Resistance to root-knot nematodes Meloidogyne spp. in woody plants

Defensive alteration of root exudate composition by grafting Prunus sp. onto resistant rootstock contributes to reducing crown gall disease

Grafting is a traditional and significant strategy to suppress soil-borne diseases, such as the crown gall disease caused by tumorigenic Agrobacterium and Rhizobium. Root exudates and the rhizosphere microbiome play critical roles in controlling crown gall disease, but their roles in suppressing crown gall disease in grafted plants remain unclear. Here, disease-susceptible cherry rootstock 'Gisela 6' and disease-resistant cherry rootstock 'Haiying 1' were grafted onto each other or self-grafted. The effect of their root exudates on the soil microbiome composition and the abundance of pathogenic Agrobacterium were studied. Grafting onto the disease-resistant rootstock helped to reduce the abundance of pathogenic Agrobacterium, accompanied by altering root exudation, enriching potential beneficial bacteria, and changing soil function. Then, the composition of the root exudates from grafted plants was analyzed and the potential compounds responsible for decreasing pathogenic Agrobacterium abundance were identified. Based on quantitative measurement of the concentrations of the compounds and testing the impacts of supplied pure chemicals on abundance and chemotaxis of pathogenic Agrobacterium and potential beneficial bacteria, the decreased valine in root exudates of the plant grafted onto resistant rootstock was found to contribute to decreasing Agrobacterium abundance, enriching some potential beneficial bacteria and suppressing crown gall disease. This study provides insights into the mechanism whereby grafted plants suppress soil-borne disease.

Host Status of Ornamental Shade Trees and Shrubs to Plant Parasitic Nematodes

Oregon leads the United States in nursery production of shade trees and is third in deciduous and broadleaf evergreen shrub production. Plant-parasitic nematodes have been implicated in problems with the growth of plants in nurseries and are also of phytosanitary risk. A greenhouse experiment was conducted to evaluate the host status of four trees (Quercus alba, Quercus garryana, Acer campestre, Thuja occidentalis) and two shrubs (Buxus sempervirens, Rhododendron catawbiense) to Meloidogyne incognita, Meloidogyne hapla, and Pratylenchus neglectus. Each plant/nematode treatment was replicated five times, and the experiment was conducted twice. Plants were inoculated with 3,000 eggs of M. incognita or M. hapla and 2,500 individuals of P. neglectus two weeks after planting. After three months, the plants were harvested, and the total density of nematodes in soil and roots for P. neglectus and the total density of second-stage juveniles (J2) in soil and eggs on roots for M. hapla and M. incognita were determined. The final nematode population (Pf) and reproductive factor (RF = Pf/initial population density) were calculated. For M. incognita and M. hapla, all of the ornamental trees and shrubs would be considered as fair to good hosts with RF values > 1. Meloidogyne incognita had the highest Pf (5,234 total J2 and eggs/pot) and RF value (28.4) on A. campestre. For P. neglectus, all of the ornamental trees and shrubs were fair to good hosts, except for B. sempervirens. Buxus sermpervirens was not a host for P. neglectus, with an RF value of almost 0. This is the first report of Q. alba, Q. garryana, and A. campestre as hosts for M. incognita, M. hapla, and P. penetrans. This is also the first report of T. occidentalis and R. catawbiense as hosts for P. penetrans and the non-host status of B. sermpervirens for P. penetrans.

Correlation of genome methylation of fig tree accessions with natural nematode and rust incidence

Abstract Commercial fig tree cultivation in Brazil involves a single cultivar, ‘Roxo-de-Valinhos’. The use of a single cultivar results in serious diseases and related problems. The aim of this study was to characterize fig accessions by analyzing the natural root-knot nematode and leaf rust incidence in relation to the epigenomic profile of the plant, since epigenetic variations affect plant–pathogen interactions. All plants were attacked by nematodes, indicating susceptibility; Meloidogyne incognita was the root-knot nematode species involved. Joint analysis of data showed that methylation and leaf rust incidence were correlated when observed in the same phenological phase, presenting initial evidence of the same factorial pressure loads in genotypes, suggesting similar behavior within these genotypes.

Detection of Resistance, Susceptibility, Tolerance, and Virulence in Plant-Nematode Interactions: Part I-Sedentary Endoparasitic Nematodes

The use of nonhost, tolerant, or resistant plants, to manage plant parasitic nematodes (PPNs), is an appealing, economic, and environmentally friendly agronomic practice, which is effective when precise information on the identification of PPN species and their virulence to target host crops is available. This chapter describes suggested protocols to evaluate the reaction of the most important crops and fruit trees to infestation by the most damaging PPN with sedentary endoparasitic habits, with the aim of assessing resistance and tolerance traits, sources of resistance in progenies from breeding programs, the reaction to nematodes of newly released cultivars, and the virulence of the most noxious PPNs. These protocols consist of classical screening techniques not involving biochemical and molecular analyses. PPN species and genera considered in this chapter include (i) the most important species of root-knot nematodes Meloidogyne spp., including also M. chitwoodi, M. enterolobii, and M. graminicola, and (ii) the cyst-forming nematodes of the genera Globodera and Heterodera, such as the potato cyst nematodes (PCNs) Globodera rostochiensis and G. pallida, and also Heterodera avenae group, H. ciceri, H. glycines, and H. schachtii. Schemes are given to identify virulence groups for most of these nematodes.

