Root-knot Nematode Resistance Research Articles

BSA-seq Identifies a Major Locus on Chromosome 6 for Root-Knot Nematode (Meloidogyne graminicola) Resistance From Oryza glaberrima.

Root-knot nematode (Meloidogyne graminicola) is one of the emerging threats to rice production worldwide that causes substantial yield reductions. There is a progressive shift of the cropping system from traditional transplanting to direct-seeded water-saving rice production that favored the development of M. graminicola. Scouting and deploying new resistance genes is an economical approach to managing the root-knot nematodes. Here, we report that the inheritance of root-knot nematode resistance in Oryza glaberrima acc. IRGC102206 is governed by a single dominant gene. Traditional mapping coupled with BSA-seq is used to map nematode resistance gene(s) using the BC1F1 population derived from a cross of O. sativa cv. PR121 (S) and O. glaberrima acc. IRGC102206 (R). One major novel genomic region spanning a 3.0-Mb interval on chromosome 6 and two minor QTLs on chromosomes 2 and 4 are the potential genomic regions associated with rice root-knot nematode resistance. Within the QTL regions, 19 putative candidate genes contain 81 non-synonymous variants. The detected major candidate region could be fine mapped to accelerate marker-assisted breeding for root-knot nematode resistance in rice.

Temperature Effects on Development of Meloidogyne Enterolobii and M. Floridensis.

Meloidogyne enterolobii and M. floridensis are virulent species that can overcome root-knot nematode resistance in economically important crops. Our objectives were to determine the effects of temperature on the infectivity of second-stage juveniles (J2) of these two species and determine differences in duration and thermal-time requirements (degree-days [DD]) to complete their developmental cycle. Florida isolates of M. enterolobii and M. floridensis were compared to M. incognita race 3. Tomato cv. BHN 589 seedlings following inoculation were placed in growth chambers set at constant temperatures of 25°C, and 30°C, and alternating temperatures of 30°C to 25°C (day–night). Root infection by the three nematode species was higher at 30°C than at 25°C, and intermediate at 30°C to 25°C, with 33%, 15%, and 24% infection rates, respectively. There was no difference, however, in the percentages of J2 that infected roots among species at each temperature. Developmental time from infective J2 to reproductive stage for the three species was shorter at 30°C than at 25°C, and 30°C to 25°C. The shortest time and DD to egg production for the three species were 13 days after inoculation (DAI) and 285.7 DD, respectively. During the experimental timeframe of 29 d, a single generation was completed at 30°C for all three species, whereas only M. floridensis completed a generation at 30°C to 25°C. The number of days and accumulated DD for completing the life cycle (from J2 to J2) were 23 d and 506.9 DD for M. enterolobii, and 25 d and 552.3 DD for M. floridensis and M. incognita, respectively. Exposure to lower (25°C) and intermediate temperatures (30°C to 25°C) decreased root penetration and slowed the developmental cycle of M. enterolobii and M. floridensis compared with 30°C.

Susceptibility of crop plants to the root-knot nematode Meloidogyne luci, a threat to agricultural productivity

The root-knot nematode (RKN) Meloidogyne luci is included in the Alert List of the European Plant Protection Organization, because it has potential negative impacts on economically important crops. Identification of plant species/cultivars resistant to M. luci is important for its management. Susceptibility of 35 commercial plant species/cultivars, from nine families to a M. luci isolate from Portugal was evaluated in pot assays, assessing root gall index (GI) and reproduction factor (Rf) 60 d after inoculation, with tomato ‘Coração-de-Boi’ used as the positive susceptible experimental control. Presence/absence of RKN resistance genes was also determined in the tomato and pepper cultivars. One cultivar of cabbage, three of lettuce, ten of pepper, one of sugar beet, and all the cultivars of Cucurbitaceae (five), Fabaceae (two) and Poaceae (one) were susceptible to M. luci (GI = 4-5; Rf = 2.1-152.3). One cultivar each of carrot, passion fruit, lettuce ‘Cocktail’, cabbage ‘Bacalan’, ‘Coração’ and ‘Lombarda’, and spinach ‘Tayto’ were resistant/hypersensitive (Rf < 1; GI > 2). The tomato ‘Actimino’, ‘Briomino’, ‘Veinal’ and ‘Vimeiro’, which carried at least one copy of the Mi-1.2 gene, were resistant to the nematode (GI = 1-2; 0.0 < Rf < 0.1). These results indicate that the tomato cultivars have potential to contribute to reduction of M. luci populations in agro-ecosystems and improve the crop yields.

