Plant virus-specific transport function. II. A factor controlling virus host range.

An experimental system allowing the transport of the dependent virus from the conducting tissues into the leaf mesophyll of a plant preinfected by the helper virus has been developed recently [Taliansky et al. (1982). Virology 122, 318-326]. The role of virus-specific transport function in the host range control of a plant virus was studied here by this technique. It was shown that Tm-2 tomato lines (in which the resistance to TMV is expressed only at the level of the intact plant, but not in protoplasts) became susceptible to TMV after preinfection with the helper virus (potato virus X, PVX). It is assumed that TMV accumulation in Tm-2 lines under these conditions becomes possible due to the transport function coded for by the helper virus (PVX), which allows TMV to spread over the resistant plant. Tm-1 tomato lines (whose protoplasts are resistant to TMV infection) remained resistant to TMV under the conditions outlined above. It was found that brome mosaic virus (BMV) can be transported from the conducting tissues and replicates in the mesophyll cells of tomato and bean plants preinfected by TMV and dolichos enation mosaic virus (DEMV), respectively. The same was true of TMV introduced into the conducting tissues of wheat plants preinfected with barley stripe mosaic virus (BSMV). It is concluded that a nonhost plant, whose resistance to a virus is due to the blockage of the transport function, can be infected systemically by this virus as a result of the complementation of transport with the helper virus.