INTERFERENCE PHENOMENA WITH PLANT AND 

 BACTERIAL VIRUSES 



F. C. Bawden 

 Rothamsted Experimental Station, Harpenden, England 



In work with plant viruses, the abihty of one virus to protect plants 

 against another is widely used to identify clinically different viruses as 

 related strains. Almost all serologically related viruses that have been 

 studied have been found to be mutually antagonistic when present to- 

 gether in plants, and a plant systematically infected with one strain 

 fails to develop symptoms characteristic of another serologically related 

 strain when reinoculated with it. The protection is not always complete; 

 in time a second strain may predominate over the first if it is one that 

 is more invasive or multiplies more rapidly. Although there is no rea- 

 son to connect the phenomenon with antibody production in plants, 

 there is evidence that the degree of protection afforded by one strain 

 against infection by another is correlated with the number of antigens 

 they have in common. One exception to the rule that serologically re- 

 lated viruses are mutually antagonistic is provided by potato virus Y 

 and tobacco veinal necrosis virus. Although they share common anti- 

 gens, tobacco plants systematically infected with one succumb to the 

 other when inoculated with it. 



The plant protection phenomenon superficially resembles the mutual 

 exclusion phenomena described with bacterial viruses, but there is one 

 striking difference. Mutual exclusion occurs with bacterial viruses that 

 are not serologically related, such as Ti and T2, whereas serologically 

 related strains of one bacteriophage, e.g., T2 and T2r, can both infect 

 the same cell, multiplying in it, and such multiple infections give new 

 strains that combine characters of each parent type. Such related strains 

 are considered not to interfere with one another, but this would seem a 

 terminology that does not accurately reflect the course of events in 

 mixed infections and one that has been used in contrast to the seemingly 

 greater interference between viruses that sho"w the phenomena of mu- 

 tual exclusion. The differences between the behaviour of serologically 

 related strains of plant and bacterial viruses are probably less than ap- 

 pear at first sight and they mainly reflect the different terminologies 

 used in the two subjects. 



If two related strains of a bacterial virus enter a bacterium, both 

 multiply, but the final yield of virus is not greater than would have 

 occurred from infection with one strain alone. Also, if a large excess of 

 one strain enters, the second may be undetectable. This is precisely 

 similar to the results obtained when plants are simultaneously infected 

 with two strains of one virus. Both will enter and multiply, and the 

 final yield of total virus will about equal that produced by either strain 



