Nematode resistance in plants: the battle underground

Parasitic nematodes infect thousands of plant species, but some plants harbor specific resistance genes that defend against these pests. Several nematode resistance genes have been cloned in plants, and most resemble other plant resistance genes. Nematode resistance is generally characterized by host plant cell death near or at the feeding site of the endoparasitic worm. The timing and localization of the resistance response varies with the particular resistance gene and nematode interaction. Although there is genetic evidence that single genes in the nematode can determine whether a plant mounts a resistance response, cognate nematode effectors corresponding to a plant resistance gene have not been identified. However, recent progress in genetics and genomics of both plants and nematodes, and developments in RNA silencing strategies are improving our understanding of the molecular players in this complex interaction. In this article, we review the nature and mechanisms of plant-nematode interactions with respect to resistance in plants.     

Molecular Transfer of Nematode Resistance Genes

Recombinant DNA techniques have been used to introduce agronomically valuable traits, including resistance to viruses, herbicides, and insects, into crop plants. Introduction of these genes into plants frequently involves Agrobacterium-mediated gene transfer. The potential exists for applying this technology to nematode control by introducing genes conferring resistance to nematodes. Transferred genes could include those encoding products detrimental to nematode development or reproduction as well as cloned host resistance genes. Host genes that confer resistance to cyst or root-knot nematode species have been identified in many plants. The best characterized is Mi, a gene that confers resistance to root-knot nematodes in tomato. A map-based cloning approach is being used to isolate the gene. For development of a detailed map of the region of the genome surrounding Mi, DNA markers genetically linked to Mi have been identified and analyzed in tomato lines that have undergone a recombination event near Mi. The molecular map will be used to identify DNA corresponding to Mi. We estimate that a clone of Mi will be obtained in 2-5 years. An exciting prospect is that introduction of this gene will confer resistance in plant species without currently available sources of resistance.     

The root knot nematode resistance gene Mi from tomato is a member of the leucine zipper, nucleotide binding, leucine-rich repeat family of plant genes.

The Mi locus of tomato confers resistance to root knot nematodes. Tomato DNA spanning the locus was isolated as bacterial artificial chromosome clones, and 52 kb of contiguous DNA was sequenced. Three open reading frames were identified with similarity to cloned plant disease resistance genes. Two of them, Mi-1.1 and Mi-1.2, appear to be intact genes; the third is a pseudogene. A 4-kb mRNA hybridizing with these genes is present in tomato roots. Complementation studies using cloned copies of Mi-1.1 and Mi-1.2 indicated that Mi-1.2, but not Mi-1.1, is sufficient to confer resistance to a susceptible tomato line with the progeny of transformants segregating for resistance. The cloned gene most similar to Mi-1.2 is Prf, a tomato gene required for resistance to Pseudomonas syringae. Prf and Mi-1.2 share several structural motifs, including a nucleotide binding site and a leucine-rich repeat region, that are characteristic of a family of plant proteins, including several that are required for resistance against viruses, bacteria, fungi, and now, nematodes.     

Aging and longevity genes

The genetics of aging has made substantial strides in the past decade. This progress has been confined primarily to model organisms, such as filamentous fungi, yeast, nematodes, fruit flies, and mice, in which some thirty-five genes that determine life span have been cloned. These genes encode a wide array of cellular functions, indicating that there must be multiple mechanisms of aging. Nevertheless, some generalizations are already beginning to emerge. It is now clear that there are at least four broad physiological processes that play a role in aging: metabolic control, resistance to stress, gene dysregulation, and genetic stability. The first two of these at least are common themes that connect aging in yeast, nematodes, and fruit flies, and this convergence extends to caloric restriction, which postpones senescence and increases life span in rodents. Many of the human homologs of the longevity genes found in model organisms have been identified. This will lead to their use as candidate human longevity genes in population genetic studies. The urgency for such studies is great: The population is graying, and this research holds the promise of improvement in the quality of the later years of life.     

Plant-nematode interactions

Root-knot nematodes and cyst nematodes are obligate, biotrophic pathogens of numerous plant species. These organisms cause dramatic changes in the morphology and physiology of their hosts. The molecular characterization of induced plant genes has provided insight into the plant processes that are usurped by nematodes as they establish their specialized feeding cells. Recently, several gene products have been identified that are secreted by the nematode during parasitism. The corresponding genes have strong similarity to microbial genes or to genes that are found in nematodes that parasitize animals. New information on host resistance genes and nematode virulence genes provides additional insight into this complex interaction.     

