How nematodes manipulate plant development pathways for infection

Nematode effector proteins: an emerging paradigm of parasitism

Phytonematodes use a stylet and secreted effectors to modify host cells and ingest nutrients to support their growth and development. The molecular function of nematode effectors is currently the subject of intense investigation. In this review, we summarize our current understanding of nematode effectors, with a particular focus on proteinaceous stylet-secreted effectors of sedentary endoparasitic phytonematodes, for which a wealth of information has surfaced in the past 10 yr. We provide an update on the effector repertoires of several of the most economically important genera of phytonematodes and discuss current approaches to dissecting their function. Lastly, we highlight the latest breakthroughs in effector discovery that promise to shed new light on effector diversity and function across the phylum Nematoda.

Effectors of root sedentary nematodes target diverse plant cell compartments to manipulate plant functions and promote infection

Sedentary plant-parasitic nematodes maintain a biotrophic relationship with their hosts over a period of several weeks and induce the differentiation of root cells into specialized feeding cells. Nematode effectors, which are synthesized in the esophageal glands and injected into the plant tissue through the syringe-like stylet, play a central role in these processes. Previous work on nematode effectors has shown that the apoplasm is targeted during invasion of the host while the cytoplasm is targeted during the induction and the maintenance of the feeding site. A large number of candidate effectors potentially secreted by the nematode into the plant tissues to promote infection have now been identified. This work has shown that the targeting and the role of effectors are more complex than previously thought. This review will not cover the prolific recent findings in nematode effector function but will instead focus on recent selected examples that illustrate the variety of plant cell compartments that effectors are addressed to in order reach their plant targets.

Exploring the host parasitism of the migratory plant-parasitic nematode Ditylenchus destuctor by expressed sequence tags analysis

The potato rot nematode, Ditylenchus destructor, is a very destructive nematode pest on many agriculturally important crops worldwide, but the molecular characterization of its parasitism of plant has been limited. The effectors involved in nematode parasitism of plant for several sedentary endo-parasitic nematodes such as Heterodera glycines, Globodera rostochiensis and Meloidogyne incognita have been identified and extensively studied over the past two decades. Ditylenchus destructor, as a migratory plant parasitic nematode, has different feeding behavior, life cycle and host response. Comparing the transcriptome and parasitome among different types of plant-parasitic nematodes is the way to understand more fully the parasitic mechanism of plant nematodes. We undertook the approach of sequencing expressed sequence tags (ESTs) derived from a mixed stage cDNA library of D. destructor. This is the first study of D. destructor ESTs. A total of 9800 ESTs were grouped into 5008 clusters including 3606 singletons and 1402 multi-member contigs, representing a catalog of D. destructor genes. Implementing a bioinformatics' workflow, we found 1391 clusters have no match in the available gene database; 31 clusters only have similarities to genes identified from D. africanus, the most closely related species to D. destructor; 1991 clusters were annotated using Gene Ontology (GO); 1550 clusters were assigned enzyme commission (EC) numbers; and 1211 clusters were mapped to 181 KEGG biochemical pathways. 22 ESTs had similarities to reported nematode effectors. Interestedly, most of the effectors identified in this study are involved in host cell wall degradation or modification, such as 1,4-beta-glucanse, 1,3-beta-glucanse, pectate lyase, chitinases and expansin, or host defense suppression such as calreticulin, annexin and venom allergen-like protein. This result implies that the migratory plant-parasitic nematode D. destructor secrets similar effectors to those of sedentary plant nematodes. Finally we further characterized the two D. destructor expansin proteins.

Microtubules and biotic interactions

Plant microtubules undergo extensive reorganization in response to symbiotic and pathogenic organisms. During the development of successful symbioses with rhizobia and mycorrhizal fungi, novel microtubule arrays facilitate the progression of infection threads and hyphae, respectively, from the plant surface through epidermal and cortical cells. During viral and nematode infections, plant microtubules appear to be commandeered by the pathogen. Viruses use plant microtubules for intra and intercellular movement, as well as for interhost transmission. Nematodes manipulate spindle and phragmoplast microtubules to enhance mitosis and partial cytokinesis during the development of syncytia and giant cells. Pathogenic bacteria, fungi and oomycetes induce a range of alterations to microtubule arrays and dynamics. In many situations, the pathogen, or the elicitor or effector proteins derived from them, induce depolymerization of plant cortical microtubule arrays. In some cases, microtubule disruption is associated with the plant defence response and resistance. In other cases, microtubule depolymerization increases plant susceptibility to the invading pathogen. The reasons for this apparent inconsistency may depend on a number of factors, in particular on the identity of the organism orchestrating the microtubule changes. Overall, the weight of evidence indicates that microtubules play an important role in both the establishment of functional symbioses and in defence against invading pathogens. Research is beginning to unravel details about the nature of both the chemical and the mechanical signals to which the plant microtubule arrays respond during biotic interactions.

