PHYSIOLOGY OF RUST RESISTANCE 1 



Michael Shaw 



University of British Columbia 



Vancouver, B.C., Canada 



.ABSTRACT 



An obligate parasite is one which can complete its life cycle in 

 nature only on the living tissue of its hosts. Some strains of 

 wheat and flax rusts have now been grown in axenic culture. The 

 two critical issues are (1) what is the biochemical or bio- 

 physical basis of obligate parasitism? (2) what is the bio- 

 chemical basis of genetically determined resistance or suscep- 

 tibility? In the absence f host tissue the germination and 

 development of rust uredospores normally stops with the forma- 

 tion of infection structures. Interaction with the host results 

 in further development of the parasite. 



In principle the following can be distinguished: Interactions 

 between (1) parasite and environment, (2) host and environment, 

 (3) "physical" interactions between parasite and host, (4) bio- 

 chemical interactions between parasite and host mediated by (a) 

 substances present before inoculation, (b) substances released 

 after inoculation, (c) substances synthesized de novo after 

 inoculation, e.g., proteins. The results of these interactions 

 are often but not always seen in the effects of infection on 

 the pattern of growth in the host. After infection respiration 

 rate, the activity of the pentose phosphate pathway and the con- 

 centrations of many constituents, including RNA and auxir are 

 increased. Water content is decreased. Such studies have 

 usally been conducted at relatively late stages in disease 

 development, but there is evidence that critical interactions 

 between host and parasite occur very shortly after inoculation. 

 Valuable insight has come from the use of (a) quantitative 

 cytophotometric techniques for the measurement of DNA, RNA, 

 total proteins, total histones, and lysine and arginine rich 

 histones, (b) microradioautography of tissue sections after 

 incubation with tritiated leucine, cytidine and uridine, (c) 

 electron microscopy to examine the response of host nuclei 

 to the presence of the parasite. 



Editor's Note: Dr. Shaw's commitments have prevented him from 

 rewriting his Study Institute paper before the publication deadline. 

 With apologies he has provided a detailed abstract of his paper. The 

 Editors have attached the Floor Discussion of Dr. Shaw's paper, since 

 most of the discussion can be referred to the abstract. 



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