Root-Knot Nematode Resistance in Gossypium hirsutum Determined by a Constitutive Defense-Response Transcriptional Program Avoiding a Fitness Penalty.

Cotton (Gossypium spp.) is the most important renewable source of natural textile fiber and one of the most cultivated crops around the world. Plant-parasitic nematode infestations, such as the southern Root-Knot Nematode (RKN) Meloidogyne incognita, represent a threat to cotton production worldwide. Host-plant resistance is a highly effective strategy to manage RKN; however, the underlying molecular mechanisms of RKN-resistance remain largely unknown. In this study, we harness the differences in RKN-resistance between a susceptible (Acala SJ-2, SJ2), a moderately resistant (Upland Wild Mexico Jack Jones, WMJJ), and a resistant (Acala NemX) cotton entries, to perform genome-wide comparative analysis of the root transcriptional response to M. incognita infection. RNA-seq data suggest that RKN-resistance is determined by a constitutive state of defense transcriptional behavior that prevails in the roots of the NemX cultivar. Gene ontology and protein homology analyses indicate that the root transcriptional landscape in response to RKN-infection is enriched for responses related to jasmonic and salicylic acid, two key phytohormones in plant defense responses. These responses are constitutively activated in NemX and correlate with elevated levels of these two hormones while avoiding a fitness penalty. We show that the expression of cotton genes coding for disease resistance and receptor proteins linked to RKN-resistance and perception in plants, is enhanced in the roots of RKN-resistant NemX. Members of the later gene families, located in the confidence interval of a previously identified QTL associated with RKN resistance, represent promising candidates that might facilitate introduction of RKN-resistance into valuable commercial varieties of cotton. Our study provides novel insights into the molecular mechanisms that underlie RKN resistance in cotton.

Missense mutation of a class B heat shock factor is responsible for the tomato bushy root-2 phenotype

The bushy root-2 (brt-2) tomato mutant has twisting roots, and slower plant development. Here we used whole genome resequencing and genetic mapping to show that brt-2 is caused by a serine to cysteine (S75C) substitution in the DNA binding domain (DBD) of a heat shock factor class B (HsfB) encoded by SolycHsfB4a. This gene is orthologous to the Arabidopsis SCHIZORIZA gene, also known as AtHsfB4. The brt-2 phenotype is very similar to Arabidopsis lines in which the function of AtHsfB4 is altered: a proliferation of lateral root cap and root meristematic tissues, and a tendency for lateral root cap cells to easily separate. The brt-2 S75C mutation is unusual because all other reported amino acid substitutions in the highly conserved DBD of eukaryotic heat shock factors are dominant negative mutations, but brt-2 is recessive. We further show through reciprocal grafting that brt-2 exerts its effects predominantly through the root genotype even through BRT-2 is expressed at similar levels in both root and shoot meristems. Since AtHsfB4 is induced by root knot nematodes (RKN), and loss-of-function mutants of this gene are resistant to RKNs, BRT-2 could be a target gene for RKN resistance, an important trait in tomato rootstock breeding.Gene & accession numbersSolycHsfB4a - Solyc04g078770.

Genetic mapping reveals complex architecture and candidate genes involved in common bean response to Meloidogyne incognita infection.