An Arabidopsis ATPase gene involved in nematode-induced syncytium development and abiotic stress responses

The beet cyst nematode Heterodera schachtii induces syncytia in the roots of Arabidopsis thaliana, which are its only nutrient source. One gene, At1g64110, that is strongly up-regulated in syncytia as shown by RT-PCR, quantitative RT-PCR, in situ RT-PCR and promoter::GUS lines, encodes an AAA+-type ATPase. Expression of two related genes in syncytia, At4g28000 and At5g52882, was not detected or not different from control root segments. Using amiRNA lines and T-DNA mutants, we show that At1g64110 is important for syncytium and nematode development. At1g64110 was also inducible by wounding, jasmonic acid, salicylic acid, heat and cold, as well as drought, sodium chloride, abscisic acid and mannitol, indicating involvement of this gene in abiotic stress responses. We confirmed this using two T-DNA mutants that were more sensitive to abscisic acid and sodium chloride during seed germination and root growth. These mutants also developed significantly smaller roots in response to abscisic acid and sodium chloride. An in silico analysis showed that ATPase At1g64110 (and also At4g28000 and At5g52882) belong to the 'meiotic clade' of AAA proteins that includes proteins such as Vps4, katanin, spastin and MSP1.

Engineered resistance and hypersusceptibility through functional metabolic studies of 100 genes in soybean to its major pathogen, the soybean cyst nematode

During pathogen attack, the host plant induces genes to ward off the pathogen while the pathogen often produces effector proteins to increase susceptibility of the host. Gene expression studies of syncytia formed in soybean root by soybean cyst nematode (Heterodera glycines) identified many genes altered in expression in resistant and susceptible roots. However, it is difficult to assess the role and impact of these genes on resistance using gene expression patterns alone. We selected 100 soybean genes from published microarray studies and individually overexpressed them in soybean roots to determine their impact on cyst nematode development. Nine genes reduced the number of mature females by more than 50 % when overexpressed, including genes encoding ascorbate peroxidase, β-1,4-endoglucanase, short chain dehydrogenase, lipase, DREPP membrane protein, calmodulin, and three proteins of unknown function. One gene encoding a serine hydroxymethyltransferase decreased the number of mature cyst nematode females by 45 % and is located at the Rhg4 locus. Four genes increased the number of mature cyst nematode females by more than 200 %, while thirteen others increased the number of mature cyst nematode females by more than 150 %. Our data support a role for auxin and ethylene in susceptibility of soybean to cyst nematodes. These studies highlight the contrasting gene sets induced by host and nematode during infection and provide new insights into the interactions between host and pathogen at the molecular level. Overexpression of some of these genes result in a greater decrease in the number of cysts formed than recognized soybean cyst nematode resistance loci.

Transcriptional changes of the root-knot nematode Meloidogyne incognita in response to Arabidopsis thaliana root signals

Root-knot nematodes are obligate parasites that invade roots and induce the formation of specialized feeding structures. Although physiological and molecular changes inside the root leading to feeding site formation have been studied, very little is known about the molecular events preceding root penetration by nematodes. In order to investigate the influence of root exudates on nematode gene expression before plant invasion and to identify new genes potentially involved in parasitism, sterile root exudates from the model plant Arabidopsis thaliana were produced and used to treat Meloidogyne incognita pre-parasitic second-stage juveniles. After confirming the activity of A. thaliana root exudates (ARE) on M. incognita stylet thrusting, six new candidate genes identified by cDNA-AFLP were confirmed by qRT-PCR as being differentially expressed after incubation for one hour with ARE. Using an in vitro inoculation method that focuses on the events preceding the root penetration, we show that five of these genes are differentially expressed within hours of nematode exposure to A. thaliana roots. We also show that these genes are up-regulated post nematode penetration during migration and feeding site initiation. This study demonstrates that preceding root invasion plant-parasitic nematodes are able to perceive root signals and to respond by changing their behaviour and gene expression.

