Northern blot analysis of Alfalfa mosaic virus (AMV) chimeric RNA accumulation in transgenic Nicotiana tabacum plants that express constitutively the Sw5‐b gene (Nt/Sw5‐b). Nicotiana tabacum wild‐type (Nt/wt) and Nt/Sw5‐b plants were inoculated as described in Fig. 3, in which the RNA 3 transcript expresses the NSm RB1(lane 1), RB2 (lane 2) and NRB (lane 3). The analysed RNAs from inoculated leaves corresponded to a mixture of total RNA extracted from the two inoculated leaves (I1 and I2) at 7 days post‐inoculation (dpi), whereas the analysed RNA from systemic leaves corresponded to a mixture of the total RNAs extracted from the upper (U) leaves U1, U2 and U3 at 14 dpi. The positions of the chimeric RNA 3 and RNA 4 are indicated in the left margin of the photographs. rRNA indicates 23S RNA loading control. NRB, non‐resistance‐breaking; RB, resistance‐breaking.