Root-knot nematodes (RKNs), particularly Meloidogyne incognita, are among the most damaging and prevalent agricultural pathogens due to their ability to infect roots of almost all crops. The best strategy for their control is through the use of resistant cultivars. However, laborious phenotyping procedures make it difficult to assess nematode resistance in breeding programs. For common bean, this task is especially challenging because little has been done to discover resistance genes or markers to assist selection. We performed genome-wide association studies and quantitative trait loci mapping to explore the genetic architecture and genomic regions underlying the resistance to M. incognita and to identify candidate resistance genes. Phenotypic data were collected by a high-throughput assay, and the number of egg masses and the root-galling index were evaluated. Complex genetic architecture and independent genomic regions were associated with each trait. Single nucleotide polymorphisms on chromosomes Pv06, Pv07, Pv08, and Pv11 were associated with the number of egg masses, and SNPs on Pv01, Pv02, Pv05, and Pv10 were associated with root-galling. A total of 216 candidate genes were identified, including 14 resistance gene analogs and five differentially expressed in a previous RNA sequencing analysis. Histochemical analysis indicated that reactive oxygen species might play a role in the resistance response. Our findings open new perspectives to improve selection efficiency for RKN resistance, and the candidate genes are valuable targets for functional investigation and gene editing approaches.

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.

Combining ability and heterosis to root-knot nematode resistance on seven genotypes of kenaf using full diallel cross analysis

A full diallel cross is confirmed to be able to advocate kenaf breeders to select breeding materials, such as inbred lines that produce the best combination with heterosis properties. The results of this study provided information about general combining abilities (GCA) of parents, specific combining ability (SCA), as well as heterosis values. This study aims to estimate the combining abilities and heterosis values of root-knot nematodes in kenaf originated from the full diallel cross. Semi-field research was conducted in an experimental garden located at Karangploso Balittas, from November 2018 to January 2019. The variance of GCA values was higher than SCA values for the number of root knot, reproductive factors, and number of egg mass. In contrast, the variance of GCA values was lower than SCA values for the number of second-stage juvenile (J2) and the number of egg per egg mass characters. KR4, K45, and KR15 genotypes indicated negative GCA values on all characters tested. Meanwhile, KR1, Kin2 and DS028 genotypes signified susceptibility with a positive GCA value. The crossbreeding combinations showed negative SCA values on the number of second-stage juvenile, the number of egg mass and the number of egg per egg mass characters of resistance to root-knot nematodes (M. incognita), i.e. KR6 x DS028. KR4, KR5 and KR15 genotipe was quite consistent with negative GCA and SCA values, as well as heterosis values for characters of resistance to root-knot nematodes on kenaf roots on the number of second-stage juvenile (J2), the number of egg mass and the number of egg per egg mass characters tested. Genotype of KR4 x KR5 and its reciprocal have negative heterobeltiosis values on number of second-stage juvenile, egg mass, and the number of eggs per egg mass evaluated characters

Evaluation of eggplant and gilo genotypes and interspecific hybrids as to root-knot nematode resistance

In Brazil, eggplant and gilo are important for the economy of small-scale farms located mainly in the southeast states and other regions, with a significant production volume year-round in the wholesale local markets. However, these species are very susceptible to root-knot nematodes, and there are few or almost none known sources of resistance. The objective of this studywas to prospect sources of resistance to root-knot nematodesin eggplant, scarlet eggplant (gilo), as well in interspecific hybrids between these species and with wildSolanumspecies, to be used as rootstocks. In the first experiment, in 2013,10 eggplant accessions, a hybrid between eggplant andgilo, and a Solanum stramonifoliumxeggplanthybrid, were evalu41atedfor theirreaction to Meloidogyne enterolobii. In the second, in 2016, 20 accessions of gilowere evaluated for their reaction to M. incognita, M. javanica,and M. enterolobii.. And in the third experiment,in 2017,one access and two experimental eggplant hybrids, and one Solanum scuticumx eggplant hybrid, were evaluated for their reaction to M. incognita, and M. enterolobii. All the trials were stablished inagreenhouse, and characters related to root infection were evaluated in a completely randomized design with six replications of one plant per pot, usinga 1.5 L pots filled with a mixedsubstrate inoculated with each nematode species.Itwas found thatall eggplant accessions were susceptible to M. incognitaand M. enterolobii, however, BER 3150 presented lower susceptibility to M. incognita. The gilogenotypes CNPH 056, CNPH 070, CNPH 220,and CNPH 363 shownbetter response to M. incognitaand M. javanicathan the susceptibility pattern, the tomato 'Rutgers'. Other giloaccessions CNPH 070, CNPH 219,and CNPH 387 showed better or equivalent response thanthe resistant tomato 'Nemadoro' for M. enterolobii.4-the BER EG203 x S. scuticuminterspecific hybrid can be recommended as a rootstock for eggplant susceptible to M. incognita, as well the wild S. stramonifoliumvar. inerme species for M. enterolobii.

