IDENTIFICATION AND CHARACTERIZATION OF AN AUXINDEGRADING ENZYME IN DOWNY MILDEW INFECTED SUNFLOWER

An auxin-metabolizing enzyme was purified from growth-retarded sunflow er plants (Helianthus annuus L.) infected with downy mildew (Plasmopar a halstedii [Farl.] Berl. et de Toni) by means of differential centrif ugation, salt precipitation and gel filtration. SDS-polyacrylamide gel electrophoresis of the final purified extract revealed a single prote in band with a mel. wt of 36 kDa. The native enzyme was characterized as a 32 kDa protein with high substrate specificity for indole-3-aceti c acid (IAA). It degraded auxin in acidic solutions without requiremen t of cofactors like hydrogen peroxide, manganese chloride or monopheno ls. In the presence of hydrogen peroxide the sunflower phytoalexin sco poletin was also metabolized. The enzyme showed strong sensitivity to cyanide and sulphhydryl reagents. 2,3,5-Triiodobenzoic acid, ferulic a cid and scopoletin inhibited IAA-oxidation. The oxidase was not found in healthy sunflower plants and its pathogenesis-related occurrence in dicates an involvement in the process which leads to growth retardatio n of downy mildew infected sunflower. This View was supported, by expe riments with intact plants. Treatment with exogenously applied phytoho rmones revealed a stimulation of elongation growth in downy mildew-inf ected sunflower with gibberellic acid, but not with auxin.