Within the vaults of the Millennium Seed Bank is the Seed Collection, which represents the greatest concentration of living seed-plant diversity on earth. The bank is a global resource for conservation and sustainable use of plants.
With around one in five plant species estimated to be threatened with extinction worldwide, seed banks are a cost-effective means of long-term ex-situ plant conservation. The collections of seeds are dried and frozen, providing a safety net against species’ extinction in the wild. The seed collections are also accessible resources for research, and for the creation of sustainable solutions to the great problems facing the world – food security, disease, climate change and biodiversity loss.
The Millennium Seed Bank at Wakehurst, with its invaluable collections, is the world leader in this kind of ex situ conservation of wild plant species, with priority given to collections of species that are most threatened and potentially most useful. The collections are curated to a high standard, to achieve and maintain high collection quality (data, viability, diversity, etc.), which is essential for maximising the usability of the collections for research, re-introduction and restoration.
The seed collections in the Millennium Seed Bank constitute the largest and most diverse wild plant species genetic resource in the world. The great majority of this collection has been collected by the associated global network, the Millennium Seed Bank Partnership (MSBP), which is active in over 80 countries and is the largest ex situ plant conservation programme in the world.
Kew was an early leader in seed banking: first developed for crops, then applied to wild species. In principle, seed banking is straightforward and relies on the seeds of most (about 90%) seed-bearing plant species surviving air-drying and then freezing, which extends the longevity of these so-called ‘orthodox’ seeds in predictable ways.
The purpose-built facility at Wakehurst is based around a vast vault for the long-term storage of seeds for research and conservation. Following collection in-country, seeds are prepared and dried (to around 4–6% moisture content, fresh-weight basis), before storage in deep-freeze chambers (-18 to -20°C) within the vault; following international standards.
At present there are more than 87,500 seed collections in the bank; representing over 39,100 species, from almost 5,800 genera and more than 360 families. That is, at least one collection each of around 12.5% of those seed-bearing species estimated to have orthodox, bankable seeds.
The challenging, high level target across the partnership to conserve 25% of bankable species by 2020 includes important priorities for wild species seed banking, now and in the future. At the same time as continuing to collect high quality seed to achieve the 25% target, there will also be increased focus on collection quality and genetic diversity of collections – breadth and depth. An average of 1–2 collections per species is not sufficient; and there is increasing focus on sub-specific taxa and appropriate eco-geographic and genetic representation, through multi-provenance species collections. For example, the almost complete coverage of the species in the UK native flora is now being supplemented by targeted, intraspecific collections.
With 60,000 to 100,000 species of plant threatened with extinction, seed collection priority is on those species most at risk – the endangered and endemic; together with those likely to be most useful in the future, through enabling human adaptation and innovation. Climate change also presents significant threats to plant biodiversity; and, as well as from dryland areas of the world, collections are being sought from ecosystems most at risk from climate change – montane, maritime and island.
Alongside these priorities are two further important themes: forestry and trees; and crop wild relatives. The MSBP is collecting seeds of 3,000 tree species, among them the world’s rarest, most threatened and most useful tree species. In the UK, the UK National Tree Seed Project is establishing a national tree seed collection to facilitate long term research into native trees and their conservation and management in the UK landscape. Collections are not only from populations across the country, but also from individual mother trees within those populations, to optimise future access to genetic diversity.
Crop wild relative species potentially hold important traits for the development of resilience; and yet they are currently under-represented in seed bank collections around the world. Kew and Global Crop Diversity Trust (‘Crop Trust’) are working on a project that is bringing collections into the bank that cover the wild relative gene pools of the world’s 29 major crops.
The collections are curated to international gene-bank standards. However, when these are not applicable to wild species collections, Kew relies on its considerable experience to develop and set global standards. These are applied across the MSB Partnership to ensure that collections are of optimum quality. All elements of the seed collection curation process are aimed at ensuring the highest quality collections; from data and identification, through the longevity and viability of each collection, to understanding how to break dormancy, so as to ensure maximum germination and so avoid selection at that stage in the life cycle.
As well as resources for research, the collections also raise research questions themselves. So-called recalcitrant species have seeds that cannot be dried and therefore cannot be banked. While current data suggests that overall about one in ten species has such seeds, the proportion rises to half, and possibly more among tree and shrub species in tropical moist forests .Research is underway at the MSB to develop novel methods for such seeds, and plans for a large-scale cryogenic storage facility, the Kew Cryosphere, are underway. Furthermore, we now know there is considerable variation among orthodox species in their expected longevities in the bank. Again, backup of sub-samples in cryo-storage is routinely used to extend storage lives of short-lived species. Indeed, it is becoming clear that there is a spectrum of seed storage behaviours, and longevities, even within species, dependent on both phylogeny and environment (maternal and post-harvest).
Finally, and above all, the collections are there to be used for many purposes, including: for all kinds of research, plant breeding, species re-introduction and vegetation restoration. Seed collection quality has a clear influence on collection usability; but, importantly MSBP partners’ seeds are duplicated at the MSB under the terms of the CBD and Nagoya Protocol, so they have final say over seed distribution. However; wherever possible and when seed numbers are sufficient, samples are distributed via the online Kew Seed List.