Cross-Protection for Plant Viruses: One Phenomenon, Several Mechanisms?

Cross-protection, discovered nearly a century ago, is a biological control method used to manage viral plant diseases. It is usually defined as a phenomenon in which a primary infection of a host by a pathogen (e.g., a mild strain) prevents its subsequent infection by a genetically related one (e.g., a severe related strain). Despite the long-standing research interest in this phenomenon from both a fundamental and an applied perspective, the mechanisms and factors—both viral and host-derived—underlying this phenomenon remain unclear, limiting its broader application. Proposed mechanisms include RNA silencing and potential roles of viral proteins, but inconsistent terminology and mixed evidence in the literature have made conclusions difficult. This review critically analyzes existing studies on plant virus cross-protection, summarizing common characteristics across systems and comparing them with proposed mechanisms. The findings suggest that cross-protection may result from different mechanisms that vary depending on the specific plant–virus interactions.