Molecular Insights in the Susceptible Plant Response to Nematode Infection

Water Stress Differentially Modulates the Expression of Tomato Cell Wall Metabolism-Related Genes in Meloidogyne incognita Feeding Sites

Microscopic observations and transcriptomic RNA-Seq analyses were applied to investigate the effect of water stress during the formation of tomato galls formation 1 and 2 weeks after inoculation with the root-knot nematode Meloidogyne incognita. Water stress affected root growth and the nematode ability to mount an efficient parasitism. The effects of water stress on the feeding site development were already observed at 1 week after nematode inoculation, with smaller giant cells, delayed development, and thinner cell walls. These features suggested changes in the expression levels of genes involved in the feeding site formation and maintenance. Gene Ontology (GO) enrichment and expression patterns were used to characterize differentially expressed genes. Water stress modified the expression profile of genes involved in the synthesis, degradation, and remodeling of the cell wall during the development of nematode feeding site. A comparison of gene expression with unstressed galls revealed that water stress intensified the up or downregulation of most genes. However, it particularly influenced the expression pattern of expansin A11 (Solyc04g081870.4.1), expansin-like B1(Solyc08g077910.3.1), a pectin acetylesterase (Solyc08g005800.4.1), and the pectin methylesterase pmeu1 (Solyc03g123630.4.1) which were upregulated in unstressed galls and repressed by water stress, at both sampling times. The expression of most members of the genes involved in cell wall metabolism, i.e., those coding for Csl, fasciclin, and COBRA proteins, were negatively influenced. Interestingly, alteration in the expression profiles of most dirigent protein genes (DIRs) and upregulation of five gene coding for Casparian strip domain protein (CASP)-like proteins were found. Gene expression analysis of galls from water stressed plants allowed us to better understand the molecular basis of M. incognita parasitism in tomato. Specific genes, including those involved in regulation of cellulose synthesis and lignification process, require further study to develop defense strategies against root-knot nematodes.

Transcriptomic and Histological Analysis of the Response of Susceptible and Resistant Cucumber to Meloidogyne incognita Infection Revealing Complex Resistance via Multiple Signaling Pathways

The root-knot nematode (RKN), Meloidogyne incognita, is a devastating pathogen for cucumber (Cucumis sativus L.) specially in production under protected environments or continuous cropping. High level RKN resistance has been identified in African horned melon Cucumis metuliferus (CM). However, the resistance mechanism remains unclear. In this study, the comparative analysis on phenotypic and transcriptomic responses in the susceptible cucumber inbred line Q24 and the resistant CM, after M. incognita infection, was performed. The results showed that, in comparison with Q24, the CM was able to significantly reduce penetration numbers of second stage juveniles (J2), slow its development in the roots resulting in fewer galls and smaller giant cells suggesting the presence of host resistance in CM. Comparative transcriptomes analysis of Q24 and CM before and after M. incognita infection was conducted and differentially expressed genes (DEGs) associated with host resistance were identified in CM. Enrichment analyses revealed most enriched DEGs in Ca²⁺ signaling, salicylic acid (SA)/jamonate signaling (JA), as well as auxin (IAA) signaling pathways. In particular, in CM, DEGs in the Ca²⁺ signaling pathway such as those for the calmodulin and calcium-binding proteins were upregulated at the early stage of M. incognita infection; genes for SA/JA synthesis/signal transduction were markedly activated, whereas the IAA signaling pathway genes were inhibited upon infection suggesting the importance of SA/JA signaling pathways in mediating M. incognita resistance in CM. A model was established to explain the different molecular mechanisms on M. incognita susceptibility in cucumber and resistance to M. incognita infection in CM.

Root-knot nematodes modulate cell walls during root-knot formation in Arabidopsis roots

Phytoparasitic nematodes parasitize many species of rooting plants to take up nutrients, thus causing severe growth defects in the host plants. During infection, root-knot nematodes induce the formation of a characteristic hyperplastic structure called a root-knot or gall on the roots of host plants. Although many previous studies addressed this abnormal morphogenesis, the underlying mechanisms remain uncharacterized. To analyze the plant-microorganism interaction at the molecular level, we established an in vitro infection assay system using the nematode Meloidogyne incognita and the model plant Arabidopsis thaliana. Time-course mRNA-seq analyses indicated the increased levels of procambium-associated genes in the galls, suggesting that vascular stem cells play important roles in the gall formation. Conversely, genes involved in the formation of secondary cell walls were decreased in galls. A neutral sugar analysis indicated that the level of xylan, which is one of the major secondary cell wall components, was dramatically reduced in the galls. These observations were consistent with the hypothesis of a decrease in the number of highly differentiated cells and an increase in the density of undifferentiated cells lead to gall formation. Our findings suggest that phytoparasitic nematodes modulate the developmental mechanisms of the host to modify various aspects of plant physiological processes and establish a feeding site.

αβ-Dehydrocurvularin isolated from the fungus Aspergillus welwitschiae effectively inhibited the behaviour and development of the root-knot nematode Meloidogyne graminicola in rice roots

Background

The root-knot nematode Meloidogyne graminicola has become a serious threat to rice production as a result of the cultivation changes from transplanting to direct seeding. The nematicidal activity of Aspergillus welwitschiae have been investigated in vitro, and the disease control efficacy of the active compound has been evaluated under greenhouse and field conditions.

Results

The active compound αβ-dehydrocurvularin (αβ-DC), isolated by nematicidal assay-directed fractionation, showed significant nematicidal activity against M. graminicola, with a median lethal concentration (LC₅₀) value of 122.2 μg mL^− 1. αβ-DC effectively decreased the attraction of rice roots to nematodes and the infection of nematodes and also suppressed the development of nematodes under greenhouse conditions. Moreover, αβ-DC efficiently reduced the root gall index under field conditions.

Conclusions

To our knowledge, this is the first report to describe the nematicidal activity of αβ-DC against M. graminicola. The results obtained under greenhouse and field conditions provide a basis for developing commercial formulations from αβ-DC to control M. graminicola in the future.

