Abstract
An auxin-metabolizing enzyme was purified from growth-retarded sunflower plants ( Helianthus annuus L.) infected with downy mildew ( Plasmopara halstedii [Farl.] Berl. et de Toni) by means of differential centrifugation, salt precipitation and gel filtration. SDS-polyacrylamide gel electrophoresis of the final purified extract revealed a single protein band with a mol. wt of 36 kDa. The native enzyme was characterized as a 32 kDa protein with high substrate specificity for indole-3-acetic acid (IAA), It degraded auxin in acidic solutions without requirement of cofactors like hydrogen peroxide, manganese chloride or monophenols. In the presence of hydrogen peroxide the sunflower phytoalexin scopoletin was also metabolized. The enzyme showed strong sensitivity to cyanide and sulphhydryl reagents. 2,3,5-Triiodobenzoic acid, ferulic acid and scopoletin inhibited IAA-oxidation. The oxidase was not found in healthy sunflower plants and its pathogenesis-related occurrence indicates an involvement in the process which leads to growth retardation of downy mildew infected sunflower. This view was supported, by experiments with intact plants. Treatment with exogenously applied phytohormones revealed a stimulation of elongation growth in downy mildew-infected sunflower with gibberellic acid, but not with auxin.
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