88 MICHAEL SHAW 



The cytophotometric results show that nuclear and nucleolar 

 enlargement are accompanied by (a) parallel decreases in 

 lysine and arginine rich histones, (b) increases in nuclear 

 and nucleolar RNA, (c) increases in nuclear and nucleolar 

 total protein, (d) no change in DNA until the nuclei finally 

 collapse, when there is a dramatic loss of DNA. These may 

 reflect a specific response to the parasite or a less specific 

 response to stress. 



The radioautographic experiments show that incorporation of 

 tritiated compounds into host nuclei is doubled at the time 

 that the cytophotometric measurements indicate a doubling or 

 tripling of nuclear RNA levels. At this time electron micro- 

 scopy indicates a marked increase in the density of the 

 diffuse chromatin and a loosening of the dense chromatin of 

 affected host nuclei. These changes may reflect an uncoiling 

 of the chromatin and presumably also the increased protein 

 content of the nuclei. They appear to be consistent with 

 the supposition that host DNA is derepressed, perhaps through 

 the loss of histones and that there is an accelerated synthesis 

 of RNA and protein in the host. The possibility also exists 

 that interaction triggers derepression in the parasite. 

 Neither of these ideas is proven. 



The effect of rust infection on the uptake of leucine-l^C and 

 on its incorporation into protein was examined, using flax 

 cotyledons. Uptake from the medium and incorporation into 

 protein was increased 1.5 to 2-fold by rust infection. This 

 effect was observed as early as 8 hours after inoculation in 

 some experiments and was highly significant at 16 hours after 

 inoculation. There was no significant difference between the 

 responses of susceptible (Bison) and resistant varieties 

 (Bomb ay i . 



Attempts to demonstrate the formation of new proteins (iso- 

 zymes) within the first 24 to 48 hours after rust infection 

 have failed. Nevertheless the possibility that this occurs 

 and that such new isozymes mediate resistance by altering 

 the balance between metabolic pathways or initiating new ones 

 must remain as a working hypothesis. 



The importance of fundamental biochemical and cell biological 

 studies on host parasite relations cannot be underemphasized. 

 The elucidation of the biochemical basis of resistance is not 

 a short-term project and the selection and breeding of 

 resistant varieties are likely to provide the most practical 

 and economic approach to disease control for many years. 



FLOOR DISCUSSION 



PERSON: Thank you very much. We are on your side, really. We 

 feel instinctively, that if the biochemists are successful they will, 

 perhaps, discover some key metabolic reaction that may actually trigger 

 the resistance response. If this could be done, it would certainly be 

 a beautiful lead for genetical and other kinds of work. Is there anyone 

 who wants to address any comment, criticism, or question to Michael 

 Shaw? 