In silico secretome analyses of the polyphagous root-knot nematode Meloidogyne javanica: a resource for studying M. javanica secreted proteins

BACKGROUND

Plant-parasitic nematodes (PPNs) that cause most damage include root-knot nematodes (RKNs) which are a major impediment to crop production. Root-knot nematodes, like other parasites, secrete proteins which are required for parasite proliferation and survival within the host during the infection process.

RESULTS

Here, we used various computational tools to predict and identify classically and non-classically secreted proteins encoded in the Meloidogyne javanica genome. Furthermore, functional annotation analysis was performed using various integrated bioinformatic tools to determine the biological significance of the predicted secretome. In total, 7,458 proteins were identified as secreted ones. A large percentage of this secretome is comprised of small proteins of ≤ 300 aa sequence length. Functional analyses showed that M. javanica secretome comprises cell wall degrading enzymes for facilitating nematode invasion, and migration by disintegrating the complex plant cell wall components. In addition, peptidases and peptidase inhibitors are an important category of M. javanica secretome involved in compatible host-nematode interactions.

CONCLUSION

This study identifies the putative secretome encoded in the M. javanica genome. Future experimental validation analyses can greatly benefit from this global analysis of M. javanica secretome. Equally, our analyses will advance knowledge of the interaction between plants and nematodes.

Peat-based hairy root transformation using Rhizobium rhizogenes as a rapid and efficient tool for easily exploring potential genes related to root-knot nematode parasitism and host response

BACKGROUND

Root-knot nematodes (RKNs) pose a worldwide threat to agriculture of many crops including cucumber. Genetic transformation (GT) has emerged as a powerful tool for exploration of plant-RKN interactions and genetic improvement of RKN resistance. However, it is usually difficult to achieve a highly efficient and stable GT protocol for most crops due to the complexity of this process.

RESULTS

Here we firstly applied the hairy root transformation system in exploring root-RKN interactions in cucumber plants and developed a rapid and efficient tool transformation using Rhizobium rhizogenes strain K599. A solid-medium-based hypocotyl-cutting infection (SHI) method, a rockwool-based hypocotyl-cutting infection (RHI) method, and a peat-based cotyledon-node injection (PCI) method was evaluated for their ability to induce transgenic roots in cucumber plants. The PCI method generally outperformed the SHI and RHI methods for stimulating more transgenic roots and evaluating the phenotype of roots during nematode parasitism. Using the PCI method, we generated the CRISPR/Cas9-mediated malate synthase (MS) gene (involved in biotic stress responses) knockout plant and the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16, a potential host susceptibility gene for RKN) promoter-driven GUS expressing plant. Knockout of MS in hairy roots resulted in effective resistance against RKNs, while nematode infection induced a strong expression of LBD16-driven GUS in root galls. This is the first report of a direct link between these genes and RKN performance in cucumber.

CONCLUSION

Taken together, the present study demonstrates that the PCI method allows fast, easy and efficient in vivo studies of potential genes related to root-knot nematode parasitism and host response.

Reaction of Commercial Cultivars of Kiwifruit to Infection by Root-knot Nematode and Its Biocontrol Using Endophytic Bacteria

Root-knot nematodes (RKN) cause considerable economic losses to kiwifruit production annually. Screening of resistant cultivars has been one of the long-standing methods to manage root-knot nematodes. Here, the reaction of the four most common commercial cultivars of kiwifruit, namely, Actinidia chinensis var. deliciosa cv. Hayward, A. chinensis var. deliciosa cv. Abbott, A. chinensis var. deliciosa cv. Bruno, and A. chinensis var. chinensis cv. Haegeum (commonly known as 'Golden' kiwifruit) to infection by the RKN, Meloidogyne incognita, was evaluated. Among examined cultivars 'Golden' was the most susceptible, having on average 52.8 galls, 56.1 egg masses per gram of root, and 642 J2 population per 200 gram of soil. 'Bruno' showed the highest resistance, with 3.3 galls, 4.1 egg masses per gram of root, and 79 J2 in 200 g of soil. Then, two potential biological control agents, namely Priestia megaterium 31.en and Agrobacterium tumefaciens 19.en were used on 'Hayward' seedlings against M. incognita and showed a significant reduction in the number of galls and egg masses on roots, juvenile population in the soil, and increased the growth parameters of the plants compared to non-treated seedlings. We demonstrated that integrated management using resistant cultivars and biological control can provide a safe and economic method to control RKN, and these resistant cultivars can be used in breeding programs.

Development and Evaluation of an AxiomTM 60K SNP Array for Almond (Prunus dulcis)

A high-density single nucleotide polymorphism (SNP) array is essential to enable faster progress in plant breeding for new cultivar development. In this regard, we have developed an Axiom 60K almond SNP array by resequencing 81 almond accessions. For the validation of the array, a set of 210 accessions were genotyped and 82.8% of the SNPs were classified in the best recommended SNPs. The rate of missing data was between 0.4% and 2.7% for the almond accessions and less than 15.5% for the few peach and wild accessions, suggesting that this array can be used for peach and interspecific peach × almond genetic studies. The values of the two SNPs linked to the RMja (nematode resistance) and SK (bitterness) genes were consistent. We also genotyped 49 hybrids from an almond F2 progeny and could build a genetic map with a set of 1159 SNPs. Error rates, less than 1%, were evaluated by comparing replicates and by detection of departures from Mendelian inheritance in the F2 progeny. This almond array is commercially available and should be a cost-effective genotyping tool useful in the search for new genes and quantitative traits loci (QTL) involved in the control of agronomic traits.

