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ARS Home » Northeast Area » Washington, D.C. » National Arboretum » Floral and Nursery Plants Research » Research » Research Project #434613

Research Project: Genetic Improvement of Nursery Crops through Functional Genomics and Breeding

Location: Floral and Nursery Plants Research

2020 Annual Report

Objective 1: Establish long-term experimental hydrangea populations, and develop information management systems and standards for hydrangea phenotyping and genetic mapping to accelerate the development of disease and stress resistant hydrangea cultivars with superior product quality. [NP301, C1, PS1A; NP301, C4, PS4A] Sub-objective 1.A. Establish and phenotype Hydrangea macrophylla experimental populations for identification of disease resistance and ornamental traits. Sub-objective 1.B. Establish, genotype, and phenotype a range-wide Hydrangea quercifolia collection for natural genetic variation for disease resistance and ornamental traits. Sub-objective 1.C. Develop new phenotyping methods for important traits and create associated data management systems for genomic and phenotypic data. Objective 2: Develop, evaluate, and release improved germplasm of hydrangea and other high-value, native, and underrepresented nursery crop species to expand market opportunities for producers, stimulate consumer interest, and mitigate the environmental footprint of nursery production. [NP301, C1, PS1B] Sub-objective 2.A. Accelerate the development of superior Hydrangea germplasm using ploidy manipulations, and wide hybridization. Sub-objective 2.B. Evaluate germplasm including Hydrangea, Osmanthus, and Hamamelis for nursery production and breeding improvement potential.

During the next five years, Hydrangea and Osmanthus selections from previous breeding cycles will be progagated and distributed to cooperators. Germplasm evalutations of new species and cultivars will ensure a broad base for future cultivar development. We will confirm the three major powdery-mildew resistance SNPs in the two most resistant and two most susceptible cultivars using real-time PCR with TaqMan probes. Controlled crosses between the most powdery mildew-resistant and most susceptible cultivar are underway. This F1 mapping population will be genotyped via RT-PCR in the same manner as the mapping parents. This population will be planted in the field for a minimum three-year observation period. During this time, natural powdery mildew incidence and/or induced powdery mildew infection will be recorded. Digital images of each plant will be obtained in regular intervals throughout the growing season. Image analysis will be used to monitor the progression of disease infection and the number and size of new inflorescences. Together, the genotype and phenotype data will be used to refine the genome-wide association analysis, determine the efficacy of marker-assisted selection for powdery mildew resistance at the seedling stage in Hydrangea, and identify plants for further gene expression analysis. For genetic and horticultural analysis of oakleaf hydrangea, we will sample up to 20 populations within each state on a longitudinal/latitudinal cline, as well as representing the full diversity of environments in which the species occurs. Within each of these populations we will sample up to 20 plants. Three to five recently expanded leaves will be collected from selected plants, stored flat in Ziploc bags and stored in coolers with unique identifiers linked with each bag. Plant morphology, location, and other notes will be recorded using the EpiCollect5 web application. Leaves will be shipped overnight via UPS to the USDA-ARS in McMinnville, TN for processing, DNA isolation, and storage. Genotyping-by-sequencing will be performed by the University of Minnesota Genomics core facility. Bioinformatic support will be provided by the ARS NEA Statistics Group. Ten populations per state on a longitudinal/latitudinal cline, as well as representing the full diversity of environments in which the species occurs, will be evaluated for current generation estimates of genetic diversity and levels of inbreeding and for common garden horticultural characterizations in Minnesota, Mississippi, and Tennessee. Each location will evaluate 25-50 individuals of each population (60 populations total) for growth, form, floral, and disease resistance traits. Over 1500 F2 seedlings derived from hybridizations between 'Snow Queen' and the compact cultivars Pee Wee and Sikes Dwarf were evaluated for compact plant habit, quality and quantity of inflorescences, disease resistance and fall foliage color. Seven plants from this population have been selected for compact form and superior floral display. Seed and seedlings from these selected plants will be used for development of disease-screening methods.

Progress Report
This is the second report for Project 8020-21000-072-00D, which began on 4/14/2018. Significant progress was made on both objectives, which fall under NP301. Under Objective 1.A., we phenotyped a large F2 mapping population of Hydrangea macrophylla for powdery mildew resistance through the first year of growth. This well-characterized population will be used for genetic mapping of powdery mildew resistance and reblooming and for determining gene-trait associations. Under Objective 1.B., we completed genotyping-by-sequencing of oakleaf hydrangea leaf samples from a range-wide collection. We established a large field trial of oakleaf hydrangea from across its native range and phenotyped plants through the first year of growth. Under Objective 1.C., we tested several platforms for analysis of digital images of plant leaves. We selected the Plant Doctor application and are currently working to automate the disease identification and scoring process. We completed development of two different protocols for hydrangea SNP genotyping, including a CAPS method for low-to-medium throughput and a KASP method for high-throughput SNP genotyping of inflorescence type. Under Objective 2.A., we completed a study of seed viability among interploid hydrangea crosses, noting that unreduced gametes and not interploid crosses led to triploid hydrangea cultivars. Plants with unique ploidy levels and phenotypes were identified in the offspring. Cultivars showing high or low numbers of unreduced gametes after growth chamber treatments were used in parents of controlled crosses. Recovering triploid progeny from these crosses would show that growth chamber treatments successfully influenced unreduced gamete formation. Under Objective 2.B., progeny from crosses between Dichroa of differing ploidy levels and hydrangeas of differing ploidy levels were evaluated with the goal of identifying hybrids with phenotypes closer to the hydrangea parent. Powdery mildew and leaf spot screening was initiated for these progeny. Under Objective 2.C., the Hamamelis germplasm trial was completed. Collections of Osmanthus and Hydrangea were phenotyped in multi-year field trials. Seedlings representing novel Osmanthus hybrids were evaluated from germination through first year’s growth.

1. Longer shelf-life for cut hydrangeas. In the floral industry, post-harvest is widespread, caused by Botrytis cinereal, pathogens of hydrangea cut flower diseases. Infection with cinereal causes flower and stem rot, which reduces the longevity, color, and texture of cut flowers, making them unsalable. A team of ARS scientists at McMinnville, Tennessee, with their collaborators at the Tennessee State University, and Rutgers University, found a post-harvest dip application of a biofungicide that worked. Applying the biofungicide Botector form the microorganism Aureobasidium pullulans, strain DSM 14940 & DSM 14941, reduced disease severity and slowed the rot of hydrangea cut flowers leading to longer post-harvest shelf life. Now, ornamental growers have a new, sustainable method to protect the quality and value of cut flowers.

Review Publications
Alexander, L.W. 2020. Ploidy level influences pollen tube growth and seed viability in interploidy crosses of Hydrangea macrophylla. Frontiers in Plant Science.