The Novel Secreted Meloidogyne incognita Effector MiISE6 Targets the Host Nucleus and Facilitates Parasitism in Arabidopsis

Greater than the sum of their parts: an overview of the AvrRps4 effector family

Phytopathogenic microbes use secreted effector proteins to increase their virulence in planta. If these effectors or the results of their activity are detected by the plant cell, the plant will mount an immune response which applies evolutionary pressure by reducing growth and success of the pathogen. Bacterial effector proteins in the AvrRps4 family (AvrRps4, HopK1, and XopO) have commonly been used as tools to investigate plant immune components. At the same time, the in planta functions of this family of effectors have yet to be fully characterized. In this minireview we summarize current knowledge about the AvrRps4 effector family with emphasis on properties of the proteins themselves. We hypothesize that the HopK1 C-terminus and the AvrRps4 C-terminus, though unrelated in sequence and structure, are broadly related in functions that counteract plant defense responses.

Meloidogyne enterolobii MeMSP1 effector targets the glutathione-S-transferase phi GSTF family in Arabidopsis to manipulate host metabolism and promote nematode parasitism

Meloidogyne enterolobii is an emerging root-knot nematode species that overcomes most of the nematode resistance genes in crops. Nematode effector proteins secreted in planta are key elements in the molecular dialogue of parasitism. Here, we show the MeMSP1 effector is secreted into giant cells and promotes M. enterolobii parasitism. Using co-immunoprecipitation and bimolecular fluorescent complementation assays, we identified glutathione-S-transferase phi GSTFs as host targets of the MeMSP1 effector. This protein family plays important roles in plant responses to abiotic and biotic stresses. We demonstrate that MeMSP1 interacts with all Arabidopsis GSTF. Moreover, we confirmed that the N-terminal region of AtGSTF9 is critical for its interaction, and atgstf9 mutant lines are more susceptible to root-knot nematode infection. Combined transcriptome and metabolome analyses showed that MeMSP1 affects the metabolic pathways of Arabidopsis thaliana, resulting in the accumulation of amino acids, nucleic acids, and their metabolites, and organic acids and the downregulation of flavonoids. Our study has shed light on a novel effector mechanism that targets plant metabolism, reducing the production of plant defence-related compounds while favouring the accumulation of metabolites beneficial to the nematode, and thereby promoting parasitism.

Counter-attack of biocontrol agents: Environmentally benign Approaches against Root-knot nematodes (Meloidogyne spp.) on Agricultural crops

Root-knot nematodes (Meloidogyne spp.) are obligate sedentary endoparasites, considered severe crop-damaging taxa among all plant-parasitic nematodes globally. Their attacks through parasitic proteins alter the physiology and machinery of the host cells to favour parasitism and reduction in crop yield. Currently, the use of excessive pesticides as a fast remedy to manage this pest is hazardous for both the environment and humans. Keeping this view in mind, there is an urgent need for developing efficient eco-friendly strategies. Bio-control as an eco-friendly is considered the best approach to manage nematodes without disturbing non-target microbes. In bio-control, living agents such as fungi and bacteria are the natural enemies of nematodes and the best substitute for pesticides. Fungi, including nematode-trapping fungi, can sense host signals and produce special trapping devices viz., constricting rings and adhesive knobs/loops, to capture nematodes and kill them. Whereas, endo-parasitic fungi kill nematodes by enzymatic secretions and spore adhesion through their hyphae. Bacteria can also control nematodes by producing antibiotic compounds, competing for nutrients and rhizosphere, production of hydrolytic enzymes viz., chitinases, proteases, lipases, and induction of systemic resistance (ISR) in host plants. Scientists throughout the world are trying to evolve environmentally benign methods that sustain agricultural production and keep nematodes below a threshold level. Whatever methods evolve, in the future the focus should be on important aspects like green approaches for managing nematodes without disturbing human health and the environment.

