Location: Floral and Nursery Plants Research
2022 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 final full year of this project, with substantial progress made towards all objectives. Under Objective 1, fifty Buxus sempervirens cultivars received from Longwood Gardens were screened for boxwood blight susceptibility using a detached leaf assay with multiple Calonectria pseudonaviculata isolates. We evaluated and selected 15 interspecific Buxus hybrids for advanced selection and propagation and have rooted at least 70% of these. Once enough plants have rooted, they will be sent to cooperating research partners for field trialing, landscape evaluation, and boxwood blight resistance evaluation. Boxwood hybrids from the 2021 breeding season have germinated and have been individually potted for further evaluation. Thirty unique controlled cross pollinations were completed for the 2022 breeding season with ripening fruit still being monitored for harvest. This was also the first attempt at using previously generated hybrids from the USNA breeding program to generate BCF1 and F2 populations. Chitalpa propagation was completed for interspecific hybrids between accessions of Catalpa bungei and Chilopsis accessions. We have 41 diploid, triploid, and tetraploid Ulmus americana selections accessioned in conjunction with the USNA Taxonomy Project, and leaf samples of 14 diploid and 1 tetraploid selection were sent to a collaborator in New York for genome sequencing. We continued to evaluate advanced selections of Prunus and Lagerstroemia for release, and we released our second interspecific adelgid resistant hemlock hybrid (‘Crossroad’) and distributed additional plant material of the first hybrid hemlock introduction, ‘Traveler’.
To address Objective 2, various genes related to plant architecture have been identified. These genes include genes in the IGT gene family such as TAC1 and LAZY1, genes in the TCP gene family such as BRC1a, genes in the GA biosynthesis and signaling pathway, such as GA20Ox, Della and SHI, and genes in the strigolactone biosynthesis and signaling pathway such as MAX and CCD genes. The corresponding homologs of these genes have been identified in available Prunus genome databases using original protein sequences as query and are being cloned in several flowering cherry species. The effects of these gene will be tested in flowering cherry using either transgenic or gene-editing technologies. We also identified plant developmental genes that could facilitate plant genetic transformation and will use them to either improve transformation efficiency or overcome transformation recalcitrance in some genotypes.
Accomplishments
1. Introduction of a new hybrid hemlock, ‘Crossroad’. The native hemlock, Tsuga canadensis, plays an important 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, DC, developed and introduced a new interspecific hybrid hemlock to the trade. A cross between the Carolina hemlock and the Chinese hemlock, ‘Crossroad’ was specifically bred for resistance to hemlock woolly adelgid, and was selected for its symmetrical, upright habit, tolerance to hemlock woolly adelgid, and moderate growth rate. It is currently being distributed for propagation by nursery cooperators, and promises to be a valuable addition to residential, commercial, and possibly forest landscapes.
Review Publications
Lattier, J., Ballard, H., Kramer, M.H., Pooler, M.R. 2022. Genome size, ploidy levels, and genetic diversity of Corylopsis germplasm collections. Genetic Resources and Crop Evolution. https://doi.org/10.1007/s10722-022-01371-0.
Gouker, F.E., Guo, Y.H., Pooler, M.R. 2022. High-resolution melting analysis enables efficient detection and differentiation of two boxwood blight pathogens by qPCR assays. PhytoFrontiers. https://doi.org/10.1094/PHYTOFR-09-21-0066-SC.
Kong, P., Li, X., Gouker, F.E., Hong, C. 2022. cDNA transcriptome of Arabidopsis reveals various defense priming induced by a broad-spectrum biocontrol agent Burkholderia sp. SSG. International Journal of Molecular Sciences. https://doi.org/10.3390/ijms23063151.
Duan, H., Maren, N.A., Da, K., Yencho, G.G., Ranney, T.G., Liu, W. 2022. Genotype-independent plant transformation. Horticulture Research. pages 9.
Duan, H., Maren, N.A., Ranney, T.G., Liu, W. 2022. New opportunities for using WUS-BBM and GRF-GIF genes to enhance genetic transformation of ornamental plants. Ornamental Plant Research. 2:4.
