Root-knot Nematode Resistance Gene Research Articles

A rapid method for detection of the root-knot nematode resistance gene, Mi-1.2, in tomato cultivars.

Molecular markers have been widely used in plant breeding to improve the accuracy and efficiency of trait selection. In particular, molecular markers are powerful in facilitating the introgression of resistance genes by circumventing costly and time-consuming infection assays. To achieve their practical use, it is important to ensure the tight linkage between the markers and the traits. Here we report a new cleaved amplified polymorphic sequence (CAPS) marker, Mi1713, for the root-knot nematode resistance gene, Mi-1.2, in cultivated tomato. The Mi1713 marker is designed in the conserved region of Mi-1.2 and its homologs in tomato and other nightshade species. Combined with a single-step procedure for preparing PCR templates, the Mi1713 marker enables rapid and reliable screening for the presence of Mi-1.2. The approach described in this study is applicable in designing CAPS markers for various genes or alleles of interest in tomato and other crops.

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.

A novel variant of Gh_D02G0276 is required for root-knot nematode resistance on chromosome 14 (D02) in Upland cotton.

MAGIC population sequencing and virus-induced gene silencing identify Gh_D02G0276 as a novel root-knot nematode resistance gene on chromosome 14 in Upland cotton. The southern root-knot nematode [RKN; Meloidogyne incognita (Kofoid & White)] remains the primary yield-limiting biotic stress to Upland cotton (Gossypium hirsutum L.) throughout the southeastern USA. While useful genetic markers have been developed for two major RKN resistance loci on chromosomes 11 (A11) and 14 (D02), these markers are not completely effective because the causative genes have not been identified. Here, we sequenced 550 recombinant inbred lines (RILs) from a multi-parent advanced generation intercross (MAGIC) population to identify five RILs that had informative recombinations near the D02-RKN resistance locus. The RKN resistance phenotypes of these five RILs narrowed the D02-RKN locus to a 30-kb region with four candidate genes. We conducted virus-induced gene silencing (VIGS) on each of these genes and found that Gh_D02G0276 was required for suppression of RKN egg production conferred by the Chr. D02 resistance gene. The resistant lines all possessed an allele of Gh_D02G0276 that showed non-synonymous mutations and was prematurely truncated. Furthermore, a Gh_D02G0276-specific marker for the resistance allele variant was able to identify RKN-resistant germplasm from a collection of 367 cotton accessions. The Gh_D02G0276 peptide shares similarity with domesticated hAT-like transposases with additional novel N- and C-terminal domains that resemble the target of known RKN effector molecules and a prokaryotic motif, respectively. The truncation in the resistance allele results in a loss of a plant nuclear gene-specific C-terminal motif, potentially rendering this domain antigenic due to its high homology with bacterial proteins. The conclusive identification of this RKN resistance gene opens new avenues for understanding plant resistance mechanisms to RKN as well as opportunities to develop more efficient marker-assisted selection in cotton breeding programs.

Genetic analyses of resistance to the peach root-knot nematode (Meloidogyne floridensis) using microsatellite markers

Most commercially important rootstocks for peach [Prunus persica (L.) Batsch] had been selected for resistance to one or more of the root-knot nematode (RKN) species: Meloidogyne incognita, M. arenaria, and M. javanica. The peach root-knot nematode, M. floridensis (MF), is a relatively newly discovered threat to peach and is not controlled by resistance genes in “Nemared,” “Nemaguard,” and “Okinawa.” The “Flordaguard” peach seedling rootstock, conventionally bred to provide resistance to MF, has solely been used for low-chill peach production in Florida for over 20 years and has already shown signs of resistance breakdown. A source of high resistance to the pathogenic MF isolate (“MFGnv14”) was identified from wild peach Prunus kansuensis Rehder (Kansu peach), thereby suggesting the potential for broadening spectrum and increasing durability of resistance in peach rootstocks through interspecific hybridization with P. kansuensis. Using 12 F2 and BC1F1 populations derived from crosses between Okinawa or Flordaguard peach and P. kansuensis populations, we examined the genetic control for MF resistance by identifying associated microsatellite markers and determining genomic location of the resistance locus. One microsatellite marker (UDP98-025) showed strong and consistent association with resistance based on root-galling index. The resistance locus was mapped on the subtelomeric region of linkage group 2, co-localizing with other previously reported RKN resistance genes in Prunus. Segregation of gall-index-based resistance observed in F2 and BC1F1 populations is compatible with the involvement of a multiallelic locus wherein a dominant (Mf1) or recessive (mf3) resistance allele is inherited from P. kansuensis, and susceptibility alleles (mf2) from peach.

