Location: Genetic Improvement for Fruits & Vegetables Laboratory
2024 Annual Report
Objectives
Objective 1: Breed improved strawberry plants that perform well for commercial growers, with emphasis on high yield, tolerance to abiotic stress, resistance to diseases, and fruit with excellent fruit quality and long shelf life.
Objective 2. Characterize important strawberry traits, their inheritance and genetic control, developmental and biochemical pathways, and metabolic processes, including the regulation of plant architecture, and fruit and flower development.
Approach
Standard plant breeding methods will be used to generate superior strawberry cultivars for traditional production practices and fruiting for the traditional short spring season. Novel evaluation practices for fruit quality and flavor will be developed and incorporated into the annual breeding cycle. A seedling screen for resistance to anthracnose crown rot, an emerging disease of worldwide importance, will be used to identify resistant strawberry plants and increase the breeding population’s average resistance to the disease. New cultivars resulting from selection based on increased disease resistance, fruit quality, yield, and shelf life will be released. To help satisfy demand for year-round availability, similar methods will be used to generate improved strawberry plants that fruit for an extended season from April through December. Because the longer-fruiting plants will face weather and pest challenges that are not problems during the traditional fruiting season, new comparison methods will be developed to facilitate identification of plants that produce fruit within the traditional season, and produce equally well outside the traditional strawberry season. Physiological, genomic, and transcriptomic analyses of novel mutant diploid strawberry lines will lead to identifying genes involved in growth and development required to meet predicted challenges, such as the trend toward completely contained growing environments for urban and off-season production. These insights into basic strawberry biology also will improve the tools breeders have to develop the necessary new cultivars. The results will help strawberry nurseries provide high-quality cultivars to growers, helping them produce strawberries more sustainably to satisfy increasing consumer demand for fresh, flavorful fruit year-round.
Progress Report
Progress was made on both Objectives, which fall under NP301 Component 1 (Crop Genetic Improvement), Problem Statements 1A (Trait discovery, analysis, and superior breeding methods) and 1B (Develop new crops, cultivars, and germplasm with improved traits).
Progress under Objective 1 focuses mainly on Problem Statement 1B (Develop new crops, cultivars, and germplasm with improved traits). In May 2024, 68 once-fruiting selections were made, and 55 repeat-fruiting selections were made in July 2024. We were disappointed that only one 2023 once-fruiting selection evaluated in observation plots and no repeat-fruiting selections were selected for virus testing and replicated evaluation. The reason many of the once-fruiting and repeat-fruiting selections died in the greenhouse in 2021-2023 was due to anthracnose crown rot caused by Colletotrichum gloeosporioides, the emerging fungal disease first identified in the once-fruiting seedling field Fall 2020. The disease has hurt our ability to advance seedling selections to observation plots in 2022 and 2023. To address this problem, we thoroughly cleaned the screenhouse section we use for propagation and allowed it to heat for ten days before reintroducing plants; those grew without symptoms. We also are using a fungicide this year on newly dug seedling selections being propagated in the greenhouse to improve survival until the daughter plants are planted in the field where no fungicide will be used.
Advancement from the observation-plot stage to the replicated-plot stage has always been slowed by virus elimination but has been particularly problematic due to the unavailability of quality distilled water in the lab, unknown to us because of poor communication during maximum teleworking and beyond. That distilled water problem was corrected Fall 2023, and three selections tested virus-negative this year so far. One of these is B3219, the first numbered breeding selection to progress to this stage from a collaboration between ARS and the University of Florida. The goal of the agreement is to combine resistance to C. gloeosporioides from UFL and flavor from ARS-Beltsville.
In May 2024, seven numbered once-fruiting selections were evaluated in replicated plots for over 100 traits contributing to marketable yield, fruit quality, disease resistance, and postharvest shelf life. Two of these selections were evaluated for the first time in replicate; both had enormous fruit with good appearance, flavor, and postharvest quality traits and will be evaluated again next year. Replicated evaluations of six repeat-fruiting selections will be made in July 2024. In July 2023, seven selections were evaluated in replicate, and one was deselected for insufficient foliage and vigor. The low-foliage selection produced a good yield of quality fruit and will be saved with at least two others with similar architecture for use as parents and possible future evaluation in a controlled-environment-agriculture growing environment. July 2023 repeat-fruiting selections were evaluated in open beds as well as under low-tunnels to determine if sufficient breeding progress had been made so that low tunnels would no longer be needed and associated costs could be reduced. All selections and the reference cultivar performed better under low tunnels. The 2024 replicated evaluations are being conducted entirely under low tunnels.
