Cryopreservation is the storage of living material in liquid nitrogen at around -196° C. Although the material is still alive, all cellular activity has stopped leaving the tissue ‘frozen in time.’ Seeds, buds, shoots and even pollen can be successfully cryopreserved.
Pros and Cons of cryopreservation
There are a number of benefits of cryopreservation:
- Its relatively low cost compared to other storage methods
- It requires no additional maintenance (compared to material stored via tissue culture or living collections)
- It uses minimal storage space
- It reduces the potential for genetic mutations or contamination by pests and diseases.
However, there are also problems with cryopreservation:
- Requirement for liquid nitrogen handling facilities and appropriately trained staff
- Difficulty in the freezing and thawing process which can damage plant tissue
- Risk of equipment failure or difficultly in obtained regular liquid nitrogen supplies
- Hazards associated with using liquid nitrogen.
Additionally, each plant species and tissue type has very specific requirements for the way in which it is prepared, frozen and thawed. So each species and tissue combination has to be tested individually before cryopreservation can be successfully applied to many plants.
Methods of storing desiccation-sensitive (recalcitrant) species
Many rainforest species are desiccation sensitive and are therefore difficult to store under standard storage conditions (15°C, 15% RH). Cryopreservation can often be used as a suitable alternative.
Cryopreservation of seeds
If seeds are to be frozen, firstly, they need to be dried to remove as much ‘free’ water as allowable. Many seeds are then treated with a cryoprotectant (similar to antifreeze in your car radiator) and other solutions to help prevent damage to the cells during storage or the freezing and thawing process. The seeds are now ready to be frozen, and may be placed directly in liquid nitrogen, or have their temperature reduced slowly in a programmable freezer.
Cryopreservation of other plant tissues
Cryopreservation of plant tissue is completed in a similar manner as seeds, however the tissue (typically generated via tissue culture) is treated with two different cryoprotectants, firstly at a lower concentration and then a higher concentration. This two-step introduction helps to reduce the ‘shock’ to plant cells associated with the strong sugars and salts present in the cryoprotectant solutions.
For more information see Plant Germplasm Conservation Guidelines by the Australian Network of Plant Conservation.