Location: Floral and Nursery Plants Research
2024 Annual Report
Objectives
Objective 1: Develop genomic resources, genetic maps, and gene-trait correlations to provide durable genetic improvement for economically valuable traits such as recurrent flowering, novel floral forms, and disease resistance in hydrangeas, including oakleaf hydrangeas that are native to North America.
Sub-Objective 1.A.: Develop gene-trait correlations for double flowers, recurrent flowering, and inflorescence shape in the model nursery crop Hydrangea macrophylla.
Sub-Objective 1.B.: Develop gene-trait correlations for powdery mildew resistance in Hydrangea macrophylla and Dichroa febrifuga.
Sub-Objective 1.C.: Develop genomic resources for identifying and characterizing native and underutilized nursery crops.
Objective 2. Breed or select new cultivars and improved germplasm of novel or underutilized woody landscape plants, such as oakleaf hydrangeas, incense-cedar (Calocedrus decurrens), manzanita (Arctostaphylos species and hybrids), and Osmanthus (Osmanthus species and hybrids) to broaden the diversity of the cultivated landscape and create marketing opportunities for growers.
Sub-Objective 2.A.: Generate genetic variation through hybridization, mutation, and ploidy manipulation in native and/or underutilized woody species including, but not limited to, Arctostaphylos, Calocedrus, Hydrangea, and Osmanthus.
Sub-Objective 2.B.: Evaluate disease resistance, growth rate, habit, and rooting ability to identify superior germplasm for nursery production and landscape use of native and/or underutilized woody species listed in Sub-Objective 2.A.
Objective 3. Improve nursery production practices, including propagation and nutrient inputs for underutilized woody plant taxa such as species listed in Objective 2, to improve nursery input efficiency and facilitate grower adoption of new taxa.
Sub-objective 3.A: Develop improved container nursery production practices to decrease nutrient runoff or improve nutrient use efficiency.
Sub-objective 3.B: Refine propagation protocols for Calocedrus decurrens and Hamamelis sp.
Approach
The research will produce novel genetic variations through hybridization and mutation, and existing genetic variations will be harnessed by collecting and cataloging wild populations of native woody species. Genomic information will be applied to current breeding lines of ornamentals to accelerate the production of high-value nursery crop cultivars. Native and underrepresented woody landscape species improved for product quality and disease resistance will stimulate consumer interest in landscape plants, provide producers with expanded marketing opportunities, and reduce the environmental footprint of nursery production by reducing the need for pesticides. A research-based nursery production protocol for these plants will facilitate adoption, provide additional marketing opportunities for the ornamental horticulture industry, and, ultimately, increase genetic diversity in cultivated landscapes. Increased consumer demand coupled with reduced costs for growers provides a strong foundation for long-term support of the U.S. nursery industry.
Progress Report
Under Objective 1.A., we completed the first-year flowering evaluation of a Hydrangea macrophylla F2 mapping population in order to identify genes for recurrent flowering. We extracted DNA from a 20-sample double-flowering hydrangea test panel and used the test panel DNA with the newly developed bigleaf hydrangea genome to identify candidate genes for inflorescence type and double-flowering. While the hypothesized long terminal repeat (LTR) was not detected, a gene in the BAM (barely any meristem) gene family was identified as a candidate for the double-flowering trait. Under Objective 1.B., we propagated 'Veitchii,' 'Bailmer,' 'Blushing Bride' and 'Nigra' bigleaf hydrangeas for powdery mildew inoculation. We extracted high-molecular weight DNA from Dichroa febrifuga; completed PacBio HiFi/CCS and Illumina sequencing of D. febrifuga, and produced a D. febrifuga raw genome assembly. RNA was extracted from seven tissue types for use in future genome annotation. Under Objective 1.C., we produced a raw genome assembly for Hydrangea arborescens using PacBio HiFi/CCS and Illumina sequencing and collected seven tissue types for RNA sequencing. We extracted and stored high-quality DNA from 215 accessions of Calocedrus, Dichroa, Hydrangea, and Osmanthus. DNA sequencing of oakleaf hydrangea populations was completed and SNP markers were identified, filtered, and used to identify unique alleles in wild oakleaf hydrangea populations.