Overexpression of the transcription factor RAP2.6 leads to enhanced callose deposition in syncytia and enhanced resistance against the beet cyst nematode Heterodera schachtii in Arabidopsis roots

BACKGROUND

Cyst nematodes invade the roots of their host plants as second stage juveniles and induce a syncytium which is their source of nutrients throughout their life. A transcriptome analysis of syncytia induced by the beet cyst nematode Heterodera schachtii in Arabidopsis roots has shown that gene expression in the syncytium is different from that of the root with thousands of genes upregulated or downregulated. Among the downregulated genes are many which code for defense-related proteins. One gene which is strongly downregulated codes for the ethylene response transcription factor RAP2.6. The genome of Arabidopsis contains 122 ERF transcription factor genes which are involved in a variety of developmental and stress responses.

RESULTS

Expression of RAP2.6 was studied with RT-PCR and a promoter::GUS line. During normal growth conditions the gene was expressed especially in roots and stems. It was inducible by Pseudomonas syringae but downregulated in syncytia from a very early time point on. Overexpression of the gene enhanced the resistance against H. schachtii which was seen by a lower number of nematodes developing on these plants as well as smaller syncytia and smaller female nematodes. A T-DNA mutant had a reduced RAP2.6 transcript level but this did not further increase the susceptibility against H. schachtii. Neither overexpression lines nor mutants had an effect on P. syringae. Overexpression of RAP2.6 led to an elevated expression of JA-responsive genes during early time points after infection by H. schachtii. Syncytia developing on overexpression lines showed enhanced deposition of callose.

CONCLUSIONS

Our results showed that H. schachtii infection is accompanied by a downregulation of RAP2.6. It seems likely that the nematodes use effectors to actively downregulate the expression of this and other defense-related genes to avoid resistance responses of the host plant. Enhanced resistance of RAP2.6 overexpression lines seemed to be due to enhanced callose deposition at syncytia which might interfere with nutrient import into syncytia.

The 8D05 parasitism gene of Meloidogyne incognita is required for successful infection of host roots

Parasitism genes encode effector proteins that are secreted through the stylet of root-knot nematodes to dramatically modify selected plant cells into giant-cells for feeding. The Mi8D05 parasitism gene previously identified was confirmed to encode a novel protein of 382 amino acids that had only one database homolog identified on contig 2374 within the Meloidogyne hapla genome. Mi8D05 expression peaked in M. incognita parasitic second-stage juveniles within host roots and its encoded protein was limited to the subventral esophageal gland cells that produce proteins secreted from the stylet. Constitutive expression of Mi8D05 in transformed Arabidopsis thaliana plants induced accelerated shoot growth and early flowering but had no visible effects on root growth. Independent lines of transgenic Arabidopsis that expressed a double-stranded RNA complementary to Mi8D05 in host-derived RNA interference (RNAi) tests had up to 90% reduction in infection by M. incognita compared with wild-type control plants, suggesting that Mi8D05 plays a critical role in parasitism by the root-knot nematode. Yeast two-hybrid experiments confirmed the specific interaction of the Mi8D05 protein with plant aquaporin tonoplast intrinsic protein 2 (TIP2) and provided evidence that the Mi8D05 effector may help regulate solute and water transport within giant-cells to promote the parasitic interaction.