Discovery of a major QTL for root-knot nematode (Meloidogyne incognita) resistance in cultivated sweetpotato (Ipomoea batatas)

Key messageUtilizing a high-density integrated genetic linkage map of hexaploid sweetpotato, we discovered a major dominant QTL for root-knot nematode (RKN) resistance and modeled its effects. This discovery is useful for development of a modern sweetpotato breeding program that utilizes marker-assisted selection and genomic selection approaches for faster genetic gain of RKN resistance.The root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] (RKN) causes significant storage root quality reduction and yields losses in cultivated sweetpotato [Ipomoea batatas (L.) Lam.]. In this study, resistance to RKN was examined in a mapping population consisting of 244 progenies derived from a cross (TB) between ‘Tanzania,’ a predominant African landrace cultivar with resistance to RKN, and ‘Beauregard,’ an RKN susceptible major cultivar in the USA. We performed quantitative trait loci (QTL) analysis using a random-effect QTL mapping model on the TB genetic map. An RKN bioassay incorporating potted cuttings of each genotype was conducted in the greenhouse and replicated five times over a period of 10 weeks. For each replication, each genotype was inoculated with ca. 20,000 RKN eggs, and root-knot galls were counted ~62 days after inoculation. Resistance to RKN in the progeny was highly skewed toward the resistant parent, exhibiting medium to high levels of resistance. We identified one major QTL on linkage group 7, dominant in nature, which explained 58.3% of the phenotypic variation in RKN counts. This work represents a significant step forward in our understanding of the genetic architecture of RKN resistance and sets the stage for future utilization of genomics-assisted breeding in sweetpotato breeding programs.

Resistance to Meloidogyne enterolobii and Meloidogyne incognita in Cultivated and Wild Cowpea

Cowpea [Vigna unguiculata (L.) Walp] is a multipurpose crop that provides nutrients for human and livestock diets, as well as regulates and supports ecosystem services. In developing countries, cowpea is exploited as a dual-purpose crop for its grain and fodder; it is cultivated primarily for grain and as a cover crop in industrialized countries. However, root-knot nematodes (RKNs) (Meloidogyne spp.) represent a threat to cowpea production worldwide. Thus, we screened the University of California, Riverside (UC-Riverside), cowpea mini-core collection for resistance to Meloidogyne incognita Kofoid and White (Chitwood) and M. enterolobii Yang and Eisenback to verify the potential of this collection to be used for improving RKN resistance in cowpeas. Both screenings showed significant genotypic variation and medium/high broad-sense heritability (H2) estimates for most traits, and several traits were also strongly correlated. For the M. incognita screening, 86.1% of accessions showed some level of resistance based on gall score (≤3), and 77.7% based on reproduction index (RI) (25 ≤ RI ≤ 50), whereas only 10.4% and 29.8% of accessions were resistant to M. enterolobii based on gall score (≤3) and RI (25 ≤ RI ≤ 50), respectively. These results demonstrate the greater virulence of M. enterolobii than M. incognita in cowpea, and that geographic origin of germplasm was not linked to sources of resistance. Among cultivars, only US-1136 showed resistance against both nematode species, whereas 12 wild/landrace germplasms exhibited resistance to M. incognita and M. enterolobii, and can be exploited for breeding resistant cowpeas.

Reproduction potential of Nigerian Meloidogyne spp. and the response of six commercial watermelon cultivars to predominant species

