Effect of rust infection on levels of uricase, allantoinase and ureides in susceptible and hypersensitive bean leaves

Abstract

The enzymatic activities involved in two sequential steps of purine catabolism and ureide biosynthesis, i.e. uricase, catalase and allantoinase, were determined in susceptible and hypersensitive bean leaf tissue after infection with Uromyces phaseoli. Uricase increased to approx. two times more than the control value 20–22 h after inoculation of the hypersensitive cv. K.W. 765 (reaction grade 2 ++) close to pathogen penetration and first haustoria formation and, 1 day later, in the susceptible cvs Golden Gate Wax, Pinto 111 (reaction grade 6) and in the hypersensitive cv. K.W. 814 (reaction grade 1). Afterwards, the highest increases in uricase activity (up to 10 times the control), mainly localized in fractions of leaf homogenate sedimenting at 2000 g and 15000 g , were detected in c.v. K.W. 765 during lesion development, indicating that a peroxisome-associated enzyme was involved. Lower increases (up to two to three times the control) were observed in the susceptible cultivars during stroma differentiation (flecks) and uredospore formation. Catalase linked to the particulate fractions increased in susceptible cultivars to the same extent as uricase from 2 to 6 days after inoculation. Also in cv. K.W. 765 it increased during lesion development, but very much less than uricase, with the consequence that a possible toxic action, due to unscavenged H 2O 2 production, could arise during hypersensitivity expression. An enhancement of allantoinase two or three times the control values during stroma differentiation was detected in susceptible cultivars. By contrast, allantoinase activity was unchanged during local lesion formation in cv. K.W. 765, indicating that the activation of this enzyme is specifically involved during the establishment of a biotrophic relationship between Phaseolus vulgaris and Uromyces phaseoli. In the susceptible cultivar, an increase in allantoic acid during the late stages of infection (uredospores differentiation) was observed. Both allantoin and allantoic acid were enhanced during lesion development in the hypersensitive cultivar. The results are consistent with the interpretation that the enhanced purine catabolism in rust-infected bean leaves is connected both to a nutritional benefit for the parasite in the compatible relationship, and to a toxic effect for the host in the incompatible one.