Systemic Resistance in Tomato Induced by Biocontrol Bacteria Against the Root-Knot Nematode, Meloidogyne javanica is Independent of Salicylic Acid Production

Salicylic acid (SA)‐mediated induction of systemic resistance by Pseudomonas aeruginosa strain 7NSK2 and P. fluorescens strain CHA0 against soil‐borne fungi and viruses have been reported. The role of SA biosynthesis in the enhancement of defence mechanism against plant‐parasitic nematodes by these bacterial strains in tomato is not known. To better understand the importance of SA in rhizobacteria‐mediated suppression of root‐knot nematodes, biocontrol potential of SA‐negative or SA‐overproducing mutants against Meloidogyne javanica was evaluated with their respective wild type counter parts. Culture supernatant of 7NSK2, CHA0 and their respective mutants caused significant mortality of M. javanica juveniles in vitro. SA deletion in 7NSK2 and SA overproduction in CHA0 did not influence bacterial efficacy to cause nematode deaths. Similarly, culture supernatants resulting from King's B liquid medium amended with FeCl₃ did not influence nematicidal activity of the bacterial strains. Strain CHA0 induced juvenile deaths more than 7NSK2 did. In pot experiments, the bacterial strains applied in unsterilized sandy loam soil markedly reduced final nematode population densities in roots and subsequent root‐knot infection in tomato seedlings. SA‐negative or overproducing derivatives prevented tomato roots in kinetics similar to those with their respective wild types. When soil iron concentration was lowered by the addition of ethylenediamine di(o‐hydroxyphenylacetic acid), nematode biocontrol by the bacterial strains (both wild type and mutants) remained unaltered. To understand the mechanism involved in rhizobacteria‐mediated suppression of root‐knot nematode in tomato, bacterial performance was assessed in a split root trial in which one‐half of the root system was treated with bacterium while the other inoculated with nematode. Compared with the controls, application of the bacterial cell suspension to one‐half of the root system lowered the populations of root‐knot nematode in non‐bacterized nematode‐treated sections indicating enhanced defence in the non‐bacterized half. With respect to nematode infection, mutants induced systemic resistance to a similar extent as that caused by the wild types in both wild type tomato and NahG tomato plants. It is concluded that fluorescent pseudomonads induce systemic resistance against root‐knot nematode via a signal transduction pathway, which is independent of SA accumulation in roots.     

The Participation of salicylic and jasmonic acids in genetic and induced resistance of tomato to Meloidogyne incognita (Kofoid and White, 1919)

Salicylic (SA) and jasmonic (JA) acids are the best known mediators of signal systems in plants. In this investigation the participation and character of interactions between SA- and JA-signals under the induced and genetic resistance of plants to nematodes was investigated on the model system tomato (Lycopersicon esculentum) and the root-knot nematode Meloidogyne incognita. This study demonstrates that application of JA and SA to tomato foliage induces systemic effects that suppress root-knot nematode infestation, inhibition of nematode reproduction, and also increased activity of LOX and PAL, the enzymes of biosynthesis of JA and SA. JA treatment did not inhibit Mz-mediated resistance, which suggests a lack of signaling conflicts between these two forms of defense.     

Pseudomonas fluorescens mediated suppression of Meloidogyne incognita infection of cowpea and tomato

Plant growth-promoting rhizobacterium, Pseudomonas fluorescens strain BICC602 suppresses root-knot nematode (Meloidogyne incognita) by enhancing defence mechanism leading to induced systemic resistance in cowpea (Vigna unguiculata) cv. L.Walp. and tomato (Solanum lycopersicum) cv. Pusa Ruby. In cowpea, the soil treatment proved more effective than foliar spray on root galling and eggs in roots. However, which factors are necessary in the induction of resistance response in plants against nematodes by BICC602 is not yet known. Salicylic acid (SA) production by some bacteria acts as endogenous signal for the activation of certain plant defence responses. In a split-root trial with tomato as a host plant and M. incognita as challenging parasite, BICC602 induces systemic resistance in tomato plants. Based on the results, it is assumed that P. fluorescens-induced resistance against M. incognita in cowpea and tomato is made either through SA-dependent or SA-independent transduction pathway.     

