Root-knot nematode parasitism and host response: molecular basis of a sophisticated interaction

Cytoskeleton reorganization, a key process in root-knot nematode-induced giant cell ontogenesis

Root-knot nematodes are plant parasitic worms that establish and maintain an intimate relationship with their host plants. RKN induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells essential for nematode growth and reproduction. Major rearrangements of the cytoskeleton occur during giant cell formation. We characterized the first plant candidate genes implicated in giant cell actin and microtubule cytoskeleton reorganization. We showed previously that formins may regulate giant cell isotropic growth by controlling the assembly of actin cables. Recently we demonstrated that a Microtubule-Associated Protein, MAP65-3, is essential for giant cell development. In the absence of functional MAP65-3, giant cells started to develop but failed to fully differentiate and were eventually destroyed. In developing giant cells, MAP65-3 was associated with a novel kind of cell plate-the giant cell mini cell plate-that separates daughter nuclei. Despite karyokinesis occurs without cell division in giant cell, we demonstrated that cytokinesis is initiated and required for successful pathogen growth and development.

Parasitism genes and host range disparities in biotrophic nematodes: the conundrum of polyphagy versus specialisation

This essay considers biotrophic cyst and root-knot nematodes in relation to their biology, host-parasite interactions and molecular genetics. These nematodes have to face the biological consequences of the physical constraints imposed by the soil environment in which they live while their hosts inhabit both above and below ground environments. The two groups of nematodes appear to have adopted radically different solutions to these problems with the result that one group is a host specialist and reproduces sexually while the other has an enormous host range and reproduces by mitotic parthenogenesis. We consider what is known about the modes of parasitism used by these nematodes and how it relates to their host range, including the surprising finding that parasitism genes in both nematode groups have been recruited from bacteria. The nuclear and mitochondrial genomes of these two nematode groups are very different and we consider how these findings relate to the biology of the organisms.

Nematode resistance

Plant-parasitic nematodes are major pests of both temperate and tropical agriculture. Many of the most damaging species employ an advanced parasitic strategy in which they induce redifferentiation of root cells to form specialized feeding structures able to support nematode growth and reproduction over several weeks. Current control measures, particularly in intensive agriculture systems, rely heavily on nematicides but alternative strategies are required as effective chemicals are withdrawn from use. Here, we review the different approaches that are being developed to provide resistance to a range of nematode species. Natural, R gene-based resistance is currently exploited in traditional breeding programmes and research is ongoing to characterize the molecular basis for the observed resistant phenotypes. A number of transgenic approaches hold promise, the best described being the expression of proteinase inhibitors to disrupt nematode digestion. The application of plant-delivered RNA interference (RNAi) to silence essential nematode genes has recently emerged as a potentially valuable resistance strategy.

Maize 9-lipoxygenase ZmLOX3 controls development, root-specific expression of defense genes, and resistance to root-knot nematodes

Root-knot nematodes (RKN) are severe pests of maize. Although lipoxygenase (LOX) pathways and their oxylipin products have been implicated in plant-nematode interactions, prior to this report there was no conclusive genetic evidence for the function of any plant LOX gene in such interactions. We showed that expression of a maize 9-LOX gene, ZmLOX3, increased steadily and peaked at 7 days after inoculation with Meloidogyne incognita RKN. Mu-insertional lox3-4 mutants displayed increased attractiveness to RKN and an increased number of juveniles and eggs. A set of jasmonic acid (JA)- and ethylene (ET)-responsive and biosynthetic genes as well as salicylic acid (SA)-dependent genes were overexpressed specifically in the roots of lox3-4 mutants. Consistent with this, levels of JA, SA, and ET were elevated in lox3-4 mutant roots, but not in leaves. Unlike wild types, in lox3-4 mutant roots, a phenylalanine ammonia lyase (PAL) gene was not RKN-inducible, suggesting a role for PAL-mediated metabolism in nematode resistance. In addition to these alterations in the defense status of roots, lox3-4 knockout mutants displayed precocious senescence and reduced root length and plant height compared with the wild type, suggesting that ZmLOX3 is required for normal plant development. Taken together, our data indicate that the ZmLOX3-mediated pathway may act as a root-specific suppressor of all three major defense signaling pathways to channel plant energy into growth processes, but is required for normal levels of resistance against nematodes.

