Different propagation levels of potato virus X (PVX) isolates in PVX coat protein expressing tobacco plants and protoplasts do not correlate with coat protein sequence similarities

We have obtained transgenic tobaccos expressing the coat protein (CP) (CP+plants) or coat protein anti‐sense RNA [CP( —) plants] of the Russian isolate of potato virus X (PVXS). When these plants were challenged with seven different PVX isolates, two CP+ lines showed delay in virus propagation, whereas CP(—) lines were not protected at inoculum concentrations of 10 μg/ml. The level of protection in CP(+) lines was different for different PVX isolates. Sequence comparisons revealed that similarity between CPs of different PVX isolates to PVXS CP do not correlate with the protection achieved against these isolates in tobaccos expressing PVXS CP. We analyzed the replication of PVX isolates S, S6111 and P551 in protoplasts derived from PVX CP+ transgenic tobaccos. In contrast to studies with intact plants, we were not able to detect any differences in propagation of these isolates in individual CP+ tobacco cells. With 1 μg PVX inoculum, in CP+ protoplasts the replication of all three isolates was inhibited, whereas at higher inoculum concentrations no difference existed between CP+ and control cells. These data indicate that two mechanisms could be involved in PVX CP‐mediated virus protection: at low virus concentrations PVX replication is inhibited, and at higher virus concentrations PVX spread is perturbed in transgenic plants.