1a. Objectives (from AD-416)
The objectives of this project focus on determining the underlying genetic bases of quality attributes and nutritive value of Solanaceous vegetables in order to facilitate their deployment in improved germplasm. Diverse Solanaceous germplasm resources exist for enhancement of cultivated forms of tomato, pepper and eggplant. Genes that may be valuable for crop improvement are often not well characterized or they may be associated with undesirable traits. Over the next 5 years we will focus on the following objectives: 1) Develop tomato germplasm with enhanced fresh- and processing-market quality. Research will focus on identification of QTL and candidate genes that contribute to fruit quality, principally fruit firmness attributes. Breeding lines and genetic stocks will be released by ARS for development of germplasm with improved firmness attributes. 2) Develop new Capsicum germplasm with improved culinary and/or ornamental quality. Genetic mechanisms underlying tissue-specific anthocyanin accumulation and fruit flavor will be characterized. Germplasm with unique tissue specificity for anthocyanin/carotenoid pigmentation in foliage, flowers and fruit will be introgressed into genotypes with novel plant habits and fruit shape, size, configuration and flavor. The inheritance of morphological characters will be determined. New novel germplasm will be released. 3) Determine the inheritance of eggplant antioxidant content, principally phenolic acids that influence postharvest quality, in populations developed from crosses of diverse accessions in the eggplant core subset.
1b. Approach (from AD-416)
(Objective 1) QTL and candidate genes that contribute to fruit quality, principally fruit firmness attributes, will be identified in an inbred backcross line population developed from parental lines originating from interspecific Solanum lycopersicum x S. galapagense crosses. These lines exhibit divergent combining ability for fruit compression, puncture resistance and fruit fresh weight. A growing database of mapped SNP markers is available and will be utilized to identify and map firmness QTL. (Objective 2; Sub-objective 2.A) Utilizing divergent genotypes and environmental treatments, we have previously demonstrated differential expression of Capsicum anthocyanin-related structural and regulatory genes in tissues that vary in anthocyanin pigmentation. A dual approach using environmental and genetic mediated modification of anthocyanin pigmentation, combined with TEV-induced silencing will provide multiple avenues to characterize regulation of tissue-specific pigmentation in Capsicum. The effect of light treatment and silencing induced by TEV infection on anthocyanin structural and regulatory gene expression will be characterized. The siRNA/miRNA populations of anthocyanin pigmented and non-pigmented tissue will be compared to evaluate the contribution of small RNAs to regulatory gene expression. Gene expression and small RNA populations will be similarly assessed in a small series of recombinant inbred lines that we have developed which vary in tissue specific anthocyanin pigmentation. (Sub-objective 2.B) Germplasm with unique specificity for anthocyanin/carotenoid pigmentation in foliage, flowers and fruit will be introgressed into genotypes with novel plant habits and fruit shape, size, configuration and flavor. Novel flavor attributes identified in exotic Capsicum germplasm will be characterized and introgressed into C. annuum. Backcross, pedigree and inbred backcross breeding will be utilized. Inheritance of morphological traits will be determined. Analyses will be performed using segregating F2, F3 and backcross populations that we have developed. QTL for flavor attributes will be identified using the principles described in Objective 1 for tomato fruit firmness. (Objective 3) Eggplant accessions with divergent fruit phenolic acid constituents and total phenolic acid content have been selected to determine the inheritance of individual classes of phenolic acid compounds and total phenolic acid content. Segregating F2 backcross populations to respective high and low phenolic acid parents will be developed and utilized to determine the inheritance of total phenolic acid content and individual classes of phenolic acids. Gene action, genetic factors and heritability estimates will be derived.
