1Istituto Patologia Vegetale, Universita’ di Napoli, 80055 Portici (NA), Italia; 2Centre de Recherche en Virologie, Institut Armand-Frappier, Laval, QC, Canada H7N4Z3

Background and objectives
Mixed infections of different virus or viral strain commonly occur in higher plants. They may increase susceptibility (synergism) or resistance (antagonism) in the plants when compared to infection by either pathogen alone. The occurrence of mixed infections has an impact in pest control strategies. The question being, which strain will take over or whether the disease will be even more pronounced or cross-protection will occur. Rapid and reliable detection of the infecting viral strain is critical for any correct intervention, however it is difficult to obtain. Therefore, replication and spread of different strains in a mixed infection have been poorly investigated so far. In a recent study, we have differentiated several strains of turnip mosaic virus (TuMV) by analysis of single-strand conformation polymorphism (SSCP) [1] of their coat protein gene (Stavolone L, Alioto D, Ragozzino A, Laliberte JF, unpublished data). This technique made it possible to detect simultaneous presence of different strains in the same plant in naturally infected hosts. To investigate interaction and competitiveness among strains of TuMV, three different isolates were mixed in different concentrations, in groups of two, and simultaneously inoculated on Brassica perviridis. Concentration and evolution of the viral strains were followed by SSCP analyses and related to symptom expression.

Results and conclusion
Three types of relationship have been observed among the TuMV strains. Firstly, the interaction between ITA1 and ITA2 is evidence of cross-protection. ITA1 completely prevented replication of ITA2, even when the latter was inoculated in a higher dose. Symptoms produced on infected B. perviridis plants were consistent with the SSCP results, the specific ITA1 symptom dominated on all inoculated plants. This demonstrates that cross-protection does not require prior infection by one virus to prevent replication of the second, and that this phenomenon is not influenced by the concentration ratio of the inoculum. The interaction between ITA1 and ITA5 is an example of partial mutual exclusion. In this case, the inoculum dose seems to influence strain competition. SSCP analysis of the a2c8 mix showed only the ITA5 electrophoretic profile, and the a8c2 mix showed the dominance of the ITA1 pattern with a faint band belonging to ITA5. This strain is probably slightly more aggressive than ITA1; in fact, when both strains are inoculated at the same concentration ITA5 is dominant on ITA1. This mutual exclusion was also confirmed by plant reactions. However, a different symptom phenotype was noted when both strains were inoculated at the same concentration. Finally, ITA2 and ITA5 present an example of co-existence and synergism. Both strains had the same relative concentration in plants inoculated with b8c2 and b5c5 mixes although, in the b5c5 mix, ITA2 reached its final concentration later than ITA5. This indicates that both strains can tolerate each other, although the replication of ITA2 seemed to be slowed down by the high initial concentration of ITA5. Synergism was observed in symptom severity which was higher than with the other two strain combinations.

In conclusion, the simultaneous presence of two viral strains in a host can have various and unpredictable consequences for symptomatology. The molecular explanation of this phenomenon is obviously still unknown, but these results indicate that strain interactions dramatically influence the mechanisms by which viruses induce plant reactions.

1. Orita M, Iwahana H, Kanazawa H et al., 1989. Proceedings of the National Academy of Sciences, USA 28, 2766-2770.