Host-delivered RNAi: an effective strategy to silence genes in plant parasitic nematodes

Root-knot nematodes (Meloidogyne spp.) are obligate, sedentary endoparasites that infect many plant species causing large economic losses worldwide. Available nematicides are being banned due to their toxicity or ozone-depleting properties and alternative control strategies are urgently required. We have produced transgenic tobacco (Nicotiana tabacum) plants expressing different dsRNA hairpin structures targeting a root-knot nematode (Meloidogyne javanica) putative transcription factor, MjTis11. We provide evidence that MjTis11 was consistently silenced in nematodes feeding on the roots of transgenic plants. The observed silencing was specific for MjTis11, with other sequence-unrelated genes being unaffected in the nematodes. Those transgenic plants able to induce silencing of MjTis11, also showed the presence of small interfering RNAs. Even though down-regulation of MjTis11 did not result in a lethal phenotype, this study demonstrates the feasibility of silencing root-knot nematode genes by expressing dsRNA in the host plant. Host-delivered RNA interference-triggered (HD-RNAi) silencing of parasite genes provides a novel disease resistance strategy with wide biotechnological applications. The potential of HD-RNAi is not restricted to parasitic nematodes but could be adapted to control other plant-feeding pests.

Physiological and genetic mapping study of tolerance to root-knot nematode in rice

The root-knot nematode Meloidogyne graminicola is an obligate biotrophic parasite and a major pest of rice (Oryza sativa) for which resistant varieties are not currently available. Quantitative trait loci (QTLs) for partial resistance to M. graminicola were identified using a mapping population based on two rice varieties, Bala x Azucena. Experiments were carried out to investigate the interactions between M. graminicola and these two varieties in terms of nematode establishment, reproduction and effect on rice yield. Nematode establishment was also assessed in the mapping population. Meloidogyne graminicola consistently caused more galling and had higher reproductive success in Azucena than in Bala. M. graminicola did not significantly reduce yield in Bala, but caused a yield reduction of almost half in Azucena, suggesting that the partial resistance to nematode establishment was related to nematode tolerance. A total of six significant or putative QTLs for nematode tolerance were detected. For two of the QTLs detected, Azucena was the donor of the tolerance alleles, suggesting it may be possible to breed plants with greater tolerance than Bala.

Genetic variability and adaptive evolution in parthenogenetic root-knot nematodes

Root-knot nematodes (RKN) of the genus Meloidogyne are biotrophic plant parasites of major agricultural importance, which exhibit very variable modes of reproduction, from classical amphimixis to mitotic parthenogenesis. This review focuses on those RKN species that reproduce exclusively by mitotic parthenogenesis (apomixis), in contrast to those that have meiotic/amphimitic events in their life cycle. Although populations of clonal organisms are often represented as being ecologically isolated and evolutionary inert, a considerable volume of literature provides evidence that asexual RKN are neither: they are widely distributed, extremely polyphagous, and amenable to selection and adaptive variation. The ancestors of the genus are unknown, but it is assumed that the parthenogenetic RKN have evolved from amphimictic species through hybridization and subsequent aneuploidization and polyploidization events. Molecular studies have indeed confirmed that the phylogenetic divergence between meiotic and mitotic RKN lineages occurred early, and have revealed an unexpected level of clonal diversity among populations within apomictic species. Laboratory experiments have shown that asexual RKN can rapidly adapt to new environmental constraints (eg host resistance), although with some fitness costs. Lastly, the molecular and chromosomal mechanisms that could contribute to genome plasticity leading to persistent genetic variation and adaptive evolution in apomictic RKN are discussed. It is concluded that RKN provide an excellent model system in which to study the dynamic nature and adaptive potential of clonal genomes.

Modulation of reactive oxygen species activities and H2O2 accumulation during compatible and incompatible tomato-root-knot nematode interactions

Here, the interaction of Melodoigyne incognita virulent and avirulent pathotypes with susceptible and Mi-resistant tomato (Solanum lycopersicon) has been studied. Significant differences in nematode penetration occurred 2 days postinoculation (dpi) and became stable from 3 dpi onwards. The hypersensitive cell response (HR) in resistant plants prevented the installation of the avirulent pathotype. The virulent pathotype overcame the Mi (nematode) resistance and induced feeding sites in root cells without triggering HR. Reactive oxygen species (ROS), visualized by subcellular reduction of nitroblue tetrazolium, accumulated in nematode penetrated cells. Quantitative analyses with dichlorofluorescein indicated that the oxidative burst occurred very early with both pathotypes, with an enhanced rate in hyper-responsive cells. Hydrogen peroxide (H(2)O(2)), detected by cerium chloride reaction, accumulated in the cell walls and especially in cells neighbouring HR. The apoplastic location of cerium perhydroxide indicated that either the plasma membrane or the cell wall was the primary site of the superoxide/H(2)O(2) generator. The data provide evidence, for the first time, for ROS-generated signals and their spatiotemporal expression in the host and nonhost interaction of tomato with nematodes.

