Seed bank
A seed bank is a specialized repository that stores seeds from diverse plant species under controlled low-temperature and low-humidity conditions to preserve genetic material for long-term conservation, primarily as a form of ex situ genebanking.[1][2] These facilities safeguard crop varieties and wild relatives against extinction risks from habitat loss, pests, diseases, and climate variability, enabling future breeding for resilient agriculture and restoration efforts.[3][4] By maintaining viable seed samples—often in duplicate from national and international genebanks—seed banks underpin global food security and biodiversity, with collections supporting research into traits like drought tolerance and disease resistance.[5][6] Prominent examples include the Svalbard Global Seed Vault in Norway, which serves as a secure backup for over one million seed samples worldwide, ensuring redundancy against localized disasters.[7][8] While effective for orthodox seeds that tolerate desiccation, challenges persist for recalcitrant species requiring alternative preservation methods, highlighting ongoing advancements in cryopreservation and tissue culture.[9]Definition and Purpose
Core Objectives
Seed banks primarily aim to conserve plant genetic resources ex situ by storing orthodox seeds—those tolerant to desiccation and low temperatures—under conditions that preserve viability for decades or centuries, thereby countering genetic erosion from habitat loss, overexploitation, and agricultural intensification. This preservation targets crop species, landraces, and wild relatives, which collectively harbor traits essential for adapting agriculture to emerging threats like pests, pathogens, and climatic shifts. For instance, the global network of genebanks holds over 7.4 million accessions as of recent inventories, ensuring a reservoir against total loss of varieties that could otherwise lead to famine or dependency on narrow genetic bases.[10][11] A key objective is to supply breeders and researchers with diverse germplasm for developing resilient cultivars, incorporating genes for drought tolerance, disease resistance, and nutritional enhancement, which underpins food security for a projected global population exceeding 9 billion by 2050. Institutions like the CGIAR genebanks facilitate this by distributing samples that have contributed to varieties yielding billions in economic value, such as rust-resistant wheat strains deployed in Africa and Asia since the 2010s.[12][13][14] Seed banks also pursue biodiversity safeguarding by maintaining underrepresented or endangered taxa, reducing extinction risks through periodic viability testing and regeneration, and supporting in situ efforts via data on adaptive traits. This dual conservation approach aligns with international frameworks like the International Treaty on Plant Genetic Resources for Food and Agriculture, emphasizing sustainable use over mere archival storage.[15][16]Scope of Preservation
Seed banks focus on preserving genetic diversity within crop species and their wild relatives, prioritizing materials that support agricultural resilience, breeding programs, and food security. Collections typically include landraces, obsolete cultivars, breeding lines, and wild progenitors, which harbor traits such as pest resistance, drought tolerance, and nutritional enhancements not found in modern elite varieties. This targeted scope reflects the causal link between genetic variation and crop improvement, as historical erosion of diversity—driven by intensive farming and selection for uniformity—has heightened vulnerability to biotic and abiotic stresses.[17][18] The preservation effort centers on orthodox seeds, which can be dried to low moisture levels (typically 3-7%) and stored at sub-zero temperatures (-18°C or lower) for decades or centuries without viability loss, enabling efficient ex situ conservation. Recalcitrant seeds, which desiccate poorly and require higher moisture and warmer conditions, are largely incompatible with standard seed bank protocols and thus underrepresented, limiting coverage to species like temperate cereals, legumes, and vegetables rather than tropical trees or wetland plants. Intermediate seeds, with partial desiccation tolerance, may receive specialized handling but remain a smaller fraction.[19] Major collections underscore this agricultural emphasis: CGIAR genebanks manage approximately 768,576 accessions across over 3,000 plant species, predominantly staple crops like wheat, rice, maize, and their wild relatives. The Svalbard Global Seed Vault, as a global backup repository, holds more than 1.3 million accessions representing over 5,000 species, with the largest shares in grains such as rice and wheat varieties. While some banks extend to forage, medicinal, and ornamental plants, the core mandate excludes most non-agronomically relevant biodiversity, as in situ habitats or other methods better suit non-seed-propagated or ecologically specialized taxa. Worldwide, over 1,750 seed banks exist, but their combined holdings—estimated in the millions—prioritize food-producing lineages over comprehensive floral representation.[20][21][22]Historical Development
Precursors and Early Initiatives