Possible utilization of weeds for the management of plant parasitic nematodes infesting some vegetable crops

Leaf extracts of noxious weeds such as Solanum xanthocarpum and Argemone maxicana were used as bare-root dip treatment for the management of three important plant-parasitic nematodes, Meloidogyne incognita, Rotylenchulus reniformis and Tylenchorhynchus brassicae infesting tomato (Lycopersicon esculantum ) and chilli (Capsicum annuum) plants. Significant reduction was observed in the root-knot development caused by M. incognita, multiplication of nematode populations of R. reniformis and T. brassicae on both the test plants. Larval penetration of second stage juveniles of M. incognita was also inhibited at various concentrations of leaf extracts and dip durations. Leaf extract of S. xanthocarpum caused relatively more inhibition in root-knot development in case of root-knot nematode, nematode multiplication of reniform and stunt nematodes than that of A. maxicana. Because of dip treatment in leaf extracts of Argemone maxicana and Solanum xanthocarpum, the plants show better growth and at the same time the populations of nematodes such as M. incognita, R. reniformis and T. brassicae significantly decreased, which naturally improved plant growth. The efficacy of root-dip treatment with respect to improvement in plant weight and reduction in root-knot development and nematode populations, increased with increasing the concentration of leaf extracts and dip durations.     

Expression of tomato salicylic acid (SA)‐responsive pathogenesis‐related genes in Mi‐1‐mediated and SA‐induced resistance to root‐knot nematodes

The expression pattern of pathogenesis‐related genes PR‐1, PR‐2 and PR‐5, considered as markers for salicylic acid (SA)‐dependent systemic acquired resistance (SAR), was examined in the roots and shoots of tomato plants pre‐treated with SA and subsequently infected with root‐knot nematodes (RKNs) (Meloidogyne incognita). PR‐1 was up‐regulated in both roots and shoots of SA‐treated plants, whereas the expression of PR‐5 was enhanced only in roots. The over‐expression of PR‐1 in the whole plant occurred as soon as 1 day after SA treatment. Up‐regulation of the PR‐1 gene was considered to be the main marker of SAR elicitation. One day after treatment, plants were inoculated with active juveniles (J2s) of M. incognita. The number of J2s that entered the roots and started to develop was significantly lower in SA‐treated than in untreated plants at 5 and 15 days after inoculation. The expression pattern of PR‐1, PR‐2 and PR‐5 was also examined in the roots and shoots of susceptible and Mi‐1‐carrying resistant tomato plants infected by RKNs. Nematode infection produced a down‐regulation of PR genes in both roots and shoots of SA‐treated and untreated plants, and in roots of Mi‐carrying resistant plants. Moreover, in resistant infected plants, PR gene expression, in particular PR‐1 gene expression, was highly induced in shoots. Thus, nematode infection was demonstrated to elicit SAR in shoots of resistant plants. The data presented in this study show that the repression of host defence SA signalling is associated with the successful development of RKNs, and that SA exogenously added as a soil drench is able to trigger a SAR‐like response to RKNs in tomato.     

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

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

Elevated CO2 changes the interactions between nematode and tomato genotypes differing in the JA pathway

Interactions between the root‐knot nematode Meloidogyne incognita and three isogenic tomato (Lycopersicon esculentum) genotypes were examined when plants were grown under ambient (370 ppm) and elevated (750 ppm) CO₂. We tested the hypothesis that, defence‐recessive genotypes tend to allocate ‘extra’ carbon (relative to nitrogen) to growth under elevated CO₂, whereas defence‐dominated genotypes allocate extra carbon to defence, and thereby increases the defence against nematodes. For all three genotypes, elevated CO₂ increased height, biomass, and root and leaf total non‐structural carbohydrates (TNC):N ratio, and decreased amino acids and proteins in leaves. The activity of anti‐oxidant enzymes (superoxide dismutase and catalase) was enhanced by nematode infection in defence‐recessive genotypes. Furthermore, elevated CO₂ and nematode infection did not qualitatively change the volatile organic compounds (VOC) emitted from plants. Elevated CO₂ increased the VOC emission rate only for defence‐dominated genotypes that were not infected with nematodes. Elevated CO₂ increased the number of nematode‐induced galls on defence‐dominated genotypes but not on wild‐types or defence‐recessive genotypes roots. Our results suggest that CO₂ enrichment may not only increase plant C : N ratio but can disrupt the allocation of plant resources between growth and defence in some genetically modified plants and thereby reduce their resistance to nematodes.     