Fine mapping of the root-knot nematode resistance gene Me1 in pepper (Capsicum annuum L.) and development of markers tightly linked to Me1

Root-knot nematodes (RKNs) can severely damage crops, including peppers, worldwide. The application of resistance genes identified in the Capsicum annuum genome may represent a safe and economically relevant strategy for controlling RKNs. Among the Me genes (Me1, Me3, Me7, and N) that have been mapped to a cluster on chromosome P9, Me1 confers a heat-stable and broad-spectrum resistance that is difficult for virulent RKNs to overcome. In this study, we developed several closely linked kompetitive allele-specific PCR (KASPar) markers, simple sequence repeat (SSR) markers, sequence characterized amplified region (SCAR) markers, and high-resolution melting (HRM) markers for the mapping of RKN-resistance genes. Analyses of 948 individuals (BC1 and F2 progenies) revealed that Me1 was located between SCAR marker 16880-1-V2 and HRM marker 16830-H-V2, with 13 and 0 recombination events with Me1, respectively. These markers were localized to a 132-kb interval, which included six genes. The development of several PCR-based markers closely linked to Me1 will be useful for the marker-assisted selection of RKN resistance in pepper cultivars. Among these markers, 16830-H-V2 and 16830-CAPS are present in the CA09g16830 gene, which is predicted to be a putative late blight resistance protein homolog R1A-3 gene. This gene appears to be a suitable Me1 candidate gene.

QTL Analysis of Transgressive Nematode Resistance in Tetraploid Cotton Reveals Complex Interactions in Chromosome 11 Regions.

Transgressive segregation in cotton (Gossypium spp.) provides an important approach to enhance resistance to the major pest root-knot nematode (RKN) Meloidogyne incognita. Our previous studies reported transgressive RKN resistance in an intraspecific Gossypium hirsutum resistant NemX × susceptible SJ-2 recombinant inbred line (RIL) population and early generations of interspecific cross Gossypium barbadense (susceptible Pima S-7) × G. hirsutum (NemX). However, the underlying functional mechanisms for this phenomenon are not known. In this study, the region of RKN resistance gene rkn1 on chromosome (Chr) 11 and its homoeologous Chr 21 was fine mapped with G. raimondii D5 genome reference sequence. Transgressive resistance was found in the later generation of a new RIL population F2:7 (Pima S-7 × NemX) and one interspecific F2 (susceptible Pima S-7 × susceptible SJ-2). QTL analysis revealed similar contributions to root-galling and egg-production resistance phenotypes associated with SSR marker CIR316 linked to resistance gene rkn1 in NemX on Chr 11 in all seven populations analyzed. In testcross NemX × F1 (Pima S-7 × SJ-2) marker allele CIR069-271 from Pima S-7 linked to CIR316 contributed 63% of resistance to galling phenotype in the presence of rkn1. Similarly, in RIL population F2:8 (NemX × SJ-2), SJ-2 markers closely linked to CIR316 contributed up to 82% of resistance to root-galling. These results were confirmed in BC1F1 SJ-2 × F1 (NemX × SJ-2), F2 (NemX × SJ-2), and F2 (Pima S-7 × SJ-2) populations in which up to 44, 36, and 15% contribution in resistance to galling was found, respectively. Transgressive segregation for resistance was universal in all intra- and inter-specific populations, although stronger transgressive resistance occurred in later than in early generations in the intraspecific cross compared with the interspecific cross. Transgressive effects on progeny from susceptible parents are possibly provided in the rkn1 resistance region of chromosome 11 by tandemly arrayed allele (TAA) or gene (TAG) interactions contributing to transgressive resistance. Complex TAA and TAG recombination and interactions in the rkn1 resistance region provide three genes and a model to study disease and transgressive resistance in polyploid plants, and novel genotypes for plant breeding.

