Two venom allergen-like proteins, HaVAP1 and HaVAP2, are involved in the parasitism of Heterodera avenae

Bursaphelenchus xylophilus Venom Allergen-Like Protein BxVAP1, Triggering Plant Defense-Related Programmed Cell Death, Plays an Important Role in Regulating Pinus massoniana Terpene Defense Responses

Bursaphelenchus xylophilus (pine wood nematode, PWN), a migratory plant-parasitic nematode, acts as an etiological agent, inflicting considerable damage to pine forests worldwide. Plant immunity constitutes a crucial factor in resisting various pathogenic invasions. The primary defensive responses of host pines against PWN infection encompass terpene accumulation, defense response-related gene expression, and programmed cell death. Venom allergen-like proteins (VAPs), as potential effectors, are instrumental in facilitating the successful colonization of PWNs. In this study, we investigated the inhibition of B. xylophilus VAP (BxVAP1) expression by RNA interference in vitro. Following BxVAP1 silencing, the reproduction rate and migration rate of the PWN population in Pinus massoniana decreased, the expression of the α-pinene synthase gene was induced, other terpene synthase and pathogenesis-related genes were inhibited and delayed, the peak times and levels of terpene-related substances were changed, and the degree of cavitation in P. massoniana was diminished. Transient expression of BxVAP1 in Nicotiana benthamiana revealed that BxVAP1 was expressed in both the cell membrane and nucleus, inducing programmed cell death and the expression of pathogen-associated molecular pattern-triggered immunity marker genes (NbAcre31 and NbPTI5). This study is the first to demonstrate that silencing the BxVAP1 gene affects host defense responses, including terpenoid metabolism in P. massoniana, and that BxVAP1 can be recognized by N. benthamiana as an effector to trigger its innate immunity, expanding our understanding of the parasitic mechanism of B. xylophilus.

A root-knot nematode effector targets the Arabidopsis cysteine protease RD21A for degradation to suppress plant defense and promote parasitism

Meloidogyne incognita is one of the most widely distributed plant-parasitic nematodes and causes severe economic losses annually. The parasite produces effector proteins that play essential roles in successful parasitism. Here, we identified one such effector named MiCE108, which is exclusively expressed within the nematode subventral esophageal gland cells and is upregulated in the early parasitic stage of M. incognita. A yeast signal sequence trap assay showed that MiCE108 contains a functional signal peptide for secretion. Virus-induced gene silencing of MiCE108 impaired the parasitism of M. incognita in Nicotiana benthamiana. The ectopic expression of MiCE108 in Arabidopsis suppressed the deposition of callose, the generation of reactive oxygen species, and the expression of marker genes for bacterial flagellin epitope flg22-triggered immunity, resulting in increased susceptibility to M. incognita, Botrytis cinerea, and Pseudomonas syringae pv. tomato (Pst) DC3000. The MiCE108 protein physically associates with the plant defense protease RD21A and promotes its degradation via the endosomal-dependent pathway, or 26S proteasome. Consistent with this, knockout of RD21A compromises the innate immunity of Arabidopsis and increases its susceptibility to a broad range of pathogens, including M. incognita, strongly indicating a role in defense against this nematode. Together, our data suggest that M. incognita deploys the effector MiCE108 to target Arabidopsis cysteine protease RD21A and affect its stability, thereby suppressing plant innate immunity and facilitating parasitism.

