While we investigate alternate storage methods for recalcitrant species, plants are often maintained in tissue culture. This process involves taking small sections of plant material and growing them in agar gel enriched with nutrients and hormones which keeps the cuttings growing.
The cuttings typically grow very quickly in tissue culture (faster than they might if growing in a pot) and all the plants are 'the same age'. Additionally the cuttings are grown under sterile conditions which is required to prevent them from being killed by bacterial or fungal contamination. Each of these conditions actually makes plant material generated under tissue culture ideal for further experimentation and storage.
More information about tissue culture can be found here.
Fortunately, because we have germinated fresh seeds as part of our normal seed testing protocols, living material is often already available to transfer into tissue culture. For other species that we were unable to germinate or obtain seeds, cuttings can alternatively be taken from plants and transferred into tissue culture. Once the material has been placed in culture, it can be maintained there indefinitely, by simply removing the cuttings, trimming them and placing them into new agar.