Role of long non coding RNA in plants under abiotic and biotic stresses

Evolutionary processes have evolved plants to cope with several different natural stresses. Basic physiological activities of crop plants are significantly harmed by these stresses, reducing productivity and eventually leading to death. The recent advancements in high-throughput sequencing of transcriptome and expression profiling with NGS techniques lead to the innovation of various RNAs which do not code for proteins, more specifically long non-coding RNAs (lncRNAs), undergirding regulate growth, development, and the plant defence mechanism transcriptionally under stress situations. LncRNAs are a diverse set of RNAs that play key roles in various biological processes at the level of transcription, post-transcription, and epigenetics. These are thought to serve crucial functions in plant immunity and response to changes in the environment. In plants, however, just a few lncRNAs have been functionally identified. In this review, we will address recent advancements in comprehending lncRNA regulatory functions, focusing on the expanding involvement of lncRNAs in modulating environmental stress responsiveness in plants.

Genetic associations with resistance to Meloidogyne enterolobii in guava (Psidium sp.) using cross-genera SNPs and comparative genomics to Eucalyptus highlight evolutionary conservation across the Myrtaceae

Tropical fruit tree species constitute a yet untapped supply of outstanding diversity of taste and nutritional value, barely developed from the genetics standpoint, with scarce or no genomic resources to tackle the challenges arising in modern breeding practice. We generated a de novo genome assembly of the Psidium guajava, the super fruit “apple of the tropics”, and successfully transferred 14,268 SNP probesets from Eucalyptus to Psidium at the nucleotide level, to detect genomic loci linked to resistance to the root knot nematode (RKN) Meloidogyne enterolobii derived from the wild relative P. guineense. Significantly associated loci with resistance across alternative analytical frameworks, were detected at two SNPs on chromosome 3 in a pseudo-assembly of Psidium guajava genome built using a syntenic path approach with the Eucalyptus grandis genome to determine the order and orientation of the contigs. The P. guineense-derived resistance response to RKN and disease onset is conceivably triggered by mineral nutrients and phytohormone homeostasis or signaling with the involvement of the miRNA pathway. Hotspots of mapped resistance quantitative trait loci and functional annotation in the same genomic region of Eucalyptus provide further indirect support to our results, highlighting the evolutionary conservation of genomes across genera of Myrtaceae in the adaptation to pathogens. Marker assisted introgression of the resistance loci mapped should accelerate the development of improved guava cultivars and hybrid rootstocks.

Bioisosterism and Scaffold Hopping in Modern Nematicide Research

The application of agrochemicals is critical to global food safety. Nowadays, environmentally friendly green agrochemicals are the trend in field crop protection. The research and development of nematicides absorbed more attention as a typical representation of agrochemicals. This review describes the origin of recently commercialized nematicides, the application of bioisosterism and scaffold hopping in the discovery and optimization of agrochemicals, especially nematicides, and novel bioisosteric design strategies for the identification of fluensulfone analogues. Pesticide repurposing, high-throughput screening, computer-aided drug design, and incorporation of known pharmacophoric fragments have been the most successful approach for the discovery of new nematicides. As outlined, the strategies of bioisosteric replacements and scaffold hopping have been very successful approaches in the search for new nematicides for sustainable crop protection. In the exploration of novel fluensulfone analogues with nematicidal activity, bioisosteric replacement of sulfone by amide, chain extension by insertion of a methylene group, and reversal of the amide group have proven to be successful approaches and yielded new and highly active fluensulfone analogues. These attempts might result in compounds with an optimal balance of steric, hydrophobic, electronic, and hydrogen-bonding properties and contribute to deal with the complex problem during the research and development of new nematicides. Further ideas are also put forward to provide new approaches for the molecular design of nematicides.

Application Potential of Bacterial Volatile Organic Compounds in the Control of Root-Knot Nematodes

Plant-parasitic nematodes (PPNs) constitute the most damaging group of plant pathogens. Plant infections by root-knot nematodes (RKNs) alone could cause approximately 5% of global crop loss. Conventionally, chemical-based methods are used to control PPNs at the expense of the environment and human health. Accordingly, the development of eco-friendly and safer methods has been urged to supplement or replace chemical-based methods for the control of RKNs. Using microorganisms or their metabolites as biological control agents (BCAs) is a promising approach to controlling RKNs. Among the metabolites, volatile organic compounds (VOCs) have gained increasing attention because of their potential in the control of not only RKNs but also other plant pathogens, such as insects, fungi, and bacteria. This review discusses the biology of RKNs as well as the status of various control strategies. The discovery of VOCs emitted by bacteria from various environmental sources and their application potential as BCAs in controlling RKNs are specifically addressed.

Microplate bioassay to examine the effects of grapevine-isolated stilbenoids on survival of root knot nematodes

OBJECTIVES

Root knot nematodes can be major pests in vineyards and cause significant yield losses over time. Control involves the use of different resistant grapevine rootstocks, but it remains unclear the mechanisms that such rootstocks possess to limit root knot nematode infections. Defense-associated compounds called stilbenoids, a type of phenolic compound, are present in relatively substantial amounts in grapevine root tissues. Therefore, experiments were performed to assess how different stilbenoid compounds impact nematode survival in microplate assays. Data generated were part of a larger effort to understand potential mechanisms that resistant grapevine rootstocks have to limit root knot infections.

