Graft-mediated PPV inoculation
To evaluate if RNA silencing-mediated resistance to PPV found in St5’-1, St5’-6, and St5’-9 transgenic plums can be transmitted to apricots grafted onto them, we utilized as inoculum source the Spanish PPV isolate 3.3 RB/GF-IVIA (AF172346.1) belonging to the PPV-D strain (García-Almodovar et al. , 2015), adopting two different virus inoculation procedures.
First, wild-type (wt) apricot buds heavily infected by PPV were grafted onto the transgenic plum rootstocks (Supplementary Figure 1), exposed to a two-month artificial winter in the cold chamber, and then transferred to the greenhouse. Eight weeks later, sprouted buds were evaluated for virus resistance (see below). In the second experimental setting, healthy apricot buds were grafted onto the transgenic plum rootstocks. After an artificial winter and sprouting of the buds, the scions were inoculated by chip-budding with new shoots of PPV-infected GF305 peaches (Supplementary Figure 3). At the end of the cycle,  plants were  again transferred to the cold chamber and evaluated for virus resistance in the following cycle, as described above.