The auxin-biosynthetic pathway from L-tryptophan to indole-3-acetic add via indole-3-acetamide (IAM), found in plant-pathogenic bacteria such as Agrobacterium tumefaciens and Pseudomonas savastanoi , has not been found in plants. We attempted to detect the enzymatic activities for this pathway in cell-free systems from various tissues of trifoliata orange ( Poncirus trifoliata Rafin.). A high level of activity of LAM hydrolase, which catalyzes the conversion of IAM to indole-3-acetic acid, was observed in a crude extract prepared from young fruits one week after full bloom. Using α-naphthaleneacetamide as a competitor of IAM hydrolase, a simple assay system was developed for the detection of the conversion of L-tryptophan to IAM (tryptophan monooxygenase activity). When this system was used to assay cell-free extracts of young fruit of P. trifoliata , the conversion of L-tryptophan to IAM was clearly demonstrated by the presence of IAM among reaction products, as demonstrated by GC/MS analysis and the incorporation of 14C-labeled L-tryptophan into an IAM fraction. This is the first report indicating the presence of an auxin-biosynthetic pathway via IAM in P. trifoliata . Furthermore, it is shown that both enzyme activities in auxin biosynthesis increased transiently during fruit development.
Copyright (C) 1993, The Japanese Society of Plant Physiologists
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