Early transcriptional responses to soybean cyst nematode HG Type 0 show genetic differences among resistant and susceptible soybeans

KEY MESSAGE

Root transcriptome profiling of three soybean cultivars and a wild relative infected with soybean cyst nematode at migratory phase revealed differential resistance pathway responses between resistant and susceptible genotypes. The soybean cyst nematode (SCN), Heterodera glycines, is the most serious pathogen of soybean production throughout the world. Using resistant cultivars is the primary management strategy against SCN infestation. To gain insight into the still obscure mechanisms of genetic resistance to nematodes in different soybean genotypes, RNA-Seq profiling of the roots of Glycine max cv. Peking, Fayette, Williams 82, and a wild relative (Glycine soja PI 468916) was performed during SCN infection at the migratory phase. The analysis showed statistically significant changes of expression beginning at eight hours after inoculation in genes associated with defense mechanisms and pathways, such as the phenylpropanoid biosynthesis pathway, plant innate immunity and hormone signaling. Our results indicate the importance of the early plant response to migratory phase nematodes in pathogenicity determination. The transcriptome changes occurring during early SCN infection included a number of genes and pathways specific to the different resistant genotypes. We observed the most extensive resistant transcriptome reaction in PI 468916, where the resistant response was qualitatively different from that of commonly used G. max varieties.

Host-specific signatures of the cell wall changes induced by the plant parasitic nematode, Meloidogyne incognita

Root-knot nematodes (Meloidogyne spp.) are an important group of plant parasitic nematodes that induce within host plant roots unique feeding site structures, termed giant cells, which supply nutrient flow to the nematode. A comparative in situ analysis of cell wall polysaccharides in the giant cells of three host species (Arabidopsis, maize and aduki bean) infected with Meloidogyne incognita has been carried out. Features common to giant cell walls of all three species include the presence of high-esterified pectic homogalacturonan, xyloglucan and pectic arabinan. The species-specific presence of xylan and mixed-linkage glucan (MLG) epitopes in giant cell walls of maize reflected that host’s taxonomic group. The LM5 galactan and LM21 mannan epitopes were not detected in the giant cell walls of aduki bean but were detected in Arabidopsis and maize giant cell walls. The LM2 arabinogalactan-protein epitope was notable for its apparent global variations in root cell walls as a response to infection across the three host species. Additionally, a set of Arabidopsis cell wall mutants were used to determine any impacts of altered cell wall structures on M. incognita infection. Disruption of the arabinogalactan-protein 8 gene had the greatest impact and resulted in an increased infection rate.

Comparative transcriptomics reveals suppressed expression of genes related to auxin and the cell cycle contributes to the resistance of cucumber against Meloidogyne incognita

BACKGROUND

Meloidogyne incognita is a devastating nematode that causes significant losses in cucumber production worldwide. Although numerous studies have emphasized on the susceptible response of plants after nematode infection, the exact regulation mechanism of M. incognita-resistance in cucumber remains elusive. Verification of an introgression line, 'IL10-1', with M. incognita-resistance provides the opportunity to unravel the resistance mechanism of cucumber against M. incognita.

RESULTS

In the present study, analyses of physiological responses and transcriptional events between IL10-1 (resistant line) and CC3 (susceptible line) were conducted after M. incognita infection. Physiological observations showed abnormal development of giant cells and M. incognita in IL10-1, which were the primary differences compared with CC3. Furthermore, Gene ontology (GO) analysis revealed that genes encoding cell wall proteins were up-regulated in IL10-1 and that the highly expressed lipid transfer protein gene (Csa6G410090) might be the principal regulator of this up-regulation. Simultaneously, analyses of gene expression profiles revealed more auxin-related genes were suppressed in IL10-1 than in those of CC3, which corresponded with the lower level of indole acetic acid (IAA) in the roots of IL10-1 than in those of CC3. Additionally, poor nucleus development as a clear indication of abnormal giant cells in IL10-1 was related to inhibition of the cell cycle. Of those genes related to the cell cycle, the F-box domain Skp2-like genes were down-regulated in IL10-1, whereas more of these genes were up-regulated in CC3.

CONCLUSIONS

All of these findings indicate that suppressed expression of genes related to auxin and the cell cycle and highly expressed cell wall proteins play important roles in the abnormal development of giant cells, which hinders the development of M. incognita, thereby causing resistance to M. incognita in IL10-1. Knowledge from this research will provide a useful foundation for developing effective strategies in M. incognita-resistance breeding.

The novel cyst nematode effector protein 30D08 targets host nuclear functions to alter gene expression in feeding sites

Cyst nematodes deliver effector proteins into host cells to manipulate cellular processes and establish a metabolically hyperactive feeding site. The novel 30D08 effector protein is produced in the dorsal gland of parasitic juveniles, but its function has remained unknown. We demonstrate that expression of 30D08 contributes to nematode parasitism, the protein is packaged into secretory granules and it is targeted to the plant nucleus where it interacts with SMU2 (homolog of suppressor of mec-8 and unc-52 2), an auxiliary spliceosomal protein. We show that SMU2 is expressed in feeding sites and an smu2 mutant is less susceptible to nematode infection. In Arabidopsis expressing 30D08 under the SMU2 promoter, several genes were found to be alternatively spliced and the most abundant functional classes represented among differentially expressed genes were involved in RNA processing, transcription and binding, as well as in development, and hormone and secondary metabolism, representing key cellular processes known to be important for feeding site formation. In conclusion, we demonstrated that the 30D08 effector is secreted from the nematode and targeted to the plant nucleus where its interaction with a host auxiliary spliceosomal protein may alter the pre-mRNA splicing and expression of a subset of genes important for feeding site formation.

