Soluble carbohydrates and invertase activity in stem rust-infected, resistant and susceptible near-isogenic wheat leaves

Seven-day-old seedlings of the near-isogenic wheat (Triticum aestivum) lines Prelude, Prelude-Sr24 and Prelude-Sr5, susceptible, moderately resistant and highly resistant to race 32 of the wheat stem rust fungus (Puccinia graminis f.sp. tritici), respectively, were inoculated with uredospores of the fungus. Healthy (non-infected control plants) and infected tissue of primary leaf blades was extracted and assayed for ethanol/water-soluble carbohydrates and invertase (E.C. 3.2.1.26) activity. In healthy leaves, sucrose, glucose, fructose, soluble starch and fructans accounted for approx. 80% of the total soluble carbohydrate content. The major carbohydrate was sucrose. Its content was 10- to 20-fold higher than those of the other four carbohydrates. Infection caused a slight increase in invertase activity in the early stage of the infection, independent of the eventual infection type of the host-pathogen interaction. A similar increase was induced in all three isolines by the injection of a fungal glycoprotein elicitor. This points to a host origin for this early invertase activation. During later stages of infection, a more marked increase in invertase activity was observed in the fully susceptible isoline, a less pronounced increase occurred in the moderately resistant isoline, while no change was detected in the highly resistant isoline. These late changes in enzyme activity caused by fungal infection correlated closely with the different growth rates the pathogen in the three cultivars. The increases in enzyme activity were accompanied by increased contents of glucose and fructose, and were followed by a decrease in sucrose content. Starch and fructan contents were unaffected by infection, except for the susceptible cultivar, in which the starch pool was depleted during the later stages of infection.