Effect of Puccinia graminis tritici on Organic Acid Content of Wheat Leaves

Although greatly increased rates of oxidative metabolism result from infection of higher plants by rust fungi (1, 6), the significance of the high rates for the development of the rust pathogen is not understood. Previous work has indicated that the increase in respiratory rates may be correlated at least partially with the development of the parasite (5, 6). At the flecking stage, when visible symptoms of infection are first noticed and only vegetative mycelium is present, there is a moderate increase in rates without any detectable change in the nature of the respiratory pathways. The subsequent larger increases in rates associated with the process of sporulation are characterized by a decline in C6/C1 ratios. This latter feature of the metabolic alterations induced by all rust infections studied suggests that an oxidative pentose, or similar, pathway may be of importanice in obligate parasitism. Shu and Ledingham (23) havre shown that the germinating spore of the wheat rust fungus has all the enzymes of the pentose pathway and the respiration is characterized by low C6/C1 ratios. Consequently, if the mycelium of the parasite is largely responsible for the increased rates of respiration via a pathway requiring a C1 decarboxylation (6), major changes should be apparent in the amounts or activity of metabolic components either directly in, or closely related to, the pathways of glucose catabolism. In contrast to other tissues in which low C,/C_ ratios have been induced (2, 17), data to be published on rust-affected plants do show increases in carbohydrate compounds which arise after decarboxylation of the aldehydic group of glucose. The effects of sodium fluoride on the respiration of safflower hypocotyls (8) suggested the cyclic (3) operation of the pentose pathway and suggest also that the tricarboxylic acid cycle (TCA) would not necessarily function at rates appreciably different from those in uninfected tissue. Similar inhibitor experiments with wheat (7, 21) and bean (7) rustaffected plants were not consistent. However, results obtained by Farkas and Kiraly (10) and Bauermeister (4) with malonic and fluoroacetic acids have been interpreted as indicating a change in respiratory pathways, presumably TCA cycle reactions, during disease development. In both instances rust-affected wheat plants were less sensitive to these inhibitors than normal tissue, but the competitive nature of malonate inhibition presents kinetic difficulties (7). A quantitative study of organic acid concentrations in wheat infected with the leaf rust fungus was made by Staples (24) while Bauermeister (4) noted changes in size of spots of organic acids separated by paper chromotography. In each instance moderate increases in malic and citric acids were observed after infections were well established and the major respiratory changes had occurred. The significant unexplained exception noted by both workers was aconitic acid which actually declined 3 to 6 days after infection. The anomalies of the changes among TCA cycle acids and the detection of moderate increases only when infections were well advanced might be explained by a reduction in TCA activity during disease development followed by an active translocation of acids from other leaves as respiration rates increased. Previous work (20) has indicated a capacity of infected tissue to accumulate exogenous metabolites. The data to be presented are part of a comparative study of organic acid changes in wheat and bean tissue infected by rust fungi in which a stage characterized by vegetative growth of the parasites (fleck stage) was compared to the sporulating stage of (levelopment. Particular attention was paid to variables such as infection intensity, ontogenic host drifts in metabolism, diurnal changes in metabolism, and environmental control, aspects which often are not assessed adequately. Since wheat plants under our conditions showed characteristics which normal bean tissue did not possess, more extensive data on bDean rust infections will be discussed elsewhere.