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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Research

2009 Annual Report

1a. Objectives (from AD-416)
Determine the genetic control of resistance to root-knot nematode (Meloidogyne incognita) and Fusarium wilt (Fon race 2) in watermelon, identify and map DNA-based markers closely linked to identified resistance loci, and release resistant watermelon lines. Utilize genomic tools to develop genetic linkage maps for watermelon and diagnostic DNA-based markers for host-plant resistance to viruses and key watermelon fruit traits. Develop inbred broccoli lines with tolerance to high temperature stress and elucidate the underlying genetic control of the tolerance. Breed high yielding, self-compatible inbred broccoli lines with high productivity and high levels of health-promoting compounds. Elucidate the genetic control of bacterial leaf spot resistance in leafy green Brassicas (B. juncea and B. rapa) and release resistant breeding lines resulting from the research.

1b. Approach (from AD-416)
Select parental lines of watermelon, broccoli, or leafy green brasscias based on phenotypic expression of resistance, tolerance, or quality traits under study. Use the selected parents to construct conventional (i.e., F2, BC1, recombinant inbred) and doubled haploid (for broccoli only) populations that segregate for the traits of interest, and then employ those populations in studies to determine mode of inheritance of each character. Utilize PCR-based markers and other genomic technologies to identify sequences linked to the studied characters and to locate controlling genes on linkage maps. Use particular markers (i.e., SSR, SRAP, SNPs, or SCARs) closely associated with traits of interest to develop tools for marker-assisted selection. Based on knowledge gained through above studies, devise breeding strategies, and applications of marker technologies to use in the further development of horticulturally enhanced lines or hybrids that express resistances and other traits of interest and that also produce high quality vegetables. Make enhanced lines available through public releases or commercial licensing. Continue ongoing searches for new resistances and tolerances among watermelon and vegetable Brassica accessions for the U.S. PI and other collections.

3. Progress Report
For the watermelon portion of this project, genetic populations segregating for resistance to root knot nematodes were developed and their responses to infection by these nematodes were evaluated. A different watermelon population segregating for resistance to Fusarium wilt disease was inoculated with the causal agent and evaluated for disease response. In addition, a diverse set of watermelon Plant Introductions (PIs) were evaluated for resistance to the same disease, and several of those showing low infection response were selected for further evaluation. Heritability studies and genetic mapping of DNA sequences associated with resistance to Zuchini Yellow Mosaic Virus (ZYMV) were conducted. We are currently using some of the DNA sequences associated with ZYMV resistance in a selection scheme that utilizes the sequence information to facilitate development of resistant watermelon lines. In other work, we identified a large number of gene sequences associated with watermelon fruit development and quality traits and located these on an existing DNA map. For the broccoli portion of this project, an additional cycle of breeding for tolerance to high temperature stress was completed, and new and improved tolerant selections were identified and advanced another generation. Replicated trials in different seasons continue to indicate that broccoli lines being developed for summer adaptation at the U.S. Vegetable Laboratory are distinct from lines developed in California, and two seed companies are currently testing hybrids derived from the ARS lines. A study conducted in cooperation with a scientist from the University of Tennessee showing that important nutritional and antioxidant compounds in broccoli called carotenoids can probably be enhanced by selective breeding was recently completed. In work aimed at developing leafy green Brassicas like mustard and turnip greens resistant to bacterial leaf spot disease, two sets of populations needed to determine how resistance is inherited have been generated.

4. Accomplishments
1. Development of watermelon lines resistant to Zucchini yellow mosaic virus (ZYMV). Because viruses like ZYMV are being transmitted by aphids that are migrating from remote fields, it is difficult to control the infections caused by these pests. Ultimately, the most effective way to control viral diseases in crops is through the development of resistant varieties. Using information already known about genes that confer resistance in a wide variety of plants, we identified DNA sequences closely associated with the gene conferring ZYMV resistance in watermelon and identified their relative location on watermelon chromosomes. These DNA sequences are proving useful by making the process of selecting resistant lines more efficient. Information from these studies is already being used by this project and plant breeders with private seed companies to develop improved watermelon varieties with ZYMV resistance.

2. The influence of genetics on expression of carotenoid levels in broccoli. Carotenoids are plant compounds that confer various health-promoting effects when they are consumed by humans, and broccoli is recognized as an excellent source of these compounds. However, very little is known about the general carotenoid profile of broccoli or about the levels of specific carotenoids and how they might vary among different varieties of the crop. In light of this, a study was conducted in cooperation with scientists at the University of Tennessee to assess carotenoid makeup of broccoli heads from different varieties grown in several different environments. In conducting this research, five different carotenoids were identified and quantified in broccoli heads, and the carotenoid called lutein was identified as the most prevalent carotenoid in the vegetable. It was also demonstrated that lutein levels in broccoli heads are largely conditioned by genetics with environment having a much less significant influence. Information from this work will be very useful to plant breeders working to modify the carotenoid content of broccoli and other related cabbage family vegetables, and results also provide a more accurate picture than previously available about levels of healthful carotenoids that consumers derive in their diet when eating broccoli.

Review Publications
King, S.R., Davis, A.R., Liu, W., Levi, A. 2008. Grafting for disease resistance. HortScience. 43(6):1673-1675.

Davis, A.R., Perkins Veazie, P.M., Hassell, R., Levi, A., King, S.R., Zhang, X. 2008. Grafting effects on vegetable quality. HortScience. 43(6):1670-1672.

Nimmakayala, P., Tomason, Y.R., Jeong, J., Vajja, G., Levi, A., Gibson, P., Reddy, U.K. 2009. Molecular Diversity in Ukrainian Melon Collection as Revealed by AFLP and Microsatellite Markers. Plant Genetic Resources, online publication doi:10.1017/S1479262108098481

Levi, A., Wechter, W.P., Davis, A.R. 2009. EST-PCR Markers Representing Watermelon Fruit Genes are Polymorphic among Watermelon Heirloom Cultivars Sharing a Narrow Genetic Base. Plant Genetic Resources. 7:16-32.

Levi, A., Thies, J., Ling, K., Simmons, A.M., Kousik, C., Hassell, R. 2009. Genetic diversity among Lagenaria siceraria accessions containing resistance to root-knot nematodes, whiteflies, ZYMV or powdery mildew. Plant Genetic Resources: Characterization and Utilization 7(3):216-226.

Last Modified: 06/26/2017
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