DATA DESCRIPTION

The percentage of surviving root knot nematodes was assessed 1, 3, and 5 days after J2 juveniles were placed into microplate wells amended with 0, 1.25, 2.5, 5, or 10 ppm of piceid, ε-viniferin, a resveratrol trimer putatively identified as miyabenol C, or a putative mixture of resveratrol tetramers putatively identified as vitisin B and hopeaphenol. Both ε-viniferin and the resveratrol tetramers significantly reduced root knot survival at the higher concentrations. These data provide insight about one potential mechanism that grapevine rootstocks might possess to combat nematodes.

The combination of two Bacillus strains suppresses Meloidogyne incognita and fungal pathogens, but does not enhance plant growth

BACKGROUND

The combination of biocontrol agents is a desirable strategy to improve control efficacy against the root-knot nematode (RKN) Meloidogyne incognita under field conditions. However, strains compatibility is generally tested in vitro and incompatible combinations are normally not further examined in experiments in planta. Therefore, there is virtually no information on the performance of incompatible strains. In this study, we evaluated two Bacillus strains previously described as incompatible in vitro for effects on plant growth and suppression of M. incognita, pathogenic fungi and nematophagous fungi.

RESULTS

Strains BMH and INV were shown to be closely related to Bacillus velezensis. These strains, when applied individually, reduced the number of galls and eggs of M. incognita by more than 90% in tomato roots. When BMH and INV were combined (BMH + INV), RKN suppression and tomato shoot weight were lower compared to single-strain applications. Additionally, metabolites in cell-free supernatants and volatile organic compounds (VOCs) from strains BMH and INV had strong effects against the plant pathogens M. incognita, Fusarium oxysporum, Rhizoctonia solani and Sclerotium rolfsiii, but not against three species of nematophagous fungi. Although strain INV and the combination BMH + INV emitted fewer VOCs than strain BMH, they were still capable of killing second-stage juveniles of M. incognita.

CONCLUSIONS

Bacillus strains BMH and INV inhibited M. incognita and fungal pathogens, and promoted tomato growth. However, strain INV emitted fewer VOCs and the combination BMH + INV did not enhance the activity of the biocontrol strains against the RKN or their capacity to promote plant growth. © 2021 Society of Chemical Industry.

Overexpression of PsoRPM3, an NBS-LRR gene isolated from myrobalan plum, confers resistance to Meloidogyne incognita in tobacco

KEY MESSAGES

We reported an NBS-LRR gene, PsoRPM3, is highly expressed following RKN infection, initiating an HR response that promotes plant resistance. Meloidogyne spp. are root-knot nematodes (RKNs) that cause substantial economic losses worldwide. Screening for resistant tree resources and identifying plant resistance genes is currently the most effective way to prevent RKN infestations. Here, we cloned a novel TIR-NB-LRR-type resistance gene, PsoRPM3, from Xinjiang wild myrobalan plum (Prunus sogdiana Vassilcz.) and demonstrated that its protein product localized to the nucleus. In response to Meloidogyne incognita infection, PsoRPM3 gene expression levels were significantly higher in resistant myrobalan plum plants compared to susceptible plants. We investigated this difference, discovering that the - 309 to - 19 bp region of the susceptible PsoRPM3 promoter was highly methylated. Indeed, heterologous expression of PsoRPM3 significantly enhanced the resistance of susceptible tobacco plants to M. incognita. Moreover, transient expression of PsoRPM3 induced a hypersensitive response in tobacco, whereas RNAi-mediated silencing of PsoRPM3 in transgenic tobacco reduced this hypersensitive response. Several hypersensitive response marker genes were considerably up-regulated in resistant myrobalan plum plants when compared with susceptible counterparts inoculated with M. incognita. PsoPR1a (a SA marker gene), PsoPR2 (a JA marker gene), and PsoACS6 (an ET signaling marker gene) were all more highly expressed in resistant than in susceptible plants. Together, these results support a model in which PsoRPM3 is highly expressed following RKN infection, initiating an HR response that promotes plant resistance through activated salicylic acid, jasmonic acid, and ethylene signaling pathways.

Genetic Diversity, Identification, and Utilization of Novel Genetic Resources for Resistance to Meloidogyne incognita in Mulberry (Morus spp.)

Mulberry (Morus spp.) is an important crop in the sericulture industry, as the leaves constitute the primary feed for the silkworm. The availability of diverse genetic sources of resistance to root-knot nematode (RKN; Meloidogyne spp.) are very scanty. To address this need, a set of 415 varied exotic and indigenous germplasm accessions were screened under glasshouse conditions. Twenty-one accessions were identified as highly resistant and 48 were resistant, with the highest numbers of highly resistant/resistant accessions being found in Morus alba. Further, 30 accessions based on rooting ability were evaluated for field resistance at four different locations with infested soil. Finally, eight germplasm accessions (BR-8, Karanjtoli-1, Hosur-C8, Nagalur Estate, Tippu, Calabresa, Thai Pecah, and SRDC-3) were identified as potential genetic sources in RKN-resistance breeding programs or as resistant rootstock for the establishment of mulberry gardens. Sixteen simple sequence repeat markers analyzed among the 77 resistant and susceptible accessions generated 55 alleles, ranging from two to five, with an average of 3.43 alleles per locus. Principal coordinates analysis grouped the accessions on the basis of susceptibility and resistance to RKN infestation. The RKN-susceptible accessions exhibited higher variability as compared with resistant accessions, and they were more dispersed. Analysis of molecular variance showed maximum molecular variance was 78% within the population, and 22% between populations. Results of this study indicate that simple sequence repeat markers are reliable for assessing genetic variability among the RKN-resistant and RKN-susceptible mulberry accessions.