Unveiling the Diversity: Plant Parasitic Nematode Effectors and Their Plant Interaction Partners

Root-knot and cyst nematodes are two groups of plant parasitic nematodes that cause the majority of crop losses in agriculture. As a result, these nematodes are the focus of most nematode effector research. Root-knot and cyst nematode effectors are defined as secreted molecules, typically proteins, with crucial roles in nematode parasitism. There are likely hundreds of secreted effector molecules exuded through the nematode stylet into the plant. The current research has shown that nematode effectors can target a variety of host proteins and have impacts that include the suppression of plant immune responses and the manipulation of host hormone signaling. The discovery of effectors that localize to the nucleus indicates that the nematodes can directly modulate host gene expression for cellular reprogramming during feeding site formation. In addition, plant peptide mimicry by some nematode effectors highlights the sophisticated strategies the nematodes employ to manipulate host processes. Here we describe research on the interactions between nematode effectors and host proteins that will provide insights into the molecular mechanisms underpinning plant-nematode interactions. By identifying the host proteins and pathways that are targeted by root-knot and cyst nematode effectors, scientists can gain a better understanding of how nematodes establish feeding sites and subvert plant immune responses. Such information will be invaluable for future engineering of nematode-resistant crops, ultimately fostering advancements in agricultural practices and crop protection. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2024.

A variant of the venom allergen-like protein, DdVAP2, is required for the migratory endoparasitic plant nematode Ditylenchus destructor parasitism of plants

The potato rot nematode, Ditylenchus destructor, poses a serious threat to numerous root and tuber crops, yet the functional characterization of effectors from this migratory endoparasitic plant nematode remains limited. Despite inhabiting distinct habitats, sedentary and migratory plant parasitic nematodes share the structurally conserved effectors, such as venom allergen-like proteins (VAPs). In this study, a variant of DdVAP2 was cloned from D. destructor. The transcription profile analysis revealed that DdVAP2 was higher expressed in D. destructor feeding on either potato or sweet potato compared to on fungus via qRT-PCR. And DdVAP2 was highly expressed at all life stages feeding on sweet potato, except for eggs. DdVAP2 was confirmed to be specifically expressed in the subventral esophageal glands of D. destructor through in situ hybridization assays. Combined with functional validation of the signal peptide of DdVAP2, it suggested that DdVAP2 could be secreted from nematode into host. Heterologous expression of DdVAP2 in Nicotiana benthamiana revealed that the protein localized in both cytosol and nuclei of plant cells. Knocking down DdVAP2 by RNAi in D. destructor resulted in infection and reproduction defects on plants. All the results suggest that DdVAP2 plays a crucial role in the interaction between D. destructor and plants by facilitating the nematode infection.

No Pairwise Interactions of GmSNAP18, GmSHMT08 and AtPR1 with Suppressed AtPR1 Expression Enhance the Susceptibility of Arabidopsis to Beet Cyst Nematode

GmSNAP18 and GmSHMT08 are two major genes conferring soybean cyst nematode (SCN) resistance in soybean. Overexpression of either of these two soybean genes would enhance the susceptibility of Arabidopsis to beet cyst nematode (BCN), while overexpression of either of their corresponding orthologs in Arabidopsis, AtSNAP2 and AtSHMT4, would suppress it. However, the mechanism by which these two pairs of orthologous genes boost or inhibit BCN susceptibility of Arabidopsis still remains elusive. In this study, Arabidopsis with simultaneously overexpressed GmSNAP18 and GmSHMT0 suppressed the growth of underground as well as above-ground parts of plants. Furthermore, Arabidopsis that simultaneously overexpressed GmSNAP18 and GmSHMT08 substantially stimulated BCN susceptibility and remarkably suppressed expression of AtPR1 in the salicylic acid signaling pathway. However, simultaneous overexpression of GmSNAP18 and GmSHMT08 did not impact the expression of AtJAR1 and AtHEL1 in the jasmonic acid and ethylene signaling pathways. GmSNAP18, GmSHMT08, and a pathogenesis-related (PR) protein, GmPR08-Bet VI, in soybean, and AtSNAP2, AtSHMT4, and AtPR1 in Arabidopsis could interact pair-wisely for mediating SCN and BCN resistance in soybean and Arabidopsis, respectively. Both AtSNAP2 and AtPR1 were localized on the plasma membrane, and AtSHMT4 was localized both on the plasma membrane and in the nucleus of cells. Nevertheless, after interactions, AtSNAP2 and AtPR1 could partially translocate into the cell nucleus. GmSNAP18 interacted with AtSHMT4, and GmSHMT4 interacted with AtSNAP2. However, neither GmSNAP18 nor GmSHMT08 interacted with AtPR1. Thus, no pairwise interactions among α-SNAPs, SHMTs, and AtPR1 occurred in Arabidopsis overexpressing either GmSNAP18 or GmSHMT08, or both of them. Transgenic Arabidopsis overexpressing either GmSNAP18 or GmSHMT08 substantially suppressed AtPR1 expression, while transgenic Arabidopsis overexpressing either AtSNAP2 or AtSHMT4 remarkably enhanced it. Taken together, no pairwise interactions of GmSNAP18, GmSHMT08, and AtPR1 with suppressed expression of AtPR1 enhanced BCN susceptibility in Arabidopsis. This study may provide a clue that nematode-resistant or -susceptible functions of plant genes likely depend on both hosts and nematode species.