Molecular Transfer of Nematode Resistance Genes

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

Selenium Nanoparticles—an Inducer of Tomato Resistance to the Root-Knot Nematode <Emphasis Type="Italic">Meloidogyne incognita</Emphasis> (Kofoid et White, 1919) Chitwood 1949

We investigated the mechanisms of action of selenium nanoparticles obtained by laser ablation for their use as an abiogenic elicitor of tomato resistance to parasitic nematodes. Selenium nanoparticles induced systemic resistance of tomatoes to the root-knot nematode, stimulated plant growth and development, was involved in the PR-6 gene expression in the roots and leaves of plants subjected to invasion, and increased the activity of proteinase inhibitors (markers of systemic resistance of plants to infection). Exogenous treatment of plants with solutions of selenium nanoparticles reduced the invasion of plants by affecting the morphological and physiological parameters of the parasites in the roots.     

Root nematode infection enhances leaf defense against whitefly in tomato

The foliar response to different herbivores sharing the same hosts is an important topic for the study of plant-insect interactions. Plants evolve local and systemic resistant strategies to cope with herbivores. Many researchers have characterized the mechanisms of leaf responses to insect infestation; however, the fact that roots serve as systemic resistance modulators to leaf herbivores has been widely ignored. Here, we report that tomato (Solanum lycopersicum) plants infected with southern root-knot nematodes (Meloidogyne incognita)—which feed on the roots to form nodules—enhanced leaf defenses against aboveground attackers, specifically, the whitefly (Bemisia tabaci). Our results show that nematode infection reduced the whitefly population abundance because of conferring a stronger SA-dependent defense pathway against whitefly than in tomato plants without nematode infection. Meanwhile, nematode-infected tomato plant also activated the foliar JA-dependent defense pathway at 4 h after whitefly infestation. However, the foliar JA-dependent defense under whitefly infestation alone was suppressed, with the JA content being nearly 30 % lower than that in tomato plants co-infected with nematodes and whiteflies. Furthermore, nematode infection significantly decreased the plant nitrogen concentration in leaves and roots. As a result, nematode infection reduced the number of whiteflies by enhancing foliar SA-dependent defense, activating JA-dependent defense and decreasing nitrogen nutrition. Our results suggest that underground nematode infection significantly enhances the defense ability of tomato plants against whitefly.     

Selection and parasite evolution: a reproductive fitness cost associated with virulence in the parthenogenetic nematode <Emphasis Type="Italic">Meloidogyne incognita</Emphasis>

Selection in plant parasites for virulence on resistant hosts and the resulting effects on parasite fitness may be considered as a driving force in host-parasite coevolution. In the present study, we tested the hypothesis that a fitness cost may be associated with nematode virulence, using the interaction between the parthenogenetic species Meloidogyne incognita and tomato as a model system. The reproductive parameters of near-isogenic lines of the nematode, selected for avirulence or virulence against the tomato Mi resistance gene, were analysed and combined into a reproductive index that was taken as a measure of fitness. The lower fitness of the virulent lines on the susceptible tomato cultivar showed for the first time that a measurable fitness cost is associated with unnecessary virulence in the nematode. Although parthenogenesis should theoretically lead to little genetic variability, such cost may impose a direct constraint on the coevolution between the plant and the nematode populations, and suggests an adaptive significance of trade-offs between selected characters and fitness-related traits. These results indicate that, although plant resistance can be broken, it might prove durable in some conditions if the virulent nematodes are counterselected in susceptible plants, which could have important consequences for the management of resistant cultivars in the field.     

