Location: Floral and Nursery Plants Research
2020 Annual Report
Objectives
Objective 1: Characterize and evaluate, breed, select, and release improved germplasm for woody landscape plants that have superior ornamental value, are tolerant of biotic and abiotic stress, and are not invasive. [NP301, C1, PS1B; C2, PS2A]
Sub-objective 1a: Characterize germplasm and develop hybrids or breeding lines in genera currently under investigation, including Buxus, Cercis, Lagerstroemia, Prunus, and Ulmus.
Sub-objective 1b: Propagate and evaluate (in-house and via cooperators) advanced selections of Buxus, Catalpa, Cercis, Nyssa, Lagerstroemia, Prunus, and Tsuga developed in previous cycles.
Sub-objective 1c: Name, release, distribute, and promote new cultivars.
Objective 2: Incorporate modern breeding tools to accelerate the creation, characterization, identification, selection, or evaluation of priority plant materials. [NP301, C1, PS1B; C2, PS2A]
Sub-objective 2a: Test genes for altered plant architecture (developed previously by ARS scientists) in several woody ornamental plant genera.
Sub-objective 2b: Use molecular markers to characterize germplasm or hybrids in Buxus and Tsuga, where phenotypic traits are ambiguous.
Additional resources in the merged project will strengthen the research in the current Objective 2:
Objective 2: Incorporate modern breeding tools to accelerate the creation, characterization, identification, selection, or evaluation of priority plant materials. [NP301, C1, PS1B; C2, PS2A]
Approach
Objective 1: For classical breeding work, parental germplasm will be collected from native habitats, botanical repositories, and commercial sources, and will be evaluated in the polyhouse or field plots. Controlled hybridizations will be carried out in the field or greenhouse by hand or by insects in pollination cages or greenhouses to produce hybrid progeny, to determine compatibility among parents, and to study breeding systems and inheritance of traits of interest. Appropriate reciprocal and test crosses will be conducted for inheritance studies. In addition to traditional evaluations and classical breeding methodologies, several techniques will be used to characterize parental germplasm and develop hybrids. This includes ploidy analysis and manipulation and creating interploid hybrids and wide hybrids in order to develop seedless selections of priority genera. Resultant progeny will be screened for ploidy and evaluated for traits of interest. Promising selections will be propagated and transplanted to the field for further evaluation.
Selections developed during previous project cycles that have performed well will also be propagated. These include elite clones of Buxus, Catalpa, Cercis, Nyssa, Lagerstroemia, Prunus, and Tsuga. Nursery cooperators, botanical gardens, or other cooperators will be chosen based on hardiness zone and production system, and at least three plants of each selection will be sent to each cooperator for evaluation. In consultation with ARS’s Office of Technology Transfer, plants selected for release will undergo stock increase by volunteer cooperators and will be released following the standard ARS administrative approval procedures. Promotional materials will be prepared and distributed. Propagation material will be sent to nurseries upon request until the cultivar is routinely available in the trade.
Objective 2: For the first few years of this five-year plan, we will focus on establishing in vitro cultures of diverse woody taxa that would have the most impact from altered plant architecture (for example maples, crapemyrtles, beech, oaks, elms, flowering cherries). We will attempt to establish many diverse taxa in culture, recognizing that some taxa won’t be successful, and then focus on those few that perform well in terms of multiplication and regeneration using updates of protocols established already in our lab. Explants will consist of shoot tips, dormant buds, or seeds. Different protocols for regeneration, including organogenesis and embryogenesis, will be tested with ARS collaborators.
Appropriate molecular markers will be used in conjunction with classical taxonomy and, when appropriate, ploidy analysis to determine genetic relationships among taxa and verify parentage of hybrids. Efforts will focus on markers in hemlock and boxwood for the first few years.
Progress Report
This is the second full year of the project with substantial progress made towards all objectives. Progress towards Objectives 1a and 1b includes acquiring, characterizing, and evaluating boxwood accessions in the USNA germplasm collection. Testing is underway to resolve taxonomic relationships using established molecular markers (objective 2a). Additional progress towards Objective 1 includes acquiring two new accessions of Buxus rugulosa through cooperators, which is the first collection of live plants in the U.S. of this taxon. Additional boxwood plant material has been acquired including Buxus wallachiana received from collaborators from Washington State, four new cultivars from European collaborators and procurement of five new boxwood accessions that will be evaluated for blight susceptibility. All newly acquired plant material is currently being propagated and is under evaluation for boxwood blight resistance. Additional work towards Objective 1a includes screening all boxwood hybrids planted at the South Farm nursery for disease resistance, which includes 461 F1 hybrid plants. Creation of new hybrids is underway including collection, stratification, and germination of seeds from controlled cross pollinations and open pollinated Buxus and Catalpa from the last year’s crosses. A total of 13 interspecific crosses have been conducted this year for Catalpa and 14 interspecific crosses of Buxus have been made. All Buxus crosses were 100% successful and seeds have been collected and will be stratified and germinated at the end of FY20.
Progress towards in vitro propagation, addressing Objective 2 includes establishment of several taxa of woody ornamental plants in tissue culture, including three additional boxwood cultivars to test for the ability to multiply and regenerate woody ornamental taxa and to create a plant tissue culture protocol for boxwood. Young shoot tips from three boxwood species were established in three different artificial growth media with one of the media outperforming the others. At least 10 additional boxwood species will be evaluated for in vitro propagation in addition to testing a new sterilization protocol and three new tissue culture proliferation protocols for woody plants that were received from cooperators.
A simple, fast, and effective method for DNA extraction from woody plant tissue was developed, which will streamline our molecular marker work and disease diagnostic assays. In addition, high-throughput diagnostic tools are being developed through implementation of artificial intelligence platforms for more accurate and efficient disease evaluations.
Accomplishments
1. Introduction of a new hybrid hemlock, ‘Traveler’. The native hemlock, Tsuga canadensis, plays an essential role in forest ecosystems as well as in cultivated landscapes; however, it is susceptible to feeding damage by the hemlock woolly adelgid, which has caused widespread loss of hemlocks in wild and cultivated settings. ARS scientists from Washington, D.C., developed and introduced the first interspecific hybrid hemlock to the trade. ‘Traveler’, a cross between the Chinese hemlock and the Carolina hemlock, was explicitly bred for resistance to hemlock woolly adelgid, and selected for its regular, slightly pendulous habit, and moderately slow growth rate. It is currently being propagated under license at cooperator nurseries and promises to be a valuable addition to residential, commercial, and forest landscapes.
Review Publications
Hoban, S., Callicrate, T., Clark, J., Dean, S., Dosmann, M., Fant, J., Gailing, O., Havens, K., Hipp, A., Kadav, P., Kramer, A., Lobdell, M., Magellan, T., Meyer, A., Pooler, M.R., Spence, E., Thompson, P., Toppila, R., Walsh, S., Westwood, M., Wood, J., Griffith, P. 2020. Taxonomic similarity does not predict necessary sample size for ex situ conservation: a comparison among five genera. Proceedings of the Royal Society B. 287:20200102. https://doi.org/10.1098/rspb.2020.0102.
Guo, Y., Kilcrease, J., Hammond, J., Pooler, M.R. 2019. Stomatal openings on boxwood leaves yield entry portals for leaf infection by Calonectria pseudonaviculata. Journal of Plant Pathology. https://doi.org/10.1007/s42161-019-00416-1.
