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:
Anthocyanin Genetics. In Capsicum, inheritance/gene expression studies revealed that different genetic mechanisms account for pigmentation in reproductive vs. vegetative tissues. We developed a miRNA library to identify regulatory elements that interact with structural/regulatory genes. Different mechanisms explained anthocyanin pigmentation in pepper vs. eggplant and related Solanaceous species. The results are relevant to evolution of anthocyanin genetics and pigmentation in Solanaceous crops. Pepper Improvement. Within Capsicum, abundant diversity for plant habit, fruit and leaf characteristics is poorly characterized. We determined the inheritance of important horticultural attributes. The results clarify/extend information on attributes for pepper improvement. Multi-state trialing, release and licensing of three ornamental pepper cultivars, Midnight Creeper, Pepper Jack and Lil’ Pumpkin, was completed. Novel peppers are very profitable in comparison to culinary peppers and address interest by wholesale and retail markets for diverse garden plants. Pepper Genetic Resources. The exotic pepper species Capsicum baccatum is an underutilized source of genes for pepper improvement. Whereas genetic marker analysis revealed that wild forms of this species formed relatedness groups consistent with geographic origin, cultivated forms exhibited intensive admixture across the distribution range. The results are important for preservation, management and utilization of germplasm resources. Fruit flavor-linked QTL were identified in C. baccatum and C. annuum/C. baccatum introgression lines for use in breeding cultivars with superior fruit flavor. The research addresses customer interest in improved product quality. Tomato Quality. Tomato color and firmness are important fruit quality determinants. Exotic sources of genes for improving color/firmness are not fully exploited. Utilizing tomato relatives, we developed inbred backcross line (IBL) mapping populations for identifying new genes responsible for tomato fruit firmness. Using populations developed to formulate a genetic model for tomato tissue-specific fruit pigmentation, IBL populations were also developed for discovery of fruit color QTL. Insufficient resources were available to complete genotyping. The mapping populations are available as a resource for gene discovery. Eggplant Improvement. Eggplant is among the top ten vegetables for antioxidants. Utilizing hybridizations between the common eggplant and African eggplant, we identified moderate heritability for phenolic acid content, indicating that progress can be made in breeding for fruit nutritive value. We identified diversity in scarlet eggplant for plant morphology, fruit shape/size and constituents and demonstrated that breeding improved scarlet eggplant is possible. Hybrids with African eggplant were superior to available eggplant rootstocks. Eggplant lacks tolerance to disease and environmental stress, leading to renewed interest in rootstock grafting for plant vigor and disease tolerance. The results are valuable for breeders and growers for development/production of superior eggplants.