Marasmius oreades agglutinin enhances resistance of Arabidopsis against plant-parasitic nematodes and a herbivorous insect

BACKGROUND

Plant-parasitic nematodes and herbivorous insects have a significant negative impact on global crop production. A successful approach to protect crops from these pests is the in planta expression of nematotoxic or entomotoxic proteins such as crystal proteins from Bacillus thuringiensis (Bt) or plant lectins. However, the efficacy of this approach is threatened by emergence of resistance in nematode and insect populations to these proteins. To solve this problem, novel nematotoxic and entomotoxic proteins are needed. During the last two decades, several cytoplasmic lectins from mushrooms with nematicidal and insecticidal activity have been characterized. In this study, we tested the potential of Marasmius oreades agglutinin (MOA) to furnish Arabidopsis plants with resistance towards three economically important crop pests: the two plant-parasitic nematodes Heterodera schachtii and Meloidogyne incognita and the herbivorous diamondback moth Plutella xylostella.

RESULTS

The expression of MOA does not affect plant growth under axenic conditions which is an essential parameter in the engineering of genetically modified crops. The transgenic Arabidopsis lines showed nearly complete resistance to H. schachtii, in that the number of female and male nematodes per cm root was reduced by 86-91 % and 43-93 % compared to WT, respectively. M. incognita proved to be less susceptible to the MOA protein in that 18-25 % and 26-35 % less galls and nematode egg masses, respectively, were observed in the transgenic lines. Larvae of the herbivorous P. xylostella foraging on MOA-expression lines showed a lower relative mass gain (22-38 %) and survival rate (15-24 %) than those feeding on WT plants.

CONCLUSIONS

The results of our in planta experiments reveal a robust nematicidal and insecticidal activity of the fungal lectin MOA against important agricultural pests which may be exploited for crop protection.

CCS52 and DEL1 function in root-knot nematode giant cell development in Xinjiang wild myrobalan plum (Prunus sogdiana Vassilcz)

Root-knot nematodes (RKNs) are highly invasive plant parasites that establish permanent feeding sites within the roots of the host plant. Successful establishment of the feeding site is essential for the survival of RKN. The formation and development of the feeding cell, also called giant cell, involve both cell division and endoreduplication. Here, we examined giant cell development and endoreduplication in Prunus sogdiana infected with the RKN. We found that feeding sites were established 3-5 days post inoculation (dpi) and matured at 21-28 dpi. The giant cells began to form 5 dpi and continued to increase in size from 7 to 21 dpi. The large numbers of dividing nuclei were observed in giant cells from 7 to 14 dpi. However, nuclear division was rarely observed after 28 days. RT-PCR and in situ hybridization analyses revealed that PsoCCS52A was abundantly expressed at 7-21 dpi and the PsoCCS52A signal observed in giant cell nucleus at 7-14 dpi. The PsoCCS52B is highly expressed at 14 dpi, and the hybridization signal was mainly in the cytoplasm of giant cells. The PsoDEL1 expression was lowest 7-21 dip, with negligible transcript detected in the giant cells. This indicates that the PsoCCS52A plays a role in the process of cell division, while the CCS52B plays a role in the development of giant cells. The PsoDEL1 plays a negative regulatory role in megakaryocyte nuclear replication. These data suggest that an increased expression of PsoCCS52A promotes nuclear division and produces a large number of polyploid nuclei, the area of giant cells and feeding sites increase, ultimately leading to the formation of galls in Prunus sogdiana.

Grapevine (Vitis spp.) rootstock stilbenoid associations with host resistance to and induction by root knot nematodes, Meloidogyne incognita

Objective

The root knot nematodes (RKN) Meloidogyne incognita can severely reduce grapevine yields over time. Grapevine rootstocks have been developed from wild Vitis species that provide resistance to nematode infections. However, the potential biochemical or mechanical mechanisms of resistance have not been thoroughly explored. Therefore, this study measured levels of stilbenoids in roots of non-infected and RKN-infected grapevines with Cabernet Sauvignon scion grafted to susceptible (O39-16) or resistant (Freedom) rootstocks. This was part of a larger effort to assess phenolic compound levels within grapevine rootstocks to determine roles of stilbenoid compounds in improving nematode resistance and overall plant health.

Results

None of the assessed compounds were consistently greater in RKN infected plants versus healthy controls. Stilbenoids putatively identified as pallidol, ɑ-viniferin, miyabenol C, and hopeaphenol were overall much greater in Freedom than O39-16 rootstocks. By contrast, the stilbenoids ampelopsin A, ω-viniferin, and vitisin B were greater in O39-16 than Freedom. O39-16 and Freedom had similar levels of other stilbenoids especially monomers and dimers. Potentially the greater levels of specific stilbenoids present in Freedom than O39-16 provided RKN resistance. If validated, breeding programs could utilize the increased presence of these compounds as a marker for increased resistance to nematodes.

Regrowing the damaged or lost body parts

Plants display extraordinary ability to revive tissues and organs lost or damaged in injury. This is evident from the root tip restoration and classical experiments in stem demonstrating re-establishment of vascular continuity. While recent studies have begun to unravel the mechanistic understanding of tissue restoration in response to injury in underground plant organs, the molecular mechanisms of the same in aerial organs remain to be ventured deeper. Here, we discuss the possibility of unearthing the regulatory mechanism that can confer universal regeneration potential to plant body and further provide a comprehensive understanding of how tissue and organ regeneration gets triggered in response to mechanical injury and later gets terminated after re-patterning and regaining the appropriate size.

