PROCESS OF INFECTION AND VIRUS SYNTHESIS 



19 



edge of U8 lesions may seriously complicate transfer of virus activity from 

 cell to cell. Infectivity measm-ements have been utilized to estimate the yield 

 of virus per cell. Typical results are shown in Fig. 8. While the points scatter 



10- 



10- 



s 10- 



10-" 



10- 



10" 



^ 



s 



-Jio^ "^ 



10 15 20 25 30 

 Cell number X lO'* 



35 40 



Fig. 8. Infectivity yield of three strains of TMV per cell, as a function of total number 

 of necrotic cells. From Rappaport and WUdman (1957). 

 Key: squares, UI; circles, U2; triangles, Holmes' rib grass. 



considerably, there seems to be a trend towards constancy of virus yield per 

 cell during the entire course of lesion expansion for Ul and U2, although 

 US appears to decrease in yield per cell as the necrosis enlarges. Evidently, 

 we can estimate that in the 16-hour period that elapses from the time a cell 

 becomes infected with strain Ul until it succumbs, between 10*-10^ new 

 virus particles are manufactured. For strain U2, the yield is about a magni- 

 tude less. A rough guess would indicate that an average doubling time of 

 around 1 hour would have to be invoked to account for the 10^-10* virus 

 particles per cell induced by U2 infection, a magnitude that is not out-of- 

 line with the estimate derived from radiation inactivation experiments. 



L. Nature of Infectious Material 



In what form is the infectious material transmitted from ceU-to-cell? In 

 the absence of any experimental evidence to the contrary, we are inclined 

 toward the notion that infection is transmitted via nucleic acid. Because of 

 the unique construction of plant cells, it seems clear that transmission by a 

 lysis or leakage mechanism, as found for phage and animal viruses, is out of 

 the question. Microscopic examination of necrotic ceUs indicates that the 

 cellulose waU remains intact, but shrunken, after the cells die. Presumably, 

 the cell wall still remains an impenetrable barrier to the passage of virus. We 