Some isolates of the nematophagous fungus Pochonia chlamydosporia promote root growth and reduce flowering time of tomato

The fungal parasite of nematode eggs Pochonia chlamydosporia is also a root endophyte known to promote growth of some plants. In this study, we analysed the effect of nine P. chlamydosporia isolates from worldwide origin on tomato growth. Experiments were performed at different scales (Petri dish, growth chamber and greenhouse conditions) and developmental stages (seedlings, plantlets and plants). Seven P. chlamydosporia isolates significantly (P < 0.05) increased the number of secondary roots and six of those increased total weight of tomato seedlings. Six P. chlamydosporia isolates also increased root weight of tomato plantlets. Root colonisation varied between different isolates of this fungus. Again P. chlamydosporia significantly increased root growth of tomato plants under greenhouse conditions and reduced flowering and fruiting times (up to 5 and 12 days, respectively) versus uninoculated tomato plants. P. chlamydosporia increased mature fruit weight in tomato plants. The basis of the mechanisms for growth, flowering and yield promotion in tomato by the fungus are unknown. However, we found that P. chlamydosporia can produce Indole‐3‐acetic acid and solubilise mineral phosphate. These results suggest that plant hormones or nutrient ability could play an important role. Our results put forward the agronomic importance of P. chlamydosporia as biocontrol agent of plant parasitic nematodes with tomato growth promoting capabilities.     

Mapping a new nematode resistance locus in Lycopersicon peruvianum

Accessions of the wild tomato species L. peruvianum were screened with a root-knot nematode population (557R) which infects tomato plants carrying the nematode resistance gene Mi. Several accessions were found to carry resistance to 557R. A L. peruvianum backcross population segregating for resistance to 557R was produced. The segregation ratio of resistant to susceptible plants suggested that a single, dominant gene was a major factor in the new resistance. This gene, which we have designated Mi-3, confers resistance against nematode strains that can infect plants carrying Mi. Mi-3, or a closely linked gene, also confers resistance to nematodes at 32°C, a temperature at which Mi is not effective. Bulked-segregant analysis with resistant and susceptible DNA pools was employed to identify RAPD markers linked to this gene. Five-hundred-and-twenty oligonucleotide primers were screened and two markers linked to the new resistance gene were identified. One of the linked markers (NR14) was mapped to chromosome 12 of tomato in an L. esculentum/L. pennellii mapping population. Linkage of NR14 and Mi-3 with RFLP markers known to map on the short arm of chromosome 12 was confirmed by Southern analysis in the population segregating for Mi-3. We have positioned Mi-3 near RFLP marker TG180 which maps to the telomeric region of the short arm of chromosome 12 in tomato.     

Interaction of vesicular-arbuscular mycorrhizal fungi and root knot nematode on cultivars of tomato and white clover susceptible to Meloidogyne hapla

The interactive effects of vesicular-arbuscular mycorrhizal (VAM) fungi and root-knot nematode (Meloidogyne hapla) were studied on nematode-susceptible cultivars of tomato (cv. Scoresby) and white clover (cv. Huia) at four levels of applied phosphate. The relative merits of simultaneous inoculation with mycorrhizal fungi and nematodes and of inoculation with mycorrhizal fungi prior to nematode inoculation were evaluated. Mycorrhizal plants were more resistant than non-mycorrhizal plants to root-knot nematode at all phosphate levels and growth benefits were generally greater in plants preinfected with mycorrhizal fungi. Nematode numbers increased with increasing levels of applied phosphate. In mycorrhizal root systems, nematode numbers increased in the lower phosphate soils; at higher phosphate levels nematode numbers were either unaffected or reduced. The numbers of juveniles and adults per gram of root were always lower in mycorrhizal treatments. Mycorrhizal root length remained unaffected by nematode inoculation. Mycorrhizal inoculation thus increased the plants' resistance to infection by M. hapla. This was probably due to some alteration in the physiology of the root system but was not entirely a result of better host nutrition and improved phosphorus uptake by mycorrhizal plants.     

Bacillus halotolerans strain LYSX1-induced systemic resistance against the root-knot nematode Meloidogyne javanica in tomato

Purpose

Determination of the nematicidal potential and mode of action of bacteria isolated from tobacco rhizosphere soil against the root-knot nematode Meloidogyne javanica in tomato plants.