Advances in Plant-Nematode Interactions with Emphasis on the Notorious Nematode Genus Meloidogyne

Plant infections by plant-parasitic nematodes (PPNs) continue to be one of the major limitations in agricultural systems. Root-knot nematodes (RKNs), belonging to the genus Meloidogyne, are one of the most important groups of PPNs worldwide. Their wide host range combined with ubiquitous presence, continues to provide challenges for their control and breeding for resistance. Although resistance to RKNs has been identified, incorporation of these resistances into crops and durability of the resistance remains challenging. In addition, progress in cloning of RKN resistance genes has been dismal. Recent identification of pattern-triggered immunity in roots against nematodes, an ascaroside as a nematode-associated molecular pattern (NAMP) and the discovery of a NAMP plant receptor, provide tools and opportunities to develop durable host resistance against nematodes including RKNs.

Evaluation of the Phytopathological Reaction of Wild and Cultivated Olives as a Means of Finding Promising New Sources of Genetic Diversity for Resistance to Root-Knot Nematodes

Olive (Olea europaea L.) is one of the most important fruit crops in the Mediterranean Basin, because it occupies significant acreage in these countries and often has important cultural heritage and landscape value. This crop can be infected by several Meloidogyne species (M. javanica, M. arenaria, and M. incognita, among others), and only a few cultivars with some level of resistance to these nematodes have been found. Innovations in intensive olive growing using high planting densities, irrigation, and substantial amounts of fertilizers could increase the nematode population to further damaging levels. To further understand the interactions involved between olive and pathogenic nematodes and in the hope of finding solutions to the agricultural risks, this research aimed to determine the reaction of important olive cultivars in Spain and wild olives to M. javanica infection, including genotypes of the same and other O. europaea subspecies. All olive cultivars tested were good hosts for M. javanica, but high levels of nematode reproduction found in three cultivars (Gordal Sevillana, Hojiblanca, and Manzanilla de Sevilla) were substantially different. In the wild accessions, O. europaea subsp. cerasiformis (genotype W147) and O. europaea subsp. europaea var. sylvestris (genotype W224) were resistant to M. javanica at different levels, with strong resistance in W147 (reproduction factor [Rf] = 0.0003) and moderate resistance in W224 (Rf = 0.79). The defense reaction of W147 to M. javanica showed a strong increase of phenolic compounds but no hypersensitive reaction.

Plant-parasitic nematode effectors - insights into their diversity and new tools for their identification

Plant-parasitic nematodes (PPNs) are a large group of obligate biotrophic pathogens that secrete molecules, called effectors, involved in parasitism. The majority of work in molecular phytonematology has focused on the root-knot and cyst nematodes, which are both sedentary endoparasitic nematodes. More recently, inexpensive sequencing technology has facilitated effector searches in PPNs with different parasitic lifestyles. Work in different PPN species suggests that effectors are diverse, and selection pressure from plant hosts has contributed to the presence of large, expanded effector gene families. The identification of promoter elements/motifs preceding effector gene sequences suggests that promoter analysis can computationally predict new putative effectors. However, until a method of genetic transformation is available for PPNs, work on characterizing effectors will be hindered.

A novel approach to determine generalist nematophagous microbes reveals Mortierella globalpina as a new biocontrol agent against Meloidogyne spp. nematodes

Root-knot nematodes (RKN) such as Meloidogyne spp. are among the most detrimental pests in agriculture affecting several crops. New methodologies to manage RKN are needed such as efficient discovery of nematophagous microbes. In this study, we developed an in vitro high-throughput method relying on the free-living nematode Caenorhabditis elegans and the infection of those nematodes with a soil slurry containing a microbiome likely to house nematophagous microbes. Nematodes were monitored for presence of infection and sub-cultured repeatedly for the purpose of isolating pure cultures of the microbe responsible for conferring the nematicidal activity. Once soil microbes were confirmed to be antagonistic to C. elegans, they were tested for pathogenicity against Meloidogyne chitwoodi. Using this methodology, the fungal isolate Mortierella globalpina was confirmed to be pathogenic in vitro against M. chitwoodi by nematode trapping via hyphal adhesion to the cuticle layer, penetration of the cuticle layer, and subsequently digestion of its cellular contents. M. globalpina was also observed to reduce disease symptomology of RKNs in vivo via significant reduction of root-galls on tomato (Solanum lycopersicum var. Rutgers).

New Data Completing the Spectrum of the Ma, RMia, and RMja Genes for Resistance to Root-Knot Nematodes (Meloidogyne spp.) in Prunus

Root-knot nematodes (RKN) (Meloidogyne spp.) are worldwide pests that affect a considerable number of plants, among which stone fruit (Prunus spp.) are severely attacked. Prevalent RKN species are Meloidogyne arenaria, M. incognita, and M. javanica in stone fruit but the emergent M. ethiopica and M. enterolobii are also reported to challenge perennial crops. In Prunus spp., the complete-spectrum resistance (R) gene Ma from plum and the more restricted-spectrum R genes RMia from peach and RMja from almond completely inhibit nematode multiplication and gall formation of the RKN species that they control. This study aimed to update the resistance spectra of these three major genes by evaluating their activity toward one isolate of the yet-untested RKN species mentioned above. To state whether a given gene controls a particular species, the principle of our experiment was to genotype with appropriate markers a number of individuals segregating for this gene and then to phenotype these individuals. A perfect matching of the genotype and the phenotype of individuals indicates that the gene of interest is active against and, thus, controls the corresponding isolate of this RKN species. Segregating materials used were an Ma F1 plum progeny, an RMia F2 peach progeny, and an RMja F2 almond progeny. In addition to previous data, our results establish a clear spectrum for each of the three genes toward isolates from both the three prevalent species and the two emerging species. Ultimately, our results reveal that (i) Ma controls all of them, (ii) RMja controls all species except M. incognita and M. floridensis, and (iii) RMia controls M. arenaria, M. incognita, and M. ethiopica but not M. javanica or M. enterolobii. Our data should have wide implications for RKN resistance management and breeding and for deciphering the molecular mechanisms of the spectrum of RKN R genes.

