Location: Southern Horticultural Research Unit
2024 Annual Report
Objectives
1. Discover and characterize superior traits from natural collections, mutants, and interspecific populations to enhance stress tolerance in woody ornamental species, such as Hibiscus.
1.A. Generate genetic information and characterize the genetic diversity and population structure of important ornamental species.
1.B. Characterize and develop improved germplasm with superior ornamental value and wide adaptation to prevalent conditions in southeastern U.S.
1.C. Use phenotypic and genomic tools to identify quantitative trait loci and candidate genes associated with important traits in Rosa and Weigela.
2. Develop and use novel tissue culture and transformation systems to improve key horticultural traits of blueberry, muscadine grape, and woody ornamental plants.
2.A. Develop a regeneration and transformation system to improve Hibiscus mutabilis.
2.B. Develop new germplasm of muscadine grapes using induced mutation, invitro ploidy manipulations, and wide hybridization.
3. Dissect the genetics and environmental factors affecting fruit quality and adaptation traits in blueberry and muscadine grape.
3.A. Use genomics resources to discover genomic regions associated with fruit quality and adaptation traits in blueberry.
3.B. Dissect the genetics and environmental factors affecting fruit quality and adaptation traits in muscadine grape.
4. Use conventional and genomic technologies to breed improved blueberry cultivars with excellent flavor and broad adaptation to biotic and abiotic stresses.
4.A Use interspecific hybridization to introduce useful genetic variability, incorporate adaptation traits into existing germplasm, and develop new rabbit-eye cultivars.
4.B Establish field trials and develop and implement effective breeding system for rapid development of improved blueberry germplasm.
Approach
Genetic diversity in Weigela. A diverse population will be characterized for horticultural traits and genotyped. Population structure, principal components, and phylogenetic analyses will be performed to assess the structure of the diversity. Principal component analysis will be performed to investigate the relationships among trait variables and the factors underlying genotype variation.
Comparative genomics of Hibiscus species. 27 genotypes from nine Hibiscus species will be sequenced using different sequencing platforms. The pan-genomes will be assembled, annotated, and phylogenetic analyses will be performed to determine the relationship between tested species.
Induction of mutation and polyploidy in Weigela. Vegetative buds from mature plants of four Weigela species and three concentrations of colchicine solution and three durations will be used in meristem culture in modified Murashige and Skoog media for the induction of polyploidy.
GWAS of important traits in Rosa. A diverse panel of 125 Rosa genotypes will be evaluated for different horticultural traits and genotyped. The SNPs data will be used to analyze the genetic diversity and locate genomic regions associated with important traits.
Develop a regeneration and transformation system to improve Hibiscus mutabilis. Agrobacterium rhizogenes transformation will be performed and gene construct containing the gene of interest and selectable marker will be used. PCR or RT-PCR test will be conducted on putative transgenic microshoots and hairy roots to select stably transformed plants.
Generation of interspecific grape hybrids. For this purpose, 14 reciprocal crosses between V. rotundifolia and seven accessions of V. mustangensis will be performed. In addition, 36 reciprocal crosses between, V. rotundifolia and nine Vitis species will be performed. F1 seeds will be soaked for 3 days in various concentrations of oryzalin to induce polyploidy. Phenotypic observations by means of visual observations, leaf thickness, and flow cytometry will be used to identify the interspecific hybrids.
Map genomic regions associated with important traits. A diverse panel of 264 SHB genotypes will be characterized for phenology and fruit quality traits and genotyped using the 3,000 DArT array platform. GWAS analysis and linkage mapping will be performed to identify genomic regions associated with traits of interest.
Dissect the genetics and environmental factors affecting fruit quality and adaptation traits in muscadine grapes. Parents and 516 individuals will be characterized for different fruit quality traits and resistance to Pierce’s disease. GBS will be used to genotype the individuals and data will be used in QTL analysis.
Progress Report
Significant progress has been on both Objective 2 and Objective 4. Under Objective 2, an in vitro micropropagation system has been optimized for Hibiscus mutabilis, a prerequisite for Agrobacterim-mediated transformation. This was achieved using various concentrations of two cytokins, BA and kinetin, and two auxins, NAA and IBA.
