6, OsCIPK16, OsCIPK31, OsEDL3, OsbZIP23, OsHVA22). ABA content significantly increased in BIP130-overexpressing and significantly decreased in bip130-RNAi plants. Therefore, BIP130 is involved in NaCl-induced antioxidant defense and enhances salt stress tolerance by modulating ABA biosynthesis and ROS scavenging in rice.

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., In this study we used a virus‐induced gene silencing technique to characterize the roles of 12 genes in controlling the hypersensitive cell death defence response in maize.

HopH1 is an effector protein of Pseudomonas syringae pv. tomato DC3000 and P. syringae pv. syringae B728a and is a homolog of the putative Zn-dependent protease effector Rip36 of Ralstonia solanacearum, which induces hypersensitive response (HR) cell death in a nonhost plant, Solanum torvum Sw. cv. Torubamubiga. Although P. syringae pv. phaseolicola (Pph) 1448A neither produces HopH1 nor induces HR cell death, hopH1-introduced Pph 1448A acquired the ability to induce HR. These results indicate that the putative Zn-protease HopH1 effector induces HR cell death in nonhost S. torvum.

Plants have evolved an innate immune system to protect themselves from pathogen invasion with the help of intracellular nucleotide-binding leucine-rich repeat (NLR) receptors, though the mechanisms remain largely undefined. RIN13 (RPM1-interacting protein 13) was previously reported to enhance disease resistance, and suppress RPM1 (a CNL-type NLR)-mediated hypersensitive response in Arabidopsis via an as yet unknown mechanism. Here, we show that RIN13 is a nuclear-localized protein, and functions therein. Overexpression of RIN13 leads to autoimmunity with high accumulation of salicylic acid (SA), constitutive expression of pathogenesis-related genes, enhanced resistance to a virulent pathogen, and dwarfism. In addition, genetic and transcriptome analyses show that SA-dependent and SA-independent pathways are both required for RIN13-mediated disease resistance, with the EDS1/PAD4 complex as an integration point. RIN13-induced dwarfism was rescued completely by either the pad4-1 or the eds1-2 mutant but partially by snc1-r1, a mutant of the TNL gene SNC1, suggesting the involvement of EDS1/PAD4 and SNC1 in RIN13 functioning. Furthermore, transient expression assays indicated that RIN13 promotes the nuclear accumulation of PAD4. Collectively, our study uncovered a signaling pathway whereby SNC1 and EDS1/PAD4 act together to modulate RIN13-triggered plant defense responses.

Pathogenesis-related (PR) proteins constitute a broad class of plant proteins with analogues found throughout nature from bacteria to higher eukaryotes. PR proteins were first noted in plants as part of the hypersensitive response, but have since been assigned an array of biological roles. The PR10/Bet v1-like proteins are a subset of PR proteins characterized by an ability to bind a wide range of lipophilic ligands, uniquely positioning them as contributors to specialized biosynthetic pathways. PR10/Bet v1-like proteins participate in the production of plant alkaloids and phenolics including flavonoids, both as general binding proteins and in special cases as catalysts. Owing initially to the perceived allergenic properties of PR10/Bet v1-like proteins, many were studied at the structural level to elucidate the basis for ligand binding. These studies provided a foundation for more recent efforts to understand higher-level structural order and how PR10/Bet v1-like proteins catalyse key reactions in plant pathways. Synthetic biology aimed at reconstituting plant-specialized metabolism in microorganisms uses knowledge of these proteins to fine-tune performance in new systems.

Erwinia amylovora is a plant pathogenic bacterium that causes the disease fire blight in a wide range of Rosaceous plants including apples and pears. This pathogen utilizes the hypersensitive response and pathogenicity (hrp)-type III secretion system (T3SS), an essential pathogenicity factor of E. amylovora, to deliver proteins from bacteria to plant apoplasts or the cytoplasm to regulate host immune responses and physiology. Proteins secreted by the E. amylovora T3SS include five effectors: DspA/E, Eop1, Eop3, Eop4 (AvrRpt2Ea), and HopPtoCEa, two harpins: HrpN and HrpW, and other proteins such as Eop2, HrpJ, and HrpK. Homologs of these T3SS-secreted proteins have been characterized in plant pathogens like Pseudomonas syringae, Xanthomonas spp., Ralstonia spp., plant symbionts like Rhizobium spp. and animal pathogens such as Yersinia spp., implicating a central role of these effector proteins in manipulating bacterial interactions with diverse host species. Translocation of effectors is crucial for T3SS-mediated disease development in hosts. It is thought to be mediated by cognate chaperones, but the translocation of E. amylovora DspA/E was recently shown to be regulated by several different chaperones as well as a previously reported harpin, HrpN, in addition to its cognate chaperone DspB/F. This suggests that the dynamics of effector translocation are more complicated than previously expected in E. amylovora. In this review, we summarize the current knowledge of E. amylovora T3SS effectors, chaperones, and harpins together with their homologs from other plant-associated bacterial species. We also discuss the roles of T3SS-secreted proteins in pathogenicity and fitness via exploring their localization, activity, and direct or indirect targets in host plants. Finally, we provide future perspectives on studying effector biology and the T3SS in E. amylovora and how this fundamental knowledge provides potential applications in preventing fire blight.

