Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK

Background and objectives
One of the most important field vegetable viruses is turnip mosaic virus (TuMV), a potyvirus with a wide host range, particularly affecting brassicas. The observation of plant-pathogen interactions reveals a degree of variation within the species. This can be seen in the range of phenotypes seen on partially genetically characterised lines of Brassica rapa [1] and B. napus [2], and is used to pathotype isolates, The four B. napus differential lines discriminate 12 pathotypes, although most isolates examined from Europe belonged to just three pathotypes [2]. Resistance is available in the B. napus lines against two of these pathotypes. We have produced a panel of monoclonal antibodies (MAbs) to discriminate between isolates of TUMV and attempted to use these as a means to predict the pathotype.

Materials and methods
A large collection of isolates (over 100) of TUMV has previously been characterised using the B. napus lines above [2]. Purified virus particles (in which the predominant protein was the viral coat protein) of three isolates (CDN 1, CZE 1, and UK 1) representing the commonest pathotypes and a fourth isolate (GK 1) were used as immunogens to raise a panel of 30 MAbs using BalbC mice. The activity of the 30 MAbs was tested against a collection of 41 TUMV isolates of diverse pathotypes and plant and geographical origins using plate-trapped antigen ELISA tests. A canonical variate analysis was used to select particular MAbs for use in distinguishing between the 41 isolates, eliminating MAbs on an iterative basis until the probability of misclassification exceeded 5%. The data was adjusted for a covariate based on the relative amount of virus present in all samples tested. In a similar exercise, the data were grouped by pathotype with the aim of eliminating MAbs until the probability of misclassification exceeded 5%.

Results and conclusions
A wide variety of specificities were found among the MAb reactions to the TUMV isolates, with 16 or 17 epitopes being recognised. No single MAb showed any ability to distinguish groups of isolates arranged according to their geographical origin or phenotype on individual differential plant lines. Not all MAbs were equally useful in distinguishing between the virus isolates, and the full panel was reduced to one of 11 MAbs while still maintaining a low probability of misclassification. A plot of the canonical variate means of the first two (most significant) roots, which accounted for 80% of the variation, indicated clustering of the isolates into three major serotypes. Isolates grouped in particular serotypes did not appear to have any other characteristics in common. When the isolates were grouped by pathotype, even the use of the full set of MAbs still had a high chance of misclassification (11%). The panel of MAbs could not therefore be used to accurately predict the pathotype.

The coat protein is considered to be one of the three most variable proteins in potyviruses. This is reflected by the large degree of immunogenic variation found. The inability of the MAbs to distinguish between some of the isolates contrasts with the ability of the resistance genes present in the differential lines to discriminate between these isolates. There is therefore no evidence that the serotypic variation in the coat protein reported here is recognised as an elicitor for the resistance genes present in the B. napus lines.

1. Provvidenti R, 1980. Journal of the American Society for Horticultural Science 105, 571-3.
2. Jenner CE, Walsh JA, 1996. Plant Pathology 45, 848-56.