Hibiscus is a large, diverse, and economically important genus because of its industrial, ornamental, and medicinal properties. Genetic diversity within this genus offers breeders opportunities to develop new and improved elite cultivars through controlled hybridization. However, challenges such as asynchronous flowering can impede these efforts. A study was conducted by ARS researchers at Poplarville, Mississippi to assess the viability of fresh and stored pollen grains from Hibiscus genotypes. Fresh pollen viability of 14 Hibiscus genotypes was evaluated. In vivo pollen viability of six Hibiscus genotypes was evaluated following controlled self-pollination at storage temperatures of 4, -20, and -80 °C for durations of two, four, and six weeks. Viability rates of fresh pollen varied significantly, from as low as 7.41% in a purported H. paramutabilis accession to 100% in H. hamabo. Viability of stored pollen grains decreased significantly when pollen was stored at -80 °C for six weeks, as the percentage of fruit sets ranged from 0 to 11%. Pollen grain characteristics, critical in taxonomic studies, were also examined to help delineate species. Included in the study was a genotype labeled H. paramutabilis that showed very low pollen viability. Cytomolecular examination found a chromosome number of 2n = 65, which is markedly different from the 2n = 82 reported for H. paramutabilis. This accession, which was acquired from two different sources and currently labeled in the United States as H. paramutabilis, does not represent a true species but is likely the result of hybridization between two Hibiscus species or is a case of naturally occurring aneuploidy. The findings reported in this study are immensely useful to Hibiscus breeders, geneticists, and taxonomists in improving breeding strategies and understanding species delineation within the genus. Seeds of interspecific diploid blueberries were germinated and treated with antimitotic agents (colchicine and oryzalin) to induce tetraploidy in interspecific blueberry hybrids. Seedlings are currently growing in the greenhouse, and their ploidy level will be later checked using flow cytometry and chromosome analysis. The artificially created tetraploid hybrids will be made available to geneticists and breeders to help avoid bottlenecks due to the narrowing of the genetic base of elite blueberry cultivars. Under Objective 4, work on conventional breeding of small fruit crops including blueberries and muscadine grapes is proceeding. Advanced blueberry and muscadine grape selections are being evaluated in trials at Poplarville, McNeill, and Perkinston, Mississippi and at Stephen F. Austin Univ., in Nacogdoches, Texas to assess performance and quality. Those showing promise are propagated and advanced for testing in grower field conditions for further evaluation as candidates for release as new cultivars. Two elite fresh market muscadine grape selections have been submitted for release as patented cultivars.
Accomplishments
1. Development of polyploid Hibiscus hamabo plants. H. hamabo is a salt-tolerant plant with yellow flowers in the summer and beautiful golden-yellow or burnt orange leaves in the fall. Unfortunately, the plant has small and often sparse flowers, so polyploidy was induced to hopefully increase flower size, thus enhancing the ornamental value of the plant. Genome size and chromosome number were determined to confirm the polyploidy nature of the converted plants (4n = 184 instead of 2n = 92). The impact of induced polyploidy on rDNA distribution/organization was also determined. Other morphological traits, including leaf color, shape, and size, trichome density and shape, and plant heigh were examined. In addition to the potential improved ornamental value of the plant, the induced polyploid plants provide the opportunity to study the impact of polyploidy on salt tolerance in H. hamabo.
2. ‘Bertha’ and ‘Ernest’ muscadine grapes.. To remain competitive and increase profitability, muscadine grape and other small fruit producers seeking alternative berry crops desire productive cultivars producing quality berries. ‘Bertha’ develops very large berries that ripen mid-season and ‘Ernest’ berries are medium size and ripen early. Both ‘Bertha’ and ‘Ernest’ have thin, edible and flavorful bronze skins and crisp pulp, traits uncommon in most currently available cultivars.
Review Publications
Clare, S.J., Driskill, M., Millar, T.R., Chagne, D., Montanari, S., Thomson, S., Espley, R.V., Munoz, P., Benevenuto, J., Zhao, D., Sheehan, M., Mengist, M.F., Rowland, L.J., Ashrafi, H., Melmaiee, K., Kulkarni, K.P., Babiker, E.M., Main, D., Olmstead, J.W., Gilbert, J.L., Havlak, P., Hung, H., Kniskern, J., Percival, D., Edger, P., Iorizzo, M., Bassil, N.V. 2024. Development of a targeted genotyping platform for reproducible results within tetraploid and hexaploid blueberry. Frontiers in Horticulture. 2. Article 1339310. https://doi.org/10.3389/fhort.2023.1339310.
Dobbs, J.T., Caballero, J.R., Ata, J.P., Babiker, E.M., Copes, W.E., Stewart, J.E. 2024. Genomic and transcriptomic comparisons of the twig blight pathogen, Passalora sequoiae with Mycosphaereallaceae foliar and conifer pathogens. Phytopathology. 114:732-742. https://doi.org/10.1094/PHYTO-08-23-0271-R.