Progress 01/15/08 to 01/14/12
Outputs
OUTPUTS: Nematode strains that differ in ability to infect tomato with the resistance gene Mi-1 were maintained in culture. DNA was extracted from these strains and assessed for differences in sequence using PCR primer sets and DNA sequence analysis. Results from research sponsored by this grant were presented in an invited talk entitled "Host resistance as a nematode management tool" at the Society of Nematologists meeting (July 2011). PARTICIPANTS: Project leader Valerie Williamson coordinated and participated in experiments. Postdoctoral scientist Stephen Gross completed bioinformatic analyses and contributed to manuscript preparation. Undergraduate student Kiho Song assisted in the laboratory and the greenhouse. TARGET AUDIENCES: This work targets basic researchers working on molecular biology of disease caused by nematodes and other organisms. The characterization of transposable elements has generated interest among those interested in genome evolution. The findings on nematode virulence are of interest to nematologists and other scientists, including those in industry, who are concerned with managing host resistance in agriculture. Processing tomato growers and farm advisors in California have been particularly interested as they search for new ways to control nematodes. PROJECT MODIFICATIONS: This report covers work carried out during a no-cost one-year extension of the original project.
Impacts
A manuscript was completed and published describing a transposable element whose loss correlated with gain of virulence on tomato with the resistance gene Mi-1. However, a mechanistic explanation for why loss or silencing of this fragment results in gain of virulence has not been determined. The effect appears to be indirect. The use of comparative transcriptome analysis with newly available sequencing techniques to further assess the material generated by this work is under consideration. Tomato growers in California continue to be concerned about the increasing loss of efficacy of Mi-1 mediated resistance. Last year we reported that we had identified resistance in wheat that was effective against Mi-1-infecting nematode isolates. We have worked with the growers to assess the potential for these resistant wheat varieties to manage the virulent nematodes.
Publications
- Gross, S.M., and Williamson, V.M. 2011. Tm1: A Mutator/Foldback transposable element family in root-knot nematodes. Plos ONE 6(9): e24534.
- Humphreys, D.A., Williamson, V.M., Salazar, L., Flores-Chaves, L., and Gomez-Alpizar, L. 2011. Presence of Meloidogyne enterolobii Yang and Eisenback (= M. mayaguensis) in guava and acerola from Costa Rica. Nematology, published on line (accepted 6 June 2011).
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Progress 01/15/10 to 01/14/11
Outputs
OUTPUTS: The loss of a gene called Cg-1 was previously correlated with gain of ability of a root-knot nematode strain to infect tomato with the resistance gene Mi-1. A DNA fragment carrying this gene and flanking sequences was cloned and sequenced. Bioinformatics analysis was carried out using the available genome sequences of two root-knot nematode species to characterize this gene and related regions of the genome. Field isolates of root-knot nematodes from several locations in California that were able to circumvent resistance were characterized using probes developed from the bioinformatic analysis. Greenhouse tests to assess the host range of these field isolates were also carried out to identify rotation options and to determine whether the gain in virulence on tomato extended to other crop species. Results from this research were presented on posters at NIFA awardee Workshops in Washington DC (March 2010) and in San Diego (December 2010). A poster was also presented at the UC Davis Plant Pathology Graduate Group fall retreat in Sept. 2010. PARTICIPANTS: Project leader Valerie Williamson coordinated work. Experiments were carried out by Stephen Gross, a postdoctoral researcher in the Williamson lab. Undergraduate students Raquel Cordero and Kiho Song assisted in the laboratory and the greenhouse. Dr. J. Dubcovsky, a wheat breeder and professor at UCDavis, collaborated on the wheat studies. TARGET AUDIENCES: This research targets basic research community working in the areas of mechanisms of evolution of parasitism and of host resistance. Practical aspects include contributions to decision making for crop production and plant resistance management. PROJECT MODIFICATIONS: A one-year no-cost extension until 1/14/12 was obtained to complete this work.
Impacts
Sequence analysis revealed that the gene Cg-1 is flanked by complex inverted repeats suggesting that the gene was within a transposable element. Bioinformatic and molecular analysis led to identification and characterization of a novel transposon family that includes Cg-1 in asexual root knot species. A master element was identified that encodes a transposase as well as two other classes of elements. Field isolates also encode the transposase but do not have same deletion spanning the Cg-1 gene. This finding suggests that the field isolates may have acquired virulence by a different mechanism than the originally studied virulent strain. Greenhouse assays for the ability of these isolates to reproduce on several different crops revealed that root-knot nematode isolates were significantly different. In contrast, few DNA polymorphisms that distinguish these isolates were identified and no clear correlation has found connecting within-species differences in host range with diagnostic molecular markers. The potential utility of another novel transposon for developing such markers is being explored. Studies on host range discovered a root-knot nematode resistance trait that had not been previously noted to be present in some wheat varieties. Using selected nearly isogenic pairs of wheat cultivars, we have been able to localize the resistance trait to a defined region in the wheat genome that was introgressed from a wild relative. This gene appears to be effective against all the Mi-virulent root-knot nematode strains tested.
Publications
- Abad, P., and Williamson, V.M. 2010. Plant nematode interaction: a sophisticated dialogue. Advances in Botanical Research. 53: 147-292.