Nuclear effectors of plant pathogens: Distinct strategies to be one step ahead

Nuclear effector proteins released by bacteria, oomycete, nematode, and fungi burden the global environment and crop yield. Microbial effectors are key weapons in the evolutionary arms race between plants and pathogens, vital in determining the success of pathogenic colonization. Secreted effectors undermine a multitude of host cellular processes depending on their target destination. Effectors are classified by their localization as either extracellular (apoplastic) or intracellular. Intracellular effectors can be further subclassified by their compartment such as the nucleus, cytoplasm or chloroplast. Nuclear effectors bring into question the role of the plant nucleus' intrinsic defence strategies and their vulnerability to effector-based manipulation. Nuclear effectors interfere with multiple nuclear processes including the epigenetic regulation of the host chromatin, the impairment of the trans-kingdom antifungal RNAi machinery, and diverse classes of immunity-associated host proteins. These effector-targeted pathways are widely conserved among different hosts and regulate a broad array of plant cellular processes. Thus, these nuclear sites constitute meaningful targets for effectors to subvert the plant defence system and acquire resources for pathogenic propagation. This review provides an extensive and comparative compilation of diverse plant microbe nuclear effector libraries, thereby highlighting the distinct and conserved mechanisms these effectors employ to modulate plant cellular processes for the pathogen's profit.

The function of plant PR1 and other members of the CAP protein superfamily in plant-pathogen interactions

The pathogenesis-related (PR) proteins of plants have originally been identified as proteins that are strongly induced upon biotic and abiotic stress. These proteins fall into 17 distinct classes (PR1-PR17). The mode of action of most of these PR proteins has been well characterized, except for PR1, which belongs to a widespread superfamily of proteins that share a common CAP domain. Proteins of this family are not only expressed in plants but also in humans and in many different pathogens, including phytopathogenic nematodes and fungi. These proteins are associated with a diverse range of physiological functions. However, their precise mode of action has remained elusive. The importance of these proteins in immune defence is illustrated by the fact that PR1 overexpression in plants results in increased resistance against pathogens. However, PR1-like CAP proteins are also produced by pathogens and deletion of these genes results in reduced virulence, suggesting that CAP proteins can exert both defensive and offensive functions. Recent progress has revealed that plant PR1 is proteolytically cleaved to release a C-terminal CAPE1 peptide, which is sufficient to activate an immune response. The release of this signalling peptide is blocked by pathogenic effectors to evade immune defence. Moreover, plant PR1 forms complexes with other PR family members, including PR5, also known as thaumatin, and PR14, a lipid transfer protein, to enhance the host's immune response. Here, we discuss possible functions of PR1 proteins and their interactors, particularly in light of the fact that these proteins can bind lipids, which have important immune signalling functions.