Volatile Organic Compounds of Bacillus pumilus Strain S1-10 Exhibit Fumigant Activity Against Meloidogyne incognita

Root-knot nematodes (RKNs) are highly specialized parasites that cause significant yield losses worldwide. In this study, we isolated Bacillus pumilus strain S1-10 from the rhizosphere soil of Zingiber officinale Rosc. plants and evaluated its fumigant activity against Meloidogyne incognita. S1-10 exhibited a strong repellent effect on second-stage juveniles (J2s) of M. incognita, and in vitro assays indicated that S1-10 volatile organic compounds (VOCs) suppressed J2 activity and egg hatching. Under greenhouse conditions, 71 and 79% reductions of nematodes and eggs were detected on plants treated with S-10 VOCs compared with controls. Ten VOCs were identified through gas chromatography and mass spectrometry (GC-MS), of which 2-(methylamino)-ethanol (2-ME) had strong fumigant activity against J2s of M. incognita, with an LC50 value of 1.5 mM at 12 h. These results indicate that S1-10 represents a potential novel biocontrol agent for RKNs.

Biotechnological Tools to Elucidate the Mechanism of Plant and Nematode Interactions

Plant-parasitic nematodes (PPNs) pose a threat to global food security in both the developed and developing worlds. PPNs cause crop losses worth a total of more than USD 150 billion worldwide. The sedentary root-knot nematodes (RKNs) also cause severe damage to various agricultural crops and establish compatible relationships with a broad range of host plants. This review aims to provide a broad overview of the strategies used to identify the morpho-physiological and molecular events that occur during RKN parasitism. It describes the most current developments in the transcriptomic, proteomic, and metabolomic strategies of nematodes, which are important for understanding compatible interactions of plants and nematodes, and several strategies for enhancing plant resistance against RKNs. We will highlight recent rapid advances in molecular strategies, such as gene-silencing technologies, RNA interference (RNAi), and small interfering RNA (siRNA) effector proteins, that are leading to considerable progress in understanding the mechanism of plant-nematode interactions. We also take into account genetic engineering strategies, such as targeted genome editing techniques, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) (CRISPR/Cas-9) system, and quantitative trait loci (QTL), to enhance the resistance of plants against nematodes.

Chromosome-scale genome assembly-assisted identification of Mi-9 gene in Solanum arcanum accession LA2157, conferring heat-stable resistance to Meloidogyne incognita

Root-knot nematodes (RKNs) are infamous plant pathogens in tomato production, causing considerable losses in agriculture worldwide. Mi-1 is the only commercially available RKN-resistance gene; however, the resistance is inactivated when the soil temperature is over 28 °C. Mi-9 in wild tomato (Solanum arcanum LA2157) has stable resistance to RKNs under high temperature but has not been cloned and applied. In this study, a chromosome-scale genome assembly of S. arcanum LA2157 was constructed through Nanopore and Hi-C sequencing. Based on molecular markers of Mi-9 and comparative genomic analysis, the localization region and candidate Mi-9 genes cluster consisting of seven nucleotide-binding sites and leucine-rich repeat (NBS-LRR) genes were located. Transcriptional expression profiles confirmed that five of the seven candidate genes were expressed in root tissue. Moreover, virus-induced gene silencing of the Sarc_034200 gene resulted in increased susceptibility of S. arcanum LA2157 to Meloidogyne incognita, and genetic transformation of the Sarc_034200 gene in susceptible Solanum pimpinellifolium conferred significant resistance to M. incognita at 25 °C and 30 °C and showed hypersensitive responses at nematode infection sites. This suggested that Sarc_034200 is the Mi-9 gene. In summary, we cloned, confirmed and applied the heat-stable RKN-resistance gene Mi-9, which is of great significance to tomato breeding for nematode resistance.

