Effect of rust infection on purine catabolism enzyme levels in wheat leaves

Abstract

Variations in purine catabolism enzymes (xanthine oxidase, uricase and allantoinase) in wheat ( Triticum aestivum) leaves were determined during compatible and incompatible interactions with a local isolate of Puccinia recondita f.sp. tritici. The cultivars Thatcher, Mentana and Leopardo as well as isolines with resistance (Lr) genes in a Thatcher background were employed. The level of xanthine oxidase enhancement was correlated with the three different infection stages: (i) from the beginning to the end of mesophyll colonization by intercellular mycelium (1-4 days post-inoculation); (ii) subepidermal structure (flecks) differentiation (5-7 days post-inoculation); and (iii) maturation of uredospore-producing uredinia (7-11 days post-inoculation). Early and consistent enhancement in xanthine oxidase was detected only during incompatible interactions whereas late enhancements were only detected in the highly compatible ones. In contrast to xanthine oxidase, uricase activity did not change early after infection, whether compatible or incompatible interactions were involved. Uricase was mostly stimulated in the compatible interactions during fungal structure differentiation (stromatic structure and uredinia) and during the same period in moderately incompatible interactions showing macroscopic necrotic symptoms but not in highly incompatible ones (pseudo-immune responses). According to previous results, allantoinase activity was absent in healthy wheat leaves. In rust-infected leaves allantoinase was evident starting from 7 days post-inoculation at the beginning of uredospore differentiation and the values reached corresponded to the level found in germinating uredospore suggesting that the fungal enzyme was being measured. Allantoinase was not detectable in rust-infected leaves 1-4 days post-inoculation with the assay method used and was hardly detectable 5-6 days postinoculation when the activity level in mycelium, calculated on the basis of chitin content, accounted for about one-third of that found during uredospore differentiation. The results obtained indicate that purine catabolism is involved during wheat leaf rust infection. Particular attention was given to the possible role of xanthine oxidase enhancement during the hypersensitive response in relation to the stage of the infection process it took place as well as the duration and intensity reached. The in vivo inhibition of host xanthine oxidase following application of allopurinol by root absorption in concentrations which did not inhibit fungal growth resulted in some inhibition of the hypersensitive response. Therefore, it was concluded that during wheat leaf rust infection, xanthine oxidase is probably responsible, at least in part, for host cell damage associated with hypersensitivity by mechanism/s possibly involving release of toxic amounts of superoxide.