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ARS Home » Southeast Area » Charleston, South Carolina » Vegetable Research » Research » Research Project #434575

Research Project: Genomics and Genetic Improvement of Disease Resistance and Horticultural Characteristics of Watermelon, Broccoli, and Leafy Brassicas

Location: Vegetable Research

2018 Annual Report

Objective 1. Develop genomic tools and use them to develop and release watermelon germplasm with improved disease resistance, combined with desirable fruit quality and other consumer- and commercially-relevant horticultural traits. Sub-objective 1.A. Utilize an identified major quantitative trait locus (QTL) for Fusarium wilt Race 2 resistance to develop sequence-based markers as selection tools to aid the incorporation of resistance into enhanced watermelon germplasm with desirable fruit characteristics. Sub-objective 1.B. Utilize the watermelon genome sequence to develop a single nucleotide polymorphism (SNP)-based linkage map for the desert watermelon (Citrullus colocynthis) and identify markers associated with resistance to Papaya ring spot virus (PRSV). Sub-objective 1.C. Develop and release watermelon germplasm with improved disease resistance from a wild watermelon type combined with improved fruit characteristics of cultivated types. Objective 2. Develop and release broccoli germplasm with improved adaptation to high temperature environments and other commercially- and consumer-relevant horticultural traits. Sub-objective 2.A. Breed and release broccoli lines with enhanced tolerance to high temperature by exploiting additional, new tolerance alleles, and identify genomic sequences associated with the tolerant phenotype. Sub-objective 2.B. Determine if elite broccoli inbreds that are vigorous and highly self-compatible can produce head yield and quality comparable to that of commercial hybrid broccoli cultivars. Objective 3. Utilize genetic diversity in leafy green Brassicas (B.) to develop germplasm with improved commercially- and consumer-relevant traits. Sub-objective 3.A. Determine mode of inheritance of resistance to Pseudomonas cannabina pv. alisalensis (Pca) in a B. rapa accession with turnip-like leaves. Sub-objective 3.B. Exploit phenotypic diversity in a unique collection of collard landraces collected from southern seed savers to develop a B. oleracea collard with resistance to Pca and another collard that expresses relatively high levels of the glucosinolate glucoraphanin.

Parental lines of watermelon, broccoli or leafy green Brassicas will be selected based on phenotypic expression of resistance, tolerance or quality traits under study. The selected parental lines will then be utilized to construct conventional (i.e., F2, BC1, recombinant inbred) and doubled haploid (for broccoli only) populations segregating for the traits of interest. These populations will in turn be used in studies to determine mode of inheritance of each character or to select new, more superior lines. Modern techniques like genotyping by sequencing or quantitative trait locus (QTL) seq will be employed to identify DNA sequences associated with the traits of interest and to locate controlling genes on genetic linkage maps. Key DNA sequences will be used to develop strategic markers, e.g. kompetitive allele specific primer (KASP) markers, that are closely linked to the traits under study and that can be used in marker-assisted selection strategies. Knowledge gained in the above studies will be applied in developing improved breeding approaches and in fine-tuning marker-assisted methods to use in the further development of enhanced horticultural lines or hybrids that express improved resistances or tolerances and other traits of interest and that also produce high quality vegetable products. The improved plant germplasm will be made available through public releases or commercial licensing. Ongoing searches for new resistances or tolerances among watermelon and vegetable Brassica accessions from the U.S. Plant Introduction and other collections will also be conducted.

Progress Report
This new project began April 2018, and continues research from in-house project No. 6080-21000-018-00D entitled “Genetic Enhancement of Watermelon, Broccoli, and Leafy Brassicas for Economically Important Traits”. For the watermelon portion of this project falling under Objective 1, this project has collaborated with a research plant pathologist at the Charleston location to initiate construction of 20 genomic DNA markers based on single nucleotide polymorphisms covering a key locus designated QFon2-9 that confers resistance to Fusarium wilt race 2 and is derived from the watermelon line USVL246FR. Also under Objective 1, genetic populations of the wild desert watermelon Citrullus colocynthis, including F1, F2 and backcross generations, are being developed to use in studies aimed at identifying markers associated with resistance to papaya ring spot virus. Lastly, research focused on developing watermelon germplasm with improved disease resistance from a wild watermelon combined with improved fruit characteristics of cultivated types has been initiated by making crosses between resistant C. colocynthis lines and the watermelon cultivar Sugar Baby. The hybrids between the different watermelons will in turn be used in a backcross scheme aided by the use of molecular markers to select for improved lines. Relative to work on Objective 2 of this project focused on broccoli, new cycles of hybridization and inbreeding were completed and resulting hybrids and lines were tested in summer field trials at Charleston. Additionally, genomic sequencing of pooled groups of lines from the same segregating population that differ in heat tolerance was conducted, and QTL analysis is underway to identify markers linked to the tolerance trait. Lastly, other elite broccoli inbreds that are highly vigorous and self-compatible and that produce high quality vegetable heads under optimal, cool-season conditions are currently being planted in field trials. These inbreds will be compared to commercial hybrid cultivars and evaluated for quality and yield of harvested vegetable heads. Evaluations of collard plant introductions for resistance to Pseudomonas cannabina alisilensis (PCA) are underway as outlined under Objective 3, which is focused on leafy green Brassicas. Collard selections with the best resistance have been identified and are being moved to the greenhouse where they will be grown to maturity, stimulated to flower, and self-pollinated. Tests of inbred lines of leafy green Brassicas related to turnip greens are ongoing and new accessions with resistance have been identified.