Location: Vegetable Research2010 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, specific genetic populations segregating for resistance to root-knot nematodes (RKNs) were evaluated for their responses to artificial inoculation and infection with RKNs. Inheritance or RKN resistance will be determined based on the results. A separate watermelon population of recombinant inbred lines (RILs) that segregates for resistance to Fusarium wilt (FW) was evaluated for response to artificial inoculation with the causal agent, Fusarium oxysporum, to determine inheritance. In addition, 134 Citrullus lanatus var. citroides plant introductions (PIs) were also evaluated for resistance to FW, and three of these accessions exhibited resistance when inoculated with F. oxysporum. Plants from the most resistant PIs were selected and self-pollinated to produce pure lines that will be used in further studies. Disease resistance gene analogs called nucleotide binding site-luecine rich repeats (NBS-LRRs) were identified in genomic DNA of cultivated watermelon species, and were subsequently located on a genetic linkage map of the species. In other work three DNA markers linked to a gene associated with resistance to zucchini yellow mosaic virus (ZYMV) were used in a marker-assisted selection (MAS) scheme. Selected breeding lines (BC4F4) showing tolerance to a ZYMV Florida strain have been developed. For the broccoli portion of this project, an additional cycle of breeding for tolerance to high temperature stress was completed, and new tolerant selections were identified and advanced another generation. Replicated trials in different seasons at Charleston, and also at Geneva, New York, indicate that broccoli lines being developed for summer adaptation at the U.S. Vegetable Laboratory are distinct from lines developed in California. Two seed companies have tested hybrids derived from ARS lines in their own trials this year. A separate study conducted in cooperation with an ARS scientist at the Children’s Nutrition Research Center in Houston has allowed us to characterize concentrations of several mineral nutrients, such as copper and iron, in broccoli and to show the relative importance of genetics versus the environment in determining actual concentrations in the harvested vegetable. In work aimed at developing leafy green Brassicas like mustard and turnip greens resistant to bacterial leaf spot disease, two sets of genetic populations that were developed for use in determining how resistance is inherited have been artificially inoculated with the causal agent and evaluated for resistance response. Efforts are also underway to produce hybrid collard seed on a pilot scale to test the feasibility of commercializing collard varieties developed by the Vegetable Laboratory program; we have been negotiating with a seed company to cooperate in this effort.
1. Disease Resistance Genes in Watermelon. Common attributes of plant genes that confer resistance to a variety of plant pathogens have been described, but it was not known if any of these types of resistance genes are present in the genetic makeup of watermelon. ARS researchers at Charleston, South Carolina identified disease resistance gene analogs, known as nucleotide binding site-leucine rich repeats (NBS-LRR), in watermelon and mapped these genes on a genetic linkage map for the crop. Three of these NBS-LRR genes were clustered closely on the same linkage group, indicating the possibility of mapping a resistance gene island. These resistance gene analogs are of great interest to geneticists and plant breeders focused on improving watermelon resistances against diseases such as Fusarium wilt or root-knot nematodes, and this information should aid in the overall effort to breed for improved watermelon resistances against damaging diseases.
Nimmakayala, P., Jeong, J., Tomason, Y., Levi, A., Perumal, R., Reddy, U.K. 2010. Molecular Phylogeny of Citrullus Species as Inferred from AFLPs and SSRs. Plant Genetic Resources. 8:16-25.