Identification, Validation and Utilization of Novel Nematode-Responsive Root-Specific Promoters in Arabidopsis for Inducing Host-Delivered RNAi Mediated Root-Knot Nematode Resistance

The root-knot nematode (RKN), Meloidogyne incognita, is an obligate, sedentary endoparasite that infects a large number of crops and severely affects productivity. The commonly used nematode control strategies have their own limitations. Of late, RNA interference (RNAi) has become a popular approach for the development of nematode resistance in plants. Transgenic crops capable of expressing dsRNAs, specifically in roots for disrupting the parasitic process, offer an effective and efficient means of producing resistant crops. We identified nematode-responsive and root-specific (NRRS) promoters by using microarray data from the public domain and known conserved cis-elements. A set of 51 NRRS genes was identified which was narrowed down further on the basis of presence of cis-elements combined with minimal expression in the absence of nematode infection. The comparative analysis of promoters from the enriched NRRS set, along with earlier reported nematode-responsive genes, led to the identification of specific cis-elements. The promoters of two candidate genes were used to generate transgenic plants harboring promoter GUS constructs and tested in planta against nematodes. Both promoters showed preferential expression upon nematode infection, exclusively in the root in one and galls in the other. One of these NRRS promoters was used to drive the expression of splicing factor, a nematode-specific gene, for generating host-delivered RNAi-mediated nematode-resistant plants. Transgenic lines expressing dsRNA of splicing factor under the NRRS promoter exhibited upto a 32% reduction in number of galls compared to control plants.

Anatomical Alterations in Plant Tissues Induced by Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPNs) interact with plants in different ways, for example, through subtle feeding behavior, migrating destructively through infected tissues, or acting as virus-vectors for nepoviruses. They are all obligate biotrophic parasites as they derive their nutrients from living cells which they modify using pharyngeal gland secretions prior to food ingestion. Some of them can also shield themselves against plant defenses to sustain a relatively long lasting interaction while feeding. This paper is centered on cell types or organs that are newly induced in plants during PPN parasitism, including recent approaches to their study based on molecular biology combined with cell biology-histopathology. This issue has already been reviewed extensively for major PPNs (i.e., root-knot or cyst nematodes), but not for other genera (viz. Nacobbus aberrans, Rotylenchulus spp.). PPNs have evolved with plants and this co-evolution process has allowed the induction of new types of plant cells necessary for their parasitism. There are four basic types of feeding cells: (i) non-hypertrophied nurse cells; (ii) single giant cells; (iii) syncytia; and (iv) coenocytes. Variations in the structure of these cells within each group are also present between some genera depending on the nematode species viz. Meloidogyne or Rotylenchulus. This variability of feeding sites may be related in some way to PPN life style (migratory ectoparasites, sedentary ectoparasites, migratory ecto-endoparasites, migratory endoparasites, or sedentary endoparasites). Apart from their co-evolution with plants, the response of plant cells and roots are closely related to feeding behavior, the anatomy of the nematode (mainly stylet size, which could reach different types of cells in the plant), and the secretory fluids produced in the pharyngeal glands. These secretory fluids are injected through the stylet into perforated cells where they modify plant cytoplasm prior to food removal. Some species do not produce specialized feeding sites (viz. Ditylenchus, Subanguina), but may develop a specialized modification of the root system (e.g., unspecialized root galls or a profusion of roots). This review introduces new data on cell types and plant organs stimulated by PPNs using sources varying from traditional histopathology to new holistic methodologies.

Gene expression analysis in Musa acuminata during compatible interactions with Meloidogyne incognita

BACKGROUND AND AIMS

Endoparasitic root-knot nematodes (RKNs) ( Meloidogyne spp.) cause considerable losses in banana ( Musa spp.), with Meloidogyne incognita a predominant species in Cavendish sub-group bananas. This study investigates the root transcriptome in Musa acuminata genotypes 4297-06 (AA) and Cavendish Grande Naine (CAV; AAA) during early compatible interactions with M. incognita .

METHODS

Roots were analysed by brightfield light microscopy over a 35 d period to examine nematode penetration and morphological cell transformation. RNA samples were extracted 3, 7 and 10 days after inoculation (DAI) with nematode J2 juveniles, and cDNA libraries were sequenced using lllumina HiSeq technology. Sequences were mapped to the M. acuminata ssp. malaccensis var. Pahang genome sequence, differentially expressed genes (DEGs) identified and transcript representation determined by gene set enrichment and pathway mapping.

KEY RESULTS

Microscopic analysis revealed a life cycle of M. incognita completing in 24 d in CAV and 27 d in 4279-06. Comparable numbers of DEGs were up- and downregulated in each genotype, with potential involvement of many in early host defence responses involving reactive oxygen species and jasmonate/ethylene signalling. DEGs revealed concomitant auxin metabolism and cell wall modification processes likely to be involved in giant cell formation. Notable transcripts related to host defence included those coding for leucine-rich repeat receptor-like serine/threonine-protein kinases, peroxidases, thaumatin-like pathogenesis-related proteins, and DREB, ERF, MYB, NAC and WRKY transcription factors. Transcripts related to giant cell development included indole acetic acid-amido synthetase GH3.8 genes, involved in auxin metabolism, as well as genes encoding expansins and hydrolases, involved in cell wall modification.

CONCLUSIONS

Expression analysis in M. acuminata during compatible interactions with RKNs provides insights into genes modulated during infection and giant cell formation. Increased understanding of both defence responses to limit parasitism during compatible interactions and effector-targeted host genes in this complex interaction will facilitate the development of genetic improvement measures for RKNs.

