Serological, molecular and immunofluorescent evidence for interference competition between isolates of Potato virus Y

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.