RESEARCH SUMMARY

Plant viruses depend largely on host factors to replicate in the cell and to propagate throughout the plant and between individual plants. Plants in turn have developed antiviral defence mechanisms that must be counteracted by viral factors. These factors appear to be preferred targets for alternative plant defences.

In our laboratory, we try to understand this complex interplay, mainly in the infection of the potyvirus Plum pox virus (PPV), the causal agent of sharka, a damaging disease of Prunus trees. We are especially interested in defence responses related to RNA silencing and its viral suppressors.

HCPro is the typical silencing suppressor of potyviruses, but other silencing suppressors of Potyviridae family members have recently been discovered. We demonstrated that serine proteinase P1b of the ipomovirus Cucumber vein yellowing virus (CVYV) suppresses silencing by sequestering siRNA, and can replace HCPro functionally in PPV infection in a host-specific manner.

Our results demonstrate that although potyviruses can exploit different sources of anti-silencing activity, their own silencing suppressors can contribute to defining the specific host range of the virus. We also showed that single amino acid changes at the N-terminal region of the capsid protein (CP) control specific PPV adaptation to Prunus persica and Nicotiana species. Our findings suggest that species-specific interactions of the CP N-terminal region with host factors have an important role in viral long distance movement, and that an unknown resistance mechanism interferes with these interactions in Nicotiana species. We showed that the N-terminal region of PPV CP is O-GlcNAcylated and phosphorylated. An O-GlcNAcylation-deficient PPV mutant infects Nicotiana clevelandii and P. persica with no apparent differences compared to wild type PPV. In contrast, PPV accumulation is significantly lower in Arabidopsis thaliana Col-0 plants infected with the mutant virus, concurring with a fine-tuning effect of CP O-GlcNAcylation on PPV infection that facilitates its adaptation to different hosts and environmental conditions.

We are interested in applying the information obtained in our research to design control strategies for sharka disease. We thus used RACE assays of viral RNA fragment accumulation in infected plants as well as deep sequencing of viral siRNA to design 20 PPV-specific amiRNA constructs. Cleavage activity on sensors and a high level of antiviral protection in agroinfiltration systems was shown for some of the amiRNA, which will be transferred to other PPV plant hosts. Finally, another target of interest is development of PPV-based plant expression vectors. To broaden the range of plants susceptible to PPV-based vectors, we developed an infectious cDNA clone of a PPV isolate of strain C, the only one infecting cherry trees in nature. Biological features of this cloned isolate are being analysed.