Plant Cyclin-Dependent Kinase Inhibitors of the KRP Family: Potent Inhibitors of Root-Knot Nematode Feeding Sites in Plant Roots

Root-knot nematodes (RKN), Meloidogyne spp., are distributed worldwide and impose severe economic damage to many agronomically important crops. The plant cell cycle machinery is considered one of the pivotal components for the formation of nematode feeding sites (NFSs) or galls. These feeding sites contain five to nine hypertrophied giant cells (GC) resulting from developmental reprogramming of host root cells by this pathogen. GC undergo synchronous waves of mitotic activity uncoupled from cytokinesis giving rise to large multinucleate cells. As development of the NFS progresses, multiple rounds of DNA synthesis occur in the nuclei of GC, coupled with nuclear and cellular expansion. These cells are highly metabolically active and provide the nematode with nutrients necessary for its development and completion of its life cycle. In Arabidopsis seven cyclin dependent kinase inhibitors (CKIs) belonging to the interactors/inhibitors of the cyclin dependent kinases (ICK) family, also referred as Kip-Related Proteins (KRPs) have been identified. Interactions of KRPs with CDK/Cyclin complexes decrease CDK activity, affecting both cell cycle progression and DNA content in a concentration-dependent manner. We performed the functional analysis of all Arabidopsis KRP gene members during RKN interaction in Arabidopsis to obtain more insight into their role during gall development. We demonstrated that three members of this family (KRP2, KRP5, and KRP6) were highly expressed in galls and were important for cell cycle regulation during NFS development as shown by their different modes of action. We also pointed out that cell cycle inhibition through overexpression of all members of the KRP family can affect NFS development and consequently compromise the nematode's life cycle. In this review we summarized our recent understanding of the KRP family of genes, and their role in controlling cell cycle progression at the RKN feeding site.

Root-Knot and Cyst Nematodes Activate Procambium-Associated Genes in Arabidopsis Roots

Developmental plasticity is one of the most striking features of plant morphogenesis, as plants are able to vary their shapes in response to environmental cues. Biotic or abiotic stimuli often promote organogenesis events in plants not observed under normal growth conditions. Root-knot nematodes (RKNs) are known to parasitize multiple species of rooting plants and to induce characteristic tissue expansion called galls or root-knots on the roots of their hosts by perturbing the plant cellular machinery. Galls contain giant cells (GCs) and neighboring cells, and the GCs are a source of nutrients for the parasitizing nematode. Highly active cell proliferation was observed in galls. However, the underlying mechanisms that regulate the symptoms triggered by the plant-nematode interaction have not yet been elucidated. In this study, we deciphered the molecular mechanism of gall formation with an in vitro infection assay system using RKN Meloidogyne incognita, and the model plant Arabidopsis thaliana. By taking advantages of this system, we performed next-generation sequencing-based transcriptome profiling, and found that the expression of procambium identity-associated genes were enriched during gall formation. Clustering analyses with artificial xylogenic systems, together with the results of expression analyses of the candidate genes, showed a significant correlation between the induction of gall cells and procambium-associated cells. Furthermore, the promoters of several procambial marker genes such as ATHB8, TDR and WOX4 were activated not only in M. incognita-induced galls, but similarly in M. javanica induced-galls and Heterodera schachtii-induced syncytia. Our findings suggest that phytoparasitic nematodes modulate the host's developmental regulation of the vascular stem cells during gall formation.

The Complex Cell Wall Composition of Syncytia Induced by Plant Parasitic Cyst Nematodes Reflects Both Function and Host Plant

Plant-parasitic cyst nematodes induce the formation of specialized feeding structures, syncytia, within their host roots. These unique plant organs serve as the sole nutrient resource for development and reproduction throughout the biotrophic interaction. The multinucleate syncytium, which arises through local dissolution of cell walls and protoplast fusion of multiple adjacent cells, has dense cytoplasm containing numerous organelles, surrounded by thickened outer cell walls that must withstand high turgor pressure. However, little is known about how the constituents of the syncytial cell wall and their conformation support its role during nematode parasitism. We used a set of monoclonal antibodies, targeted to a range of plant cell wall components, to reveal the microstructures of syncytial cell walls induced by four of the most economically important cyst nematode species, Globodera pallida, Heterodera glycines, Heterodera avenae and Heterodera filipjevi, in their respective potato, soybean, and spring wheat host roots. In situ fluorescence analysis revealed highly similar cell wall composition of syncytia induced by G. pallida and H. glycines. Both consisted of abundant xyloglucan, methyl-esterified homogalacturonan and pectic arabinan. In contrast, the walls of syncytia induced in wheat roots by H. avenae and H. filipjevi contain little xyloglucan but are rich in feruloylated xylan and arabinan residues, with variable levels of mixed-linkage glucan. The overall chemical composition of syncytial cell walls reflected the general features of root cell walls of the different host plants. We relate specific components of syncytial cell walls, such as abundant arabinan, methyl-esterification status of pectic homogalacturonan and feruloylation of xylan, to their potential roles in forming a network to support both the strength and flexibility required for syncytium function.

Biochar-amended potting medium reduces the susceptibility of rice to root-knot nematode infections

BACKGROUND

Biochar is a solid coproduct of biomass pyrolysis, and soil amended with biochar has been shown to enhance the productivity of various crops and induce systemic plant resistance to fungal pathogens. The aim of this study was to explore the ability of wood biochar to induce resistance to the root-knot nematode (RKN) Meloidogyne graminicola in rice (Oryza sativa cv. Nipponbare) and examine its histochemical and molecular impact on plant defense mechanisms.

RESULTS

A 1.2 % concentration of biochar added to the potting medium of rice was found to be the most effective at reducing nematode development in rice roots, whereas direct toxic effects of biochar exudates on nematode viability, infectivity or development were not observed. The increased plant resistance was associated with biochar-primed H2O2 accumulation as well as with the transcriptional enhancement of genes involved in the ethylene (ET) signaling pathway. The increased susceptibility of the Ein2b-RNAi line, which is deficient in ET signaling, further confirmed that biochar-induced priming acts at least partly through ET signaling.

CONCLUSION

These results suggest that biochar amendments protect rice plants challenged by nematodes. This priming effect partially depends on the ET signaling pathway and enhanced H2O2 accumulation.