Opposite Beet Cyst Nematode Infection Phenotypes of Transgenic Arabidopsis Between Overexpressing GmSNAP18 and AtSNAP2 and Between Overexpressing GmSHMT08 and AtSHMT4

The rhg1-a GmSNAP18 (an α-SNAP) and Rhg4 GmSHMT08 are two major cloned genes conferring soybean cyst nematode resistance in Peking-type soybeans, but the application of α-SNAPs and SHMTs in cyst nematode management remains elusive. In this study, GmSNAP18 and GmSHMT08, together with their orthologs in Arabidopsis, AtSNAP2 (an α-SNAP) and AtSHMT4, were individually transformed into Arabidopsis Col-0 to generate the transgenic lines, and the growth of transgenic plants, beet cyst nematode (BCN) infection phenotypes, and AtSNAP2, AtSHMT4, and AtPR1 expression patterns were analyzed using Arabidopsis-BCN compatible interaction system, in addition with protein-protein interaction assay. Pulldown and BiFC assays revealed that GmSNAP18 and GmSHMT08 interacted with AtSHMT4 and AtSNAP2, respectively. Plant root growth was not impacted by overexpression of GmSNAP18 and AtSNAP2. However, overexpression of GmSHMT08 and AtSHMT4 both increased plant height, additionally, overexpression of GmSHMT08 decreased rosette leaf size. Overexpression of GmSNAP18 and GmSHMT08 both suppressed AtPR1 expression and significantly enhanced BCN susceptibility, while overexpression of AtSNAP2 and AtSHMT4 both substantially boosted AtPR1 expression and remarkably enhanced BCN resistance, in transgenic Arabidopsis. Overexpression of GmSNAP18 reduced, while overexpression of AtSNAP2 unaltered AtSHMT4 expression. Overexpression of GmSHMT08 and AtSHMT4 both suppressed AtSNAP2 expression in transgenic Arabidopsis. Thus, different expression patterns of AtPR1 and AtSHMT4 are likely associated with opposite BCN infection phenotypes of Arabidopsis between overexpressing GmSNAP18 and AtSNAP2, and between overexpressing GmSHMT08 and AtSHMT4; and boosted AtPR1 expression are required for enhanced BCN resistance in Arabidopsis. All these results establish a basis for extension of α-SNAPs and SHMTs in cyst nematode management.

Beet cyst nematode HsSNARE1 interacts with both AtSNAP2 and AtPR1 and promotes disease in Arabidopsis

INTRODUCTION

Plant parasitic cyst nematodes secrete a number of effectors into hosts to initiate formation of syncytia and infection causing huge yield losses.

OBJECTIVES

The identified cyst nematode effectors are still limited, and the cyst nematode effectors-involved interaction mechanisms between cyst nematodes and plants remain largely unknown.

METHODS

The t-SNARE domain-containing effector in beet cyst nematode (BCN) was identified by In situ hybridization and immunohistochemistry analyses. The mutant of effector gene was designed by protein structure modeling analysis. The functions of effector gene and its mutant were analyzed by genetic transformation in Arabidopsis and infection by BCN. The protein-protein interaction was analyzed by yeast two hybrid, BiFC and pulldown assays. Gene expression was assayed by quantitative real-time PCR.

RESULTS

A t-SNARE domain-containing BCN HsSNARE1 was identified as an effector, and its mutant HsSNARE1-M1 carrying three mutations (E141D, A143T and -148S) that altered regional structure from random coils to α-helixes was designed and constructed. Transgenic analyses indicated that expression of HsSNARE1 significantly enhanced while expression of HsSNARE1-M1 and highly homologous HgSNARE1 remarkably suppressed BCN susceptibility of Arabidopsis. HsSNARE1 interacted with AtSNAP2 and AtPR1 via its t-SNARE domain and N-terminal, respectively, while HsSNARE1-M1/HgSNARE1 could not interact with AtPR1 but bound AtSNAP2. AtSNAP2, AtSHMT4 and AtPR1 interacted pairwise, but neither HsSNARE1 nor HsSNARE1-M1/HgSNARE1 could interact with AtSHMT4. Expression of HsSNARE1 significantly suppressed while expression of HsSNARE1-M1/HgSNARE1 considerably induced both AtSHMT4 and AtPR1 in transgenic Arabidopsis infected with BCN. Overexpression of AtPR1 significantly suppressed BCN susceptibility of Arabidopsis.