The reproduction potential of nine single-species Meloidogyne populations and 16 mixed-species communities, recovered from watermelon rhizospheres in South-West Nigeria, were determined in roots of tomato (cv. Tropimech) in a glasshouse. The host responses of six commercial watermelon cultivars were also assessed under glasshouse conditions to single-species populations of Meloidogyne incognita, M. javanica and M. enterolobii. Reproduction potential of different Meloidogyne populations and the host response of the cultivars were assessed based on the number of egg masses, final nematode population (Pf), reproduction factor (Rf) and relative susceptibility values (S%). No significant interaction (P < 0.05) existed between replicated reproduction potential experiments for Meloidogyne populations, while a significant interaction (P < 0.05) was apparent between the two host status experiments. Rfs varied in between 4 and 46 in comparing single and mixed community populations of Meloidogyne spp. All watermelon cultivars were susceptible to the predominant single-species populations of Meloidogyne spp.: ‘Charleston Gray’ had the highest Rf (14.7) for M. enterolobii, while ‘Sugar Dragon’ had the lowest (2.1) for M. incognita. The results suggest that cultivars with the lowest Rf and S% for the respective Meloidogyne spp. should be the preferred choice of producers to avoid uncontrolled population build-up of Meloidogyne spp. The data generated from this study provide valuable and useful information to watermelon growers and breeders focusing on root-knot nematode resistance breeding to enable sustainable cultivation of the crop.

Single nucleotide polymorphism‐based haplotypes associated with Meloidogyne javanica resistance in Brazilian soybean germplasm

AbstractThe root‐knot nematode (RKN) species Meloidogyne javanica (MJ) causes substantial root damage and yield loss in susceptible soybean [Glycine max (L.) Merr.] cultivars. In this study, a genome wide association study (GWAS) was undertaken to identify genomic regions controlling RKN resistance in a soybean association‐mapping panel, using single nucleotide polymorphism (SNP) markers and haplotype information. This study was carried out with a sample (n = 124) from the Coodetec soybean gene bank, including some commercial Brazilian varieties. For the phenotypic evaluations MJ inoculum was obtained from roots of cultivar CD 206, a highly susceptible soybean used to maintain MJ for studies at Coodetec. The cultivars CD 208 and BRS CONQUISTA were used as resistant checks while CD 206, R7, and Nidera NA 5909 RG were used as susceptible checks and all 124 lines were genotyped with 6,000 SNP markers. MJ resistance determinants in the soybean panel were linked to SNPs with significant effects, with nine SNPs distributed along chromosomes 7, 13, and 20. The observed average MJ resistance in the group of soybean entries with haplotype GCT, ACC, ACT, and GCC in the markers Gm20_44446670_G_A, Gm13_29739984_C_A and Gm07_8166605_T_C, respectively, represents moderately resistant genotypes. Gm13_29739984_C_A and Gm07_8166605_T_C discriminated resistant and susceptible cultivars better than the marker Gm20_44446670_G_A. These results are particularly promising for soybean‐breeding programs, by not only identifying new resistance‐associated haplotypes, but also suggesting new genetic mechanisms in RKN‐resistance sources.

Evaluation of root-knot nematode resistance assays for sugarcane accession lines in Australia.

Root-knot nematode (Meloidogyne javanica) is an important pathogen of sugarcane and can cause significant yield loss in sandy soil. Resistance to root-knot nematode is not common in commercial cultivars of sugarcane. In order to find new sources of resistance for root-knot nematodes, Sugar Research Australia (SRA) has screened new sets of accession lines derived from introgression breeding between wild relatives of sugarcane and commercial parents, and sugarcane accession lines from advanced stages of the core selection programs. This study aimed to determine the most suitable assessment parameter for comparing resistance of accession lines and cultivars. Eight screening trials were conducted from 2012 to 2019. Three parameters were measured on individual plants grown in pots inoculated with an initial population of M. javanica eggs, (i) number of eggs per plant, (ii) number of eggs per g of roots, and (iii) visual ratings of galls on roots. Resistance parameter, eggs per plant was repeatable among trials and had less variations within a trial than the other parameters. Eggs per g of roots was negatively correlated (r = −0.19 to −0.74, p < 0.05) with root biomass in seven out of eight trials, and with shoot biomass in four out of seven trials (r = −0.24 to −0.47, p < 0.001). Visual rating of galls showed inconsistent (positive or negative) correlations with shoot and root biomass. No significant correlations were found between number of eggs per plant and shoot or root biomass of test accession lines. Based on repeatability and variability of resistance parameters, eggs per plant was the most suitable parameter to compare and select sugarcane accession lines for resistance to M. javanica.

