High-throughput sequencing of small RNAs reveals the role of different plant viruses in the activation of RNA silencing-related genes and the induced resistance to Plum pox virus on peach by 'Garrigues' almond grafting

Plum pox virus (PPV) causes the serious sharka disease in Prunus trees. Peach (P. persica (L.) Batsch) trees are severely affected by PPV and no definitive source of genetic resistance has been identified at this moment. Previous results showed that PPV-resistant 'Garrigues' almond was able to transfer its resistance to 'GF305' peach through grafting, preventing these trees from PPV infection and reducing symptomatology and viral load in PPV-infected plants. Recent work tried to identify genes responsible for this effect by studying mRNA expression through RNAseq data in peach and almond plants, before and after grafting, and before and after PPV infection. In our work, we used peach and almond samples, but focused the high-throughput analyses on small RNAs (sRNAs) expression. We studied massive sequencing data and found an interesting pattern of sRNAs overexpression linked to antiviral defense genes that suggested activation of these genes followed by downregulation to basal levels. We also discovered that 'Garrigues' almond plants were infected by different plant viruses that were transferred to peach plants. The large amounts of viral sRNAs found in grafted peaches indicated a strong RNA silencing antiviral response and led us to postulate that these plant viruses could be collaborating by cross-protection in the observed 'Garrigues' effect.