New approaches to control plant parasitic nematodes

Plant parasitic nematodes are a serious threat for crop production worldwide. This review summarizes our understanding of plant nematode interactions and presents new alternatives for nematode control in the field. Breeding for resistance has been a major goal for many important crop species like soybean, potato, tomato and sugar-beet. As a result numerous nematode-resistance genes have been identified, two of which have been cloned recently, Hs1 ^pro-1 from sugar-beet, giving resistance to the beet cyst nematode Heterodera schachtii, and Mi from tomato, giving resistance to the root-knot nematode Meloidogyne incognita. Also artificial resistance genes, coding for nematotoxic proteins or causing rapid death of feeding cells, have been elucidated. In the future, genetic engineering of nematode resistance will become more and more important for plant breeding. Transformation techniques will allow genes to be quickly introduced into susceptible breeding lines and then combined with each other to produce plant varieties with durable resistance. Received: 26 August 1998 / Received revision: 16 December 1998 / Accepted: 21 December 1998     

Genetic approaches to sustainable pest management in sugar beet (Beta vulgaris)

Sugar beet (Beta vulgaris) is an important arable crop, traditionally used for sugar extraction, but more recently, for biofuel production. A wide range of pests, including beet cyst nematode (Heterodera schachtii), root‐knot nematodes (Meloidogyne spp.), green peach aphids (Myzus persicae) and beet root maggot (Tetanops myopaeformis), infest the roots or leaves of sugar beet, which leads to yield loss directly or through transmission of beet pathogens such as viruses. Conventional pest control approaches based on chemical application have led to high economic costs. Development of pest‐resistant sugar beet varieties could play an important role towards sustainable crop production while minimising environmental impact. Intensive Beta germplasm screening has been fruitful, and genetic lines resistant to nematodes, aphids and root maggot have been identified and integrated into sugar beet breeding programmes. A small number of genes responding to pest attack have been cloned from sugar beet and wild Beta species. This trend will continue towards a detailed understanding of the molecular mechanism of insect–host plant interactions and host resistance. Molecular biotechnological techniques have shown promise in developing transgenic pest resistance varieties at an accelerated speed with high accuracy. The use of transgenic technology is discussed with regard to biodiversity and food safety.     

Two novel loci underlie natural differences in Caenorhabditis elegans abamectin responses

Parasitic nematodes cause a massive worldwide burden on human health along with a loss of livestock and agriculture productivity. Anthelmintics have been widely successful in treating parasitic nematodes. However, resistance is increasing, and little is known about the molecular and genetic causes of resistance for most of these drugs. The free-living roundworm Caenorhabditis elegans provides a tractable model to identify genes that underlie resistance. Unlike parasitic nematodes, C. elegans is easy to maintain in the laboratory, has a complete and well annotated genome, and has many genetic tools. Using a combination of wild isolates and a panel of recombinant inbred lines constructed from crosses of two genetically and phenotypically divergent strains, we identified three genomic regions on chromosome V that underlie natural differences in response to the macrocyclic lactone (ML) abamectin. One locus was identified previously and encodes an alpha subunit of a glutamate-gated chloride channel (glc-1). Here, we validate and narrow two novel loci using near-isogenic lines. Additionally, we generate a list of prioritized candidate genes identified in C. elegans and in the parasite Haemonchus contortus by comparison of ML resistance loci. These genes could represent previously unidentified resistance genes shared across nematode species and should be evaluated in the future. Our work highlights the advantages of using C. elegans as a model to better understand ML resistance in parasitic nematodes.     

The root-knot nematode resistance gene Mi-1.2 of tomato is responsible for resistance against the whitefly Bemisia tabaci

The tomato gene Mi-1.2 confers resistance against root-knot nematodes and some isolates of potato aphid. Resistance to the whitefly Bemisia tabaci previously has been observed in Mi-bearing commercial tomato cultivars, suggesting that Mi, or a closely linked gene, is responsible for the resistance. The response of two biotypes of B. tabaci to tomato carrying the cloned Mi was compared with that of the isogenic untransformed tomato line Moneymaker. Our results indicate that Mi-1.2 is responsible for the resistance in tomato plants to both B- and Q- biotypes. Mi-1.2 is unique among characterized resistance genes in its activity against three very different organisms (root-knot nematodes, aphids, and whiteflies). These pests are among the most important on tomato crops worldwide, making Mi a valuable resource in integrated pest management programs.     