Nikolai Vavilov, a Soviet botanist and geneticist, initiated one of the earliest systematic efforts in seed preservation during the 1910s and 1920s, driven by the recognition that crop genetic diversity was essential for developing famine-resistant varieties. Beginning expeditions in 1916, Vavilov traveled to over 60 countries, collecting more than 250,000 seed accessions from wild and cultivated plants, which formed the basis of the world's largest genebank at the time.[23] His work emphasized centers of origin for crops, where genetic variation was highest, to enable breeding programs that could address agricultural vulnerabilities exposed by events like the 1891 Russian famine.[24] In 1921, Vavilov established the Institute of Plant Industry (later renamed the N.I. Vavilov Institute of Plant Genetic Resources) in Leningrad, which housed his growing collection in controlled storage to maintain seed viability for regeneration and research. By 1933, the institute held at least 148,000 viable seeds and tubers, stored under rudimentary but deliberate conditions to preserve germplasm for future use in Soviet agriculture.[25] These efforts predated formalized international seed banking protocols and focused on ex situ conservation to safeguard against loss from monoculture expansion and environmental pressures, though Vavilov's Lysenkoist opposition led to his 1940 arrest and death in prison in 1943.[24] During the 1941–1944 Siege of Leningrad, institute staff heroically protected the collection from looting and consumption, with at least nine scientists starving to death while guarding the seeds, ensuring over 80% of the stored materials survived intact.[26] This demonstrated the causal importance of dedicated preservation infrastructure, as the intact seeds later supported post-war crop recovery and global breeding efforts. Earlier ad hoc preservation, such as 19th-century U.S. government seed distribution programs starting in 1839, lacked systematic long-term viability testing and focused more on dissemination than archival storage.[27] These initiatives laid foundational principles for modern seed banks, including geographic prioritization of collections and the imperative of duplicate storage, though they operated without the cryogenic or standardized viability protocols developed later. Vavilov's repository influenced subsequent institutions by proving that large-scale seed archiving could mitigate biodiversity erosion from industrialization and political instability.[28]Establishment of Modern Institutions
The establishment of dedicated modern seed banks accelerated in the mid-20th century, driven by concerns over genetic erosion from selective breeding and agricultural modernization. These institutions shifted from ad hoc collections to systematic, long-term storage facilities emphasizing viability testing and duplication for security.[28] A pivotal early example was the United States Department of Agriculture's National Seed Storage Laboratory, constructed in 1958 in Fort Collins, Colorado, to centralize and protect germplasm from plant introduction stations, collectors, and public breeders amid Cold War-era priorities for food security.[29] Dedicated on December 5, 1958, it pioneered controlled low-temperature storage protocols for orthodox seeds, influencing global standards.[30] In parallel, international agricultural research networks formalized genebanks during the 1960s and 1970s as ex situ conservation complements to in situ efforts. The Consultative Group on International Agricultural Research (CGIAR), established in 1971, integrated seed preservation into centers like the International Rice Research Institute (founded 1960) and the International Maize and Wheat Improvement Center (1966), conserving thousands of accessions to support breeding for developing regions.[31] These facilities emphasized accessibility under benefit-sharing agreements, amassing over 700,000 accessions across CGIAR genebanks by the late 20th century.[32] Regional initiatives further expanded the model, such as the Nordic Gene Bank's permafrost storage facility in Svalbard, operational from the 1980s, which duplicated collections to mitigate risks like institutional failure observed in earlier efforts.[28] This era's institutions prioritized empirical viability monitoring and causal factors in seed deterioration, laying groundwork for standardized global protocols despite varying national capacities.[17]Classification and Types
Structural Categories
Seed banks are classified structurally into short-term, medium-term, and long-term storage facilities, each designed with specific environmental controls to match the intended preservation duration and seed viability needs.[33] These categories reflect differences in infrastructure, such as temperature regulation systems, humidity controls, and containment structures, which ensure seeds remain dormant without deterioration. Short-term facilities prioritize accessibility for immediate use in breeding or distribution, while medium- and long-term ones emphasize cryogenic or refrigerated vaults to extend longevity for conservation.[34] Short-term storage structures, often integrated into research labs or field stations, maintain seeds under ambient or mildly controlled conditions to support operations lasting 1 to 3 years. Typical parameters include temperatures of 15–20°C and relative humidity (RH) of 40–50%, using simple shelving or bins without advanced cooling. These facilities facilitate rapid seed multiplication and testing, as seen in agricultural extension programs where seeds are held briefly before planting.