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The U.S. Department of Agriculture (USDA) has performed European pear (Pyrus communis L.) scion breeding for over a century. The breeding program started in the early 1900s by Merton B. Waite in the Washington D.C. area and the program's main goal was to develop host resistance to the devastating disease fire blight, caused by Erwinia amylovora. Most of the historic European pear cultivars being produced in the U.S. were susceptible to fire blight, prompting a need to breed for resistance. More than six generations of USDA breeders have continued this effort to breed disease-resistant European pears. Along with fire blight resistance, the pear breeding programs sought improved fruit quality, cold hardiness, and resistance to psylla (Cacopsylla pyricola Foërster), a significant insect pest of pear. Herein, we discuss the history of the program through each generation of breeder(s). We also present breeding aims, parental selection, and releases. In total, the program has released ten named pear varieties between 1938 and 2022. [ABSTRACT FROM AUTHOR]

Plant genetic resources (PGR) underpin the security of global agriculture. Rapid global climate change presents formidable challenges for ex situ and in situ PGR management programs that operate over extended timeframes. The U.S. Department of Agriculture National Plant Germplasm System (NPGS) maintains more than 605,000 PGR accessions representing over 16,300 plant species in 20 genebank locations. These PGR are maintained in cold storage as seeds and vegetative tissues that must be periodically regenerated; or as actively growing plants in fields, greenhouses, screenhouses, and in vitro; or in some cases within in situ reserves. The complicated relationships between crops and their growth environments present unique challenges regarding PGR maintenance under a changing climate. Here we present potential effects of climate change on ex situ PGR maintenance and reproductive success, pollinators and beneficial insects, pathogens and pests, infrastructure considerations, and wild populations within NPGS in situ reserves. We provide a novel tool that summarizes past US temperature and precipitation data alongside future climate projections to guide ongoing planning for the effects of climate change for NPGS genebanks throughout the United States. A series of case studies exemplify instances where climate change has already impacted NPGS PGR management. Ongoing improvements to NPGS PGR management in response to climate change require continued observations of the current effects, careful and innovative planning, and creative approaches to ensure that PGR are successfully conserved for future generations. Core Ideas: Plant genetic resources (PGR) are crucial for continued crop improvement that underpins global food security.Climate change can threaten PGR safeguarded by the USDA‐ARS National Plant Germplasm System (NPGS).Approaches developed by the NPGS to adapt its PGR management operations to climate change are reviewed.The new NPGS Climate Futures Application delivers key estimates for future climatic conditions at NPGS locations.The NPGS's PGR management operations can adjust to climate change using current experience and those estimates. [ABSTRACT FROM AUTHOR]

Cryopreservation has emerged as a low-maintenance, cost-effective solution for the long-term preservation of vegetatively propagated crops. Shoot tip cryopreservation often makes use of vitrification methods that employ highly concentrated mixtures of cryoprotecting agents; however, little is understood as to how these cryoprotecting agents protect cells and tissues from freezing. In this study, we use coherent anti-Stokes Raman scattering microscopy to directly visualize where dimethyl sulfoxide (DMSO) localizes within Mentha × piperita shoot tips. We find that DMSO fully penetrates the shoot tip tissue within 10 min of exposure. Variations in signal intensities across images suggest that DMSO may interact with cellular components, leading to its accumulation in specific regions. [ABSTRACT FROM AUTHOR]

Citrus is among the most economically important fruit crops. Its vast species diversity and global production was observed by N.I. Vavilov during his international plant explorations from the early to mid-1900s. Currently, ex situ citrus collections located around the world conserve and protect citrus genetic resources, as revealed in a survey conducted in 2021. Responses were received from 43 collections in 27 countries, of which 35 provided data regarding collection composition, management practices, and security, as well as other information. The six largest citrus collections have between 1000 and 1735 accessions. The largest accession holdings are mandarins and sweet oranges, although all citrus fruit types are maintained: mandarin, sweet orange, lemon, pummelo, grapefruit, hybrids, lime, sour orange, citron, kumquat, papeda, finger lime, and crop wild relatives. Diseases pose significant threats to collections, though some collections are maintained in a clean-plant state as a result of intensive sanitation efforts. National and regional quarantine regulations often limit the export and import of citrus plants or propagative materials, thus limiting the availability of materials at an international level. Resources, both financial and human, are necessary to ensure the long-term safety and security of citrus collections on a global scale. Future efforts to develop citrus genebanking communities will provide opportunities for improved conservation, as well as collaborations and training. [ABSTRACT FROM AUTHOR]

Societal Impact Statement: Historic North American apple (Malus domestica) orchards that thrived in the late 19th and early 20th centuries, with cultivar compositions unlike today's orchards, are vanishing. There are several reasons for this loss: tree aging, cost of tree maintenance, and urbanization. Many groups have collected local knowledge regarding the history and horticulture of apples using both phenotypic and genotypic identification methods. Some of these groups have joined with scientists to form the collaborative "Historic Fruit Tree Working Group of North America" to facilitate the conservation of heirloom apple cultivars in North America through documentation, identification, collaboration, and education. [ABSTRACT FROM AUTHOR]