Characterisation of root-knot nematode resistance in black oat, Avena strigosa, ‘Terara’

Summary The mechanism of root-knot nematode (RKN) resistance in black oat ‘Terara’ and its stability under high temperatures were examined. To investigate the suppressive mechanism, we observed root invasion and development of RKN in ‘Terara’. Juveniles of Meloidogyne incognita, M. arenaria and M. javanica successfully invaded roots of ‘Terara’ 3 days after inoculation (DAI), whereas juveniles of M. hapla were hardly observed in black oat roots. The total number of M. incognita and the ratio of successfully developing juveniles were significantly lower in roots of ‘Terara’ than a susceptible cultivar from 7 DAI. A number of M. incognita developed to sausage-shaped juveniles in ‘Terara’; however, juveniles beyond that stage were seldom observed. Histological observation of feeding sites suggested the poor development of giant cells was involved in this arrested development. Juveniles of M. arenaria and M. javanica also only developed to the sausage-shaped stage at 21 DAI. These observations indicated that ‘Terara’ and resistant common oat cultivars suppressed RKN in a similar way. The effect of high temperature on resistance was evaluated using ‘Terara’ cultivated at 30°C throughout the experiment and at 32°C for the initial 5 DAI. The number of egg masses of M. incognita under the 32°C treatment was slightly higher than the 25°C control. However, it was still suppressed to 4.9% of the susceptible cultivar.

Reproduction of Meloidogyne arenaria race 2 on flue-cured tobacco with putative resistance derived from Nicotiana repanda.

Chemical controls for root-knot nematodes are increasingly restricted due to environmental and human health concerns. Host resistance to these nematodes is key to flue-cured tobacco production in Virginia. Resistance to Meloidogyne incognita races 1 and 3, and race 1 of M. arenaria is imparted by the gene Rk1, which is widely available in commercial flue-cured tobacco. Rk2 imparts increased resistance to M. javanica when stacked with Rk1 and is becoming commercially available. The efficacy of Rk2 against M. arenaria race 2, which is increasingly prevalent in Virginia, is unclear. Greenhouse trials were conducted in 2017 to determine how potential resistance derived from N. repanda compares to the root-knot nematode resistance afforded by Rk1 and Rk2. Trials were arranged in a completely randomized block design and included an entry with traits derived from N. repanda, a susceptible entry and entries possessing Rk1 and/or Rk2. Data collected after 60 days included percent root galling, egg mass counts, and egg counts. Root galling and reproduction were significantly lower on the entry possessing traits derived from N. repanda relative to other entries, suggesting that the N. repanda species may hold a novel source of root-knot nematode resistance for commercial flue-cured tobacco cultivars.

Susceptibility of Flordaguard peach rootstock to a resistant-breaking population of Meloidogyne floridensis and two populations of Meloidogyne arenaria.

Cultivar Flordaguard is suggested as a root-knot nematode (RKN) resistant rootstock for Florida peaches, however, RKN disease has been observed on this rootstock in peach orchards. Our goal was to confirm whether the RKN resistance breaking isolates of M. floridensis and M. arenaria indeed could infect and reproduce on the peach rootstock cv. Flordaguard in both laboratory and field studies. Root galling occurred on all peach cultivars evaluated including Flordaguard, Flordaglo, Okinawa, and Lovell, in the presence of the RKN resistance-breaking isolates of M. floridensis (MfGnv14) and two M. arenaria isolates (Ma1 and Ma2). These rootstocks showed varying degrees of susceptibility (to a lesser extent in Okinawa) to these three RKN resistance-breaking isolates. The importance of nematode inoculum concentrations in differentiating between resistance and susceptible plants was demonstrated, and thus are an important factor to consider in nematode resistance breeding programs. In host differential tests the peach-originated isolates of M. floridensis and M. arenaria behaved similarly with the vegetable-originated isolates of M. floridensis on tomato, peanut, watermelon, and tobacco, but showed variable host responses on cotton and pepper. The two M. arenaria isolates from peach reproduced on pepper but not on peanut. To our knowledge this is the first report of M. arenaria race 3 infecting Flordaguard and pepper in Florida. Soil and root samples collected from cv. Flordaguard infected trees at two commercial peach orchards showed that M. floridensis and M. arenaria were established on the rootstock.

