PROCESS OF INFECTION AND VIRUS SYNTHESIS 7 



of the susceptible site. The explanation is that, at high concentrations of 

 virus, there is an insufficiency of susceptible sites to accommodate all the 

 virus that may be present. With low concentrations, there is not enough virus 

 to ensure infection of each susceptible site. 



Kleczkowski (1950) has proposed that the leaf varies in regional suscept- 

 ibility to infection. His hypothesis is derived from mathematical analysis 

 of the infectivity-dilution curve, which mdicates that the logarithm of the 

 minimal effective concentration is normally distributed. One interpretation 

 of this finding might be that some susceptible sites are infected with a single 

 virus particle, whereas other susceptible sites require the cooperation of 

 several particles to institute the infection. However, this interpretation seems 

 to be eliminated by an exclusion phenomenon, which will be discussed later 

 in this chapter. Another explanation might be that the susceptible sites 

 differ in size so that there is a greater probability of virus making contact 

 with a larger site. 



The number of susceptible sites produced by rubbing can be appreciably 

 increased by the use of abrasives incorporated with the inoculum (see 

 Yarwood (1957) for references). However, even by this means, a maximum 

 of around 10^ susceptible sites per N. glutinosa leaf is about all that can be 

 engendered, because more drastic and extensive injury results only in killing 

 host cells before they reveal the process of virus reproduction. 



For maximum lesion production, it is necessary for the virus to be present 

 at the instant the susceptible site is produced. Rubbing the leaf first and then 

 applying the inoculum later will result in fewer lesions than having virus 

 present at the time of rubbing. Evidently, the susceptible site is in an 

 optimum condition to accept the virus for only an instant and then rapidly 

 loses the capacity. There are reports (AUington and Laird, 1954) that test 

 plants deficient in potassium can stdl be infected with TMV for appreciable 

 periods of time after rubbing the leaf. Yarwood (1957) has compUed the ex- 

 perimental results of comparison of different buffers, apphcators, abrasives, 

 host plants, viruses, etc. The reasons why one set of conditions wiU produce 

 a greater, or lesser, number of lesions than another set are not verv well 

 understood. 



C. Specific Infectivity of Tobacco Mosaic Virus 



Under most circumstances, 10""-^ grams of TMV per miUihter is about the 

 minimum dilution of TMV that will start an infection either in the form of 

 local lesions or as a systemic infection. Thus, the difficult question of why 

 each infection requires in the neighborhood of 10^ virus particles is posed; 

 in this connection, TMV is more infective than any other plant virus. 



Various mterpretations have been advanced to account for the low specific 

 infectivity of plant \dnises. One of the arguments is that all virus particles 