The root-knot nematode effector MiMSP32 targets host 12-oxophytodienoate reductase 2 to regulate plant susceptibility

To establish persistent infections in host plants, herbivorous invaders, such as root-knot nematodes, must rely on effectors for suppressing damage-induced jasmonate-dependent host defenses. However, at present, the effector mechanisms targeting the biosynthesis of biologically active jasmonates to avoid adverse host responses are unknown. Using yeast two-hybrid, in planta co-immunoprecipitation, and mutant analyses, we identified 12-oxophytodienoate reductase 2 (OPR2) as an important host target of the stylet-secreted effector MiMSP32 of the root-knot nematode Meloidogyne incognita. MiMSP32 has no informative sequence similarities with other functionally annotated genes but was selected for the discovery of novel effector mechanisms based on evidence of positive, diversifying selection. OPR2 catalyzes the conversion of a derivative of 12-oxophytodienoate to jasmonic acid (JA) and operates parallel to 12-oxophytodienoate reductase 3 (OPR3), which controls the main pathway in the biosynthesis of jasmonates. We show that MiMSP32 targets OPR2 to promote parasitism of M. incognita in host plants independent of OPR3-mediated JA biosynthesis. Artificially manipulating the conversion of the 12-oxophytodienoate by OPRs increases susceptibility to multiple unrelated plant invaders. Our study is the first to shed light on a novel effector mechanism targeting this process to regulate the susceptibility of host plants.

Rooting Out the Mechanisms of Root-Knot Nematode-Plant Interactions

Root-knot nematodes (RKNs; Meloidogyne spp.) engage in complex parasitic interactions with many different host plants around the world, initiating elaborate feeding sites and disrupting host root architecture. Although RKNs have been the focus of research for many decades, new molecular tools have provided useful insights into the biological mechanisms these pests use to infect and manipulate their hosts. From identifying host defense mechanisms underlying resistance to RKNs to characterizing nematode effectors that alter host cellular functions, the past decade of research has significantly expanded our understanding of RKN-plant interactions, and the increasing number of quality parasite and host genomes promises to enhance future research efforts into RKNs. In this review, we have highlighted recent discoveries, summarized the current understanding within the field, and provided links to new and useful resources for researchers. Our goal is to offer insights and tools to support the study of molecular RKN-plant interactions.

In planta RNAi approach targeting three M. incognita effector genes disturbed the process of infection and reduced plant susceptibility

Meloidogyne incognita is the most economically important species of the root-knot nematode complex causing damage to several crops worldwide. During parasitism in host plants, M. incognita secretes several effector proteins to suppress the plant immune system, manipulate the plant cell cycle, and promote parasitism. Several effector proteins have been identified, but their relationship with plant parasitism by M. incognita has not been fully confirmed. Herein, the Minc01696, Minc00344, and Minc00801 putative effector genes were evaluated to assess their importance during soybean and Nicotiana tabacum parasitism by M. incognita. For this study, we used in planta RNAi technology to overexpress dsRNA molecules capable of producing siRNAs that target and downregulate these nematode effector genes. Soybean composite roots and N. tabacum lines were successfully generated, and susceptibility level to M. incognita was evaluated. Consistently, both transgenic soybean roots and transgenic N. tabacum lines carrying the RNAi strategy showed reduced susceptibility to M. incognita. The number of galls per plant and the number of egg masses per plant were reduced by up to 85% in transgenic soybean roots, supported by the downregulation of effector genes in M. incognita during parasitism. Similarly, the number of galls per plant, the number of egg masses per plant, and the nematode reproduction factor were reduced by up to 83% in transgenic N. tabacum lines, which was also supported by the downregulation of the Minc00801 effector gene during parasitism. Therefore, our data indicate that all three effector genes can be a target in the development of new biotechnological tools based on the RNAi strategy in economically important crops for M. incognita control.