CONCLUSIONS

This work identified a t-SNARE-domain containing cyst nematode effector HsSNARE1 and deciphered a molecular mode of action of the t-SNARE-domain containing cyst nematode effectors that HsSNARE1 promotes cyst nematode disease by interaction with both AtSNAP2 and AtPR1 and significant suppression of both AtSHMT4 and AtPR1, which is mediated by three structure change-causing amino acid residues.

The Meloidogyne javanica effector Mj2G02 interferes with jasmonic acid signalling to suppress cell death and promote parasitism in Arabidopsis

Plant-parasitic nematodes can cause devastating damage to crops. These nematodes secrete effectors that suppress the host immune responses to enhance their survival. In this study, Mj2G02, an effector from Meloidogyne javanica, is described. In situ hybridization and transcriptional analysis showed that Mj2G02 was highly expressed in the early infection stages and exclusively expressed in the nematode subventral oesophageal gland cells. In planta RNA interference targeting Mj2G02 impaired M. javanica parasitism, and Mj2G02-transgenic Arabidopsis lines displayed more susceptibility to M. javanica. Using an Agrobacterium-mediated transient expression system and plant immune response assays, we demonstrated that Mj2G02 localized in the plant cell nuclei and could suppress Gpa2/RBP-1-induced cell death. Moreover, by RNA-Seq and quantitative reverse transcription PCR analyses, we showed that Mj2G02 was capable of interfering with the host jasmonic acid (JA) signalling pathway. Multiple jasmonate ZIM-domain (JAZ) genes were significantly upregulated, whereas the JAR1 gene and four JA-responsive genes, MYC3, UPI, THI2.1, and WRKY75, were significantly downregulated. In addition, HPLC analysis showed that the endogenous jasmonoyl-isoleucine (JA-Ile) level in Mj2G02-transgenic Arabidopsis lines was significantly decreased compared to that in wildtype plants. Our results indicate that the M. javanica effector Mj2G02 suppresses the plant immune response, therefore facilitating nematode parasitism. This process is probably mediated by a JA-Ile reduction and JAZ enhancement to repress JA-responsive genes.

The Meloidogyne graminicola effector MgMO289 targets a novel copper metallochaperone to suppress immunity in rice

Recent studies have reported that plant-parasitic nematodes facilitate their infection by suppressing plant immunity via effectors, but the inhibitory mechanisms remain poorly understood. This study found that a novel effector MgMO289 is exclusively expressed in the dorsal esophageal gland of Meloidogyne graminicola and is up-regulated at parasitic third-/fourth-stage juveniles. In planta silencing of MgMO289 substantially increased plant resistance to M. graminicola. Moreover, we found that MgMO289 interacts with a new rice copper metallochaperone heavy metal-associated plant protein 04 (OsHPP04), and that rice cytosolic COPPER/ZINC -SUPEROXIDE DISMUTASE 2 (cCu/Zn-SOD2) is the target of OsHPP04. Rice plants overexpressing OsHPP04 or MgMO289 exhibited an increased susceptibility to M. graminicola and a higher Cu/Zn-SOD activity, but lower O2•- content, when compared with wild-type plants. Meanwhile, immune response assays showed that MgMO289 could suppress host innate immunity. These findings reveal a novel pathway for a plant pathogen effector that utilizes the host O2•--scavenging system to eliminate O2•- and suppress plant immunity.