The USDA-ARS National Plant Germplasm System (NPGS) apple collection in Geneva, NY, USA maintains accessions of the primary Malus domestica (Suckow) Borkh. progenitor species M. sieversii (Ledeb.) M. Roem., M. orientalis Uglitzk., and M. sylvestris (L.) Mill. Many of these accessions originated from seeds that were collected from wild populations in the species' centers of diversity. Some of these accessions have fruit phenotypes that suggest recent M. domestica hybridization, which if true would represent crop contamination of wild species populations and mislabeled species status of NPGS accessions. Pedigree connections and admixture between M. domestica and its progenitor species can be readily identified with apple SNP array data, despite such arrays not being designed for these purposes. To investigate species purity, most (463 accessions) of the NPGS accessions labeled as these three progenitor species were genotyped using the 20K apple SNP array. DNA profiles obtained were compared with a dataset of more than 5000 unique M. domestica apple cultivars. Only 212 accessions (151 M. sieversii, 26 M. orientalis, and 35 M. sylvestris) were identified as "pure" species representatives because their DNA profiles did not exhibit genotypic signatures of recent hybridization with M. domestica. Twenty-one accessions (17 M. sieversii, 1 M. orientalis, and 3 M. sylvestris) previously labeled as wild species were instead fully M. domestica. Previously unrealized hybridization and admixture between wild species and M. domestica was identified in 230 accessions (215 M. sieversii, 9 M. orientalis, and 6 M. sylvestris). Among these species-mislabeled accessions, 'Alexander', 'Gold Reinette', 'Charlamoff', 'Rosmarina Bianca', and 'King of the Pippins' were the most frequently detected M. domestica parents or grandparents. These results have implications for collection management, including germplasm distribution, and might affect conclusions of previous research focused on these three progenitor species in the NPGS apple collection. Specifically, accessions received from the NPGS for breeding and genomics, genetics, and evolutionary biology research might not be truly representative of their previously assigned species. [ABSTRACT FROM AUTHOR]

Plant cryopreservation technologies are used within gene banks for the long‐term preservation of vegetatively propagated collections. Surface‐sterilized plant tissues grown in the field, greenhouse/screenhouse, growth chamber, or in vitro are the source of shoot tips subjected to vitrification‐based cryopreservation methods. Here, we describe the methods used to minimize microbial contamination during the tissue culture initiation process. We also discuss the occurrence and possible elimination of endophytes after extended in vitro culture and during recovery after liquid nitrogen exposure. We describe two case studies in which bacterial endophytes were observed in Citrus gene bank accessions during recovery after cryopreservation. These were identified using the MinION Oxford Nanopore system and Kirby–Bauer disc diffusion assays to examine the bacterial responses to antibiotic exposure. The methods used in this case study could be applied to identify endophytes to better target antimicrobial treatments of plant tissue collections. [ABSTRACT FROM AUTHOR]

Diseases caused by plant pathogens such as viruses, viroids and phytoplasmas cause huge economic losses of agricultural production and limit the safe movement of plant materials across borders. The use of pathogen‐free planting materials provides a strategy for efficient management of these diseases and facilitates the global exchange of genetic resources. Shoot tip cryotherapy is a novel biotechnology method that uses cryogenic procedures to eradicate plant pathogens from the diseased plants. Combining thermotherapy or chemotherapy with shoot tip cryotherapy has further enhanced pathogen eradication efficiency. This review provides updated and comprehensive information on shoot tip cryotherapy and the combination of thermotherapy or chemotherapy with shoot tip cryotherapy for pathogen eradication. Prospects are proposed for future studies. [ABSTRACT FROM AUTHOR]

Gene banks have a critical role in preserving genetic diversity for many conservation uses. A key challenge in this task is maintaining both the genetic integrity and the viability of a collection during regeneration and long-term storage. In this study we examine the interacting effects of sampling and viability decline from seed aging on genetic diversity. To do this, we developed a model that simulates sampling during regeneration of individual genotypes using two commonly used but contrasting regeneration protocols. Critically, the model simulates a realistic nonlinear decline in viability across time, and the combined effects of sampling and mortality were evaluated for their effect on allelic diversity. In addition, the model was used to contrast the kinetics of allelic richness among rounds of regeneration in accessions with different heterozygosity levels (e.g., cultivars vs. wild) and different breeding systems (outcrossing vs. inbred). Results underscore a significant interaction between regeneration timing and regeneration size. Regeneration procedures should consider viability kinetics and intra-accession diversity to best manage genetic erosion. These changes highlight the trade-off faced by curators between the logistics of increasing regeneration sampling size and improving the technical procedures for seed storage and monitoring. doi: 10.2135/cropsci2010.04.0236

Methods for collecting genetic diversity from in situ populations are important tools for plant conservation. Many quantitative collection strategies for sampling populations without a priori information regarding the ecology, reproductive biology, or population genetic structure of the taxa have been proposed, but their different assumptions regarding the collection scale and the basis for diversity often make them difficult to compare. Understanding the limitations of the different strategies enables collectors to make more informed choices when implementing conservation and restoration projects or collecting for germplasm improvement. We compare two genetically based strategies under a common set of assumptions and extend the probabilistic arguments of the strategies to accommodate rare alleles, multiple loci, and multiple populations. The recommendations of many models are based on a single locus, but larger numbers of individuals must be collected to assure with a high probability (>0.95) the acquisition of alleles at multiple independent loci within a population. Sampling from multiple populations linked by gene flow may offset this increase. Additionally, the likelihood of capturing rare alleles remains high when sampling for multiple loci or across multiple populations. Comparison of the models provides germplasm collectors with a basis to evaluate risks of over- and undersampling to capture genetic diversity within a species.