Mycopathologia 148: 87–95, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 87 Cytokinins in pathogenesis and disease resistance of Pyrenophora teres-barley and Dreschslera maydis-maize interactions during early stages of infection Renu Angra-Sharma & DK Sharma Lab # 11, Botany Department, University of Rajasthan, Jaipur 302 004, India Received 18 May 1999; accepted 22 June 2000 Abstract Infection of Hordeum vulgare L. by Pyrenophora teres and of Zea mays by Dreschslera maydis were characterized by ‘green island’ formation, higher cytokinin levels and accumulation of metabolites in the infected areas. Higher cytokinin concentrations of the order 6-Y,Y-dimethylallylaminopurine > zeatinriboside > zeatin > dihydrozeat- inriboside were detected at infection sites of susceptible hosts. By virtue of these cytokinins, infection sites may be acting as metabolic sinks helping proliferation of the pathogen. Existence of translocatory sinks at infection zones was confirmed from autoradiographic studies, where, accumulation of labeled metabolites was prominent at infection sites of susceptible hosts. Upon infection the lower cytokinin levels of resistant hosts decreased further with progress of infection. In the infected resistant hosts the concentrations of zeatin/zeatinriboside were the maximum among the four identified cytokinins. The pathogen is also capable of secreting cytokinins as evident from quantification of cytokinins in culture filtrate extracts using HPLC. Since detached leaves were used in the experiments the increase/decrease of various cytokinin levels may be attributed to pathogen influence. The increase in cytokinin levels in the susceptible host may be aiding the growth of the pathogen on one hand, while the decrease in the infected resistant host may signal the host to activate defenses against a potential pathogen at the early stage of infection. Key words: Autoradiography, barley, cytokinins, Dreschslera maydis, green islands, HPLC, maize, Pyrenophora teres. Introduction Cytokinins are 6 N-substituted derivatives of adenine. In healthy plants cytokinins are synthesized in roots and exported via the transpiration stream to the shoot, where depending on the organ they are either utilized or metabolized [1]. In the infected plants, infection sites of Pyrenophora teres-barley and Dreschslera maydis-maize interactions have been reported to re- tain chlorophyll longer than in uninfected zones [2, 3]. These areas by virtue of their higher cytokinin activity become metabolic sinks and thus exhibit a reduced ex- pression of senescence symptoms and accumulation of carbohydrates, thereby aiding the growth and prolifer- ation of the growing pathogens P. teres and D. maydis [4, 5]. Extracts from spores of several microorganisms have been shown to contain cytokinins [6] which is also true for P. teres and D. maydis [2, 3]. Therefore, cytokinin imbalance constitutes an important attack mechanism in which the pathogen either produces or catabolizes the hormone directly or induces the host to do so. The knowledge of cytokinin involvement in patho- genesis, disease resistance and translocation espe- cially during early stages of infection is of poten- tial importance for devising control measures. Un- fortunately, the involvement of cytokinins in disease resistance and translocation of metabolites has been mainly concluded from the studies on the effect of exogenously applied cytokinins to healthy plants [7], probably due to complexity of disease phenomena. Moreover, the few studies that have been conducted to