90 MICHAEL SHAW 



LOEGERING: Actually, there is one illustration of where this is 

 true, and that is Watson's work on progressive mutation. Perhaps the 

 reason this has never been seen in nature is because the final step in 

 this progressive mutation is the only one that would be observed. This 

 is a little bit of evidence, and as I said this morning I don't see why 

 the strict gene-for-gene relationship has to be, only it has been. I 

 would like to ask Dr. Shaw a question. What do you think of this idea 

 which can be put in this way: Does the pathogen change the differentia- 

 tion of the host cell back to meristematic development? 



SHAW: We know that it does. Yarwood recorded years ago that there 

 was a hypertrophy of bean leaf tissue at the infection sites of the bean 

 rust. Recently, I'm not sure where it was, somebody recorded mitotic 

 figures in the host that indicate there is induced division there. In 

 the wheat system, of course, this doesn't occur. But we all know various 

 rust systems where there is a great deal of cell division induced in the 

 host. 



LOEGERING: This is not the question I was asking. 



SHAW: Oh, I'm sorry. 



LOEGERING: It has been postulated that the physiological state of 

 an infected cell with a haustorium in it is equivalent to a meristematic 

 cell. Is there anybody who's compared these two types of tissue instead 

 of comparing the susceptible with the resistant? 



SHAW: No, I don't think anybody has done that. I was trying to 

 say that there were certain similarities, and in some cases, you do get 

 induction of meristematic activity as a result of infection. Certainly, 

 you observe the induction of cell division, and this is one character- 

 istic of the meristematic cell. 



SCHUTT: On one of your first slides, you showed data on water con- 

 tent, dry weight, etc. Does the data vary according to the resistance or 

 susceptibility of wheat to rust? And, did you observe these same dif- 

 ferences before inoculation? 



SHAW: That figure simply referred to the changes in water status 

 of the tissue under the influence of the parasite. Yes, the water 

 content changes vary with the degree of susceptibility. No, I did not 

 observe the same differences before inoculation. 



ZUFA: In one instance you mentioned that the invaded cells of 

 the resistant plant died. 



SHAW: That happens eventually. 



ZUFA: We call that hypersensitivity. In other cases, evidently 

 the parasite lives together with the host in a kind of a symbiosis, 

 sometimes for many years. Would any changes occur in the tissue of such 

 a host tree and could it be considered resistant to the disease? 



SHAW: I would guess that if you had a fungus living in contact 

 with the host tissue, that host tissue would certainly not be the same 

 as it is in the absence of the parasite, no matter how long the two 

 stay together. 