Conferring root-knot nematode resistance via host-delivered RNAi-mediated silencing of four Mi-msp genes in Arabidopsis

The root-knot nematode (RKN) Meloidogyne incognita is considered one of the most damaging pests among phytonematodes. The majority of nematode oesophageal gland effector genes are indispensable in facilitating M. incognita parasitization of host plants. We report the effect of host-delivered RNAi (HD-RNAi) silencing of four selected M. incognita effector genes, namely, Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24, in Arabidopsis thaliana. Mi-msp5, Mi-msp18 and Mi-msp24, which are dorsal gland genes, were found to be maximally expressed in the adult female stage, whereas Mi-msp3, which is a sub-ventral gland gene, was maximally expressed in an earlier stage. In transgenic plants expressing dsRNA, the reduction in the number of galls on roots was 89 %, 78 %, 86 % and 89 % for the Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24 RNAi events, respectively. Moreover, gene transcript abundance was significantly reduced in RKN females feeding on dsRNA-expressing lines by up to 60 %, 84 %, 31 % and 61 % for Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24, respectively. Furthermore, the M. incognita reproduction factor was reduced up to 71-, 344-, 107- and 114-fold in Arabidopsis plants expressing Mi-msp3, Mi-msp5, Mi-msp18 and Mi-msp24 dsRNA constructs, respectively. This study provides a set of potential target genes to curb nematode infestation in economically important crops via the HD-RNAi approach.

Tomato Mi-1.2 gene confers resistance to Meloidogyne luci and M. ethiopica

The root knot nematode (RKN) Meloidogyne luci, recently added to the European Plant Protection Organization Alert List, represents a potential threat to the production of important solanaceous crops. RKN resistance in tomato (Solanum lycopersicum L.) is conferred by a single dominant gene. The objectives were to screen the RKN resistance Mi-1.2 gene in 27 tomato hybrids and cultivars and assess the ability of M. luci and M. ethiopica to reproduce on these accessions. Thirteen Mimi tomato genotypes, two MiMi and twelve lacking the Mi-1.2 gene (mimi) were identified. Tomato accessions Reconquista (MiMi) and Vimeiro F1, Paipai, Sahel, Agora F1, Amaral, Valoasis RZ F1 and SV1917 (Mimi) displayed a resistant reaction to both RKN species with a reproduction factor (Rf) ranging from 0.00 to 0.14. Significant differences in M. luci and M. ethiopica reproduction were detected among and within the Mi-1.2 allelic conditions, which may reflect a dosage effect of the gene as well as an influence of the plant’s genetic background, respectively. Results revealed that resistant tomato accessions carrying the resistant Mi gene have potential to be employed as an alternative to the use of nematicides aiming to control M. luci, and M. ethiopica and the most common RKN species.

Candidate genes underlying the quantitative trait loci for root-knot nematode resistance in a Cucumis hystrix introgression line of cucumber based on population sequencing

The southern root-knot nematode (RKN), Meloidogyne incognita (Kofoid & White) Chitwood, is one of most destructive species of plant parasitic nematodes, causing significant economic losses to numerous crops including cucumber (Cucumis sativus L. 2n = 14). No commercial cultivar is currently available with resistance to RKN, severely hindering the genetic improvement of RKN resistance in cucumber. An introgression line, IL10-1, derived from the interspecific hybridization between the wild species Cucumis hystrix Chakr. (2n = 24, HH) and cucumber, was identified with resistance to RKN. In this study, an ultrahigh-density genetic linkage bin-map, composed of high-quality single-nucleotide polymorphisms (SNPs), was constructed based on low-coverage sequences of the F2:6 recombinant inbred lines derived from the cross between inbred line IL10-1 and cultivar ‘Beijingjietou’ CC3 (hereinafter referred to as CC3). Three QTLs were identified accounting for 13.36% (qRKN1-1), 9.07% and 9.58% (qRKN5-1 and qRKN5-2) of the resistance variation, respectively. Finally, four genes with nonsynonymous SNPs from chromosome 5 were speculated to be the candidate RKN-resistant related genes, with annotation involved in disease resistance. Though several gaps still exist on the bin-map, our results could potentially be used in breeding programs and establish an understanding of the associated mechanisms underlying RKN resistance in cucumber.