Citrus Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (Las), is one of the most destructive citrus diseases worldwide, yet how Las causes HLB is poorly understood. Here we show that a Las-secreted protein, SDE15 (CLIBASIA_04025), suppresses plant immunity and promotes Las multiplication. Transgenic expression of SDE15 in Duncan grapefruit (Citrus × paradisi) suppresses the hypersensitive response induced by Xanthomonas citri ssp. citri (Xcc) and reduces the expression of immunity-related genes. SDE15 also suppresses the hypersensitive response triggered by the Xanthomonas vesicatoria effector protein AvrBsT in Nicotiana benthamiana, suggesting that it may be a broad-spectrum suppressor of plant immunity. SDE15 interacts with the citrus protein CsACD2, a homolog of Arabidopsis (Arabidopsis thaliana) ACCELERATED CELL DEATH 2 (ACD2). SDE15 suppression of plant immunity is dependent on CsACD2, and overexpression of CsACD2 in citrus suppresses plant immunity and promotes Las multiplication, phenocopying overexpression of SDE15. Identification of CsACD2 as a susceptibility target has implications in genome editing for novel plant resistance against devastating HLB.

Defence‐related LsGRP1 is a leaf‐specific plant class II glycine‐rich protein (GRP) involved in salicylic acid‐induced systemic resistance against grey mould caused by necrotrophic Botrytis elliptica in lily (Lilium) cultivar Stargazer. The C‐terminal region of LsGRP1 (LsGRP1C) can inhibit fungal growth in vitro via a mechanism of inducing fungal apoptosis programmed cell death (PCD). In this study, the role of LsGRP1 in induced defence mechanism was investigated using LsGRP1‐silenced Stargazer lily and LsGRP1‐transgenic Arabidopsis thaliana. LsGRP1 silencing in lily was found to slightly inhibit plant growth and greatly increase the susceptibility to B. elliptica by suppressing callose deposition and early reactive oxygen species (ROS) accumulation. In contrast, LsGRP1‐transgenic Arabidopsis showed higher resistance to Botrytis cinerea and also to Pseudomonas syringae pv. tomato DC3000 as compared to the wild type, accompanied with the enhancement of callose deposition and ROS accumulation. Additionally, LsGRP1 silencing increased plant cell death caused by B. elliptica secretion and reduced pathogen‐associated molecular pattern (PAMP)‐triggered defence activation in Stargazer lily. Consistently, LsGRP1 expression boosted PAMP‐triggered defence responses and effector recognition‐induced hypersensitive response in Arabidopsis. Moreover, fungal apoptosis PCD triggered by LsGRP1 in an LsGRP1C‐dependent manner was demonstrated by leaf infiltration with LsGRP1C‐containing recombinant proteins in Stargazer lily. Based on these results, we presume that LsGRP1 plays roles in plant defence via functioning as a pathogen‐inducible switch for plant innate immune activation and acting as a fungal apoptosis PCD inducer to combat pathogen attack., LsGRP1 from Lilium exhibits a dual function in plant defence, which involves innate immune activation and fungal apoptosis induction, to protect plants from necrotrophic and hemibiotrophic pathogens.

Black pepper (Piper nigrum L.) is an important spice crop with high economic value. However, its production is severely hampered by the oomycete pathogen, Phytophthora capsici. Integrative disease management strategies have been developed to control the pathogen, but the pathogen is in the phase of evolving its virulence. Absolute resistance against Phytophthora rot was not reported in black pepper germplasm. However, Piper colubrinum, a wild species is reported as resistant. Resistance proteins are involved in continuous surveillance of pathogen entry and activation of plant defense signalling pathways for an effective hypersensitive response to prevent pathogen invasion. In this study, a sequence-based homology approach using the conserved nucleotide-binding site (NBS) of known plant resistance genes was used to isolate Resistance Gene Analogs (RGA) and assess their transcript level during Phytophthora infection. The RGA transcript level was evaluated in resistant wild species (P. colubrinum), two moderately resistant black pepper genotypes (IISR Sakthi and 04-P24-1), and one susceptible genotype (Subhakara). The identified RGAs of black pepper were found to be of non-TIR R gene class with NBS motifs. The expressions of six PnRGAs were assessed employing qRT-PCR at different time points after challenging with highly aggressive Phytophthora isolate. The kinetics of differential expression post-infection with P. capsici indicates the differential timing and magnitude of pathogen recognition in resistant P. colubrinum and moderately resistant black pepper genotypes compared to susceptible genotype. In silico analysis revealed that differentially expressed P. nigrum RGAs function through ADP phosphorylation, which is a key process in pathogen recognition. The identification of P. nigrum RGAs induced by P. capsici should provide valuable information for cloning and characterization of resistance genes.

