4 S. G. WILDMAK 



B. Exclusion of Some Plant Viruses from Dividing Cells and Embryonic 



Tissues 



One of the striking features of T]\IV infection is the apparent exclusion of 

 the virus from meristems and embryonic tissues. Even though a plant may 

 be so thoroughly infected with the virus that symptoms are apparent in the 

 flower petals, some unknown process operates to exclude the virus from the 

 seed which will develop from the flower. To the writer's knowledge, there is 

 no confirmed report that TMV infection is ever transmitted through the 

 seeds. Equally interesting is the apparent exclusion of the virus from the 

 dividing cells of the meristem of plants (Beimett, 1940). However, once 

 mitoses are completed and the growth by expansion has commenced, the 

 virus can readily be demonstrated to be present, although the amount of 

 virus synthesized in young versus old cells is not strikingly different, accord- 

 ing to the experience of this laboratory. Exclusion from embryonic tissue is 

 not a general property of plant viruses, for some, like lettuce mosaic, are 

 transmitted with high frequency via seed. 



The activity of TMV also seems to be mdifferent to the internal state of the 

 protoplasm. This indifference is illustrated by the fact that the virus seems 

 to multiply during any stage of leaf development after mitoses are completed. 

 As the cells in the leaf expand, there is a rapid increase in protein (Dorner 

 et al., 1958) and the leaf may be said to be in a stage of net protein synthesis. 

 The rise in protein ceases when the leaf stops expanding. Thereafter, as the 

 leaf ages, there is a steady decline in protein, or a condition of net protein 

 loss. However, the virus does not distinguish between these phases of leaf 

 development, for it will infect and multiply in the leaf practically up to the 

 time the leaf withers and falls off the plant. 



C. Strains of Plant Viruses and Host Reactions 



Like other viruses, such as the phage T-even and T-odd series, TMV and 

 many other plant viruses consist of a complex of many different strains, each 

 being primarily distinguished from other strains by the specific symptoms 

 produced in a host plant. Indeed, with TMV, it has been said that there are 

 as many strains as investigators who have sought to identify them. However, 

 for the purposes of our discussion, there are only two strains that have been 

 subjected to investigation that faUs within the compass of this article. One 

 of these is the common strain, or Ul, as designated in this laboratory, and 

 the other is U2 (Siegel and Wildman, 1954). When a N. tabacum var. Turkish 

 Samsun plant is infected with Ul, a characteristic mottling symptom de- 

 velops on leaves above the leaf which serves to initiate the infection. Simi- 

 larly, U2 also produces a symptom in this kind of tobacco, but the spnptom 

 is less noticeable than Ul. In other varieties of N. tabacum plants, Ul may 