Bacterial artificial chromosome library construction of root-knot nematode resistant pepper genotype HDA149 and identification of clones linked to Me3 resistant locus

Pepper (Capsicum annuum. L.) is a widely cultivated vegetable crop worldwide and has the second largest planting area and the first largest vegetable output and value in China. Pepper root-knot nematode (Meloidogyne spp.) is one of the most serious pests of pepper, which caused huge losses every year. Previous studies showed that the Me3 gene is resistant to a wide range of Meloidogyne species, including M. arenaria, M. javanica, and M. incognita. HDA149, a double haploid pepper genotype, harboring the root-knot nematode resistance gene Me3, was used to construct bacterial artificial chromosome library (BAC) via the vector of CopyControl™ pCC1 in this study. The library consists of 210 200 BAC clones and is equivalent to 5.3 pepper genomes. The average insert size is 95 kb, and most of them are 90–120 kb; but the empty clones are less than 3%. In order to screen the BAC library easily, 550 super pools with 384 BAC clones of each pool were further developed in this study. Specific primers from Me3 gene locus were used for BAC library screening, and more than 20 positive BAC clones were obtained. Then the selected positive BAC clones were analyzed by restriction enzyme digestion, BAC-end sequencing, marker development, and new positive BAC clones exploration, respectively. Finally, the contig with total length of about 300 kb linked to the Me3 locus was constructed based on chromosome walking strategy, which made a solid foundation for the cloning of the important root-knot nematode resistance gene Me3.

Physical mapping of NBS-coding resistance genes to the Me-gene cluster on chromosome P9 reveals markers tightly linked to the N gene for root-knot nematode resistance in pepper

Natural root-knot nematode resistance genes are unique resources to control this major pest in pepper (Capsicum annuum). Although four genes (Me1, Me3, Me7 and N) conferring broad-spectrum resistance were mapped to a cluster in a 28-cm interval on chromosome P9, limited markers targeting this region were available. In the present study, the Me-gene cluster was structurally annotated for resistance genes to develop markers targeting the N gene. As a result, the Me-gene cluster (4.07 Mb in size) was found to contain three resistance gene hotspots. In addition, a SSR maker tightly linked to the N gene (0.8 cM away) was developed for marker-assisted selection in pepper.

Evidence for a Second RKN Resistance Gene in Peanut.

ABSTRACT Root-knot nematode (RKN), [Meloidogyne arenaria (Neal) Chitwood race 1] can result in highly significant yield losses in peanut (Arachis hypogaea L.) production. Fortunately, very high levels of RKN nematode resistance have been identified and incorporated from wild species into newly developed peanut cultivars. In 2011-12 at Tifton, GA, a field site was artificially inoculated with M. arenaria race 1. A susceptible cultivar was used to uniformly increase the peanut-specific race 1 nematode population during the summer and fall; whereas, hairy vetch (Vicia villosa Roth) was used for the same purpose each winter as a susceptible cover crop. During 2013 and 2014, space-planted F2 and F3 populations from cross combinations involving A. hypogaea susceptible × resistant parental lines derived from ‘COAN’ were evaluated, respectively. Several past inheritance studies had suggested a single dominant gene, Rma, controlled the resistance. However in this study, the occurrence of a second recessive gene (r...

Inheritance and Identification of a Major Quantitative Trait Locus (QTL) that Confers Resistance to Meloidogyne incognita and a Novel QTL for Plant Height in Sweet Sorghum