3. Progress Report
Pepper Flavor Genetics. A wealth of genetic diversity is present in cultivated and wild pepper species. The pepper, Capsicum baccatum, is poorly characterized and genes that may be valuable for crop improvement have not been identified. Utilizing chemical analyses and molecular markers, we characterized fruit flavor constituents in breeding populations, mapped markers for identification of flavor genes, and advanced breeding lines for development of improved cultivars. The research addresses consumer interest in improved product quality and will provide material for the seed industry to develop improved cultivars. New Pepper Germplasm. New pepper germplasm is needed to enrich the commercial pepper gene pool which lacks genetic diversity. Utilizing the rich genetic diversity available in pepper landraces and related species, we are developing new cultivars with novel fruit, foliage, and plant growth habit. Building upon our award winning and patented cultivars, five potential releases with novel attributes were advanced to cooperator trials for culinary and ornamental applications. The cooperator continues to work with a commercial propagator to bring licensed releases to market. Novel peppers have the highest per unit value of any pepper product and have become an alternative high-value profitable crop for pot-plant and transplant production. Anthocyanin Gene Regulation. Color is an important characteristic of culinary and ornamental plants. In breeding for color, conventional approaches rely upon visual selection for color and color intensity. New methods are needed to ensure continued progress. Utilizing peppers that differ in fruit, flower and foliar pigmentation, we previously described differential expression of structural and regulatory genes coincident with anthocyanin pigmentation in fruits and flowers. Differential regulatory gene expression did not occur in foliage. We have now identified and are evaluating additional regulatory genes. We are evaluating small pepper RNAs to identify regulatory elements. The results are important for elucidating the genetic control of pigment biosynthesis in reproductive vs. vegetative tissue and development of new plants with enhanced pigmentation. Tomato Fruit Quality. Tomato fruit color and firmness are important product quality determinants. Utilizing tomato mapping populations that we developed to identify new genes responsible for tomato fruit firmness and color, we completed digital image analysis of selected populations developed to evaluate the genetic control of pigment accumulation in distinct tissues that make up a tomato fruit. Completion of an additional tomato population segregating for fruit texture attributes was accomplished in order to improve the robustness of additional research planned to identify molecular markers linked to texture related genes. A new high throughput marker assay is now available to conduct this marker work within cultivated tomato. Gene identification will afford new opportunities for fruit quality improvement.
1. Pepper genetic diversity. The pepper species Capsicum baccatum is recognized as a potentially valuable source of genes for pepper quality improvement. However, this germplasm is poorly characterized and genes that may be valuable for crop improvement have not been identified. ARS scientists in Beltsville, MD, and a CRADA partner previously utilized molecular markers to characterize genetic diversity and population structure in the USDA, ARS Capsicum baccatum germplasm collection. They have now completed further characterization of this collection using morphological and chemical attributes and available locale and climatic indices. Population structure analyses were consistent with those obtained using molecular markers. The analyses further revealed the presence of attributes consistent with domestication events and adaptation to varied climatic conditions. The results are important for 1) preservation and management of crop germplasm resources and 2) use in pepper crop improvement programs for selection of genetically informative accessions with genes for improved product quality and adaptation to varied production environments.
2. Utilization of exotic eggplant species for eggplant improvement. Eggplant often lacks tolerance to important diseases and environmental stresses. This has led to renewed interest in rootstock grafting to ensure plant vigor and disease tolerance. Utilizing diverse exotic eggplant hybrids developed for research on genetic control of eggplant fruit phytonuturients, ARS scientists in Beltsville, MD, evaluated exotic African eggplant relatives for their value as new eggplant rootstocks. Working with a collaborator at the University of Valencia, Spain, they identified rootstocks that are superior to available eggplant rootstocks. These rootstocks improved plant vigor, disease tolerance, and fruit yield without negative effects on fruit quality. The results will be valuable for vegetable breeders and growers for development and production of superior eggplants.
Albrecht, E., Saftner, R.A., Stommel, J.R., Zhang, D. 2011. Genetic diversity and population structure of Capsicum baccatum genetic resources. Genetic Resources and Crop Evolution. 59:517-538.