Content and Metabolism of Indole-3-acetic Acid (IAA) in Healthy and Rust-Infected Wheat Leaf Segments

Abstract

Abstract The content of IAA in stem rust-infected susceptible wheat leaves shows a highly pronounced maximum 5-6 days after inoculation, shortly prior to the onset of sporulation. This auxin increase can not only be caused by a reduced degradation of IAA. Considerable amounts of IAA are also found in urediospores and germlings; the IAA is in part released by them into the germination medium. IAA applied exogenously to wheat leaves is channelled into two different degradation path­ways: (a) into the peroxidase-catalysed decarboxylation which leads to indole-3-methanol and subsequent products as well as into (b) a non-decarboxylative path which leads to a number of oxindolic compounds. Furthermore, IAA conjugates such as IAAglc and IAAsp are formed. The formation of the products is characteristically dependent upon the concentration of the IAA applied. In rust fungus-infected wheat leaves, all IAA metabolites occur which are known in healthy leaves. The mode of their formation after “feeding” of radioactively-labelled IAA leads to the conclusion that the main part of the IAA in the infected leaves is present in a pool which does not permit a rapid exchange with the IAA taken up. The results lead to the hypothesis that IAA is present, to a major extent, in the structures of the fungus and is probably also produced by it.