Southern root-knot nematodes (Meloidogyne incognita) are a pest on many economically important row crop and vegetable species and management relies on chemicals, plant resistance, and cultural practices such as crop rotation. Little is known about the inheritance of resistance to M. incognita or the genomic regions associated with resistance in sorghum (Sorghum bicolor). In this study, an F2 population (n = 130) was developed between the resistant sweet sorghum cultivar 'Honey Drip' and the susceptible sweet cultivar 'Collier'. Each F2 plant was phenotyped for stalk weight, height, juice Brix, root weight, total eggs, and eggs per gram of root. Strong correlations were observed between eggs per gram of root and total eggs, height and stalk weight, and between two measurements of Brix. Genotyping-by-sequencing was used to generate single nucleotide polymorphism markers. The G-Model, single marker analysis, interval mapping, and composite interval mapping were used to identify a major quantitative trait locus (QTL) on chromosome 3 for total eggs and eggs per gram of root. Furthermore, a new QTL for plant height was also discovered on chromosome 3. Simple sequence repeat markers were developed in the total eggs and eggs per gram of root QTL region and the markers flanking the resistance gene are 4.7 and 2.4 cM away. These markers can be utilized to move the southern root-knot nematode resistance gene from Honey Drip to any sorghum line.

Calcium is involved in the R Mc1 (blb)-mediated hypersensitive response against Meloidogyne chitwoodi in potato.

Functional characterization of the Columbia root-knot nematode resistance gene R Mc1 ( blb ) in potato revealed the R gene-mediated resistance is dependent on a hypersensitive response and involves calcium. The resistance (R) gene R Mc1(blb) confers resistance against the plant-parasitic nematode, Meloidogyne chitwoodi. Avirulent and virulent nematodes were used to functionally characterize the R Mc1(blb)-mediated resistance mechanism in potato (Solanum tuberosum). Histological observations indicated a hypersensitive response (HR) occurred during avirulent nematode infection. This was confirmed by quantifying reactive oxygen species activity in response to avirulent and virulent M. chitwoodi. To gain an insight into the signal transduction pathways mediating the R Mc1(blb)-induced HR, chemical inhibitors were utilized. Inhibiting Ca(2+) channels caused a significant reduction in electrolyte leakage, an indicator of cell death. Labeling with a Ca(2+)-sensitive dye revealed high Ca(2+) levels in the root cells surrounding avirulent nematodes. Furthermore, the calcium-dependent protein kinase (CDPK), StCDPK4 had a higher transcript level in R Mc1(blb) potato roots infected with avirulent nematodes in comparison to roots infected with virulent M. chitwoodi. The results of this study indicate Ca(2+) plays a role in the R Mc1(blb)-mediated resistance against M. chitwoodi in potato.

Inheritance and Mapping ofMj-2, a New Source of Root-knot Nematode (Meloidogyne javanica) Resistance in Carrot

Root-knot nematodes limit carrot production around the world by inducing taproot forking and galling deformities that render carrots unmarketable. In warmer climates, Meloidogyne javanica and Meloidogyne incognita are most prevalent. In F2 and F3 progeny from the cross between an Asian carrot resistant to M. javanica, PI 652188, and a susceptible carrot, resistance response was incompletely dominant with a relatively high heritability (H (2) = 0.78) and provided evidence for a single gene, designated Mj-2, contributing to resistance. Molecular markers linked to the previously described root-knot nematode resistance gene, Mj-1 on chromosome 8 derived from "Brasilia," demonstrated that Mj-2 does not map to that same locus but is on the same chromosome.

Marker analysis for detection of the Mi-1.2 resistance gene in tomato hybrid rootstocks and cultivars