The dynamics of cereal cyst nematode infection differ between susceptible and resistant barley cultivars and lead to changes in (1,3;1,4)-β-glucan levels and HvCslF gene transcript abundance

Heterodera avenae (cereal cyst nematode, CCN) infects the roots of barley (Hordeum vulgare) forming syncytial feeding sites. In resistant host plants, relatively few females develop to maturity. Little is known about the physiological and biochemical changes induced during CCN infection. Responses to CCN infection were investigated in resistant (Rha2) and susceptible barley cultivars through histological, compositional and transcriptional analysis. Two phases were identified that influence CCN viability, including feeding site establishment and subsequent cyst maturation. Syncytial development progressed faster in the resistant cultivar Chebec than in the susceptible cultivar Skiff, and was accompanied by changes in cell wall polysaccharide abundance, particularly (1,3;1,4)-β-glucan. Transcriptional profiling identified several glycosyl transferase genes, including CELLULOSE SYNTHASE-LIKE F10 (HvCslF10), which may contribute to differences in polysaccharide abundance between resistant and susceptible cultivars. In barley, Rha2-mediated CCN resistance drives rapid deterioration of CCN feeding sites, specific changes in cell wall-related transcript abundance and changes in cell wall composition. During H. avenae infection, (1,3;1,4)-β-glucan may influence CCN feeding site development by limiting solute flow, similar to (1,3)-β-glucan during dicot cyst nematode infections. Dynamic transcriptional changes in uncharacterized HvCslF genes, possibly involved in (1,3;1,4)-β-glucan synthesis, suggest a role for these genes in the CCN infection process.

Suppression of NGB and NAB/ERabp1 in tomato modifies root responses to potato cyst nematode infestation

Plant-parasitic nematodes cause significant damage to major crops throughout the world. The small number of genes conferring natural plant resistance and the limitations of chemical control require the development of new protective strategies. RNA interference or the inducible over-expression of nematicidal genes provides an environment-friendly approach to this problem. Candidate genes include NGB, which encodes a small GTP-binding protein, and NAB/ERabp1, which encodes an auxin-binding protein, which were identified as being up-regulated in tomato roots in a transcriptome screen of potato cyst nematode (Globodera rostochiensis) feeding sites. Real-time reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization confirmed the localized up-regulation of these genes in syncytia and surrounding cells following nematode infection. Gene-silencing constructs were introduced into tomato, resulting in a 20%-98% decrease in transcription levels. Nematode infection tests conducted on transgenic plants showed 57%-82% reduction in the number of G. rostochiensis females in vitro and 30%-46% reduction in pot trials. Transmission electron microscopy revealed a deterioration of cytoplasm, and degraded mitochondria and plastids, in syncytia induced in plants with reduced NAB/ERabp1 expression. Cytoplasm in syncytia induced in plants with low NGB expression was strongly electron translucent and contained very few ribosomes; however, mitochondria and plastids remained intact. Functional impairments in syncytial cytoplasm of silenced plants may result from NGB's role in ribosome biogenesis; this was confirmed by localization of yellow fluorescent protein (YFP)-labelled NGB protein in nucleoli and co-repression of NGB in plants with reduced NAB/ERabp1 expression. These results demonstrate that NGB and NAB/ERabp1 play important roles in the development of nematode-induced syncytia.

Computational discovery of soybean promoter cis-regulatory elements for the construction of soybean cyst nematode-inducible synthetic promoters

Computational methods offer great hope but limited accuracy in the prediction of functional cis-regulatory elements; improvements are needed to enable synthetic promoter design. We applied an ensemble strategy for de novo soybean cyst nematode (SCN)-inducible motif discovery among promoters of 18 co-expressed soybean genes that were selected from six reported microarray studies involving a compatible soybean-SCN interaction. A total of 116 overlapping motif regions (OMRs) were discovered bioinformatically that were identified by at least four out of seven bioinformatic tools. Using synthetic promoters, the inducibility of each OMR or motif itself was evaluated by co-localization of gain of function of an orange fluorescent protein reporter and the presence of SCN in transgenic soybean hairy roots. Among 16 OMRs detected from two experimentally confirmed SCN-inducible promoters, 11 OMRs (i.e. 68.75%) were experimentally confirmed to be SCN-inducible, leading to the discovery of 23 core motifs of 5- to 7-bp length, of which 14 are novel in plants. We found that a combination of the three best tools (i.e. SCOPE, W-AlignACE and Weeder) could detect all 23 core motifs. Thus, this strategy is a high-throughput approach for de novo motif discovery in soybean and offers great potential for novel motif discovery and synthetic promoter engineering for any plant and trait in crop biotechnology.

NEMATIC: a simple and versatile tool for the in silico analysis of plant-nematode interactions

Novel approaches for the control of agriculturally damaging nematodes are sorely needed. Endoparasitic nematodes complete their life cycle within the root vascular cylinder, inducing specialized feeding cells: giant cells for root-knot nematodes and syncytia for cyst nematodes. Both nematodes hijack parts of the transduction cascades involved in developmental processes, or partially mimic the plant responses to other interactions with microorganisms, but molecular evidence of their differences and commonalities is still under investigation. Transcriptomics has been used to describe global expression profiles of their interaction with Arabidopsis, generating vast lists of differentially expressed genes. Although these results are available in public databases and publications, the information is scattered and difficult to handle. Here, we present a rapid, visual, user-friendly and easy to handle spreadsheet tool, called NEMATIC (NEMatode-Arabidopsis Transcriptomic Interaction Compendium; http://www.uclm.es/grupo/gbbmp/english/nematic.asp). It combines existing transcriptomic data for the interaction between Arabidopsis and plant-endoparasitic nematodes with data from different transcriptomic analyses regarding hormone and cell cycle regulation, development, different plant tissues, cell types and various biotic stresses. NEMATIC facilitates efficient in silico studies on plant-nematode biology, allowing rapid cross-comparisons with complex datasets and obtaining customized gene selections through sequential comparative and filtering steps. It includes gene functional classification and links to utilities from several databases. This data-mining spreadsheet will be valuable for the understanding of the molecular bases subjacent to feeding site formation by comparison with other plant systems, and for the selection of genes as potential tools for biotechnological control of nematodes, as demonstrated in the experimentally confirmed examples provided.

