PROCESS OF INFECTION AND VIRUS SYNTHESIS 21 



is iiiactivated by a systemic TMV antiserum in a maimer similar to systemic 

 virus. The vital question of whether the specific infectivity, i.e., the number 

 of particles per unit of infectivity, is the same or different for lesion versus 

 systemic virus has not yielded a confident answer, although the results of 

 one trial indicate no striking differences. We are inclined to believe in the 

 identity of the two materials, but we wouJd still like to see further experi- 

 mental evidence to support this belief. 



Thus, intensive study of TMV in its local lesion host has provided a reason- 

 able working model for the intracellular behavior of a plant virus. The 

 question now is how far these findings will apply to the systemic virus-host 

 system, which is the subject of the next section. 



IV. Behavior of Plant Viruses in a Systemic Host 



A. Quantities of Virus Synthesized 



Vastly more effort has been expended on miderstanding the behavior of 

 TMV in systemic hosts than in local lesion hosts. The reasons are not far to 

 seek. In a matter of a few days after a leaf has been rubbed with virus, 

 symptoms will make their appearance on leaves above the leaf that was 

 inoculated. Stanley (1937), shortly after his discovery that TMV could be 

 isolated and crystalhzed, was able to show a miUionfold increase in virus 

 infectivity, as well as a concomitant increase in viras protein in the short 

 span of 4 days after infection of tobacco leaves with TMV. In a matter of a 

 few weeks, milligram quantities of virus can be readily extracted from each 

 leaf showing symptoms (Cohen et at., 1957). As far as the leaf that was 

 inoculated is concerned, no very conspicuous symptoms will appear, although 

 the leaf -will also contain milligram quantities of virus some 14 days after 

 inoculation. Thus, the ease with wliich newly formed virus can be measured 

 is responsible for the emphasis that has been placed on the systemic system. 

 In the experience of this laboratory, we have extracted as much as 70 mg. of 

 virus per leaf within 20 days after infection, although an average value would 

 be in the neighborhood of 5-10 mg. virus per leaf. Since the leaf weighed 

 about 5 grams at the time of extraction, about 4 grams of which was water, 

 the concentration of virus rose from about 10~^ mg./ml. in the inoculum to 

 around 10 mg./ml. in the leaf durmg 20 days of infection. On any scale of 

 magnitude, TMV synthesis in the systemic host is in a class by itself. It is 

 a remarkable facet of the T]\IV problem that a plant can endure such an 

 enormous spithesis of a foreign, virus nucleoprotein AA-ithout the plant 

 showing distress. There is one strain of TMV which produces no symptoms 

 whatsoever, and yet, extraction of the leaf two weeks after inoculation will 

 yield aroxmd 10** particles of T^IV per cell! 