Sustainable alternatives to 1,3-dichloropropene for controlling root-knot nematodes and fungal pathogens in melon crops in Mediterranean soils: Efficacy and effects on soil quality

The control of agricultural pests is key to maintain economically viable crops. Increasing environmental awareness, however, is leading to more restrictive European policies regulating the use of certain pesticides due to their impact on human health and the soil system. Given this context, we evaluated the efficacy of three alternatives to the soil fumigant 1,3-dichloropropene (1,3-D), which is currently banned in Europe: two non-fumigant nematicides [oxamyl (OX) and fenamiphos (FEN)] and the soil fumigant dimethyl disulfide (DMDS). We analysed the efficiency of these pesticides against root-knot nematodes and soil fungal pathogens (determined by qPCR) as well as the soil biological quality after treatments application (estimated by enzyme activities). Among treatments, 1,3-D and DMDS significantly reduced nematode populations. FEN was more effective in sandy soil, while OX had no effect in any soil. OX and FEN had no effect on fungal pathogens, whereas DMDS reduced the abundance of Rhizoctonia solani and Fusarium solani at the root level in clay-loam soil. Soil quality decreased after treatment application but then recovered throughout the experiment, indicating the possible dissipation of the pesticides. Our findings support DMDS as a potential sustainable alternative for controlling root-knot nematodes and fungal pathogens due to its effectiveness in both studied soils, although its negative impact on soil biological quality in sandier soils must be taken into account. Main finding of the work. DMDS is a reliable alternative to 1,3-D for controlling agricultural pest but its inhibitory effect on soil enzyme activities varied according to the soil characteristics.

Genome-wide identification and functional prediction of tobacco lncRNAs responsive to root-knot nematode stress

Root-knot nematodes (RKNs, Meloidogyne spp.) are destructive plant parasites with a wide host range. They severely reduce crop quality and yield worldwide. Tobacco is a versatile model plant organism for studying RKNs-host interactions and a key plant material for molecular research. Long noncoding RNAs (lncRNAs) play critical roles in post transcriptional and transcriptional regulation in a wide range of biological pathways, especially plant development and stress response. In the present study, we obtained 5,206 high-confidence lncRNAs based on RNA sequencing data. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that the target genes of these lncRNAs are mainly involved in plant biotic and abiotic stresses, plant hormone signal transduction, induced systemic resistance, plant-type hypersensitive response, plant-type cell wall organization or biogenesis. The 565 differentially expressed lncRNAs found to be involved in nematode stress response were validated by quantitative PCR using 15 randomly-selected lncRNA genes. Our study provides insights into the molecular mechanisms of RKNs-plant interactions that might help preventing nematode damages to crops.

Ma Orthologous Genes in Prunus spp. Shed Light on a Noteworthy NBS-LRR Cluster Conferring Differential Resistance to Root-Knot Nematodes

Root-knot nematodes (RKNs) are considerable polyphagous pests that severely challenge plants worldwide and especially perennials. The specific genetic resistance of plants mainly relies on the NBS-LRR genes that are pivotal factors for pathogens control. In Prunus spp., the Ma plum and RMja almond genes possess different spectra for resistance to RKNs. While previous works based on the Ma gene allowed to clone it and to decipher its peculiar TIR-NBS-LRR (TNL) structure, we only knew that the RMja gene mapped on the same chromosome as Ma. We carried out a high-resolution mapping using an almond segregating F2 progeny of 1448 seedlings from resistant (R) and susceptible (S) parental accessions, to locate precisely RMja on the peach genome, the reference sequence for Prunus species. We showed that the RMja gene maps in the Ma resistance cluster and that the Ma ortholog is the best candidate for RMja. This co-localization is a crucial step that opens the way to unravel the molecular determinants involved in the resistance to RKNs. Then we sequenced both almond parental NGS genomes and aligned them onto the RKN susceptible reference peach genome. We produced a BAC library of the R parental accession and, from two overlapping BAC clones, we obtained a 336-kb sequence encompassing the RMja candidate region. Thus, we could benefit from three Ma orthologous regions to investigate their sequence polymorphism, respectively, within plum (complete R spectrum), almond (incomplete R spectrum) and peach (null R spectrum). We showed that the Ma TNL cluster has evolved orthologs with a unique conserved structure comprised of five repeated post-LRR (PL) domains, which contain most polymorphism. In addition to support the Ma and RMja orthologous relationship, our results suggest that the polymorphism contained in the PL sequences might underlie differential resistance interactions with RKNs and an original immune mechanism in woody perennials. Besides, our study illustrates how PL exon duplications and losses shape TNL structure and give rise to atypical PL domain repeats of yet unknown role.

