PPV accumulates at a lower level in wild-type scions of the highly resistant transgenic rootstocks
As shown in Figure 1, depending on the cycle taken into consideration and the transgenic rootstock line, a certain number of wt scions were infected by the virus. RT-qPCR analysis was conducted on a random subset of PPV-positive wt scions from the four transgenic rootstocks. The PPV titer of the apricot scions grafted onto the resistant plum rootstocks St5’-1 and St5’-9 was significantly lower than those found in apricot scions grafted onto the susceptible St5’-7 line (Figure 2). In addition, apricots grafted onto the tolerant St5’-6 line had a lower virus titer than St5’-7, although differences were not significant according to a one-tail Dunnett’s test (Figure 2). The above data, together with those reported in Figure 1 and Table 2, pointed out that the RNA silencing mechanism reduces the virus titer progressively till eliminating the virus from the wt scions grafted on the transgenic resistant PPV plants.