Under Objective 2.A., we collected three new Osmanthus fragrans accessions and made 40 controlled crosses among Osmanthus and Chionanthus varieties and seedlings. We tested 56 microsatellite primers on an Osmanthus diversity panel and genotyped 84 Osmanthus breeding parents and offspring, resulting in the identification of novel Osmanthus hybrids. We collected at least 1000 open-pollinated seeds each of Hydrangea arborescens, H. macrophylla, H. paniculata, and H. quercifolia We collected over 800 open-pollinated seeds from O. armatus (446), O. fragrans (0), O. ×fortunei (255), and O. heterophyllus (160) for use in mutation breeding and genetic variation experiments. Under objective 2.B., we continued ongoing container evaluations of Osmanthus and Hydrangea hybrids; completed the third and final year of field evaluation of Osmanthus species and cultivars and of oakleaf hydrangea form and flowering selections; and pruned the oakleaf hydrangea range-wide test. We initiated the Year 2 bigleaf hydrangea cultivar screening for tolerance to full sun exposure. In collaboration with San Diego Botanic Garden, we collected over 30,000 incense cedar seeds from four provenances and have planned expeditions to collect seed from an additional eight populations in fall 2024.
Under Objective 3.A., we completed two greenhouse studies assessing the availability of iron and phosphorus to ornamental plants growing in ferrous sulfate-amended peat as well as the impact of irrigation rate on the efficacy of iron-charged peat to retain phosphorus. We conducted the second of two experiments evaluating the effect of irrigation frequency and container color on root zone temperature, shoot and root growth, and controlled-release fertilizer longevity when growing a shrub rose and red-osier dogwood in full sun. Under objective 3.B., we assessed the effect of cutting harvest timing on rooting success of ten witchhazel cultivars. Rooting success of three incense cedar selections was evaluated using three rooting hormone concentrations and, in a separate experiment, 17 incense cedar selections were screened for cutting propagation potential.
Accomplishments
1. High-quality genome sequences reveal genes for economically important flower traits in hydrangea. Bigleaf hydrangea (Hydrangea macrophylla) is one of the most valuable species in the nursery trade, yet few genomic resources are available for this crop or closely related Asterid species. ARS scientists in McMinnville, Tennessee, co-led a team of eight institutions that developed the first, complete DNA blueprint for two varieties of bigleaf hydrangea that identified the genetic loci responsible for flower structure and single vs. double flowers. These markers contribute to efficient selection of promising new varieties by reducing the time needed to characterize these traits from 2 years to 4 weeks after seed germination. In addition, these newly characterized hydrangea genomes were useful in comparative studies to decipher evolutionary timelines in the Asterid family. These genomic resources are immediately useful for breeding applications and technology development, have been shared by request with national and international stakeholders.
Review Publications
Shreckhise, J.H., Altland, J.E. 2024. A base layer of ferrous sulfate-amended pine bark reduces phosphorus leaching from nursery containers. Agronomy. https://doi.org/10.3390/agronomy14040757.
Fields, J.S., Nackley, L.L., Shreckhise, J.H., Bampasidou, M., Contreras, R.N., Kantrovich, A.J., Knuth, M.J., Owen Jr, J.S., White, S.A. 2024. How natural resources, consumer perceptions, and labor are transforming the U.S. nursery industry. HortTechnology. https://doi.org/10.1094/PDIS-01-24-0005-PDN.
Oksel, C., Avin, F.A., Liyanapathiranage, P., Shreckhise, J.H., Baysal-Gurel, F. 2024. First report of Achromobacter xylosoxidans causing bacterial stem and leaf blight on Cyrilla arida in Tennessee and the United States. Plant Disease. https://doi.org/10.1094/PDIS-01-24-0005-PDN.
Neupane , S., Baysal-Gurel , F., Alexander, L.W. 2023. Evaluation of Hydrangea cultivars for tolerance against root rot caused by Fusarium oxysporum. Plant Disease. https://doi.org/10.1094/PDIS-11-22-2712-RE.
Wu, X., Simpson, S.A., Youngblood, R., Scheffler, B.E., Alexander, L.W., Hulse-Kemp, A.M. 2023. Whole genome sequencing reveals genes implicated with important flower traits in bigleaf hydrangea (Hydrangea macrophylla). Horticulture Research. https://doi.org/10.1093/hr/uhad217.
Jennings, C., Baysal-Gurel, F., Alexander, L.W. 2024. Powdery mildew of bigleaf hydrangea: biology, control, and breeding strategies for resistance. Horticulturae. https://doi.org/10.3390/horticulturae10030216.
Alexander, L.W., Wu, X., Gouker, F.E. 2024. Production and verification of novel Osmanthus hybrids. Frontiers in Horticulture. https://doi.org/10.3389/fhort.2024.1382450.