Long-term preservation of potato leafroll virus, potato virus S, and potato spindle tuber viroid in cryopreserved shoot tips.

Availability of and easy access to diverse plant viruses and viroids is a prerequisite in applied and basic studies related to viruses and viroids. Long-term preservation of viruses and viroids is difficult. A protocol was described for long-term preservation of potato leafroll virus (PLRV), potato virus S (PVS), and potato spindle tuber viroid (PSTVd) in cryopreserved shoot tips of potato cv. Zihuabai. Shoot regrowth levels following cryopreservation were higher in 1.5 mm-shoot tips (58-60%) than in 0.5-mm-ones (30-38%). All shoots recovered from 0.5-mm-shoot tips were PVS- and PSTVd-preserved, but none of them were PLRV-preserved. Cryopreservation of 1.5-mm-shoot tips resulted in 35% and 100% of PLRV- and PVS- and PSTVd-preserved shoots. Studies on cell survival patterns and virus localization provided explanations to the varying PLRV-preservation frequencies produced by cryopreservation of the two sizes of shoot tips. Although micropropagation efficiencies were low after 12 weeks of subculture following cryopreservation, similar efficiencies were obtained after 16 weeks of subculture in pathogen-preserved shoots recovered from cryopreservation, compared with the diseased in vitro stock shoots (the control). Pathogen concentrations in the three pathogens-preserved shoots analyzed by qRT-PCR were similar to those in micropropagated shoots. The three pathogens cryopreserved in shoot tips were readily transmitted by grafting and mechanical inoculation to potato plants. PLRV, PVS, and PSTVd represent a diverse range of plant viruses and viroid in terms of taxonomy and infectious ability. Therefore, shoot tip cryopreservation opens a new avenue for long-term preservation of the virus and viroid.