A Venom Allergen-Like Protein, RsVAP, the First Discovered Effector Protein of Radopholus similis That Inhibits Plant Defense and Facilitates Parasitism

Radopholus similis is a migratory endoparasitic nematode that is extremely harmful to host plants. Venom allergen-like proteins (VAPs) are members of the cysteine-rich secretory protein family that are widely present in plants and animals. In this study, we cloned a VAP gene from R. similis, designated as RsVAP. RsVAP contains an open reading frame of 1089 bp encoding 362 amino acids. RsVAP is specifically expressed in the esophageal gland, and the expression levels of RsVAP are significantly higher in juveniles than in other life stages of R. similis. This expression pattern of RsVAP was consistent with the biological characteristics of juveniles of R. similis, which have the ability of infection and are the main infection stages of R. similis. The pathogenicity and reproduction rate of R. similis in tomato was significantly attenuated after RsVAP was silenced. In tobacco leaves transiently expressing RsVAP, the pathogen-associated molecular pattern-triggered immunity (PTI) induced by a bacterial flagellin fragment (flg22) was inhibited, while the cell death induced by two sets of immune elicitors (BAX and Gpa2/RBP-1) was repressed. The RsVAP-interacting, ras-related protein RABA1d (LeRabA1d) was identified in tomato hosts by yeast two-hybrid and co-immunoprecipitation assays. RsVAP may interact with LeRabA1d to affect the host defense response, which in turn facilitates nematode infection. This study provides the first evidence for the inhibition of plant defense response by a VAP from migratory plant-parasitic nematodes, and, for the first time, the target protein of R. similis in its host was identified.

Expression interference of a number of Heterodera avenae conserved genes perturbs nematode parasitic success in Triticum aestivum

The cereal cyst nematode, Heterodera avenae is distributed worldwide and causes substantial damage in bread wheat, Triticum aestivum. This nematode is extremely difficult to manage because of its prolonged persistence as unhatched eggs encased in cysts. Due to its sustainable and target-specific nature, RNA interference (RNAi)-based strategy has gained unprecedented importance for pest control. To date, RNAi strategy has not been exploited to manage H. avenae in wheat. In the present study, 40 H. avenae target genes with different molecular function were rationally selected for in vitro soaking analysis in order to assess their susceptibility to RNAi. In contrast to target-specific downregulation of 18 genes, 7 genes were upregulated and 15 genes showed unaltered expression (although combinatorial soaking showed some of these genes are RNAi susceptible), suggesting that a few of the target genes were refractory or recalcitrant to RNAi. However, RNAi of 37 of these genes negatively altered nematode behavior in terms of reduced penetration, development and reproduction in wheat. Subsequently, wheat plants were transformed with seven H. avenae target genes (that showed greatest abrogation of nematode parasitic success) for host-induced gene silencing (HIGS) analysis. Transformed plants were molecularly characterized by PCR, RT-qPCR and Southern hybridization. Production of target gene-specific double- and single-stranded RNA (dsRNA/siRNA) was detected in transformed plants. Transgenic expression of galectin, cathepsin L, vap1, serpin, flp12, RanBPM and chitinase genes conferred 33.24-72.4 % reduction in H. avenae multiplication in T₁ events with single copy ones exhibiting greatest reduction. A similar degree of resistance observed in T₂ plants indicated the consistent HIGS effect in the subsequent generations. Intriguingly, cysts isolated from RNAi plants were of smaller size with translucent cuticle compared to normal size, dark brown control cysts, suggesting H. avenae developmental retardation due to HIGS. Our study reinforces the potential of HIGS to manage nematode problems in crop plant.

