2012 Annual Report
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):
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.
Pepper Flavor Genetics. A wealth of genetic diversity is present in cultivated and wild pepper (Capsicum) species. The exotic pepper species Capsicum baccatum is recognized as a potentially valuable source of genes for pepper quality improvement. This germplasm is poorly characterized and genes that may be valuable for crop improvement have not been identified. Utilizing morphological and chemical analyses and molecular markers, we continued characterization of fruit flavor constituents and genotyping of segregating populations for identification of flavor related genes. Breeding lines were advanced for use in developing cultivars with improved flavor. The research addresses consumer interest in improved product quality and will provide new plant material for use by the vegetable seed industry to develop improved pepper cultivars. New value-added cultivars typically afford superior marketability for vegetable producers. The research is cooperative with the ARS Food Quality Laboratory in Beltsville and private industry.
New Pepper Germplasm. New pepper germplasm is needed to enrich the commercial pepper gene pool which lacks diverse new cultivars to meet market demands for novel culinary, ornamental, and dual-purpose cultivars. Utilizing the rich genetic diversity available in pepper landraces and related species, new pepper cultivars with novel fruit, foliage, and plant growth habit are being developed. Building upon our previous award winning and patented cultivars, new culinary and dual-purpose culinary/ornamental breeding lines were advanced in the breeding program and two licensed cultivars with novel character attributes and plant ideotypes were submitted for commercial seed increase. Analysis of miRNA libraries is continuing for identification of regulatory elements that influence pepper quality attributes. Novel peppers have the highest per unit value of any pepper product and have become a profitable crop for greenhouse pot plant and transplant production and an innovative way for farmers to produce a high value alternative crop.
In additional research, segregating lines in a tomato mapping population were characterized for fruit color. The mapping population provides a resource for identification of genes that influence tomato fruit texture and color. Research to further assess the impact of genotype x environment interaction on eggplant fruit phenolic acid composition and concentration was initiated with ARS and university collaborators. Knowledge and tools developed from gene expression studies targeting anthocyanin accumulation in pepper have been extended to a novel eggplant mutant identified by a collaborator.
Genetic improvement of eggplant fruit quality. Common eggplant, Solanum melongena, is one of the most important vegetable crops in the world and has been the subject of considerable efforts in breeding for yield and quality. A related cultivated eggplant species, the scarlet eggplant (Solanum aethiopicum), has received little attention for genetic improvement. ARS scientists collaborated on a project led by scientists at the University of Valencia, Spain using traditional breeding, molecular markers, and food chemical assays, to evaluate breeding behavior and fruit quality attributes of the scarlet eggplant in order to assess its value in eggplant breeding. Considerable diversity was evident within scarlet eggplant for plant morphology, fruit shape and size, and fruit constituents. The results indicate that breeding for improved scarlet eggplant is possible and that traits can be transferred between scarlet eggplant and common eggplant via breeding. These results will be of value to eggplant breeders and to specialty crop vegetable growers.
Pepper pigmentation genetics. Knowledge of the inheritance of pepper fruit pigmentation has focused on simple inheritance studies wherein color classes or simple presence and absence of pigments are described. Little information is available on the inheritance of pigment concentration within reported color classes. ARS scientists carried out studies to characterize variation for pepper fruit pigment concentration. The results demonstrate that fruit color can be controlled by just one or several genes but that variation for concentration of individual pigments is inherited in a complex fashion, with numerous genes. The results will be useful to breeders in identifying genes that influence pigment composition and concentration and in breeding improved pepper cultivars.
Stommel, J.R., Albrecht, E. 2012. Genetics. In: Russo, V.M., editor. Peppers: botany, production and uses. Cambridge, MA: CABI. p. 29-57.
Prohens, J., Plazas, M., Raigon, M.D., Stommel, J.R., Vilanova, S. 2012. Characterization of interspecific hybrids and backcross generations from crosses between two cultivated eggplants (Solanum melongena and S. aethiopicum Kumba group)and implications for eggplant breeding. Euphytica. 186:517-538.