Plant lesion mimic mutants (lmms) generally possess autoimmunity and hypersensitive response (HR)-like cell death in the absence of biotic or abiotic stress. They have attracted much attention because they are useful tools for deciphering the interaction between defense signaling and growth. Recent studies have identified more than 30 lmms involved in the plant immune response and cell death in rice. Genes underlying these lmms, coding for diverse types of proteins, mainly regulate transcription, protein translation and modification, vesicular trafficking and catalyzation of metabolism. Here, we presented an overview of the most recent advances on the study of lmms in rice and proposed a perspective on potential utilization of LMM genes in agriculture.

Bacterial wilt caused by Ralstonia solanacearum is an important disease of pepper (Capsicum annuum), an economically important solanaceous vegetable worldwide, in particular, under high temperature (HT) conditions. However, the molecular mechanism underlying pepper immunity against bacterial wilt remains poorly understood. Herein, CaCBL1, a putative calcineurin B-like protein, was functionally characterized in the pepper response to R. solanacearum inoculation (RSI) under HT (RSI/HT). CaCBL1 was significantly upregulated by RSI at room temperature (RSI/RT), HT, or RSI/HT. CaCBL1-GFP fused protein targeted to whole epidermal cells of Nicotiana benthamiana when transiently overexpressed. CaCBL1 silencing by virus-induced gene silencing significantly enhanced pepper susceptibility to RSI under RT or HT, while its transient overexpression triggered hypersensitive response mimic cell death and upregulation of immunity-associated marker genes, including CabZIP63, CaWRKY40, and CaCDPK15, the positive regulators in the pepper response to RSI or HT found in our previous studies. In addition, by chromatin immunoprecipitation PCR and electrophoretic mobility shift assay, CaCBL1 was found to be directly targeted by CaWRKY40, although not by CaWRKY27 or CaWRKY58, via the W-box-2 within its promoter, and its transcription was found to be downregulated by silencing of CaWRKY40 while it was enhanced by its transient overexpression. These results suggest that CaCBL1 acts as a positive regulator in pepper immunity against R. solanacearum infection, constituting a positive feedback loop with CaWRKY40.

Previous studies have shown that the hypersensitive response and pathogenicity (hrp)-type III secretion system (T3SS) is activated by the bacterial alarmone (p)ppGpp, which mediates the stringent response in nutrient starvation conditions in Erwinia amylovora. ATP and GTP, the precursors of (p)ppGpp biosynthesis, are synthesized from AMP/adenine and GMP/guanine, respectively, by the de novo pathway of purine biosynthesis. In this study, we examined the role of the purA/purB and guaA/guaB genes, which are involved in the AMP/adenine and GMP/guanine biosynthesis, respectively. Our results showed that growth of the purA, purB, and guaAB mutants was much slower in minimal media, but could be restored by addition of exogenous AMP/adenine and GMP/guanine, respectively. However, mixed results were obtained for their contribution to virulence on immature pear fruits. These results suggested that the de novo purine biosynthesis pathway might be required for proliferation, colonization and infection of E. amylovora.

Two phospholipase C2s in Nicotiana benthamiana, PLC2-1 and PLC2-2, complement each other during disease resistance responses and play important roles the induction of PAMP-triggered immunity., Phospholipid signaling plays an important role in plant immune responses against phytopathogenic bacteria in Nicotiana benthamiana. Here, we isolated two phospholipase C2 (PLC2) orthologs in the N. benthamiana genome, designated as PLC2-1 and 2-2. Both NbPLC2-1 and NbPLC2-2 were expressed in most tissues and were induced by infiltration with bacteria and flg22. NbPLC2-1 and NbPLC2-2 (NbPLC2s) double-silenced plants showed a moderately reduced growth phenotype. The induction of the hypersensitive response was not affected, but bacterial growth and the appearance of bacterial wilt were accelerated in NbPLC2s-silenced plants when they were challenged with a virulent strain of Ralstonia solanacearum that was compatible with N. benthamiana. NbPLC2s-silenced plants showed reduced expression levels of NbPR-4, a marker gene for jasmonic acid signaling, and decreased jasmonic acid and jasmonoyl-L-isoleucine contents after inoculation with R. solanacearum. The induction of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) marker genes was reduced in NbPLC2s-silenced plants after infiltration with R. solanacearum or Pseudomonas fluorescens. Accordingly, the resistance induced by flg22 was compromised in NbPLC2s-silenced plants. In addition, the expression of flg22-induced PTI marker genes, the oxidative burst, stomatal closure, and callose deposition were all reduced in the silenced plants. Thus, NbPLC2s might have important roles in pre- and post-invasive defenses, namely in the induction of PTI.