A role for LATERAL ORGAN BOUNDARIES-DOMAIN 16 during the interaction Arabidopsis-Meloidogyne spp. provides a molecular link between lateral root and root-knot nematode feeding site development

Plant endoparasitic nematodes induce the formation of their feeding cells by injecting effectors from the esophageal glands into root cells. Although vascular cylinder cells seem to be involved in the formation of root-knot nematode (RKN) feeding structures, molecular evidence is scarce. We address the role during gall development of LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16), a key component of the auxin pathway leading to the divisions in the xylem pole pericycle (XPP) for lateral root (LR) formation. Arabidopsis T-DNA tagged J0192 and J0121 XPP marker lines, LBD16 and DR5::GUS promoter lines, and isolated J0192 protoplasts were assayed for nematode-dependent gene expression. Infection tests in LBD16 knock-out lines were used for functional analysis. J0192 and J0121 lines were activated in early developing galls and giant cells (GCs), resembling the pattern of the G2/M-transition specific ProC yc B 1;1 :CycB1;1(NT)-GUS line. LBD16 was regulated by auxins in galls as in LRs, and induced by RKN secretions. LBD16 loss of function mutants and a transgenic line with defective XPP cells showed a significantly reduced infection rate. The results show that genes expressed in the dividing XPP, particularly LBD16, are important for gall formation, as they are for LR development.

On the track of transfer cell formation by specialized plant-parasitic nematodes

Transfer cells are ubiquitous plant cells that play an important role in plant development as well as in responses to biotic and abiotic stresses. They are highly specialized and differentiated cells playing a central role in the acquisition, distribution and exchange of nutrients. Their unique structural traits are characterized by augmented ingrowths of invaginated secondary wall material, unsheathed by an amplified area of plasma membrane enriched in a suite of solute transporters. Similar morphological features can be perceived in vascular root feeding cells induced by sedentary plant-parasitic nematodes, such as root-knot and cyst nematodes, in a wide range of plant hosts. Despite their close phylogenetic relationship, these obligatory biotrophic plant pathogens engage different approaches when reprogramming root cells into giant cells or syncytia, respectively. Both nematode feeding-cells types will serve as the main source of nutrients until the end of the nematode life cycle. In both cases, these nematodes are able to remarkably maneuver and reprogram plant host cells. In this review we will discuss the structure, function and formation of these specialized multinucleate cells that act as nutrient transfer cells accumulating and synthesizing components needed for survival and successful offspring of plant-parasitic nematodes. Plant cells with transfer-like functions are also a renowned subject of interest involving still poorly understood molecular and cellular transport processes.

Nematode feeding sites: unique organs in plant roots

Although generally unnoticed, nearly all crop plants have one or more species of nematodes that feed on their roots, frequently causing tremendous yield losses. The group of sedentary nematodes, which are among the most damaging plant-parasitic nematodes, cause the formation of special organs called nematode feeding sites (NFS) in the root tissue. In this review we discuss key metabolic and cellular changes correlated with NFS development, and similarities and discrepancies between different types of NFS are highlighted.

Enhanced levels of plant cell cycle inhibitors hamper root-knot nematode-induced feeding site development

Root-knot nematodes (RKN) are highly specialized, obligatory plant parasites. These animals reprogram root cells to form large, multinucleate, and metabolically active feeding cells (giant cells) that provide a continuous nutrient supply during 3-6 weeks of the nematode's life. The establishment and maintenance of physiologically fully functional giant cells are necessary for the survival of these nematodes. As such, giant cells may be useful targets for applying strategies to reduce damage caused by these nematodes, aiming the reduction of their reproduction. We have recently reported the involvement of cell cycle inhibitors of Arabidopsis, named Kip-Related Proteins (KRPs), on nematode feeding site ontogeny. Our results have demonstrated that this family of cell cycle inhibitors can be envisaged to efficiently disrupt giant cell development, based on previous reports which showed that alterations in KRP concentration levels can induce cell cycle transitions. Herein, we demonstrated that by overexpressing KRP genes, giant cells development is severely compromised as well as nematode reproduction. Thus, control of root-knot nematodes by modulating cell cycle-directed pathways through the enhancement of KRP protein levels may serve as an attractive strategy to limit damage caused by these plant parasites.

An insight into critical endocycle genes for plant-parasitic nematode feeding sites establishment

Root-knot and cyst nematodes are biotrophic parasites that invade the root apex of host plants and migrate toward the vascular cylinder where they cause the differentiation of root cells into galls (or root-knots) containing hypertrophied multinucleated giant-feeding cells, or syncytia, respectively. The precise molecular mechanisms that drive the formation of such unique nematode feeding sites are still far-off from being completely understood. The diverse gene expression changes occurring within the host cells suggest that both types of plant-parasitic nematodes modulate a variety of plant processes. Induction and repression of genes belonging to the host cell cycle control machinery have shown to be essential to drive the formation of such specialized nematode feeding cells. We demonstrate that nematodes usurp key components regulating the endocycle in their favor. This is illustrated by the involvement of anaphase-promoting complex (APC) genes (CCS52A and CCS52B), the endocycle repressor DP-E2F-like (E2F/DEL1) gene and the ROOT HAIRLESS 1 PROTEIN (RHL1), which is part of a multiprotein complex of the toposiomerase VI, in the proper formation of nematode feeding sites. Altering the expression of these genes in Arabidopsis plants by down- or overexpressing strategies strongly influences the extent of endoreduplication in both types of nematode feeding site leading to a disturbance of the nematode's life cycle and reproduction.

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.

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.

RNA interference in plant parasitic nematodes: a summary of the current status

SUMMARYRNA interference (RNAi) has emerged as an invaluable gene-silencing tool for functional analysis in a wide variety of organisms, particularly the free-living model nematode Caenorhabditis elegans. An increasing number of studies have now described its application to plant parasitic nematodes. Genes expressed in a range of cell types are silenced when nematodes take up double stranded RNA (dsRNA) or short interfering RNAs (siRNAs) that elicit a systemic RNAi response. Despite many successful reports, there is still poor understanding of the range of factors that influence optimal gene silencing. Recent in vitro studies have highlighted significant variations in the RNAi phenotype that can occur with different dsRNA concentrations, construct size and duration of soaking. Discrepancies in methodology thwart efforts to reliably compare the efficacy of RNAi between different nematodes or target tissues. Nevertheless, RNAi has become an established experimental tool for plant parasitic nematodes and also offers the prospect of being developed into a novel control strategy when delivered from transgenic plants.