In tomato, the REX-1 marker is typically used to detect the root-knot nematode resistance gene Mi-1.2 in Solanum lycopersicum cultivars with introgression from S. peruvianum (Sp). However, using this marker, false positives have been reported in hybrid tomato rootstocks with introgressions from other Solanum species. We evaluated the reliability of available PCR-based markers Mi23, PMi and PM3 for detection of root-knot nematode resistance in a set of tomato hybrid rootstocks, cultivars and in several accessions of different Solanum species. Results showed that none of the tested markers was able to identify unequivocally lines with resistant phenotype. The Mi23 marker amplified from the susceptible tomato cvs Ailsa Craig and Chatham was Sp-like, while the PMi marker was highly polymorphic in several accessions of Solanum species and did not distinguish between a susceptible accession of S. peruvianum (LA1336) and two resistant accessions/clones (PI270435-2R2 and PI270435-3MH). In addition, the PM3 marker generated amplification products from the susceptible tomato cv. Tyrmes and susceptible accessions of S. chilense (LA1969 and LA2746) that were also Sp-like. Fingerprinting Mi-1.2 gene homologues (MiGHs) using primers flanking intron-1 in the tomato hybrid rootstocks and nematode-resistant and susceptible cvs indicated a great variation in the number and nature of the MiGHs. In silico analysis with the existing Mi-1.2 gene-specific primers C1/2, IMO-F1/R1 and VIGS-F indicated that they were not specific for the Mi-1.2 gene as they could amplify a fragment with a similar size from MiGHs from S. lycopersicum as well as other MiGHs from S. peruvianum. A newly designed Mi-1.2 gene-specific primer, Pau-do, used with an existing primer, C2S4, amplified an identical fragment of 1494 bp from cDNA of nematode-resistant tomato cv. Motelle and rootstocks Beaufort and Maxifort. Overall, this paper provides information to breeders, scientists and growers on the limited reliability of the available molecular tools to characterise root-knot nematode resistance in hybrid tomato rootstocks. The results show that, as most of the current markers are not universal and can lead to the appearance of false positives for root-knot nematode resistance, the resistance response of hybrid tomato rootstocks will still need to be evaluated through infection tests.

English

Several root-knot nematode ( Meloidogyne spp.) resistance genes have been discovered in different stone fruit crops. However, none of them has yet been cloned and they were only located on the chromosomes. In this study, a candidate root-knot nematode resistance gene (designated as psoRPM1) was isolated from the individual plant of Xinjiang wild myrobalan plum ( Prunus sogdiana ) by degenerate PCR amplification combined with the RACE technique. The gene had a typical NBS-LRR structure and high homology with Mi-1.2 (root-knot nematode resistance genes in tomato). The expression of psoRPM1 gene increased in the roots of resistant wild myrobalan plum material 12, 24 and 48 h after inoculation with root-knot nematodes and the expression of psoRPM1 gene was maximum 12 h after inoculation. But in susceptible plant, the psoRPM1 gene expression remained low both before and after inoculation. This result suggested that the psoRPM1 gene was constitutively expressed gene in the wild myrobalan plum. In-situ hybridization results showed that the psoRPM1 gene mainly expressed in both phloem and cortex parenchyma of root 12 h after inoculation in resistant plant. Furthermore, the psoRPM1 gene only expressed in phloem 48 h after inoculation in resistant plant. The result suggested that the psoRPM1 gene played a role in keeping nematodes off the cortex when nematodes began to infect the plant’s roots. After root-knot nematodes entering into cortex parenchyma, the psoRPM1 gene mainly played defense function in phloem of pericycle. Using the gene gun bombarding into onion epidermal cells, the result was that psoRPM1 protein was located in cytomembrane and might be interacted with other proteins in cytomembrane to locate Key words: Xingjiang wild myrobalan plum (Prunus sogdiana), root-knot nematodes (Meloidogyne incognita), gene, in-situ, gene location.

SSR markers for marker assisted selection of root-knot nematode (Meloidogyne incognita) resistant plants in cotton (Gossypium hirsutum L)

Cotton (Gossypium hirsutum L) cultivars highly resistant to the southern root-knot nematode (RKN) [Meloidogyne incognita (Kofoid and White) Chitwood] are not available. Resistant germplasm lines are available; however, the difficulty of selecting true breeding lines has hindered applied breeding and no highly resistant cultivars are available to growers. Recently, molecular markers on chromosomes 11 and 14 have been associated with RKN resistance, thus opening the way for marker assisted selection (MAS) in applied breeding. Our study aimed to determine the utility of these markers for MAS. Cross one was RKN resistant germplasm M240 RNR × the susceptible cultivar, FM966 and is representative of the initial cross a breeder would make to develop a RKN resistant cultivar. Cross two consists of Clevewilt 6 × Mexico Wild (PI563649), which are the two lines originally used to develop the first highly RKN resistant germplasm. Mexico Wild is photoperiodic. We phenotyped the F2 of cross one for gall index and number of RKN eggs per plant and genotyped each plant for CIR 316 (chromosome 11) and BNL 3661 (chromosome 14). From this, we verified that MAS was effective, and the QTL on chromosome 14 was primarily associated with a dominant RKN resistance gene affecting reproduction. In the first F2 population of cross two, we used MAS to identify 11 plants homozygous for the markers on chromosomes 11 and 14, and which also flowered in long days. Progeny of these 11 plants were phenotyped for RKN gall index and egg number and confirmed as RKN highly resistant plants. Generally about 7–10 generations of RKN phenotyping and progeny testing were required to develop the original RKN highly resistant germplasms. Our results show that commercial breeders should be able to use the markers in MAS to rapidly develop RKN resistant cultivars.

