Double infections of related or unrelated viruses frequently occur in single plants, the viral agents being inoculated into the host plant simultaneously (co-infection) or sequentially (super-infection). Plants attacked by viruses activate sophisticated defence pathways which operate at different levels, often at significant fitness costs, resulting in yield reduction in crop plants. The occurrence and severity of the negative effects depend on the type of within-host interaction between the infecting viruses. Unrelated viruses generally interact with each other in a synergistic manner, whereas interactions between related viruses are mostly antagonistic. These can incur substantial fitness costs to one or both of the competitors. A relatively well-known antagonistic interaction is cross-protection, also referred to as super-infection exclusion. This type of interaction occurs when a previous infection with one virus prevents or interferes with subsequent infection by a homologous second virus. The current knowledge on why and how one virus variant excludes or restricts another is scant. Super-infection exclusion between viruses has predominantly been attributed to the induction of RNA silencing, which is a major antiviral defence mechanism in plants. There are, however, presumptions that various mechanisms are involved in this phenomenon. This review outlines the current state of knowledge concerning the molecular mechanisms behind antagonistic interactions between plant viruses. Harmful or beneficial effects of these interactions on viral and host plant fitness are also characterized. Moreover, the review briefly outlines the past and present attempts to utilize antagonistic interactions among viruses to protect crop plants against destructive diseases.

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The influence of co‐infection on concentration and accumulation of genetically different isolates of Potato virus Y (PVY) in potato and tobacco plants and the efficiency of transmission by Myzus persicae of PVY isolates from doubly versus singly infected plants were evaluated. The vector ability to simultaneously transmit two virus isolates was examined. Eight PVY isolates represented three strain groups: PVYO (pathotype and serotype O), PVYNW (pathotype N and serotype O), and PVYNTN (pathotype and serotype N). Different diagnostic methods, including DAS‐ELISA, multiplex RT‐PCR, aphid transmission tests and bioassays, were applied to detect the presence of PVY isolates in source and assay plants. Significant reductions in concentrations of certain PVY isolates during co‐infection with other isolates were found both in potato and tobacco plants. The observed effects were both isolate‐ and host‐dependent in form. The highest rates of virus transmission by single aphids were recorded with PVYNTN isolates, and the lowest ones with PVYO isolates. Individual aphids of M. persicae were able to simultaneously transmit two PVY isolates. The frequency of transmission was generally low, but it reached as high as 20% for one of the isolate combinations. The findings presented in the work provide proof for antagonistic within‐plant interactions between isolates of PVY, with some implications of these interactions for virus transmission by aphid vectors. Consequently, this research contributes to a better understanding of the epidemiology of the disease caused by PVY.

Under natural field conditions, the abundance of plant viruses sharing the same host frequently results in mixed infections. Viruses co‐occurring inside host tissues often interact with one another synergistically or antagonistically. This paper addresses within‐host interactions among genetically different isolates of Potato virus Y (PVY), a serious pathogen that attacks solanaceous crops, particularly potato and tobacco. Manifold aspects of the relationships between six PVY isolates representing PVYO, PVYN‐Wi/PVYN:O and PVYNTN strains have been characterized. Plants of tobacco cv. Samsun and of two potato cultivars were inoculated both simultaneously and sequentially with different inoculation sequences and different time intervals between inoculations. Symptoms exhibited by doubly inoculated plants varied, primarily depending on the isolates involved as well as on the order of inoculation. Significant reductions of the titres of PVYN:O and PVYN‐Wi isolates in plants inoculated sequentially or simultaneously with PVYNTN isolates were recorded. These reductions were most probably due to inhibition of replication and accumulation of one isolate by another. In some combinations, a pre‐existing infection with PVYNTN prevented infection with the secondary virus. By contrast, challenge inoculation by PVYN:O, PVYN‐Wi or PVYO had no marked effects on the titre of the pre‐inoculated PVYNTN isolate. Microscopic visualization revealed a tendency of strains towards separation from each other in host tissues. These results provide strong experimental evidence for the existence of antagonistic interactions among PVY isolates. Moreover, these results demonstrate the occurrence of complete or at least partial cross‐protection among PVY isolates, with NTN isolates being evidently more fit than their counterparts.

Cross‐protection is a phenomenon that occurs when infection with a mild virus prevents or suppresses the harmful effects of subsequent infection by a related severe virus. Isolates of Potato virus M (PVM, genus Carlavirus, family Betaflexiviridae), like isolates of other viruses, are likely to occur in plants in multiple infections, but the knowledge about intrahost interactions between PVM isolates and their impact upon the host response to infection is none. Here, we report that pre‐infection with a mild isolate I‐38 successfully protected Datura metel plants against destructive effects of infection by a severe isolate Uran. The latter was not detected by reverse transcription‐PCR in plants pre‐inoculated 7 days earlier by I‐38, which indicates that it was excluded by host defence mechanisms triggered by the low virulent isolate. Our results provide the first evidence for the occurrence of cross‐protection between PVM isolates within a common host plant.