Synergistic interaction of CLAVATA1, CLAVATA2, and RECEPTOR-LIKE PROTEIN KINASE 2 in cyst nematode parasitism of Arabidopsis

Plant-parasitic cyst nematodes secrete CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (CLE)-like effector proteins. These proteins act as ligand mimics of plant CLE peptides and are required for successful nematode infection. Previously, we showed that the CLV2/CORYNE (CRN) heterodimer receptor complex is required for nematode CLE signaling. However, there was only a partial reduction in nematode infection when this signaling was disrupted, indicating that there might be additional nematode CLE receptors. In this study, we demonstrate that CLV1 and RECEPTOR-LIKE PROTEIN KINASE 2/TOADSTOOL2 (RPK2), two additional receptors that can transmit the CLV3 signal independent of CLV2/CRN for shoot apical meristem maintenance, also play a role in nematode CLE perception. Localization studies showed that both receptors are expressed in nematode-induced syncytia. Infection assays with clv1 and rpk2 single mutants revealed a decrease in both nematode infection and syncytium size. Significantly, further reduction in nematode infection was observed when rpk2 was combined with clv1 and clv2 mutants. Taken together, our results indicate that parallel signaling pathways involving CLV1, CLV2, and RPK2 are important for nematode parasitism.

Phloem development in nematode-induced feeding sites: the implications of auxin and cytokinin

Sedentary plant parasitic nematodes such as root-knot nematodes and cyst nematodes induce giant cells or syncytia, respectively, in their host plant's roots. These highly specialized structures serve as feeding sites from which exclusively the nematodes withdraw nutrients. While giant cells are symplastically isolated and obtain assimilates by transporter-mediated processes syncytia are massively connected to the phloem by plasmodesmata. To support the feeding sites and the nematode during their development, phloem is induced around syncytia and giant cells. In the case of syncytia the unloading phloem consists of sieve elements and companion cells and in the case of root knots it consists exclusively of sieve elements. We applied immunohistochemistry to identify the cells within the developing phloem that responded to auxin and cytokinin. Both feeding sites themselves did not respond to either hormone. We were able to show that in root knots an auxin response precedes the differentiation of these auxin responsive cells into phloem elements. This process appears to be independent of B-type Arabidopsis response regulators. Using additional markers for tissue identity we provide evidence that around giant cells protophloem is formed and proliferates dramatically. In contrast, the phloem around syncytia responded to both hormones. The presence of companion cells as well as hormone-responsive sieve elements suggests that metaphloem development occurs. The implication of auxin and cytokinin in the further development of the metaphloem is discussed.

Manipulation of plant cells by cyst and root-knot nematode effectors

A key feature of sedentary plant-parasitic nematodes is the release of effector proteins from their esophageal gland cells through their stylets into host roots. These proteinaceous stylet secretions have been shown to be crucial for successful parasitism by mediating the transition of normal root cells into specialized feeding sites and by negating plant defenses. Recent technical advances of purifying mRNA from esophageal gland cells of plant-parasitic nematodes coupled with emerging sequencing technologies is steadily expanding our knowledge of nematode effector repertoires. Host targets and biological activities of a number of nematode effectors are continuously being reported and, by now, a first picture of the complexity of sedentary nematode parasitism at the molecular level is starting to take shape. In this review, we highlight effector mechanisms that recently have been uncovered by studying the host-pathogen interaction. These mechanisms range from mediating susceptibility of host plants to the actual triggering of defense responses. In particular, we portray and discuss the mechanisms by which nematode effectors modify plant cell walls, negate host defense responses, alter auxin and polyamine signaling, mimic plant molecules, regulate stress signaling, and activate hypersensitive responses. Continuous molecular characterization of newly discovered nematode effectors will be needed to determine how these effectors orchestrate host signaling pathways and biological processes leading to successful parasitism.

Copy number variation of multiple genes at Rhg1 mediates nematode resistance in soybean

The rhg1-b allele of soybean is widely used for resistance against soybean cyst nematode (SCN), the most economically damaging pathogen of soybeans in the United States. Gene silencing showed that genes in a 31-kilobase segment at rhg1-b, encoding an amino acid transporter, an α-SNAP protein, and a WI12 (wound-inducible domain) protein, each contribute to resistance. There is one copy of the 31-kilobase segment per haploid genome in susceptible varieties, but 10 tandem copies are present in an rhg1-b haplotype. Overexpression of the individual genes in roots was ineffective, but overexpression of the genes together conferred enhanced SCN resistance. Hence, SCN resistance mediated by the soybean quantitative trait locus Rhg1 is conferred by copy number variation that increases the expression of a set of dissimilar genes in a repeated multigene segment.

