PROCESS OF INFECTION AND VIRUS SYNTHESIS 23 



appeared in the proximal cell during tlie entire experimental period, which 

 extended to 216 hours. However, a crystal inclusion showed up in the 

 proximal cell some 150 hours after infection. (It has been known, since 

 Sheffield's experiments in 1939, that crystalline inclusions contain virus 

 infectivity.) Zech was also able to follow similar changes in the cells radiating 

 from the basal cell of the leaf hair and concluded that the changes were 

 indicative of virus activity spreading slowly, in all directions, away from the 

 basal cell. 



The insensitivity of the TMV infectivity assay precludes the possibihty 

 for directly correlatmg infectivity measurements with microscopic observa- 

 tions of the leaf hair cells. Consequently, Zech was forced to resort to rubbing 

 virus on the entire surface of leaves, and then, measuring for virus activity 

 contained in leaf discs punched out at random from the infected leaves. In 

 this way, he reached the conclusion that appearance of new infectivity was 

 considerably delayed beyond the time when he could get microscopic evi- 

 dence of virus activity in the hair ceUs. He postulates that the infective 

 process begins by the virus entering an "ecKpse" period of a minimum dura- 

 tion of 42 hours, and that, furthermore, the agent responsible for the transfer 

 of virus activity from ceU-to-ceU is different in state from the TMV virus 

 particles observed in the extracellidar state. 



Utilising the ultraviolet hght microscope, Zech (1954; Zech and Vogt- 

 Kohne, 1955) have made additional fascinating observations of TMV act- 

 ivity in leaf hair cells. He finds that 4-20 hours after infection, the nucleus 

 of the hair ceU becomes surrounded by UV-absorbhig material. This first 

 stage is followed by increased UV absorj^tion in the cytoplasm, accompanied 

 by a diffusion of the material that was concentrated around the nucleus. 

 This stage is accompanied by a reduction both ia the volume of the nucleus 

 and its contained UV-absorbuig material. With the appearance of X-bodies 

 between 42-220 hours following infection, which is also the period before 

 infectivity can be ascertained by the method described above, the nucleus 

 begins to regain some of its lost volume. When the infection period progresses 

 to the point where crystals arise out of collapsed X-bodies, the nucleus and 

 nucleolus may swell up to four times their original size. 



Zech's results, at first sight, would seem to indicate a distinct difference 

 in the process of infection of TMV in the systemic host compared to the local 

 lesion host, particularly in terms of the 42-houj minimum echpse period for 

 the systemic infection. In the local lesion system, multiphcation occurs 7-9 

 hours after infection, and only about 16 hours elapse from the time the ^arus 

 invades a new cell until the cell becomes necrotic, after about 10* new par- 

 ticles of virus activity have been manufactured. Analysis of the results of 

 Goodcluld and Cohen, and Yarwood may help to throw some hght on this 

 seeming great difference. 



