Mechanisms of cross-protection.

Abstract Cross-protection is a phenomenon in which mild or attenuated strains of a virus protect plants against secondary infections with a more severe strain of the same virus. Though this phenomenon was described in the 1920s, the mechanism(s) behind it have been poorly understood for many years. This review gives an historic overview of the discovery of cross-protection and the hypotheses that were initially put forward to explain the phenomenon. The involvement of the viral coat proteins (CPs) in the mechanism of cross-protection as well as the involvement of RNA silencing (thought to have evolved as an antiviral defence response of plants) are discussed. The author comes to the conclusion that in some cases cross-protection appears to require the CP of protective strains to prevent successful infection with a challenging strain but that in other cases cross-protection may operate either by highly localized RNA silencing or direct competition between strains for host cell resources.