Toward in planta studies of persistent fungal viruses in a model plant.

A model plant, Nicotiana benthamiana, was examined as a host for persistent fungal viruses capable of crossing organismal kingdoms. Protoplasts of N. benthamiana were transfected with a mixture of virions of a betapartitivirus, Rosellinia necatrix partitivirus 18 (RnPV18), and an alphapartitivirus, RnPV19, and were then subjected to plantlet regeneration. Primary RT-PCR-based screening showed that nearly 100% of the resulting calli tested positive for RnPV18, whereas approximately 90% were positive for RnPV19. However, secondary screening performed at a later stage of tissue culture revealed that only 6% of the calli retained RnPV19, whereas approximately 33% retained RnPV18. These results suggest that the calli were chimeric, consisting of virus-infected and virus-free sectors, and that the partitiviruses were progressively lost during callus maintenance. It is also possible that these fungal partitiviruses were unable to fully adapt to, or counteract, host defense responses sufficiently to establish stable infection. We succeeded in obtaining RnPV18-positive calli and suspension cultures that maintained the virus at detectable levels, as shown by northern blotting, after prolonged subculture for at least 9 months. High-throughput small RNA analyses revealed both similarities and differences in virus-derived small RNA profiles among protoplasts, calli, and suspension cultures. Viral genome analyses further revealed developmental stage-specific and stage-independent substitutions compared with the RnPV18 genomic sequence maintained in the original fungal host. Interestingly, a C-to-U mutation in the RNA-dependent RNA polymerase-encoding region of RnPV18 was detected much more frequently in a particular line, designated B21, than in another stably RnPV18-infected line, P8, irrespective of whether the virus was maintained in suspension cultures or calli. This may explain why virus accumulation in B21 calli and suspension cultures was much lower than that in P8 cell cultures, as RNA-seq analyses showed 159 K counts per million for P8 versus 44 K counts per million for B21. Taken together, this study provides a platform for investigating partitiviruses and other ubiquitous persistent viruses, which are generally difficult to inoculate experimentally.