Acidovorax citrulli Type III Effector AopP Suppresses Plant Immunity by Targeting the Watermelon Transcription Factor WRKY6

Acidovorax citrulli (Ac) is the causal agent of bacterial fruit blotch (BFB), and BFB poses a threat to global watermelon production. Despite its economic importance, the molecular mechanisms underlying Ac pathogenicity and virulence are not well understood, particularly with regard to its type III secreted effectors. We identify a new effector, AopP, in Ac and confirm its secretion and translocation. AopP suppresses reactive oxygen species burst and salicylic acid (SA) content and significantly contributes to virulence. Interestingly, AopP interacts with a watermelon transcription factor, ClWRKY6, in vivo and in vitro. ClWRKY6 shows typical nuclear localization, and AopP and ClWRKY6 co-localize in the nucleus. Ac infection, SA, and the pathogen-associated molecular pattern flg22 ^Ac promote ClWRKY6 production, suggesting that ClWRKY6 is involved in plant immunity and SA signaling. Furthermore, ClWRKY6 positively regulates PTI and SA production when expressed in Nicotiana benthamiana. Importantly, AopP reduces ClWRKY6 mRNA and ClWRKY6 protein levels, suggesting that AopP suppresses plant immunity by targeting ClWRKY6. In summary, we identify a novel effector associated with the virulence mechanism of Ac, which interacts with the transcription factor of the natural host, watermelon. The findings of this study provide insights into the mechanisms of watermelon immune responses and may facilitate molecular breeding for bacterial fruit blotch resistance.

Identification and Functional Analysis of AopN, an Acidovorax Citrulli Effector that Induces Programmed Cell Death in Plants

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli, seriously affects watermelon and other cucurbit crops, resulting in significant economic losses. However, the pathogenicity mechanism of A. citrulli is not well understood. Plant pathogenic bacteria often suppress the plant immune response by secreting effector proteins. Thus, identifying A. citrulli effector proteins and determining their functions may improve our understanding of the underlying pathogenetic mechanisms. In this study, a novel effector, AopN, which is localized on the cell membrane of Nicotiana benthamiana, was identified. The functional analysis revealed that AopN significantly inhibited the flg22-induced reactive oxygen species burst. AopN induced a programmed cell death (PCD) response. Unlike its homologous protein, the ability of AopN to induce PCD was dependent on two motifs of unknown functions (including DUP4129 and Cpta_toxin), but was not dependent on LXXLL domain. More importantly, the virulence of the aopN mutant of A. citrulli in N. benthamiana significantly decreased, indicating that it was a core effector. Further analysis revealed that AopN interacted with watermelon ClHIPP and ClLTP, which responds to A. citrulli strain Aac5 infection at the transcription level. Collectively, these findings indicate that AopN suppresses plant immunity and activates the effector-triggered immunity pathway.

Analyses of the Root-Knot Nematode (Meloidogyne graminicola) Transcriptome during Host Infection Highlight Specific Gene Expression Profiling in Resistant Rice Plants

The plant-parasitic nematode Meloidogyne graminicola causes considerable damages to rice (Oryza sativa) culture. Resistance to M. graminicola in the related species Oryza glaberrima reduces root penetration by juveniles and stops further nematode development. M. graminicola genes expressed during O. sativa infection were previously characterized but no information is available about the molecular dialogue established with a resistant plant. We compared the M. graminicola transcriptomes of stage-two juveniles (J2s) before and during infection of susceptible or resistant rice. Among 36,121 M. graminicola genes surveyed, 367 were differentially expressed during infection of resistant or susceptible plants. Genes encoding cell wall-degrading enzymes, peptidases and neuropeptides were expressed for a longer time in resistant plants compared to susceptible plants. Conversely, genes related to nematode development were not activated in the resistant host. The majority of M. graminicola effector genes had similar expression patterns, whatever the host genotype. However, two venom allergen-like protein (VAP)-encoding genes were specifically induced in resistant plants and Mg-VAP1 silencing in J2s reduced their ability to colonize roots. This study highlighted that M. graminicola adapts its gene expression to the host susceptibility. Further investigation is required to assess the role of Mg-VAPs in the rice-nematode interaction.