Transcriptional reprogramming by root knot and migratory nematode infection in rice

Rice is one of the most important staple crops worldwide, but its yield is compromised by different pathogens, including plant-parasitic nematodes. In this study we have characterized specific and general responses of rice (Oryza sativa) roots challenged with two endoparasitic nematodes with very different modes of action. Local transcriptional changes in rice roots upon root knot (Meloidogyne graminicola) and root rot nematode (RRN, Hirschmanniella oryzae) infection were studied at two time points (3 and 7 d after infection, dai), using mRNA-seq. Our results confirm that root knot nematodes (RKNs), which feed as sedentary endoparasites, stimulate metabolic pathways in the root, and enhance nutrient transport towards the induced root gall. The migratory RRNs, on the other hand, induce programmed cell death and oxidative stress, and obstruct the normal metabolic activity of the root. While RRN infection causes up-regulation of biotic stress-related genes early in the infection, the sedentary RKNs suppress the local defense pathways (e.g. salicylic acid and ethylene pathways). Interestingly, hormone pathways mainly involved in plant development were strongly induced (gibberellin) or repressed (cytokinin) at 3 dai. These results uncover previously unrecognized nematode-induced expression profiles related to their specific infection strategy.

A root-knot nematode-secreted protein is injected into giant cells and targeted to the nuclei

Root-knot nematodes (RKNs) are obligate endoparasites that maintain a biotrophic relationship with their hosts over a period of several weeks and induce the differentiation of root cells into specialized feeding cells. Nematode effectors synthesized in the oesophageal glands and injected into the plant tissue through the syringe-like stylet certainly play a central role in these processes. In a search for nematode effectors, we used comparative genomics on expressed sequence tag (EST) datasets to identify Meloidogyne incognita genes encoding proteins potentially secreted upon the early steps of infection. We identified three genes specifically expressed in the oesophageal glands of parasitic juveniles that encode predicted secreted proteins. One of these genes, Mi-EFF1 is a pioneer gene that has no similarity in databases and a predicted nuclear localization signal. We demonstrate that RKNs secrete Mi-EFF1 within the feeding site and show Mi-EFF1 targeting to the nuclei of the feeding cells. RKNs were previously shown to secrete proteins in the apoplasm of infected tissues. Our results show that nematodes sedentarily established at the feeding site also deliver proteins within plant cells through their stylet. The protein Mi-EFF1 injected within the feeding cells is targeted at the nuclei where it may manipulate nuclear functions of the host cell.

The role of flavonoids in root-rhizosphere signalling: opportunities and challenges for improving plant-microbe interactions

The flavonoid pathway produces a diverse array of plant compounds with functions in UV protection, as antioxidants, pigments, auxin transport regulators, defence compounds against pathogens and during signalling in symbiosis. This review highlights some of the known function of flavonoids in the rhizosphere, in particular for the interaction of roots with microorganisms. Depending on their structure, flavonoids have been shown to stimulate or inhibit rhizobial nod gene expression, cause chemoattraction of rhizobia towards the root, inhibit root pathogens, stimulate mycorrhizal spore germination and hyphal branching, mediate allelopathic interactions between plants, affect quorum sensing, and chelate soil nutrients. Therefore, the manipulation of the flavonoid pathway to synthesize specifically certain products has been suggested as an avenue to improve root-rhizosphere interactions. Possible strategies to alter flavonoid exudation to the rhizosphere are discussed. Possible challenges in that endeavour include limited knowledge of the mechanisms that regulate flavonoid transport and exudation, unforeseen effects of altering parts of the flavonoid synthesis pathway on fluxes elsewhere in the pathway, spatial heterogeneity of flavonoid exudation along the root, as well as alteration of flavonoid products by microorganisms in the soil. In addition, the overlapping functions of many flavonoids as stimulators of functions in one organism and inhibitors of another suggests caution in attempts to manipulate flavonoid rhizosphere signals.

