Cryopreservation of Rubus viruses in raspberry shoot tips via droplet-vitrification: assessment of viral preservation, localization, and post-thaw transmission capacity

We report the successful cryopreservation of three economically important Rubus viruses: raspberry bushy dwarf virus (RBDV), black raspberry necrosis virus (BRNV), and Rubus yellow net virus (RYNV), using shoot tip cryopreservation in four raspberry cultivars. Virus-infected shoot tips (approximately 1.0 mm in length) containing 3-4 leaf primordia (LPs) were cryopreserved using the droplet-vitrification technique. In the cultivars 'Zlatá Královna (ZK)' and 'Tulameen (TUM)', over 90% of shoot tips survived, and more than 90% regenerated into whole shoots. All three viruses were successfully preserved in the cryopreserved tissues, with recovery rates varying depending on virus type and cultivar: RBDV was recovered at rates of 86% in 'ZK' and 87% in 'TUM'; BRNV at 66% in 'ZK' and 45% in 'TUM'; and RYNV at 96%, 94%, and 86% in 'Fairview', 'Stiora', and 'ZK', respectively. To investigate viral localization in shoot tips, in situ hybridization was used. RBDV and RYNV infected a broad range of meristematic tissues, including the apical dome and LPs, whereas BRNV showed a more limited distribution. Virus distribution varied not only among virus species but also across raspberry cultivars, suggesting genotype-specific patterns of virus localization. Post-cryopreservation viral activity was verified using micrografting and aphid transmission assays. RBDV, BRNV, and RYNV were all successfully transmitted to healthy plants via micrografting, indicating the preservation of viral infectivity. Furthermore, BRNV was effectively transmitted by large raspberry aphids from cryopreserved materials, confirming vector-mediated transmission capacity post-thaw. Overall, this study demonstrates that shoot tip cryopreservation via droplet-vitrification is a reliable and effective strategy for preservation of biologically active Rubus viruses. This approach offers a valuable biotechnological tool for virus maintenance in support of diagnostic, breeding, and virology research.