PineappleDB: an online pineapple bioinformatics resource

BACKGROUND

A world first pineapple EST sequencing program has been undertaken to investigate genes expressed during non-climacteric fruit ripening and the nematode-plant interaction during root infection. Very little is known of how non-climacteric fruit ripening is controlled or of the molecular basis of the nematode-plant interaction. PineappleDB was developed to provide the research community with access to a curated bioinformatics resource housing the fruit, root and nematode infected gall expressed sequences.

DESCRIPTION

PineappleDB is an online, curated database providing integrated access to annotated expressed sequence tag (EST) data for cDNA clones isolated from pineapple fruit, root, and nematode infected root gall vascular cylinder tissues. The database currently houses over 5600 EST sequences, 3383 contig consensus sequences, and associated bioinformatic data including splice variants, Arabidopsis homologues, both MIPS based and Gene Ontology functional classifications, and clone distributions. The online resource can be searched by text or by BLAST sequence homology. The data outputs provide comprehensive sequence, bioinformatic and functional classification information.

CONCLUSION

The online pineapple bioinformatic resource provides the research community with access to pineapple fruit and root/gall sequence and bioinformatic data in a user-friendly format. The search tools enable efficient data mining and present a wide spectrum of bioinformatic and functional classification information. PineappleDB will be of broad appeal to researchers investigating pineapple genetics, non-climacteric fruit ripening, root-knot nematode infection, crassulacean acid metabolism and alternative RNA splicing in plants.

Loss of susceptibility as an alternative for nematode resistance

Among plant pathogens, sedentary endoparasitic nematodes are one of the most damaging pests in global agriculture. These obligate parasites interact with their hosts in a quite unique and intriguing way. They induce the redifferentiation of root cells into specialized feeding cells essential for nematode growth and reproduction; thus, nematodes have evolved the ability to exploit plant genes and hijack host functions for their own requirements. Various approaches to engineer plants with resistance to parasitic nematodes have been pursued, most focusing on the introduction of resistance genes. An alternative strategy to achieve resistance is to exploit the susceptibility of plant disease. Better knowledge of the plant response during the compatible interaction should allow the identification of targets to engineer resistance to parasitic nematodes in crop species.

Using laser capture microdissection to study gene expression in early stages of giant cells induced by root-knot nematodes

SUMMARY Root-knot nematodes (Meloidogyne spp.) are economically important plant parasites that induce specific feeding cells called giant cells in host roots. Study of molecular events involved in induction and differentiation of giant cells has been limited because it is difficult to obtain pure cytoplasm specifically from the highly specialized cells. In this work, laser capture microdissection (LCM) was used to collect cytoplasmic contents from paraffin-embedded sections of 4 day post-inoculation giant cells in tomato roots. Total RNA was isolated from the sections, and used in RT-PCR to investigate expression of cell cycle genes in giant cells. Two D-type cyclin genes, LeCycD3;2 and LeCycD3;3, were expressed at higher levels in giant cells compared with other cell-cycle-related cyclin genes, suggesting that the induction of the G1 phase of the cell cycle may be triggered in response to stimulation by the infecting nematode. LCM provides a powerful new tool to study the molecular basis of host-pathogen interactions at the cellular or subcellular level.

Secretions of plant-parasitic nematodes: a molecular update

The interaction between sedentary endoparasitic nematodes and plants is fascinating, because these animals have developed an ingenious way to manipulate the plant's gene regulation and metabolism to their own advantage. They are able to form highly specialized feeding structures in the plant root to satisfy their nutritional demands for development and reproduction. This ability makes them extremely successful parasites with severe consequences for agriculture. Triggered by these economical losses, detailed studies of the parasitic interaction have been performed, which resulted in an extensive descriptive knowledge. However, the underlying biochemical and molecular events of this intimate relationship have still not been elucidated. It is generally accepted that secretions produced by the nematode are responsible for the dramatic alteration of specific cells in the host plant. In the past few years, the identification of genes coding for secreted proteins was a breakthrough in plant nematode research. However, the available information is still too limited to allow the formulation of a comprehensive model, mainly because the sequences of many of these genes are novel with no similar sequence found in the existing databases. A new challenge in the coming years will be the functional analysis of these putative parasitism genes.