Plant-derived auxin plays an accessory role in symptom development upon Rhodococcus fascians infection

The biotrophic phytopathogen Rhodococcus fascians has a profound impact on plant development, mainly through its principal virulence factors, a mix of synergistically acting cytokinins that induce shoot formation. Expression profiling of marker genes for several auxin biosynthesis routes and mutant analysis demonstrated that the bacterial cytokinins stimulate the auxin biosynthesis of plants via specific targeting of the indole-3-pyruvic acid (IPA) pathway, resulting in enhanced auxin signaling in infected tissues. The double mutant tryptophan aminotransferase 1-1 tryptophan aminotransferase related 2-1 (taa1-1 tar2-1) of Arabidopsis (Arabidopsis thaliana), in which the IPA pathway is defective, displayed a decreased responsiveness towards R. fascians infection, although bacterial colonization and virulence gene expression were not impaired. These observations implied that plant-derived auxin was employed to reinforce symptom formation. Furthermore, the increased auxin production and, possibly, the accumulating bacterial cytokinins in infected plants modified the polar auxin transport so that new auxin maxima were repetitively established and distributed, a process that is imperative for symptom onset and maintenance. Based on these findings, we extend our model of the mode of action of bacterial and plant signals during the interaction between R. fascians and Arabidopsis.

Comparing systemic defence-related gene expression changes upon migratory and sedentary nematode attack in rice

Complex defence signalling pathways, controlled by different hormones, are known to be involved in the reaction of plants to a wide range of biotic and abiotic stress factors. Here, we studied the differential expression of genes involved in stress and defence responses in systemic tissue of rice infected with the root knot nematode (RKN) Meloidogyne graminicola and the migratory root rot nematode Hirschmanniella oryzae, two agronomically important rice pathogens with very different lifestyles. qRT-PCR revealed that all investigated systemic tissues had significantly lower expression of isochorismate synthase, a key enzyme for salicylic acid production involved in basal defence and systemic acquired resistance. The systemic defence response upon migratory nematode infection was remarkably similar to fungal rice blast infection. Almost all investigated defence-related genes were up-regulated in rice shoots 3 days after root rot nematode attack, including the phenylpropanoid pathway, ethylene pathway and PR genes, but many of which were suppressed at 7 dpi. Systemic shoot tissue of RKN-infected plants showed similar attenuation of expression of almost all studied genes already at 3 dpi, with clear attenuation of the ethylene pathway and methyl jasmonate biosynthesis. These results provide an interesting starting point for further studies to elucidate how nematodes are able to suppress systemic plant defence mechanisms and the effect in multitrophic interactions.

Role of nematode peptides and other small molecules in plant parasitism

Molecular, genetic, and biochemical studies are demonstrating an increasingly important role of peptide signaling in nematode parasitism of plants. To date, the majority of nematode-secreted peptides identified share similarity with plant CLAVATA3/ESR (CLE) peptides, but bioinformatics analyses of nematode genomes have revealed sequences homologous to other classes of plant peptide hormones that may be utilized by these pests. Extracellular host receptors for secreted nematode peptides are beginning to be identified and their roles in parasitism elucidated. Here, we outline recent advances from studies of biologically active nematode-secreted peptides that function as molecular mimics of endogenous plant peptides to promote parasitism. Several strategies are being used to exploit this information to provide new targets for engineering nematode resistance.

A plethora of virulence strategies hidden behind nuclear targeting of microbial effectors

Plant immune responses depend on the ability to couple rapid recognition of the invading microbe to an efficient response. During evolution, plant pathogens have acquired the ability to deliver effector molecules inside host cells in order to manipulate cellular and molecular processes and establish pathogenicity. Following translocation into plant cells, microbial effectors may be addressed to different subcellular compartments. Intriguingly, a significant number of effector proteins from different pathogenic microorganisms, including viruses, oomycetes, fungi, nematodes, and bacteria, is targeted to the nucleus of host cells. In agreement with this observation, increasing evidence highlights the crucial role played by nuclear dynamics, and nucleocytoplasmic protein trafficking during a great variety of analyzed plant-pathogen interactions. Once in the nucleus, effector proteins are able to manipulate host transcription or directly subvert essential host components to promote virulence. Along these lines, it has been suggested that some effectors may affect histone packing and, thereby, chromatin configuration. In addition, microbial effectors may either directly activate transcription or target host transcription factors to alter their regular molecular functions. Alternatively, nuclear translocation of effectors may affect subcellular localization of their cognate resistance proteins in a process that is essential for resistance protein-mediated plant immunity. Here, we review recent progress in our field on the identification of microbial effectors that are targeted to the nucleus of host plant cells. In addition, we discuss different virulence strategies deployed by microbes, which have been uncovered through examination of the mechanisms that guide nuclear localization of effector proteins.