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Production of yam mosaic virus (ymv)-free Dioscorea opposita plants by cryotherapy of shoot-tips.

In the present study, Yam mosaic virus (YMV) could be efficiently eliminated by cryotherapy in Dioscorea opposita. Shoot apices were precultured for 16 h with 0.3 M sucrose, encapsulated in sodium alginate and dehydrated for 4 h prior to direct immersion in liquid nitrogen. Up to 90 percent of the plants regenerated from cryopreserved shoot tips were YMV-free, whereas only 40% of those regenerated using meristem culture were YMV-free. YMV-free yam plantlets could be propagated in vitro through nodal stem culture, with sequential subculturing at 6-week intervals on medium containing 0.5 mg per liter kinetin. The microtubers formed at the bottom and axil of the explants, incubated at 30 degreeC after being chilled (4 degree C) for 3 months, could be sprouted successfully under in vivo conditions. Healthy plants were established without any damaging symptoms of the virus. Thus, cryotherapy provides an alternative method for efficient elimination of yam viruses, and could be simultaneously used for long-term storage of yam germplasm and for the production of virus-free plants.