Characterization of Five Meloidogyne incognita Effectors Associated with PsoRPM3

Root-knot nematodes (RKNs) are devastating parasites that invade thousands of plants. In this study, five RKN effectors, which might interact with Prunussogdiana resistance protein PsoRPM3, were screened and identified. In situ hybridisation results showed that MiCal, MiGST_N_4, MiEFh and MiACPS are expressed in the subventral oesophageal glands (SvG), and MiTSPc hybridization signals are found in the dorsal esophageal gland (DG) of Meloidogyne incognita in the pre-J2. RT-qPCR data indicated that the expression of MiCal, MiGST_N_4, MiEFh, and MiACPS genes are highly expressed in M. incognita of pra-J2 and J3/J4 stages. The expression of MiTSPc increased significantly in the female stage of M. incognita. Moreover, all effectors found in this study localize in the cytoplasm and nucleus when transiently expressed in plant cells. In addition, MiGST_N_4, MiEFh, MiACPS and MiTSPc can elicit the ROS burst and strong hypersensitive response (HR), as well as significant ion leakage. Our data suggest that MiGST_N_4, MiEFh, MiACPS and MiTSPc effectors may be involved in triggering the immune response of the host plant.

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.

Minc00344 and Mj-NULG1a effectors interact with GmHub10 protein to promote the soybean parasitism by Meloidogyne incognita and M. javanica

Several economically important crops are susceptible to root-knot nematode (RKNs). Meloidogyne incognita and M. javanica are the two most reported species from the RKN complex, causing damage to several crops worldwide. The successful outcome of the Meloidogyne-plant interaction is associated with molecular factors secreted by the nematode to suppress the plant's immune response and promote nematode parasitism. In contrast, several plant factors are associated with defense against nematode infection. In this study, we identified and characterized the specific interaction of Minc00344 and Mj-NULG1a effectors with soybean GmHub10 (Glyma.19G008200) protein in vitro and in vivo. An Arabidopsis thaliana T-DNA mutant of AtHub10 (AT3G27960, an orthologous gene of GmHub10) showed higher susceptibility to M. incognita. Thus, since soybean and A. thaliana Hub10 proteins are involved in pollen tube growth and indirect activation of the defense response, our data suggest that effector-Hub10 interactions could be associated with an increase in plant susceptibility. These findings indicate the potential of these effector proteins to develop new biotechnological tools based on RNA interference and the overexpression of engineered Hub10 proteins for the efficient management of RKN in crops.

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.

Recent applications of biotechnological approaches to elucidate the biology of plant-nematode interactions

Plant-parasitic nematodes are a major threat to food security. The most economically important species have remarkable abilities to manipulate host physiology and immunity. This review highlights recent applications of biotechnological approaches to elucidate the underlying biology on both sides of the interaction. Their obligate biotrophic nature has hindered the development of simple nematode transformation protocols. Instead, transient or stable expression of the effector (native or tagged) in planta has been instrumental in elucidating the biology of plant-nematode interactions. Recent progress in the development of functional genetics tools 'in nematoda' promises further advances. Finally, we discuss how effector research has uncovered novel protein translocation routes in plant cells and may reveal additional unknown biological processes in the future.

Comparative Genomics Reveals Novel Target Genes towards Specific Control of Plant-Parasitic Nematodes

Plant-parasitic nematodes cause extensive annual yield losses to worldwide agricultural production. Most cultivated plants have no known resistance against nematodes and the few bearing a resistance gene can be overcome by certain species. Chemical methods that have been deployed to control nematodes have largely been banned from use due to their poor specificity and high toxicity. Hence, there is an urgent need for the development of cleaner and more specific control methods. Recent advances in nematode genomics, including in phytoparasitic species, provide an unprecedented opportunity to identify genes and functions specific to these pests. Using phylogenomics, we compared 61 nematode genomes, including 16 for plant-parasitic species and identified more than 24,000 protein families specific to these parasites. In the genome of Meloidogyne incognita, one of the most devastating plant parasites, we found ca. 10,000 proteins with orthologs restricted only to phytoparasitic species and no further homology in protein databases. Among these phytoparasite-specific proteins, ca. 1000 shared the same properties as known secreted effectors involved in essential parasitic functions. Of these, 68 were novel and showed strong expression during the endophytic phase of the nematode life cycle, based on both RNA-seq and RT-qPCR analyses. Besides effector candidates, transcription-related and neuro-perception functions were enriched in phytoparasite-specific proteins, revealing interesting targets for nematode control methods. This phylogenomics analysis constitutes a unique resource for the further understanding of the genetic basis of nematode adaptation to phytoparasitism and for the development of more efficient control methods.