<Emphasis Type="Italic">CaMi</Emphasis>, a root-knot nematode resistance gene from hot pepper (<Emphasis Type="Italic">Capsium annuum</Emphasis> L.) confers nematode resistance in tomato

Several root-knot nematode (Meloidogyne spp.) resistance genes have been discovered in different pepper (Capsium annuum L.) lines; however, none of them has yet been cloned. In this study, a candidate root-knot nematode resistance gene (designated as CaMi) was isolated from the resistant pepper line PR 205 by degenerate PCR amplification combined with the RACE technique. Expression profiling analysis revealed that this gene was highly expressed in roots, leaves, and flowers and expressed at a lower level in stems and was not detectable in fruits. To verify the function of CaMi, a sense vector containing the genomic DNA spanning the full coding region of CaMi was constructed and transferred into root-knot nematode susceptible tomato plants. Sixteen transgenic plants carrying one to five copies of T-DNA inserts were generated from two nematode susceptible tomato cultivars. RT-PCR analysis revealed that the expression levels of CaMi gene varied in different transgenic plants. Nematode assays showed that the resistance to root-knot nematodes was significantly improved in some transgenic lines compared to untransformed susceptible plants, and that the resistance was inheritable. Ultrastructure analysis showed that nematodes led to the formation of galls or root knots in the susceptible lines while in the resistant transgenic plants, the CaMi gene triggered a hypersensitive response (HR) as well as many necrotic cells around nematodes. Rugang Chen and Hanxia Li are contributed equally to this work.     

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.     

Transgenic approaches to microbial disease resistance in crop plants

Recent progress in the genetic dissection of plant disease resistance signaling pathways has opened a number of new avenues towards engineering pathogen resistance in crops. Genes controlling race-specific and broad-spectrum resistance responses have been cloned, and novel induced resistance pathways have been identified in model and crop systems. Advances continue to be made in identification of antifungal proteins with effects inhibitory to either pathogen development or accumulation of associated mycotoxins.     

The MI-1-mediated pest resistance requires Hsp90 and Sgt1

The tomato (Solanum lycopersicum) Mi-1 gene encodes a protein with putative coiled-coil nucleotide-binding site and leucine-rich repeat motifs. Mi-1 confers resistance to root-knot nematodes (Meloidogyne spp.), potato aphids (Macrosiphum euphorbiae), and sweet potato whitefly (Bemisia tabaci). To identify genes required in the Mi-1-mediated resistance to nematodes and aphids, we used tobacco rattle virus (TRV)-based virus-induced gene silencing (VIGS) to repress candidate genes and assay for nematode and aphid resistance. We targeted Sgt1 (suppressor of G-two allele of Skp1), Rar1 (required for Mla12 resistance), and Hsp90 (heat shock protein 90), which are known to participate early in resistance gene signaling pathways. Two Arabidopsis (Arabidopsis thaliana) Sgt1 genes exist and one has been implicated in disease resistance. Thus far the sequence of only one Sgt1 ortholog is known in tomato. To design gene-specific VIGS constructs, we cloned a second tomato Sgt1 gene, Sgt1-2. The gene-specific VIGS construct TRV-SlSgt1-1 resulted in lethality, while silencing Sgt1-2 using TRV-SlSgt1-2 did not result in lethal phenotype. Aphid and root-knot nematode assays of Sgt1-2-silenced plants indicated no role for Sgt1-2 in Mi-1-mediated resistance. A Nicotiana benthamiana Sgt1 VIGS construct silencing both Sgt1-1 and Sgt1-2 yielded live plants and identified a role for Sgt1 in Mi-1-mediated aphid resistance. Silencing of Rar1 did not affect Mi-1-mediated nematode and aphid resistance and demonstrated that Rar1 is not required for Mi-1 resistance. Silencing Hsp90-1 resulted in attenuation of Mi-1-mediated aphid and nematode resistance and indicated a role for Hsp90-1. The requirement for Sgt1 and Hsp90-1 in Mi-1-mediated resistance provides further evidence for common components in early resistance gene defense signaling against diverse pathogens and pests.     

Signatures of selection in sheep bred for resistance or susceptibility to gastrointestinal nematodes

Background

Gastrointestinal nematodes are one of the most serious causes of disease in domestic ruminants worldwide. There is considerable variation in resistance to gastrointestinal nematodes within and between sheep breeds, which appears to be due to underlying genetic diversity. Through selection of resistant animals, rapid genetic progress has been demonstrated in both research and commercial flocks. Recent advances in genome sequencing and genomic technologies provide new opportunities to understand the ovine host response to gastrointestinal nematodes at the molecular level, and to identify polymorphisms conferring nematode resistance.