Gene expression profiling of resistant and susceptible soybean lines infected with soybean cyst nematode

Soybean cyst nematode (SCN) is the most devastating pathogen of soybean. Information about the molecular basis of soybean-SCN interactions is needed to assist future development of effective management tools against this pathogen. Toward this end, soybean transcript abundance was measured using the Affymetrix Soybean Genome Array in a susceptible and a resistant reaction of soybean to SCN infection. Two genetically related soybean sister lines TN02-226 and TN02-275, which are resistant and susceptible, respectively, to the SCN race 2 infection were utilized in these experiments. Pairwise comparisons followed by false discovery rate analysis indicated that the expression levels of 162 transcripts changed significantly in the resistant line, of which 84 increased while 78 decreased. However, in the susceptible line, 1,694 transcripts changed significantly, of which 674 increased while 1,020 decreased. Comparative analyses of these transcripts indicated that a total of 51 transcripts were in common between resistance and susceptible responses. In this set, 42 transcripts increased in the resistant line, but decreased in the susceptible line. Quantitative real-time reverse-transcription polymerase chain reaction confirmed the results of microarray analysis. Of the transcripts to which a function could be assigned, genes were associated with metabolism, cell wall modification, signal transduction, transcription, and defense. Microarray analyses examining two genetically related soybean lines against the same SCN population provided additional insights into the specific changes in gene expression of a susceptible and a resistant reaction beneficial for identification of genes involved in defense.

Arabidopsis peroxidase AtPRX53 influences cell elongation and susceptibility to Heterodera schachtii

Cyst nematodes establish and maintain feeding sites (syncytia) in the roots of host plants by altering expression of host genes. Among these genes are members of the large gene family of class III peroxidases, which have reported functions in a variety of biological processes. In this study, we used Arabidopsis-Heterodera schachtii as a model system to functionally characterize peroxidase 53 (AtPRX53). Promoter assays showed that under non-infected conditions AtPRX53 is expressed mainly in the root, the hypocotyl and the base of the pistil. Under infected conditions, the AtPRX53 promoter showed upregulation at the nematode penetration sites and in their migration paths. Interestingly, strong GUS activity was observed in H. schachtii-induced syncytia during the early stage of infection and remained strong in the syncytia of third-stage juveniles. Also, AtPRX53 showed upregulation in response to wounding and jasmonic acid treatments. Manipulation of AtPRX53 expression through overexpression and knockout mutation affected both plant morphology and nematode susceptibility. While AtPRX53 overexpression lines exhibited short hypocotyls, aberrant flower development and reduced nematode susceptibility to H. schachtii, the atprx53 mutant showed long hypocotyls and a 3-carpel silique phenotype as well as a non significant increase of nematode susceptibility. Taken together these data, therefore, indicate diverse roles of AtPRX53 in the wound response, flower development and syncytium formation.

Mycorrhizal symbiosis stimulates endoreduplication in angiosperms

Symbiotic and parasitic relationships can alter the degree of endoreduplication in plant cells, and a limited number of studies have documented this occurrence in root cells colonized by arbuscular mycorrhizal (AM) fungi. However, this phenomenon has not been tested in a wide range of plant species, including species that are non-endopolyploid and those that do not associate with AM fungi. We grew 37 species belonging to 16 plant families, with a range of genome sizes and a range in the degree of endopolyploidy. The endoreduplication index (EI) was compared between plants that were inoculated with Glomus irregulare and plants that were not inoculated. Of the species colonized with AM fungi, 22 of the 25 species had a significant increase in endopolyploid root nuclei over non-mycorrhizal plants, including species that do not normally exhibit endopolyploidy. Changes in the EI were strongly correlated (R(2) = 0.619) with the proportion of root length colonized by arbuscules. No change was detected in the EI for the 12 non-mycorrhizal species. This work indicates that colonization by symbiotic fungi involves a mechanism to increase nuclear DNA content in roots across many angiosperm groups and is likely linked to increased metabolism and protein production.

How nematodes manipulate plant development pathways for infection

Sedentary plant-parasitic nematodes establish long term relationships with their hosts. Root vascular cells are transformed into large multinucleate feeding cells from which the nematodes feed for more than one month. Recent transcriptome analyses suggest that feeding cells are different from other plant cell types. Their development, however, remains poorly understood, despite new evidence that appears to confirm previously proposed models, such as the important role of auxin. From the analysis of nematode effector proteins that interact with plant proteins, it has become clear that nematodes manipulate many aspects of plant development, including auxin transport and plant cell differentiation pathways. These studies are also revealing roles for effectors in the inhibition of plant stress and defense responses to establish feeding cells. In the coming years breakthroughs can be expected in our understanding of plant-nematode interactions from the functional analysis of nematode effector genes as well as the involved plant genes.

The F-actin cytoskeleton in syncytia from non-clonal progenitor cells

The actin cytoskeleton of plant syncytia (a multinucleate cell arising through fusion) is poorly known: to date, there have only been reports about F-actin organization in plant syncytia induced by parasitic nematodes. To broaden knowledge regarding this issue, we analyzed F-actin organization in special heterokaryotic Utricularia syncytia, which arise from maternal sporophytic tissues and endosperm haustoria. In contrast to plant syncytia induced by parasitic nematodes, the syncytia of Utricularia have an extensive F-actin network. Abundant F-actin cytoskeleton occurs both in the region where cell walls are digested and the protoplast of nutritive tissue cells fuse with the syncytium and also near a giant amoeboid in the shape nuclei in the central part of the syncytium. An explanation for the presence of an extensive F-actin network and especially F-actin bundles in the syncytia is probably that it is involved in the movement of nuclei and other organelles and also the transport of nutrients in these physiological activity organs which are necessary for the development of embryos in these unique carnivorous plants. We observed that in Utricularia nutritive tissue cells, actin forms a randomly arranged network of F-actin, and later in syncytium, two patterns of F-actin were observed, one characteristic for nutritive cells and second-actin bundles-characteristic for haustoria and suspensors, thus syncytia inherit their F-actin patterns from their progenitors.