UvHrip1, an effector secreted by Ustilaginoidea virens, suppresses basal defense and promotes disease development in Arabidopsis thaliana

Rice false smut (RFS), caused by Ustilaginoidea virens, is one of the most detrimental rice fungal diseases and pose a severe threat to rice production and quality. Effectors in U. virens often act as a set of essential virulence factors that play crucial roles in the interaction between host and the pathogen. Thus, the functions of each effector in U. virens need to be further explored. Here, a conserved small secreted hypersensitive response-inducing protein (hrip) was named UvHrip1. Functional validation was investigated to prove that UvHrip1 suppressed cell death symptom and ROS accumulation in Nicotiana benthamiana triggered by Burkholderia glumae. We performed transgenic technology to demonstrate UvHrip1 remarkably inhibited pathogen-associated molecular pattern-induced defense responses in Arabidopsis seedlings and plants, including the expression of defense-response genes. Furthermore, disease progression caused by the type III secretion system-defective mutant from Pseudomonas syringae pv. tomato DC3000 was strongly facilitated in transgenic Arabidopsis ectopic expressing UvHrip1. Our data demonstrated UvHrip1 suppresses plant innate immunity and promoting disease multiplication in Arabidopsis.

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.

BSA-Seq Discovery and Functional Analysis of Candidate Hessian Fly (Mayetiola destructor) Avirulence Genes

The Hessian fly (HF, Mayetiola destructor) is a plant-galling parasite of wheat (Triticum spp.). Seven percent of its genome is composed of highly diversified signal-peptide-encoding genes that are transcribed in HF larval salivary glands. These observations suggest that they encode effector proteins that are injected into wheat cells to suppress basal wheat immunity and redirect wheat development towards gall formation. Genetic mapping has determined that mutations in four of these genes are associated with HF larval survival (virulence) on plants carrying four different resistance (R) genes. Here, this line of investigation was pursued further using bulked-segregant analysis combined with whole genome resequencing (BSA-seq). Virulence to wheat R genes H6, Hdic, and H5 was examined. Mutations associated with H6 virulence had been mapped previously. Therefore, we used H6 to test the capacity of BSA-seq to map virulence using a field-derived HF population. This was the first time a non-structured HF population had been used to map HF virulence. Hdic virulence had not been mapped previously. Using a structured laboratory population, BSA-seq associated Hdic virulence with mutations in two candidate effector-encoding genes. Using a laboratory population, H5 virulence was previously positioned in a region spanning the centromere of HF autosome 2. BSA-seq resolved H5 virulence to a 1.3 Mb fragment on the same chromosome but failed to identify candidate mutations. Map-based candidate effectors were then delivered to Nicotiana plant cells via the type III secretion system of Burkholderia glumae bacteria. These experiments demonstrated that the genes associated with virulence to wheat R genes H6 and H13 are capable of suppressing plant immunity. Results are consistent with the hypothesis that effector proteins underlie the ability of HFs to survive on wheat.

A MIF-like effector suppresses plant immunity and facilitates nematode parasitism by interacting with plant annexins