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Progress 01/15/09 to 01/14/10
Outputs
OUTPUTS: The published sequence of the genome of the root-knot nematode Meloidogyne incognita, which is closely related to M. javanica, the species under investigation in this proposal, has been used to identify sequences resembling the candidate avirulence gene Cg-1 and an inverted repeat family flanking this locus. PCR analysis has been used to identify and characterize DNA sequence flanking Cg-1. An arrayed BAC library of the genome of the avirulent nematode strain used in these studies has been obtained. Screening the library by PCR has led to the identification of a clone carrying Cg-1 and sequence 3' to this gene. Host range tests and DNA analysis have been carried out on Mi-virulent root-knot nematodes obtained from several different counties in California. Invited talks on this research were presented at the Tomato Breeders' Roundtable, Sacramento, CA in July 2009 and at the Entomological Society of America meeting in December 2009. PARTICIPANTS: Project leader Valerie Williamson coordinated work. Experiments were carried out by Stephen Gross, postdoctoral researcher, and Rushi Shah, Junior Specialist. Undergraduate students Raquel Cordero and Herschel Espiritu assisted in the laboratory and the greenhouse. TARGET AUDIENCES: This research targets crop production, plant resistance management, and researchers working on host resisteance. Publications target plant nematologists and molecular biologists working on plant nematode interactions as well as evolutionary geneticists. We have presented this work to tomato growers, breeders, farm advisors and seed companies. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The published genomic sequence of the root-knot nematode M. incognita lacks the sequence corresponding to the candidate avirulence gene Cg-1 as well as flanking sequence even though this sequence is present in the genome as determined by PCR and sequence analysis. This discrepancy indicates that the published genome sequence is incomplete. PCR analysis has revealed that the deletion in the Mi-1-infecting nematode strain VW5 spans at least 3 kb. Using primers designed from the inverted repeats flanking Cg-1, a putative autonomous element that encodes a transposase-like protein was identified and demonstrated to be expressed in several isolates of root-knot nematode. Examination of host ranges of nematodes virulent and avirulent on Mi-tomato indicates that virulence on Mi is not correlated with a general gain in virulence on resistant hosts or with a loss of Cg-1 sequence.
Publications
- Bird, D. McK., Williamson, V.M., Abad, P., McCarter, J., Danchin, E.G.J., Castagnone-Sereno, P., and Opperman, C.H. 2009. The Genomes of root-knot nematodes. Ann. Rev. Phytopathology, 47:333-352.
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Progress 01/15/08 to 01/14/09
Outputs
OUTPUTS: The sequence of the genome of the root-knot nematode Meloidogyne incognita has been published, and we have used this resource to investigate the candidate avirulence gene Cg-1 from M. javanica. A combination bioinformatics studies, PCR analyses, and sequence analysis have proved useful for investigating Cg-1 and related sequences. We have also compared the virulent and avirulent near-isogenic pairs of nematode strains for ability to reproduce on tomato without the nematode resistance gene Mi-1 to determine whether there was a fitness cost to gain of virulence. An invited talk on this research was presented at the 5th International Congress of Nematology in Brisbane, Australia, and a poster was presented at the NRI Awardee Workshop held in conjunction with the Entomology Society of America meeting in Reno Nevada. PARTICIPANTS: Project leader Valerie Williamson coordinated work. Experiments were carried out by Stephen Gross, postdoctoral researcher, and Rushi Shah, Junior Specialist. Undergraduate students Bay Ho, Raquel Cordero, and Herschel Espiritu assisted in the laboratory and the greenhouse. TARGET AUDIENCES: This research targets a wide spectrum of audiences. Publications target plant nematologists and molecular biologists working on plant nematode interactions as well as evolutionary geneticists. We also present this work to tomato growers and seed companies mainly through interactions with the California Tomato Research Institute. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Comparison of the genomic region of the root-knot nematode M. incognita that carries the sequence most similar to our avirulent nematode strain indicates that Cg-1 is flanked by inverted repeats and has a structure resembling a non-autonomous transposable element. Using primers designed from the inverted repeats, we have isolated a putative autonomous element that encodes a transposase-like protein. Interestingly, homologs of this protein are found in aphids, but not in other nematode species, suggesting that it may have been acquired by horizontal gene transfer. In two experiments, we have found that virulent nematodes reproduce less well than avirulent nematodes on tomato without the resistance gene Mi-1, suggesting that virulence is associated with a loss of fitness.
Publications
- Gleason, C.A., Liu, Q.L., and Williamson, V.M. 2008. Silencing a candidate nematode effector gene corresponding to the tomato resistance gene Mi-1 leads to acquisition of virulence. Molecular Plant Microbe Int., 21:576-585.
- Westerdahl, B.B., and Williamson, V.M. 2008. Update on resistance-breaking nematodes. California Processing Tomatoes, published by California Tomato Research Institute 29: 1-5.
- Williamson, V.M., and Roberts, P.A. Mechanisms and genetics of resistance, In: Root-knot nematodes, R.N. Perry, M. Moens, and J. Starr, eds. CABI Publishing, UK. 2009. in press.
- Bird, D.McK., Opperman, C.H., and Williamson, V.M. Plant infection by root-knot nematodes. In: Cell Biology of Plant-Nematode Interactions, R.H. Berg and C.G. Taylor, eds. Springer, Heidelberg. 2009. in press.
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