Polyamine Metabolism in an Incompatible Interaction between Barley and the Powdery Mildew Fungus, Blumeria graminis f. sp. hordei

In the incompatible interaction between the barley cultivar Hart (mlo allele) and the powdery mildew fungus Blumeria graminis DC. Speer f. sp. hordei Marchal, free spermidine and conjugated forms of putrescine and spermidine were found to accumulate 1–3 days following inoculation. The increase in free spermidine could be accounted for by increased activities of the biosynthetic enzymes ornithine decarboxylase (EC 4.1.1.17) and S‐adenosylmethionine decarboxylase (EC 4.1.1.50). Activities of the transferase enzymes putrescine hydroxycinnamoyl transferase (EC 2.3.1.‐) and tyramine feruloyl‐CoA transferase (EC 2.3.1.‐) were also increased in leaves of Hart inoculated with powdery mildew, as were activities of the polyamine catabolic enzymes diamine oxidase (DAO, EC 1.4.3.6) and polyamine oxidase (PAO, EC 1.4.3.4). The possible role of conjugated polyamines, and increases in DAO and PAO, in penetration based resistance to powdery mildew infection is discussed.