SNP markers tightly linked to root knot nematode resistance in grapevine (Vitis cinerea) identified by a genotyping-by-sequencing approach followed by Sequenom MassARRAY validation

Plant parasitic nematodes, including root knot nematode Meloidogyne species, cause extensive damage to agriculture and horticultural crops. As Vitis vinifera cultivars are susceptible to root knot nematode parasitism, rootstocks resistant to these soil pests provide a sustainable approach to maintain grapevine production. Currently, most of the commercially available root knot nematode resistant rootstocks are highly vigorous and take up excess potassium, which reduces wine quality. As a result, there is a pressing need to breed new root knot nematode resistant rootstocks, which have no impact on wine quality. To develop molecular markers that predict root knot nematode resistance for marker assisted breeding, a genetic approach was employed to identify a root knot nematode resistance locus in grapevine. To this end, a Meloidogyne javanica resistant Vitis cinerea accession was crossed to a susceptible Vitis vinifera cultivar Riesling and results from screening the F1 individuals support a model that root knot nematode resistance, is conferred by a single dominant allele, referred as MELOIDOGYNE JAVANICA RESISTANCE1 (MJR1). Further, MJR1 resistance appears to be mediated by a hypersensitive response that occurs in the root apical meristem. Single nucleotide polymorphisms (SNPs) were identified using genotyping-by-sequencing and results from association and genetic mapping identified the MJR1 locus, which is located on chromosome 18 in the Vitis cinerea accession. Validation of the SNPs linked to the MJR1 locus using a Sequenom MassARRAY platform found that only 50% could be validated. The validated SNPs that flank and co-segregate with the MJR1 locus can be used for marker-assisted selection for Meloidogyne javanica resistance in grapevine.

Host-Induced Silencing of Two Pharyngeal Gland Genes Conferred Transcriptional Alteration of Cell Wall-Modifying Enzymes of Meloidogyne incognita vis-à-vis Perturbed Nematode Infectivity in Eggplant

The complex parasitic strategy of Meloidogyne incognita appears to involve simultaneous expression of its pharyngeal gland-specific effector genes in order to colonize the host plants. Research reports related to effector crosstalk in phytonematodes for successful parasitism of the host tissue is yet underexplored. In view of this, we have used in planta effector screening approach to understand the possible interaction of pioneer genes (msp-18 and msp-20, putatively involved in late and early stage of M. incognita parasitism, respectively) with other unrelated effectors such as cell-wall modifying enzymes (CWMEs) in M. incognita. Host-induced gene silencing (HIGS) strategy was used to generate the transgenic eggplants expressing msp-18 and msp-20, independently. Putative transformants were characterized via qRT-PCR and Southern hybridization assay. SiRNAs specific to msp-18 and msp-20 were also detected in the transformants via Northern hybridization assay. Transgenic expression of the RNAi constructs of msp-18 and msp-20 genes resulted in 43.64-69.68% and 41.74-67.30% reduction in M. incognita multiplication encompassing 6 and 10 events, respectively. Additionally, transcriptional oscillation of CWMEs documented in the penetrating and developing nematodes suggested the possible interaction among CWMEs and pioneer genes. The rapid assimilation of plant-derived carbon by invading nematodes was also demonstrated using 14C isotope probing approach. Our data suggests that HIGS of msp-18 and msp-20, improves nematode resistance in eggplant by affecting the steady-state transcription level of CWME genes in invading nematodes, and safeguard the plant against nematode invasion at very early stage because nematodes may become the recipient of bioactive RNA species during the process of penetration into the plant root.

Transcriptomic and histological responses of African rice (Oryza glaberrima) to Meloidogyne graminicola provide new insights into root-knot nematode resistance in monocots

BACKGROUND AND AIMS

The root-knot nematode Meloidogyne graminicola is responsible for production losses in rice ( Oryza sativa ) in Asia and Latin America. The accession TOG5681 of African rice, O. glaberrima , presents improved resistance to several biotic and abiotic factors, including nematodes. The aim of this study was to assess the cytological and molecular mechanisms underlying nematode resistance in this accession.

METHODS

Penetration and development in M. graminicola in TOG5681 and the susceptible O. sativa genotype 'Nipponbare' were compared by microscopic observation of infected roots and histological analysis of galls. In parallel, host molecular responses to M. graminicola were assessed by root transcriptome profiling at 2, 4 and 8 d post-infection (dpi). Specific treatments with hormone inhibitors were conducted in TOG5681 to assess the impact of the jasmonic acid and salicylic acid pathways on nematode penetration and reproduction.

KEY RESULTS

Penetration and development of M. graminicola juveniles were reduced in the resistant TOG5681 in comparison with the susceptible accession, with degeneration of giant cells observed in the resistant genotype from 15 dpi onwards. Transcriptome changes were observed as early as 2 dpi, with genes predicted to be involved in defence responses, phenylpropanoid and hormone pathways strongly induced in TOG5681, in contrast to 'Nipponbare'. No specific hormonal pathway could be identified as the major determinant of resistance in the rice-nematode incompatible interaction. Candidate genes proposed as involved in resistance to M. graminicola in TOG5681 were identified based on their expression pattern and quantitative trait locus (QTL) position, including chalcone synthase, isoflavone reductase, phenylalanine ammonia lyase, WRKY62 transcription factor, thionin, stripe rust resistance protein, thaumatins and ATPase3.

CONCLUSIONS

This study provides a novel set of candidate genes for O. glaberrima resistance to nematodes and highlights the rice- M. graminicola pathosystem as a model to study plant-nematode incompatible interactions.