The plant patent application for ‘USDA Lumina’ was reviewed, revised, and resubmitted. A plant patent application for ‘Keepsake’ in Canada is under review and requires field evaluation by the examiner. The Canadian nursery, Production Lareault, is cooperating by making its fields available to the Canadian examiner this summer.
Progress under Objective 2 focuses mainly on Problem Statement 1A. The F2 progeny from a cross combining 3 genes involved in runnering were genotyped for the DELLA gene and the gibberellin oxidase gene. The results show that the RUN1 gene acts epistatically on the DELLA and gibberellin oxidase genes. Bulk sequence analysis and QTLseq analysis showed that the gene which is mutated to result in non-runnering in the run1 mutant is a TCP transcription factor (TCP9) that is also implicated in branching in dicots and monocots. This transcription factor was named BRANCH1 in Arabidopsis.
A molecular marker was developed for genotyping Fragaria vesca TCP9 in order to substantiate the epistatic effect of this gene on the DELLA gene and the gibberellin oxidase gene.
Two EMS-induced mutants of F. vesca were characterized genetically, phenotypically, and physiologically. One mutant, fruitless 1, resulting from mutation of a single gene, does not develop a critical cell layer in the developing anther, and thus, cannot produce pollen. The other mutant, short inflorescence 1, also results from mutation of a single gene. This mutation has a pleiotropic effect in that both the inflorescence and the runner length are short. Microscopic measurements showed that cell length is affected in this mutant. Mutant inflorescences and runners elongate in response to application of gibberellin.
Accomplishments
1. Discovery of the complicated genetic control for important traits can guide genetic research and breeding methods for strawberry. Breeding for key traits in numerous crops has been accelerated through indirect selection of those traits using DNA genetic markers. The practicality of this breeding strategy depends on simple genetic inheritance of those traits. ARS researchers in Beltsville, Maryland, developed the woodland strawberry, Fragaria vesca, as a laboratory reference model for the more genetically complex commercial strawberry F. xananassa. Using mutants of F. vesca, they found that inheritance of two horticultural traits, elongation of flower stems and elongation of stolons, is nonetheless quite complicated. Those traits are determined by a single gene that controls two other genes, a phenomenon termed epistasis. The important discovery in F. vesca compliments earlier ARS research at Beltsville showing that a key trait, repeat fruiting, in the commercial strawberry also is epistatically controlled. These discoveries made in both the genetically simpler and more complex strawberry species suggest that epistasis may be involved in several other important traits, and that breeders should allow for the possibility as they plan cross-pollinations and determine appropriate family size. It may be helpful to record family segregation ratios in the first year of breeding for any new trait to determine if epistasis may be involved. These discoveries also suggest that indirect selection by DNA genetic markers might be inefficient and impractical for breeding at least some key traits in strawberry. Consequently, breeders and geneticists might need to develop alternative approaches for applying DNA-based tools to strawberry breeding and assess the relative value of devising more effective direct selection methods for horticultural traits, perhaps incorporating new high-throughput image analyses, spectral analyses and related methods.
Review Publications
Turechek, W., Myhrene, O., Slovin, J.P., Peres, N. 2020. The use of aerated steam as a heat treatment for managing angular leaf spot in strawberry nursery production and its effect on plant yield. PhytoFrontiers. 1:104-119. https://doi.org/10.1094/PHYTOFR-08-20-0012-R.
Islam, N., Krishnan, H.B., Slovin, J.P., Natarajan, S.S. 2023. Metabolic profiling of a fast neutron soybean mutant reveals increased abundance of isoflavones. Journal of Agricultural and Food Chemistry. 71(26):9994-10003. https://doi.org/10.1021/acs.jafc.3c01493.
Slovin, J.P., Dougherty, L.E. 2023. Abnormal pollen development in the fragaria vesca mutant fruitless 1. HortScience. 58(12):1488-1491. https://doi.org/10.21273/HORTSCI16961-22.
Islam, N., Krishnan, H.B., Slovin, J.P., Natarajan, S.S. 2023. Metabolite profiling of a fast neutron soybean mutant reveals an increased abundance of isoflavones. Food Chemistry. 71(26):9994-10003. https://doi.org/10.1021/acs.jafc.3c01493.
Slovin, J.P., Booker, J.C. 2024. The short inflorescence mutation in diploid strawberry fragaria vesca affects inflorescence architecture and runner elongation. HortScience. 59(4):465-470. https://doi.org/10.21273/HORTSCI17652-23.