Plant Proteins and Processes Targeted by Parasitic Nematode Effectors

Sedentary endoparasitic nematodes, such as root-knot nematodes (RKN; Meloidogyne spp.) and cyst nematodes (CN; Heterodera spp. and Globodera spp.) cause considerable damage to agricultural crops. RKN and CN spend most of their life cycle in plant roots, in which they induce the formation of multinucleate hypertrophied feeding cells, called "giant cells" and "syncytia," respectively. The giant cells result from nuclear divisions of vascular cells without cytokinesis. They are surrounded by small dividing cells and they form a new organ within the root known as a root knot or gall. CN infection leads to the fusion of several root cells into a unique syncytium. These dramatically modified host cells act as metabolic sinks from which the nematode withdraws nutrients throughout its life, and they are thus essential for nematode development. Both RKN and CN secrete effector proteins that are synthesized in the oesophageal glands and delivered to the appropriate cell in the host plant via a syringe-like stylet, triggering the ontogenesis of the feeding structures. Within the plant cell or in the apoplast, effectors associate with specific host proteins, enabling them to hijack important processes for cell morphogenesis and physiology or immunity. Here, we review recent findings on the identification and functional characterization of plant targets of RKN and CN effectors. A better understanding of the molecular determinants of these biotrophic relationships would enable us to improve the yields of crops infected with parasitic nematodes and to expand our comprehension of root development.

Host-induced silencing of Mi-msp-1 confers resistance to root-knot nematode Meloidogyne incognita in eggplant

RNA interference (RNAi)-based host-induced gene silencing (HIGS) is emerging as a novel, efficient and target-specific tool to combat phytonematode infection in crop plants. Mi-msp-1, an effector gene expressed in the subventral pharyngeal gland cells of Meloidogyne incognita plays an important role in the parasitic process. Mi-msp-1 effector is conserved in few of the species of root-knot nematodes (RKNs) and does not share considerable homology with the other phytonematodes, thereby making it a suitable target for HIGS with minimal off-target effects. Six putative eggplant transformants harbouring a single copy RNAi transgene of Mi-msp-1 was generated. Stable expression of the transgene was detected in T₁, T₂ and T₃ transgenic lines for which a detrimental effect on RKN penetration, development and reproduction was documented upon challenge infection with nematode juveniles. The post-parasitic nematode stages extracted from the transgenic plants showed long-term RNAi effect in terms of targeted downregulation of Mi-msp-1. These findings suggest that HIGS of Mi-msp-1 enhances nematode resistance in eggplant and protect the plant against RKN parasitism at very early stage.

Resistance to Cereal Cyst Nematodes in Wheat and Barley: An Emphasis on Classical and Modern Approaches

Cereal cyst nematodes (CCNs) are among the most important nematode pests that limit production of small grain cereals like wheat and barley. These nematodes alone are estimated to reduce production of crops by 10% globally. This necessitates a huge enhancement of nematode resistance in cereal crops against CCNs. Nematode resistance in wheat and barley in combination with higher grain yields has been a preferential research area for cereal nematologists. This usually involved the targeted genetic exploitations through natural means of classical selection breeding of resistant genotypes and finding quantitative trait luci (QTLs) associated with resistance genes. These improvements were based on available genetic diversity among the crop plants. Recently, genome-wide association studies have widely been exploited to associate nematode resistance or susceptibility with particular regions of the genome. Use of biotechnological tools through the application of various transgenic strategies for enhancement of nematode resistance in various crop plants including wheat and barley had also been an important area of research. These modern approaches primarily include the use of gene silencing, exploitation of nematode effector genes, proteinase inhibitors, chemodisruptive peptides and a combination of one or more of these approaches. Furthermore, the perspective genome editing technologies including CRISPR-Cas9 could also be helpful for improving CCN resistance in wheat and barley. The information provided in this review will be helpful to enhance resistance against CCNs and will attract the attention of the scientific community towards this neglected area.