Process of Infection by Tobacco Mosaic Virus' 



IRVING RAPPAPORT, A. SIEGEL and S. G. WILDMAN 



Department of Botany, lUiiversity of California, Los Angeles, California 



IN THE PAST FEW YEARS, information from various fields of virus research has 

 converged to provide us with a rather general and exciting picture of viruses 

 and their mode of replication. This is particularly true for the bacteriophages 

 and some animal viruses. In the realm of plant viruses, though, our facts are 

 somewhat out of balance. We know considerably more of their extracellular 

 properties than we do of their intracellular behavior. The high yields and ex- 

 ceptional degree of purity with which a virus such as tobacco mosaic (TMV) 

 can be prepared has attracted many investigators to this unique rod. As a 

 result, the in vitro characteristics of the virus as a macromolecular nucleoprotein 

 are very well-defined. Its size, shape, molecular weight, chemical composition, 

 and numerous other properties associated with its extracellular existence, have 

 been determined with great precision. What is lacking to balance this type of 

 information, however, is a more intimate knowledge of the host-virus relation- 

 ship. Within its host, TMV exerts a profound influence on the synthetic path- 

 ways of the cell. In terms of the vast number of foreign particles each cell is 

 triggered into producing, TMV stands alone. Somewhere between 100,000 and 

 half a million virus rods can be recovered from a single infected plant cell, 

 compared to the hundreds or possible thousands of virus particles produced by 

 animal or bacterial cells. 



In seeking more information on the intracellular behavior of TMV^ our 

 research efforts have been limited to a mild and a severe strain of TMV and to 

 one host — Nicotiana glulinosa. X. glnlinosa is known as a local lesion host . When 

 virus is rubbed on its leaves, small local lesions appear, usually within 2 days. 

 These necrotic areas are composed of cells that died as a result of having sup- 

 ported virus replication. Because the lesions on N . glulinosa are discrete and 

 obvious, they serve a three-fold purpose. First of all, the number of lesions 

 obtained is related to the concentration of virus applied, and so forms the basis 

 of the well known plant virus bio-assay. Second, since the area of virus activity 

 is so clearly defined, the rate of spread of the infection from cell to cell can be 

 observed and measured; and third, the entire spectrum of virus activity, from 



' These studies were supported, in part, by Contract AT(ii-i)-34, Project 8 of the Atomic 

 Energy Commission, and USPHS Grant G-4124 from the Division of Research Grants, Na- 

 tional Institutes of Health. 