Methods

Antagonistic bacteria were isolated from rhizosphere soil of tobacco infested with root-knot nematodes. Culture filtrate was used to examine nematicidal activity and ovicidal action of bacterial strains. Biocontrol of M. javanica and growth of treated tomato plants were assessed in pot experiments. To clarify whether secondary metabolites of bacteria in tomato roots induced systemic resistance to M. javanica, bacterial culture supernatants and second-stage juvenile nematodes were applied to spatially separated tomato roots using a split-root system. Bacterial strains were identified by 16S rDNA and gyrB gene sequencing and phylogenetic analysis.

Results

Of the 15 bacterial strains isolated, four (LYSX1, LYSX2, LYSX3, and LYSX4) demonstrated nematicidal activity against second-stage juveniles of M. javanica, and strain LYSX1 showed the greatest antagonistic activity; there was dose-dependent variability in nematicidal activity and inhibition of egg mass hatching by strain LYSX1. In vivo application of LYSX1 to tomato seedlings decreased the number of egg masses and galls and increased the root and shoot fresh weight. Treatment of half of the split-root system with LYSX1 reduced nematode penetration to the other half by 41.64%. Strain LYSX1 was identified as Bacillus halotolerans.

Conclusion

Bacillus halotolerans LYSX1 is a potential microbe for the sustainable biocontrol of root-knot nematodes through induced systemic resistance in tomato.

     

Effects of dual inoculation of mycorrhiza and endophytic,rhizospheric or parasitic bacteria on the root-knot nematode disease of tomato

The effects of mycorrhisation and inoculation with soil bacteria on the disease caused by Meloidogyne incognita on tomato were studied in pots under greenhouse conditions. Efficacy in promoting plant growth and reducing disease severity and final nematode densities were evaluated for two arbuscular mycorrhizal fungi (AMF; Funneliformis mosseae and Rhizophagus irregularis), three soil bacteria with different living strategies (the endophyte Bacillus megaterium, a rhizospheric Pseudomonas putida and the hyperparasite of nematodes Pasteuria penetrans) and combinations of the fungi and bacteria. In M. incognita-infested plants, F. mosseae increased tomato growth more than R. irregularis, and plants inoculated with B. megaterium presented higher shoot fresh weight than with P. putida or P. penetrans, but dual inoculation did not improve tomato growth more than single inoculations. Disease severity and final nematode densities were reduced by F. mosseae compared to non-mycorrhizal plants. B. megaterium and P. penetrans reduced both the root galling and the final nematode densities compared to treatments without bacteria. P. penetrans reduced final nematode densities more than B. megaterium or P. putida. Dual inoculation of AMF and P. penetrans showed the highest efficacy in reducing the final nematode densities in tomato.     

Different mechanisms of Trichoderma virens‐mediated resistance in tomato against Fusarium wilt involve the jasmonic and salicylic acid pathways

In the present study, we investigated the role of Trichoderma virens (TriV_JSB100) spores or cell‐free culture filtrate in the regulation of growth and activation of the defence responses of tomato (Solanum lycopersicum) plants against Fusarium oxysporum f. sp. lycopersici by the development of a biocontrol–plant–pathogen interaction system. Two‐week‐old tomato seedlings primed with TriV_JSB100 spores cultured on barley grains (BGS) or with cell‐free culture filtrate (CF) were inoculated with Fusarium pathogen under glasshouse conditions; this resulted in significantly lower disease incidence in tomato Oogata‐Fukuju plants treated with BGS than in those treated with CF. To dissect the pathways associated with this response, jasmonic acid (JA) and salicylic acid (SA) signalling in BGS‐ and CF‐induced resistance was evaluated using JA‐ and SA‐impaired tomato lines. We observed that JA‐deficient mutant def1 plants were susceptible to Fusarium pathogen when they were treated with BGS. However, wild‐type (WT) BGS‐treated tomato plants showed a higher JA level and significantly lower disease incidence. SA‐deficient mutant NahG plants treated with CF were also found to be susceptible to Fusarium pathogen and displayed low SA levels, whereas WT CF‐treated tomato plants exhibited moderately lower disease levels and substantially higher SA levels. Expression of the JA‐responsive defensin gene PDF1 was induced in WT tomato plants treated with BGS, whereas the SA‐inducible pathogenesis‐related protein 1 acidic (PR1a) gene was up‐regulated in WT tomato plants treated with CF. These results suggest that TriV_JSB100 BGS and CF differentially induce JA and SA signalling cascades for the elicitation of Fusarium oxysporum resistance in tomato.     

