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Azospirillum brasilense is a Gramnegative, nitrogen-fixing bacterium found in the rhizosphere of many plant species. The plant growth promotion observed after inoculation with A. brasilense is mainly caused by biosynthesis and secretion of bacterial IAA (Ref. 6). At least three pathways for IAA biosynthesis exist in A. brasilense, two trytophan-dependent pathways (IAM and IPyA) and one tryptophan-independent pathway7. Our recent data indicate that indole-3-pyruvate decarboxylase (encoded by the ipdC gene) is a common enzyme for both the IPyA pathway and the tryptophan-independent pathway (the precursor of which is so far unknown), suggesting that both pathways merge at the IPyA intermediate (branched IPyA pathway). Expression of the A. brasilense ipdC gene is upregulated by IAA (Ref. 8); this is the first description of a bacterial gene that is specifically induced by auxins. The