The Ma gene for complete-spectrum resistance to Meloidogyne species in Prunus is a TNL with a huge repeated C-terminal post-LRR region.

Root-knot nematode (RKN) Meloidogyne species are major polyphagous pests of most crops worldwide, and cultivars with durable resistance are urgently needed because of nematicide bans. The Ma gene from the Myrobalan plum (Prunus cerasifera) confers complete-spectrum, heat-stable, and high-level resistance to RKN, which is remarkable in comparison with the Mi-1 gene from tomato (Solanum lycopersicum), the sole RKN resistance gene cloned. We report here the positional cloning and the functional validation of the Ma locus present at the heterozygous state in the P.2175 accession. High-resolution mapping totaling over 3,000 segregants reduced the Ma locus interval to a 32-kb cluster of three Toll/Interleukin1 Receptor-Nucleotide Binding Site-Leucine-Rich Repeat (LRR) genes (TNL1-TNL3), including a pseudogene (TNL2) and a truncated gene (TNL3). The sole complete gene in this interval (TNL1) was validated as Ma, as it conferred the same complete-spectrum and high-level resistance (as in P.2175) using its genomic sequence and native promoter region in Agrobacterium rhizogenes-transformed hairy roots and composite plants. The full-length cDNA (2,048 amino acids) of Ma is the longest of all Resistance genes cloned to date. Its TNL structure is completed by a huge post-LRR (PL) sequence (1,088 amino acids) comprising five repeated carboxyl-terminal PL exons with two conserved motifs. The amino-terminal region (213 amino acids) of the LRR exon is conserved between alleles and contrasts with the high interallelic polymorphisms of its distal region (111 amino acids) and of PL domains. The Ma gene highlights the importance of these uncharacterized PL domains, which may be involved in pathogen recognition through the decoy hypothesis or in nuclear signaling.

Mapping resistance gene analogs (RGAs) in cultivated tetraploid cotton using RGA-AFLP analysis

Diseases cause significant losses in cotton production throughout the US Cotton Belt. Growing resistant cultivars can significantly improve cotton yields and effectively reduce production inputs. Disease resistance (R) genes have been isolated in numerous plant species and the R genes with domains of nucleotide binding sites (NB) and leucine rich repeats (LRR) represent the largest R gene family. Degenerate primers designed based on conserved motifs of plant disease resistance genes were used alone or in combination with AFLP primers to analyze disease resistance gene analogs (RGAs) in a recombinant inbred line (RIL) population of Pima (Gossypium barbadense) 3–79 and Upland cotton (G. hirsutum) line NM 24016. Eighty-eight polymorphic RGA markers were amplified by 8 pairs of RGA degenerate primers, while 131 polymorphic RGA-AFLP markers were produced from six pairs of RGA-AFLP primer combinations. Of the 219 polymorphic RGA and RGA-AFLP markers that were identified, 212 were assigned to 18 chromosomes and linkage groups based on existing SSR markers that are on known chromosomes. However, the RGA and RGA-AFLP markers are not evenly distributed among chromosomes in that 189 RGA and RGA-AFLP markers (88%) are assigned onto three “giant” chromosomes, i.e., C6, C12, and C15, suggesting RGA clusters in the cotton genome. Several RGA and RGA-AFLP markers were mapped to the same linkage group carrying a root-knot nematode resistance gene. The identification and mapping of RGA and RGA-AFLP markers provide a framework to facilitate marker-assisted selection of disease resistance in cotton breeding and to understand the physical relationship of cotton resistance genes.

