Azospirillum brasilense Indole-3-Acetic Acid Biosynthesis: Evidence for a Non-Tryptophan Dependent Pathway. E. Prinsen. University of Antwerp (UIA), Department of Biology, Universiteitsplein 1, B-2610 Wilrijk, Belgium. A. Costacurta(2), K. Michiels(2), J. Vanderleyden(2), and H. Van Onckelen(1). (1)University of Antwerp (UIA), Department of Biology, Universiteitsplein 1, B-2610 Wilrijk, Belgium. (2)F. A. Janssens Laboratory of Genetics, Catholic University of Leuven, W. de Croylaan 42, B-3001 Heverlee, Belgium.. MPMI 6:609-615. Accepted 1 June 1993. Copyright 1993 The American Phytopathological Society.

Bacteria of the nitrogen-fixing genus Azospirillum live in association with roots of many plants. Bacterial phytohormone synthesis is proposed to influence the host plant root proliferation. Analysis of tryptophan (Trp), indole-3-acetamide (IAM), and indole-3-acetic acid (IAA) synthesis of the mutant Azospirillum brasilense strain SpM7918 showed an indoleacetamide accumulation concomitant with reduced indoleacetic acid synthesis. The IAA deficiency, and IAM accumulation could be reversed with a specific cosmid from an A. brasilense Sp245 library. The identity of the indoleacetic acid and indoleacetamide produced was confirmed by HPLC with on-line mass spectrometry. Specific radioactivities of tryptophan, indoleacetamide, and indoleacetic acid formed during ³H-IAM and ³H-Trp feeding experiments revealed multiple IAA biosynthetic pathways in Azospirillum: the indoleacetamide pathway, a second tryptophan-dependent, and a tryptophan-independent pathway, the latter being predominant in case no tryptophan was supplied to the medium. This report is the first to demonstrate tryptophan-independent indoleacetic acid synthesis in bacteria.