Expression of Arabidopsis pathogenesis-related genes during nematode infection

The expression pattern of pathogenesis-related genes PR-1 to PR-5 was examined in the roots and leaves of Arabidopsis thaliana plants on infection with beet-cyst (Heterodera schachtii) and root-knot (Meloidogyne incognita) nematodes. During H. schachtii parasitism of Arabidopsis, the expression of PR-1, PR-2 and PR-5, which are considered to be markers for salicylic acid (SA)-dependent systemic acquired resistance (SAR), was induced in both roots and leaves of infected plants. In addition, the expression of PR-3 and PR-4, which are used as markers for jasmonic acid (JA)-dependent SAR, was not altered in roots, but in the leaves of H. schachtii-infected plants, the expression PR-3 was induced, whereas the expression of PR-4 was down-regulated. During M. incognita infection of Arabidopsis, the expression of PR-1, PR-2 and PR-5 was highly induced in roots, as was PR-3 to a lesser extent, but the expression of PR-4 was not altered, indicating that infection with M. incognita activated both SA- and JA-dependent SAR in roots. However, all PRgenes examined (PR-1 to PR-5) were down-regulated in the leaves of M. incognita-infected plants, suggesting the suppression of both SA- and JA-dependent SAR. Furthermore, constitutive expression of a single PR in Arabidopsis altered the transcription patterns of other PR genes, and the over-expression of PR-1 reduced successful infection by both H. schachtii and M. incognita, whereas the over-expression of PR-3 reduced host susceptibility to M. incognita but had no effect on H. schachtii parasitism. The results suggest that fundamental differences in the mechanisms of infection by beet-cyst and root-knot nematodes differentially regulate PR protein production and mobilization within susceptible host plants.

Comparative sequence analysis of the potato cyst nematode resistance locus H1 reveals a major lack of co-linearity between three haplotypes in potato (Solanum tuberosum ssp.)

The H1 locus confers resistance to the potato cyst nematode Globodera rostochiensis pathotypes 1 and 4. It is positioned at the distal end of chromosome V of the diploid Solanum tuberosum genotype SH83-92-488 (SH) on an introgression segment derived from S. tuberosum ssp. andigena. Markers from a high-resolution genetic map of the H1 locus (Bakker et al. in Theor Appl Genet 109:146-152, 2004) were used to screen a BAC library to construct a physical map covering a 341-kb region of the resistant haplotype coming from SH. For comparison, physical maps were also generated of the two haplotypes from the diploid susceptible genotype RH89-039-16 (S. tuberosum ssp. tuberosum/S. phureja), spanning syntenic regions of 700 and 319 kb. Gene predictions on the genomic segments resulted in the identification of a large cluster consisting of variable numbers of the CC-NB-LRR type of R genes for each haplotype. Furthermore, the regions were interspersed with numerous transposable elements and genes coding for an extensin-like protein and an amino acid transporter. Comparative analysis revealed a major lack of gene order conservation in the sequences of the three closely related haplotypes. Our data provide insight in the evolutionary mechanisms shaping the H1 locus and will facilitate the map-based cloning of the H1 resistance gene.

The Arabidopsis bHLH25 and bHLH27 transcription factors contribute to susceptibility to the cyst nematode Heterodera schachtii

Successful cyst nematode parasitism depends on the formation and maintenance of feeding sites (syncytia) in host roots, and these processes are highly regulated by the interaction between the cyst nematode and the host. Using an integrated research approach and the Arabidopsis-Beta vulgaris (sugar beet) cyst nematode (Heterodera schachtii) pathosystem, we have determined that the two Arabidopsis basic helix-loop-helix transcription factors bHLH25 and bHLH27 positively influence cyst nematode parasitism. Promoter studies indicated that as early as 1 day post-inoculation, both transcription factor genes were upregulated in developing syncytia, whereas in non-infected plants, these two promoters were not found to be active in the same cells. By using yeast two-hybrid analyses and bimolecular fluorescence complementation assays, we documented that the two bHLH transcription factors can dimerize in planta. Transgenic Arabidopsis plants overexpressing either one or both of the bHLH genes exhibited altered morphology of roots and shoots, as well as an increased susceptibility to H. schachtii. bhlh25 or bhlh27 single mutants were without strong phenotypes, presumably because of functional redundancies in this gene family. However, the bhlh25 bhlh27 double mutant was less susceptible to H. schachtii, confirming an important conducive role of the co-expression of both transcription factor genes for cyst nematode parasitism. Our results document an example of pathogen-induced ectopic co-expression of two regulatory genes to enhance pathogen success, although these transcription factors apparently do not function in concert in non-infected plants. This is an intriguing biological phenomenon that highlights the complexity of obligate biotrophic plant-pathogen interactions, like those of cyst nematodes.

Activation of geminivirus V-sense promoters in roots is restricted to nematode feeding sites

Obligate sedentary endoparasitic nematodes, such as the root-knot and cyst nematodes, elicit the differentiation of specialized nematode nurse or feeding cells [nematode feeding sites (NFS), giant cells and syncytia, respectively]. During NFS differentiation, marked changes in cell cycle progression occur, partly similar to those induced by some geminiviruses. In this work, we describe the activation of V-sense promoters from the Maize streak virus (MSV) and Wheat dwarf virus (WDV) in NFS formed by root-knot and cyst nematodes. Both promoters were transiently active in microinjection experiments. In tobacco and Arabidopsis transgenic lines carrying promoter-beta-glucuronidase fusions, the MSV V-sense promoter was activated in the vascular tissues of aerial plant parts, primarily leaf and cotyledon phloem tissue and some floral structures. Interestingly, in roots, promoter activation was restricted to syncytia and giant cells tested with four different nematode populations, but undetectable in the rest of the root system. As the activity of the promoter in transgenic rootstocks should be restricted to NFS only, the MSV promoter may have utility in engineering grafted crops for nematode control. Therefore, this study represents a step in the provision of some of the much needed additional data on promoters with restricted activation in NFS useful in biotechnological nematode control strategies.