SOME SALIENT POINTS CONCERNING PLANT VIRUSES 



C. A. Knight 

 Virus Laboratory, University of California, Berkeley 



Chemical Nature of the Isolated Viruses. — (See Knight, Ann. Rev. 

 Microbiol, 1949, 3„ 121.) The plant viruses isolated and highly purified 

 thus far appear to consist solely of nucleic acid and protein combined 

 to form specific nucleoproteins. Examples are tobacco mosaic virus, to- 

 bacco necrosis, tomato bushy stunt, Southern bean mosaic, turnip yel- 

 low mosaic, tobacco ringspot, and alfalfa mosaic. 



The amounts of nucleic acid found in plant viruses range from 6% 

 for the TMV group to about 40% for tobacco ringspot virus. All appear 

 to be pentose nucleic acids. Most are bound rather firmly to their pro- 

 tein components. 



The protein components of plant viruses appear to be acidic, rather 

 than basic as in the case of classical sperm nucleoproteins. Analyses of 

 4 or 5 different viruses have revealed no unusual quantities or types of 

 amino acids. 



Crystallinity. — About a half-dozen of plant viruses (counting strains 

 of TMV as only one instance) have been obtained in the crystalline 

 state. Aside from the important implications concerning the funda- 

 mental nature of viruses associated with the first crystallization of a 

 virus, no particular significance seems to attach to this property. At 

 present, the so-called internal crystallinity of individual TMV rods, as 

 revealed by X-ray analysis, appears to be of potentially greater signifi- 

 cance than the orderly arrangement of numerous particles into para- 

 crystalline aggregates. The X-ray studies indicate the presence of a 

 repeat unit in the individual virus particles having dimensions of an 

 approximately 11 A cube (Bernal and Fankuchen, /. Gen. Physiol., 

 1941, 25, 111). The demonstration of such units provides a definite 

 basis for speculation concerning the possible mechanics of virus repro- 

 duction. 



Size and Shape of Plant Viruses. — Each virus appears to possess a 

 characteristic size and shape. The shapes revealed thus far by electron 

 microscopy include globular, rod-like and filamentous forms. The sizes 

 range from about 1 6 m^i for alfalfa mosaic virus to those of some other 

 viruses whose one dimension, at least, is several hundred millimicrons. 

 Some speculation is occasioned in the case of TMV by the occurrence 

 of rods of different lengths in preparations of this virus. However, the 



