Plant genebanks, such as the USDA-ARS National Plant Germplasm System (NPGS), conserve vast collections of crops. They are saved for use in breeding and research programs. These collections are primarily conserved as seeds. The genetic diversity of wild species (crop wild relatives) of some fruit crops, such as apples, can be stored in gene banks as seeds. This ensures that researchers can turn to the wild varieties to help with breeding programs.
But, many varieties of crops are best saved in other ways besides seeds. Collections of fruit tree varieties are maintained as field or greenhouse collections in gene banks. These fruit variety collections are conserved as actively growing plants. Seeds of fruit trees are the result of pollinations from another apple tree’s pollen. Thus, the seeds of the tree are not identical to that of the mother tree and will likely not produce desirable fruit. To maintain the genetic identity of a fruit tree variety, fruit trees are propagated from maternal tissues, such as by grafting or by rooted cuttings. This preserves the genetic heritage we value.
Often, seed crops can be successfully placed into long-term reduced-temperature storage for safe-keeping. The gene bank collections of fruit trees that are maintained entirely in fields or greenhouses are vulnerable to adverse environments, pests, and diseases.
Researchers developed methods to store non-seed tissues, such as dormant buds or growing shoot tips, of fruit varieties. These are harvested and processed so that they can be placed into liquid nitrogen, which is around -330°F! This is called cryopreservation, and it is successful for long-term storage.
When needed, the dormant buds or shoot tips are removed from liquid nitrogen and allowed to warm. They can then be grown into plants that are identical to the original cultivars.
Cryopreservation isn’t as simple as nipping a bud or shoot and freezing it. Successful cryopreservation requires that water be removed from the living cells to prevent ice crystals from forming in the frozen state.
In some cases, such as for dormant apple buds, the plant tissues can be dehydrated, packaged, and then slow-cooled to about -22oF prior to liquid nitrogen exposure. For recovery, the dormant buds are warmed, rehydrated, and grafted onto rootstocks.
For other less cold-hardy fruit crops, such as citrus, researchers use shoot tips which are the growing parts of branches, instead of dormant buds. These small (1 mm) shoot tips are then treated with solutions that protect cells and remove water. Then, they are exposed to liquid nitrogen. For recovery, the shoot tips are warmed, rinsed, rehydrated, and grown using sterile tissue culture techniques.
These methods are actively being used at the National Laboratory for Genetic Resources Preservation in Fort Collins, Colorado to cryopreserve priceless field and greenhouse collections of fruit cultivars. These cryopreserved collections can be safely preserved for the long-term, and then recovered and grown into trees that are identical to the original varieties when they are needed.
Answered by Gayle Volk, US Department of Agriculture
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