PA-566, A Root-knot Nematode-resistant, Pimento-type Pepper

PA-566 is a new pimento-type pepper (Capsicum annuum L.) released 30 Apr. 2010 by the Agricultural Research Service of the U.S. Department of Agriculture. PA566 is the product of a backcross breeding program to incorporate the N root-knot nematode resistance gene into a ‘Pimiento L’type genetic background. ‘Pimiento L’ is a root-knot nematode-susceptible, pimentotype cultivar that is widely grown in the southern states where it can produce good yields under high-temperature conditions. The dominant N gene conditions a high level of resistance to the southern root-knot nematode [Meloidogyne incognita (Chitwood) Kofoid and White], the peanut root-knot nematode [M. arenaria (Neal) Chitwood], and the tropical root-knot nematode [M. javanica (Treub) Chitwood]. The release of PA-566 will provide pepper breeders interested in developing both open-pollinated and F1 hybrid pimentotype cultivars access to a near-cultivar quality parental line that is homozygous for the N root-knot nematode resistance gene.

AFLP and SRAP markers linked to the mj gene for root-knot nematode resistance in cucumber

Root-knot nematodes (Meloidogyne spp.) are an important worldwide pest of cucumber (Cucumis sativus L.). Molecular markers linked to the Javanese root-knot nematode (M. javanica) resistance gene mj in cucumber may aid marker assisted selection. One-hundred AFLP (EcoRI-MseI) and 112 SRAP were used to screen resistant and susceptible parents for polymorphisms to develop molecular markers linked to the mj gene. Of the 100 AFLP primers, 92 produced bands and two yielded candidate markers (E-ATT/M-CAA and E-AAC/M-CTG). These two bands were cut off from polyacrylamide gel, cloned and sequenced. Primers designed from the sequences did not yield polymorphic bands between the parents. In addition, the sequences did not contain any restriction site or indel to be used to convert them to CAPS or SCAR markers. The two sequences obtained from polymorphic AFLP markers were used primarily to design D1F, D1R, D17F and D17R primers. SRAP forward and reverse primers were used in combination with these four specific primers to search for polymorphisms between parents. Of the 112 primer combinations 11 yielded polymorphisms between parents. MapMaker Exp 3.0 software was used to analyze the 11 markers. Two markers were identified that flanked the mj gene at distance of 16.3 and 19.3 cM. The results indicated that these markers should be useful to develop molecular markers flanking the mj gene.

Effect of the Mi gene on reproduction of Meloidogyne hispanica on tomato genotypes

AbstractThe root-knot nematode resistance (Mi) gene was screened in 25 tomato genotypes of Solanum lycopersicum, by amplification of REX-1 and Mi23 markers. Ten heterozygous tomato genotypes (Mimi), nine homozygous (MiMi) at the Mi locus and six lacking the Mi gene for resistance to root-knot nematode were identified using the marker REX-1. The results obtained with Mi23 marker confirmed the Mi gene status of the tomato genotypes, except for genotype Valouro RZ F1 that was homozygous (MiMi) and heterozygous (Mimi) at the Mi locus when using the REX-1 and Mi23 markers, respectively. The pathogenicity of Meloidogyne hispanica on the 25 tomato genotypes was assessed 60 days after inoculation with 5000 eggs on the basis of root gall index (GI) and reproduction factor (Rf). All the tomato genotypes were susceptible (excellent or good hosts), with GI > 4 and Rf > 2, except the genotype Rapit (Mimi), considered as resistant/hypersensitive (poor host). In this genotype, the nematode induced galls (GI = 4) on its roots and a small number of eggs were produced (Pf = 3085 ± 485). Significant differences in reproduction were detected between the Mi allelic conditions and genotypes within Mi allelic conditions. The increasing number of Mi alleles (0, 1 or 2) is associated with decreasing Rf, which suggests a possible dosage effect of the Mi gene. The variability observed in the Rf values for MiMi tomato genotypes may reflect an influence of the genetic background of the plants containing the Mi gene. Ten of the 25 tomato genotypes with Mi gene are commercially available. However, only Rapit can be used to control the three most common Meloidogyne spp. and inhibit the increasing of M. hispanica populations, and may have potential to be included in an integrated pest management programme. However, it is advisable to evaluate the pathogenicity of local populations of this nematode species associated with different environmental factors.