SENESCENCE-ASSOCIATED GENE 13 (SAG13) of Arabidopsis is a widely conserved gene of unknown function that has been extensively used as a marker of plant senescence. SAG13 induction occurs during plant cell death processes, including senescence and hypersensitive response, a type of programmed cell death that occurs in response to pathogens. This implies that SAG13 expression is regulated through at least two different signaling pathways affecting these two different processes. Our work highlights a contrasting role for SAG13 in regulating resistance against disease-causing biotrophic bacterial and necrotrophic fungal pathogens with contrasting infection strategies. We provide further evidence that SAG13 is not only induced during oxidative stress but also plays a role in protecting the plant against other stresses. SAG13 is also required for normal seed germination, seedling growth, and anthocyanin accumulation. The work presented here provides evidence for the role of SAG13 in regulating multiple plant processes including senescence, defense, seed germination, and abiotic stress responses. SAG13 is a valuable molecular marker for these processes and is conserved in multiple plant species, and this knowledge has important implications for crop improvement.

Successful plant defence responses to pathogen challenges are based on fast and specific pathogen recognition and plant reaction mechanisms. Elicitins, proteinaceous elicitors secreted by the Phytophthora and Pythium species, were first described in Phytophthora culture filtrates as proteins able to induce a hypersensitive response (HR) and resistance in tobacco at low concentrations. Later, they were classified as microbial-associated molecular patterns (MAMPs) able to induce defences in a variety of plant species. In this review, we present a comprehensive summary of the actual knowledge on the representative elicitins and their structure, perception and activation of plant signalling pathways. The current research of elicitins has been focused on a detailed understanding of the molecular mechanisms of the elicitin recognition by plant cells. Moreover, the possibility of elicitin involvement in the establishment and enhancement of plant host resistance to a broad spectrum of pathogens has been intensively studied.

Blackleg, (stem canker, Leptosphaeria maculans), is the most economically important disease on oilseed rape (Brassica napus). Studies were undertaken to determine the effect of three different L. maculans isolates with different virulence/avirulence patterns on the expression of qualitative resistance and senescence on cotyledons of 11 B. napus genotypes. There were significant differences between genotypes (P Scotia > Amber x Commanche DH Line. Westar, Drummonds Purple Top, and Parkside were the most susceptible. L. maculans isolates D5 and D13 were the most pathogenic, showing the greatest disease index and lesion diameters. Genotypes such as LSF0530, Scotia, Duell and Barrel responded with a distinct hypersensitive response, whereas Parkside, Drummonds Purple Top, Amber x Commanche DH Line and Westar all showed a distinct yellow halo around the lesions, indicative of cotyledon senescence likely involving programmed cell death (PCD). However, expression of the distinct yellow halo symptom was not observed in all B. napus genotypes infected by one specific L. maculans isolate nor in one specific B. napus genotype challenged with all the isolates. We believe that this is the first report, i), showing the involvement of a distinct yellow halo (senescence/PCD) associated with both ‘typical’ lesions and with an expression of HR in cotyledons of winter-type B. napus genotypes challenged with L. maculans isolates and, ii), that expression of the distinct yellow halo is clearly dependent on the interaction between particular isolate virulence(s) x particular winter-type B. napus genotype combinations.

Betasatellites (DNA β) are circular ssDNA molecules that are associated with monopartite geminiviruses and exert a positive effect on the viral infection. Betasatellites encode one protein, named βC1, on the complementary strand; βC1 functions as a pathogenicity factor and RNA silencing suppressor. In this report, we describe the identification of another betasatellite-encoded protein, βVl, which also contributes to symptom development. The βVl open reading frame can be found on the viral strand of approximately 40% of reported betasatellite sequences, and is conserved in position and sequence. The presence of the βVl transcript was observed in plants infected with Tomato yellow leaf curl China virus (TYTCCNV) along with its associated betasatellite Tomato yellow leaf curl China betasatellite (TYTCCNB). Mutant viruses unable to produce βVl showed reduced virulence and decreased viral load. Ectopic expression of the TYTCCNB-PV1 gene in Nicotiana benthamiana plants from a PVX-based vector resulted in leaf mosaic and chlorosis. We further demonstrated that the βVl protein could elicit hypersensitive response (HR)-type cell death in N. benthamiana leaves. Our results uncover a novel betasatellite-encoded protein that contributes to the virus infection, and this discover gives us a more complete view of the plant-geminivirus interaction landscape.