Correlation between the Structure of Plant Steroids and Their Effects on Phytoparasitic Nematodes

The effects of certain plant steroids (belonging to furostanol glycosides or glycoalkaloids) and -ecdysone on growth and development of phytoparasitic nematodes were studied. It was shown using an experimental system including tomato Lycopersicon esculentum Mill. and root-knot nematode, Meloidogyne incognita Kofoid et White, that a steroid molecule exhibits significant nematicidal activity if it contains a carbohydrate moiety and an additional heterocycle in the steroid core. The maximum nematicidal activity is inherent in glycosides containing chacotriose as the carbohydrate moiety of the molecule. Some compounds tested in this work could be used for protecting plants against phytoparasitic nematodes.     

Lower incidence and severity of tomato virus in elevated CO2 is accompanied by modulated plant induced defence in tomato

Elevation in atmospheric CO₂ concentration broadly affects plant phenology and physiology, and these effects may alter the performance of plant viruses. The effects of elevated CO₂ on the susceptibility of tomato plants to Tomato yellow leaf curl virus (TYLCV) were examined for two successive years in open top chambers (OTC) in the field. We experimentally tested the hypothesis that elevated CO₂ would reduce the incidence and severity of TYLCV on tomato by altering plant defence strategies. Our results showed that elevated CO₂ decreased TYLCV disease incidence (by 14.6% in 2009 and 11.8% in 2010) and decreased disease severity (by 20.0% in 2009 and 10.4% in 2010). Elevated CO₂ also decreased the level of TYLCV coat protein in tomato leaves. Regardless of virus infection, elevated CO₂ increased plant height and aboveground biomass. Additionally, elevated CO₂ increased the leaf C:N ratio of tomato, but decreased soluble protein content in leaves. Notably, elevated CO₂ increased the salicylic acid (SA) level in uninfected and infected plants. In contrast, elevated CO₂ reduced jasmonic acid (JA) in uninfected plants while it increased JA and abscisic acid (ABA) in virus‐infected plants. Furthermore, combined exogenous SA and JA application enhanced resistance to TYLCV more than application of either SA or JA alone. Our results suggest that the modulated antagonistic relationship between SA and JA under elevated CO₂ makes a great contribution to increased tomato resistance to TYLCV, and the predicted increases in tomato productivity may be enhanced by reduced plant virus susceptibility under projected rising CO₂ conditions.     

Differential expression of root-knot nematode resistance genes in tomato and pepper: evidence with Meloidogyne incognita virulent and avirulent near-isogenic lineages

Reproduction of artificially selected near isogenic Meloidogyne incognita lineages virulent and avirulent against the Mi resistance gene of tomato was assessed on host and resistant lines and cultivars of pepper. Egg mass production following inoculation of individual potted seedlings with second-stage juveniles was studied in experiments conducted in controlled environment. Artificially selected Mi-virulent nematode populations were unable to develop on resistant pepper lines PM 217 and PM 687. This suggests that the genetic systems governing resistance to root-knot nematodes are differently expressed in tomato and pepper, in spite of the very close phylogenetic relationships and structural genomic homologies occurring between these two vegetable crops. Moreover, these artificially selected nematode populations were also found unable to develop on the susceptible pepper cultivars California Wonder and Doux Long des Landes, while their pathogenicity was not significantly affected on susceptible tomatoes. Due to the existence of naturally virulent Meloidogyne populations, these results enhance the need for a better understanding of the mechanisms involved, in order to develop new forms of management of plant resistance to root-knot nematodes.