Results

Divergent lines of Romney and Perendale sheep, selectively bred for high and low faecal nematode egg count, were genotyped using the Illumina® Ovine SNP50 BeadChip. The resulting genome-wide SNP data were analysed for selective sweeps on loci associated with resistance or susceptibility to gastrointestinal nematode infection. Population differentiation using F_ST and Peddrift revealed sixteen regions, which included candidate genes involved in chitinase activity and the cytokine response. Two of the sixteen regions identified were contained within previously identified QTLs associated with nematode resistance.

Conclusions

In this study we identified fourteen novel regions associated with resistance or susceptibility to gastrointestinal nematodes. Results from this study support the hypothesis that host resistance to internal nematode parasites is likely to be controlled by a number of loci of moderate to small effects.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-637) contains supplementary material, which is available to authorized users.     

水稻广谱抗稻瘟病基因研究进展

稻瘟病是水稻生产中的最严重病害之一,由于稻瘟菌小种的高度变异性,垂直抗性基因难以持续控制稻瘟病的危害,因此,克隆和利用广谱持久抗瘟基因被认为是解决稻瘟病问题最经济有效的策略。本文从广谱抗源的筛选与利用,广谱抗瘟基因的定位、克隆与应用等方面对水稻广谱抗稻瘟病基因研究取得的进展进行了概述,并介绍了广谱抗性分子机理的最新研究进展。基于国内外稻瘟病抗性基因研究的现状及趋势,以及我国丰富的抗瘟水稻种质资源,克隆越来越多的广谱抗瘟基因具有重要的理论与应用价值。     

转基因改良植物抗真菌病害的策略及其进展

随着近几年来生物技术的迅猛发展,一系列新的广谱持久性抗病基因已经被鉴定、克隆并应用于转基因改良抗病性的实践。概述目前植物抗真菌病害的基因工程策略及其研究进展：（1）基于寄主－病原菌相互识别和信号传导体系的基因工程策略;（2）基于抗真菌蛋白的基因工程策略。     

Drug resistance in nematodes of veterinary importance: a status report

Reports of drug resistance have been made in every livestock host and to every anthelmintic class. In some regions of world, the extremely high prevalence of multi-drug resistance (MDR) in nematodes of sheep and goats threatens the viability of small-ruminant industries. Resistance in nematodes of horses and cattle has not yet reached the levels seen in small ruminants, but evidence suggests that the problems of resistance, including MDR worms, are also increasing in these hosts. There is an urgent need to develop both novel non-chemical approaches for parasite control and molecular assays capable of detecting resistant worms.     

Diversity of bacteria associated with grassland soil nematodes of different feeding groups

The bacterial diversity associated with soil nematodes and its relationship with their feeding habits are as yet poorly understood. In the present study the diversity and abundance of bacteria from nematodes and their surrounding soil were analysed and compared. The nematodes were collected from a grassland soil and sorted into bacterial, fungal, plant, predatory and omnivore feeding groups and assigned to taxonomic groups. Total DNA was extracted from the nematodes and partial bacterial 16S rRNA genes were PCR amplified, cloned and sequenced. The abundance and composition of bacterial taxa differed between and within feeding groups. The lowest bacterial diversity was found in the predatory nematodes Prionchulus sp., whereas the highest bacterial diversity was associated with the bacterial-feeding nematode Acrobeles sp. The soil had a more diverse bacterial community than the communities found in the nematode groups. The 16S rRNA gene sequences of bacteria associated with nematodes did not overlap with those detected in soil as determined using the cloning screening approach. However, bacterial sequences identified from nematodes could be detected in the soil with targeted PCR. Our data suggest that the nematodes do not feed on the most abundant bacteria present in soil. Furthermore, several nematodes contained suspected bacterial symbionts and parasites.     

Toward practical, DNA-based diagnostic methods for parasitic nematodes of livestock--bionomic and biotechnological implications

Parasitic nematodes of livestock have a major economic impact worldwide. In spite of the health problems caused by nematodes and advances toward the development of vaccines and new therapeutic agents against some of them, relatively limited attention has been paid to the need for improved, practical methods of diagnosis. Accurate diagnosis and genetic characterization of parasitic nematodes of livestock are central to their effective control, particularly given the current, serious problems with anthelmintic resistance in nematode populations. Traditional diagnostic techniques have considerable limitations, and there have been some advances toward the development of molecular-diagnostic tools. This article provides a brief account of the significance of parasitic nematodes (order Strongylida), reviews the techniques that have been evaluated or used for diagnosis and describes developments in polymerase chain reaction (PCR)-based methods for the specific diagnosis of nematode infection/s and the genetic characterisation of the causative agents. The advances made in recent years provide a solid foundation for the development of practical, highly sensitive and specific diagnostic tools for epidemiological investigations and for use in control programmes.