deterioration reported in overseas studies may be applied totropical chip piles.

Section II - Virology

Virus Characterization and Identification

Adrian Gibbs

Research School of Biological Sciences,Australian National UniversitY, Canberra, ACT 2601

Classification and identification are both parts of theprocess of data acquisition, storage and retrieval, and whenapplied to plant viruses are of great practical importance toplant pathologists. This is because, if a newly found viruscan be shown to be related to a previously described groupof viruses, then much can reasonably be predicted about thebehaviour of the new isolate by analogy with the betterstudied members of the group, even to the extent of pre­dicting its vector and ecology in the field, and hence theways in which its spread might be limited.

Classification starts with the investigation and descrip­tion of the properties of the individuals that are to becompared and grouped; for each virus it is best to describethe properties of several carefully cloned isolates. The great­er the number of properties investigated and used as a basisfor the classification the more likely is it that informationfrom all parts of the genome will have been sampled andthe more useful the classification will be. The isolates arethen most often grouped using two levels of taxonomiccategories, the virus 'species' and the virus group. Thesecategories are difficult to define but I consider a 'species'to be a collection of strains whose known properties areso similar that there is no value in giving them separatenames, and a virus group is a collection of 'species' thatis useful to define by their shared properties becausethe group thus formed helps with problems such as virusidentification and the prediction of the properties ofnewly isol ated vi ruses.

Strains of virus 'species' share most of their propertiesand differ from one another in only a restricted range ofgenetically unstable properties such as the severity ofsymptoms they cause, whereas the different 'species' ofa group differ in a greater range of more stable propertiessuch as their host range; and viruses in different groups maydiffer from one another in any property. Harrison et al.(Virology 45, 356, 1971) Iist about 50 properties that areuseful for characterizing viruses. These properties are oftwo types. One type can be measured in absolute termsusing analytical tools; such properties as the composition anddimensions of the virus particle~. Most properties of thistype are genetically stable and useful for defining, anddistinguishing between, different groups of viruses. Otherproperties are assessed indirectly using the reactions of livingorganisms to compare viruses with one another; such proper­ties as host range and symptoms of a virus, its serologicalspecificity, and its behaviour in cross protection tests.These tests are very specific but, with the exception ofserological behaviour, we have little idea what viral propertiesare being compared in these tests; however, they aregenetically unstable and hence useful both to group anddistinguish between related viruses.

In the early days of plant virology only the indirectbiological tests were available, and hence virus identificationconsisted of comparing these 'differentiating characters' ofa new isolate with those of all previously describedviruses or, in other words, it was only possible to ask the

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question "Is this new virus the same as, or very closelyrelated to, a previously described virus?" Nowadays,however, with a better understanding of virus propertiesand virus classification, viruses can be more logically andspeedily identified by first assessing the 'grouping characters'of the virus, so as to answer the question "To what groupdoes this virus belong?" and then determining the 'differ­entiating characters' that are best suited to distinguishingbetween different members of that group.

The relative usefulness of different characters in virusclassification and identification can be seen most clearlywith the tobamoviruses, as a great range of their propertieshas been studied; from their host ranges and the symptomsthey cause, to the composition of their particles and thesequence of amino acids in their coat proteins. It seemsthat the single most useful character for classifying a newlyisolated tobamovirus is the amino acid composition of itscoat protein, but this is not necessarily the most usefulfor other virus groups.

Various data storage and retrieval aids are now availableto help with classification and identification, but for variousreasons I believe the most usefu I is the polyclave and havestarted to prepare a computer-based punched-card polyclavefor plant virus identification.

Plant Reaction to Virus Infection

J.W. Randles

Waite Agricultural Research Institute, Glen Osmond, S.A. 5064

Virus infection leads to changes in host plant bio­chemistry, physiology, cell components and,growth patterns,which are usually expressed ultimately as the symptoms ofdisease. These changes are examined in relation to timeafter inoculation of the plant, with attention being givento the inoculated leaf, the first systemically infected leaf,leaves which are produced after systemic invasion of theplant, and to symptomless tissue.

Inoculated leaves, soon after commencement of virusreplication, show increased respiration, decreased net rateof photosynthesis, and increased activity of hydrolyticenzymes such as ribonuclease. Most extensive studies havebeen done with the first systemically infected leaf becauseit combines the advantages of infection without wounding,and rapid virus synthesis in a leaf past the cell division stage.With four viruses so far studied, chloroplast ribosomal RNAsynthesis ceases as rapid virus synthesis commences, andwith two of these, lettuce necrotic yellows virus (LNYV)and tomato spotted wilt virus (TSWV), the concentrationof chloroplast ribosomes shows a rapid decline which isfollowed by a less marked decline in the cytoplasmicribosome concentration. The synthesis and concentrationof the chloroplast-associated Fraction 1 protein shows aparallel decline, and these changes together with ultra­structural aberrations and cessation of chlorophyll synthesissuggest that the chloroplast may be the specific site forpathogenesis. Because the chloroplast does not appear to bedirectly involved in virus synthesis perhaps one long termapproach to ameliorating virus disease could be to attemptto block the development of damage to chloroplasts.Protein synthesis is reduced in LNYV-infected leaf within1 day of symptom appearance, and soluble amino acidsaccumulate.

Leaves on old infected plants, which are infected asthey develop, show the most severe effects of infection whenthey are young (Boucher, personal communication). Hydro­lytic enzymes have higher activity than in uninfected tissue;enzymes of respiration, and enzymes dependent on chloro­plast function have reduced activity. As these leavesmature, the activity of some of the respiratory and