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Plant reaction to virus infection

Classification and identification are both parts of the process of data acquisition, storage and retrieval, and when applied to plant viruses are of great practical importance to plant pathologists. This is because, if a newly found virus can be shown to be related to a previously described group of viruses, then much can reasonably be predicted about the behaviour of the new isolate by analogy with the better studied members of the group, even to the extent of predicting its vector and ecology in the field, and hence the ways in which its spread might be limited. Classification starts with the investigation and description of the properties of the individuals that are to be compared and grouped; for each virus it is best to describe the properties of several carefully cloned isolates. The greater the number of properties investigated and used as a basis for the classification the more likely is it that information from all parts of the genome will have been sampled and the more useful the classification will be. The isolates are then most often grouped using two levels of taxonomic categories, the virus 'species' and the virus group. These categories are difficult to define but I consider a 'species' to be a collection of strains whose known properties are so similar that there is no value in giving them separate names, and a virus group is a collection of 'species' that is useful to define by their shared properties because the group thus formed helps with problems such as virus identification and the prediction of the properties of newly isol ated vi ruses. Strains of virus 'species' share most of their properties and differ from one another in only a restricted range of genetically unstable properties such as the severity of symptoms they cause, whereas the different 'species' of a group differ in a greater range of more stable properties such as their host range; and viruses in different groups may differ from one another in any property. Harrison et al. (Virology 45, 356, 1971) Iist about 50 properties that are useful for characterizing viruses. These properties are of two types. One type can be measured in absolute terms using analytical tools; such properties as the composition and dimensions of the virus particle~. Most properties of this type are genetically stable and useful for defining, and distinguishing between, different groups of viruses. Other properties are assessed indirectly using the reactions of living organisms to compare viruses with one another; such properties as host range and symptoms of a virus, its serological specificity, and its behaviour in cross protection tests. These tests are very specific but, with the exception of serological behaviour, we have little idea what viral properties are being compared in these tests; however, they are genetically unstable and hence useful both to group and distinguish between related viruses. In the early days of plant virology only the indirect biological tests were available, and hence virus identification consisted of comparing these 'differentiating characters' of a new isolate with those of all previously described viruses or, in other words, it was only possible to ask the