[35] Such setups minimize costs but limit viability for sensitive species, focusing on orthodox seeds that tolerate moderate drying. Medium-term facilities employ refrigerated rooms or chambers at 5–10°C and 20–30% RH, structured with insulated walls, dehumidifiers, and monitoring sensors to preserve seeds for 5 to 15 years. These are common in national genebanks for working collections used in crop improvement, allowing periodic regeneration without full loss of genetic integrity. For instance, structures may include modular cold rooms with backup power to prevent fluctuations that could trigger premature germination.[36] This category balances accessibility and durability, suitable for intermediate seed behaviors where ultra-low temperatures are unnecessary.[34] Long-term storage infrastructures feature deep-freeze vaults at -10°C to -20°C (or lower) and moisture content below 5%, often built as secure bunkers with multilayered sealing, permafrost utilization, or liquid nitrogen systems for indefinite viability spanning decades to centuries. Exemplified by facilities like the Svalbard Global Seed Vault, these emphasize redundancy, such as duplicate power supplies and seismic-resistant designs, to safeguard base collections against global threats.[37] Structural robustness here prioritizes minimal human intervention, with seeds stored in airtight foil packets to combat oxidation and pests.[38]| Category | Temperature Range | Relative Humidity | Expected Viability | Primary Use Case |
|---|---|---|---|---|
| Short-term | 15–20°C | 40–50% | 1–3 years | Breeding, distribution, testing |
| Medium-term | 5–10°C | 20–30% | 5–15 years | Crop improvement, working collections |
| Long-term | -10°C to -20°C | <5% (seed MC) | Decades+ | Genetic conservation, backup |
Seed Viability Considerations
Orthodox seeds, which tolerate desiccation and low temperatures, form the basis for long-term storage in most seed banks, with viability extended by reducing moisture content to 5-10% and storing at subfreezing temperatures such as -18°C.[39] Under these conditions, deterioration rates slow due to minimized metabolic activity and oxidative damage, allowing half-lives (P50, the time for viability to decline by 50%) to exceed ambient storage estimates of 5-10 years for many species.[40] [41] Initial seed quality, including vigor at acquisition, critically influences longevity, as lower starting viability accelerates aging regardless of storage parameters.[42] Recalcitrant seeds, lacking desiccation tolerance, maintain high moisture levels (often >20%) and succumb rapidly to drying or freezing, with viability typically limited to weeks or months even in moist, cool environments.[43] This sensitivity arises from ongoing metabolic processes that promote deterioration, rendering conventional seed banking infeasible and necessitating alternatives like short-term moist storage or cryopreservation of excised embryonic tissues.[44] Intermediate seeds bridge these categories, enduring moderate drying but aging faster than orthodox types, often with viabilities of around 5 years at -20°C.[45] Storage environment factors—temperature, relative humidity, and equilibrium moisture content—interact predictably with seed physiology, as quantified by viability equations that model germination decline over time based on these variables.[46] Genetic determinants, such as repair mechanisms for DNA damage, further modulate species-specific longevity, independent of environmental controls.[47] In practice, genebanks determine storage behavior pre-deposition via desiccation and viability tests to classify seeds and select protocols, followed by periodic germination assessments to monitor decline and trigger regeneration.[42] Failure to account for these considerations risks irrecoverable genetic loss, underscoring the need for empirical validation over assumptions of uniform durability.Operational Mechanisms
Storage Protocols
Storage protocols for seeds in genebanks are tailored to seed behavior categories—orthodox, intermediate, and recalcitrant—to maximize viability longevity. Orthodox seeds, comprising the majority of collections, can withstand desiccation to low moisture contents and subfreezing temperatures, enabling extended storage periods. Protocols begin with rapid drying post-harvest, typically within 3–5 days of collection, at 5–20°C and 10–25% relative humidity (RH) to reach equilibrium moisture contents of 5–14% (wet basis). Seeds are then packaged in hermetically sealed, moisture-proof containers such as aluminum foil-laminated packets or laminated plastic bags with a metal foil barrier, minimizing gas and moisture exchange. For long-term base collections, storage maintains -18 ± 3°C and 15 ± 3% RH in the storage atmosphere, though sealed packaging stabilizes internal conditions based on initial moisture. These conditions support viability for decades to centuries, species-dependent, with examples like slash pine retaining 66% germination after 50 years at 4°C, though subfreezing is preferred for optimal longevity.[35]| Seed Type | Moisture Content | Temperature | Expected Longevity |
|---|---|---|---|
| Orthodox | 5–10% | -18 to -20°C | Decades to centuries |
| Intermediate | 12–15% | > -20°C (e.g., 0–5°C for short-term) | Few years |
| Recalcitrant | 25–50% (moist) | 12–20°C (tropical) or -3 to +4°C (temperate) | Months to 3–5 years |