Cryopreservation of Rice Tissue Cultures

The biotechnological improvement of rice is largely dependent on the maintenance of dedifferentiated cultures as either callus and/or suspension cultures. For example, the production of transgenic plants of rice (Oryza sativa L.) either by direct DNA uptake into protoplasts or by particle bombardment is dependent on embryogenic callus or cell suspension cultures from which fertile plants can be regenerated (1 ,2 ). However, over time the morphogenic competence of dedifferentiated rice cultures declines (3 ). Therefore, new cultures have to be regularly initiated and characterized in order to maintain a constant supply of embryogenic cells. This approach is highly problematic, particularly with Indica, Varietal Group 1 (4 ), rice varieties (5 ). Cryopreservation of embryogenic cells provides a more efficient means of ensuring a constant supply of competent cells for genetic manipulation. The recovery of embryogenic rice cultures after cryogenic storage that were capable of plant regeneration has been reported by several groups (6 –9 ). Embryogenic callus, and more commonly, suspension cultures from a range of different rice varieties have been cryopreserved, including Indica (Varietal Group 1) varieties (9 ), Japonica (Varietal Group 6) varieties (7 ,10 ). Transgenic rice suspension cultures have also been successfully recovered from cryogenic storage (6 ).