THE BIOCHEMISTRY OF PLANT VIRUSES 35 



I. Introduction 



The biochemical study of plant viruses is still very largely in its early 

 stages. For many years the plant viruses were regarded as unusual, some- 

 what ethereal, entities having properties which were more characteristic of 

 the supernatural than of the chemical laboratory. Nowadays, largely owing 

 to the pioneer work of Stanley and of Bawden and Pirie, the plant viruses 

 are commonplace chemicals used for calibrating physical apparatus of 

 various kinds and used as sources of nucleic acids. They are now being 

 investigated by all manner of methods from X-ray crystallography and 

 electron microscopy to enzymatic and immunological tests, and yet we still 

 do not know why viruses cause disease. Generally it is thought that the 

 viruses cause the diseases in plants by sequestering the synthetic potentialities 

 of the plant. Nothing could be further from the truth. The damage caused by 

 virus infection to plants may range from negligible, or only detectable by 

 careful statistical analysis, to complete death of the plant, while the amount 

 of virus in a plant may range from grams per kilogram to minute amounts. 

 Often, in fact usually, severity of disease is inversely correlated with the 

 quantity of virus produced. It is quite evident that the severity of the disease 

 has httle or no connection with the amount of virus which is being synthesized 

 or which is present at any one time. As the viruses evidently produce their 

 effects by some kind of chemical mechanism, it is obvious that a mere study 

 of the constitution of viruses will not give us more than a very superficial 

 idea, if any, of the way m which viruses produce their pathological 

 effects. 



The study of plant viruses has followed a more or less logical development. 

 They were first characterized as nucleoproteins at a time when it had become 

 accepted that the enzymes were protein in nature, and the general interest 

 in the viruses was largely centered around morphology. Much labor was 

 expended, and still is being expended, in determining the morphology of 

 viruses by indirect methods, such as viscometry, light scattering, flow 

 birefringence, diffusion, and so on. Some of this produced useful information, 

 but much of it is now purely of historical interest, because much of the 

 morphological work is better done by electron microscopy. 



The protein components of viruses attracted much attention for many 

 years, and attempts were made to cause mutations and otherwise to interfere 

 with virus activity by modifymg the proteins. Nowadays it is believed that 

 the nucleic acids are the prime factors and much the same is being done 

 with them. The protein parts of viruses are not being neglected, however, 

 and an aU-out assault is at present being made on the chemical composition 

 of the simpler viruses. Far from being a routine task, this is proving interesting 

 and uideed surprising. A short time ago it was thought that viruses contained 

 only protein and nucleic acid. Now odd structures are being recognized at 



