PROCESS OF INFECTION AND VIRUS SYNTHESIS 29 



not known. Of the 10'^ to 10^ cells on tlie surface of a leaf, all of which seem 

 to be able to support virus activity, only about 10^ of them may be infected 

 at one time under the usual conditions of virus inoculation. 



2. Attachment of extracellular virus to the host protoplast is an irrevers- 

 ible and practically instantaneous process. An exclusion phenomenon 

 operates at the site of attachment such that only a single infectious entity is 

 responsible for the processes which will ultimately lead to the formation of a 

 lesion. 



3. After attachment to the host cell protoplasm, the virus undergoes se- 

 quential changes in its sensitivity to inactivation by radiation, which are 

 considered to involve the release of nucleic acid from the virus particle. 

 MultipHcation of the nucleic acid then ensues. 



4. Once multiplication is underway, newly formed, infectious units invade 

 adjacent cells by way of the protoplasmic strands which intercoimect all of 

 the cells of the leaf. The spread of infectious units is continuous. After 

 the virus has left the cell which served as the focus of infection, about 16 

 hours elapse between the time of infection and the necrosis of another 

 cell in a local lesion host. During this interval, around 10* rod-shaped par- 

 ticles are synthesized in the infected cell. Of the large number of particles 

 manufactured, only a fraction of these possess the information necessary to 

 initiate a new round of virus reduplication as the cycle of extracellular 

 -^ intracellular -> extracellular virus is repeated in a new host plant. 



Acknowledgements 



I am greatly indebted to my former and present associates for much of the infor- 

 mation and notions contained in this paper, and for permitting me to use some of 

 their unpublished findings. In this connection, I must single out Dr. Albert Siegel 

 and Dr. Irving Rappaport for special mention, since they have provided so many of the 

 ideas on which this concept of plant virus activity is based. Lideed, tliis paper merely 

 fills in the details of a paper previously published by them (Rappaport et al., 1956). 



I also express my appreciation for the moral and financial support that I have received 

 from the Division of Biology and Medicine, U.S. Atomic Energy Commission, The 

 National Science Foundation, Office of Naval Research, the U.S. Public Health Service, 

 and the Cancer Committee and the Research Committee of the University of California. 



References 



Allington, W. B., and Laird, E. F. (1954). Phytopathology 44, 297. 



Bald, J. G. (1937). Ann. Appl Biol. 24, 33. 



Bald, J. G. (1950). In "Viruses 1950" (M. Delbruck, ed.), p. 17. Calif. Inst. Technol, 



Pasadena, Calif. 

 Bawden, P. C, and Kleczkowski, A. (1952). Nature 169, 90. 

 Bawden, F. C, and Kleczkowski, A. (1953). J. Gen. Microbiol. 8, 145. 