Plant-parasitic nematodes secrete numerous effectors to facilitate parasitism, but detailed functions of nematode effectors and their plant targets remain largely unknown. Here, we characterized four macrophage migration inhibitory factors (MIFs) in Meloidogyne incognita resembling the MIFs secreted by human and animal parasites. Transcriptional data showed MiMIFs are up-regulated in parasitism. Immunolocalization provided evidence that MiMIF proteins are secreted from the nematode hypodermis to the parasite surface, detected in plant tissues and giant cells. In planta MiMIFs RNA interference in Arabidopsis decreased infection and nematode reproduction. Transient expression of MiMIF-2 could suppress Bax- and RBP1/Gpa2-induced cell death. MiMIF-2 ectopic expression led to higher levels of Arabidopsis susceptibility, suppressed immune responses triggered by flg22, and impaired [Ca2+]cyt influx induced by H2O2. The immunoprecipitation of MiMIF-2-interacting proteins, followed by co-immunoprecipitation and bimolecular fluorescence complementation validations, revealed specific interactions between MiMIF-2 and two Arabidopsis annexins, AnnAt1 and AnnAt4, involved in the transport of calcium ions, stress responses, and signal transduction. Suppression of expression or overexpression of these annexins modified nematode infection. Our results provide functional evidence that nematode effectors secreted from hypodermis to the parasite cuticle surface target host proteins and M. incognita uses MiMIFs to promote parasitism by interfering with the annexin-mediated plant immune responses.

DnaJ molecules as potential effectors in Meloidogyne arenaria. An unexplored group of proteins in plant parasitic nematodes

Plant pathogenic organisms secrete proteins called effectors that recognize, infect and promote disease within host cells. Bacteria, like Pseudomona syringae, use effectors with DnaJ function to disrupt plant defenses. DnaJ proteins (also called Hsp40) are a group of co-chaperone molecules, which assist in the folding of proteins. Despite the described role of DnaJs as effectors in several groups of pathogens, this group of proteins has never been correlated with the infection process in plant parasitic nematodes. In this study, we analyze the importance of DnaJ for plant parasitic nematodes. To do that, we compare the number of DnaJ proteins in nematodes with different lifestyles. Then, we predict the secreted DnaJ proteins in order to detect effector candidates. We found that Meloidogyne species have more secreted DnaJs than the rest of the nematodes analyzed in the study. Particularly, M. arenaria possess the highest proportion of secreted DnaJ sequences in comparison to total DnaJ proteins. Furthermore, we found in this species at least five sequences with a putative nuclear localization signal, three of them with a serine rich region with an unknown function. Then, we chose one of these sequences (MG599854) to perform an expression analysis. We found that MG599854 is over-expressed from 3 days post inoculation onwards in tomato plants. Moreover, MG599854 seems to be enough to produce cell death in Nicotiana benthamiana under transient expression conditions. In concordance with our results, we propose that DnaJ proteins are a potential source of effector proteins in plant parasitic nematodes.

A Novel G16B09-Like Effector From Heterodera avenae Suppresses Plant Defenses and Promotes Parasitism

Plant parasitic nematodes secrete effectors into host plant tissues to facilitate parasitism. In this study, we identified a G16B09-like effector protein family from the transcriptome of Heterodera avenae, and then verified that most of the members could suppress programmed cell death triggered by BAX in Nicotiana benthamiana. Ha18764, the most homologous to G16B09, was further characterized for its function. Our experimental evidence suggested that Ha18764 was specifically expressed in the dorsal gland and was dramatically upregulated in the J4 stage of nematode development. A Magnaporthe oryzae secretion system in barley showed that the signal peptide of Ha18764 had secretion activity to deliver mCherry into plant cells. Arabidopsis thaliana overexpressing Ha18764 or Hs18764 was more susceptible to Heterodera schachtii. In contrast, BSMV-based host-induced gene silencing (HIGS) targeting Ha18764 attenuated H. avenae parasitism and its reproduction in wheat plants. Transient expression of Ha18764 suppressed PsojNIP, Avr3a/R3a, RBP-1/Gpa2, and MAPK kinases (MKK1 and NPK1Nt)-related cell death in Nicotiana benthamiana. Co-expression assays indicated that Ha18764 also suppressed cell death triggered by four H. avenae putative cell-death-inducing effectors. Moreover, Ha18764 was also shown strong PTI suppression such as reducing the expression of plant defense-related genes, the burst of reactive oxygen species, and the deposition of cell wall callose. Together, our results indicate that Ha18764 promotes parasitism, probably by suppressing plant PTI and ETI signaling in the parasitic stages of H. avenae.