Physiological evidence for differently regulated tryptophan-dependent pathways for indole-3-acetic acid synthesis in Azo
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O R I GI N A L P A P E R
R. CarrenÄo-Lopez á N. Campos-Reales á C. Elmerich B. E. Baca
Physiological evidence for differently regulated tryptophan-dependent pathways for indole-3-acetic acid synthesis in Azospirillum brasilense Received: 13 January 2000 / Accepted: 20 July 2000 / Published online: 8 September 2000 Ó Springer-Verlag 2000
Abstract Disruption of ipdC , a gene involved in indole3-acetic acid (IAA) production by the indole pyruvate pathway in Azospirillum brasilense Sp7, resulted in a mutant strain that was not impaired in IAA production with lactate or pyruvate as the carbon source. A tryptophan auxotroph that is unable to convert indole to tryptophan produced IAA if tryptophan was present but did not synthesise IAA from indole. Similar results were obtained for a mutant strain with additional mutations in the genes ipdC and trpD . This suggests the existence of an alternative Trp-dependent route for IAA synthesis. On gluconate as a carbon source, IAA production by the ipdC mutant was inhibited, suggesting that the alternative route is regulated by catabolite repression. Using permeabilised cells we observed the enzymatic conversion of tryptamine and indole-3-acetonitrile to IAA, both in the wild-type and in the ipdC mutant. IAA production from tryptamine was strongly decreased when gluconate was the carbon source. Key words Azospirillum brasilense á Indole-3-acetic acid synthesis á ipdC gene á Tryptamine á Indole-3acetonitrile
Introduction Azospirillum is a gram-negative, nitrogen-®xing bacterium that lives in soil and in association with the roots of grasses and cereals. This bacterium increases plant R. CarrenÄo-Lopez á C. Elmerich (&) Unite de Physiologie Cellulaire, CNRS URA D2172, DeÂpartement des Biotechnologies, Institut Pasteur, 25±28, Rue du Dr. Roux, 75724 Paris Cedex, France E-mail: [email protected] Tel.: +33-1-45688817 Fax: +33-1-45688790 R. CarrenÄo-Lopez á N. Campos-Reales á B. E. Baca Centro de Investigaciones MicrobioloÂgicas, Universidad AutoÂnoma de Puebla, Apdo. Postal 1622, C. P. 72000, Puebla, Pue. MeÂxico
growth primarily by colonising the root surface, causing an increase in the proliferation of the root hairs and root system of the host plant. This eect is thought to result from the production of auxin-like compounds, such as indole-3-acetic acid (IAA), by the bacterium, because application of IAA mimics the eect of inoculation with the bacteria (for a review see Costacurta and Vanderleyden 1995). No mutant strain that is totally devoid of IAA production has yet been described in Azospirillum brasilense (Hartmann and Zimmer 1994). There are several biosynthetic pathways for IAA in prokaryotes (Fig. 1) and a given bacterial species may use more than one pathway (Hartmann and Zimmer 1994; Patten and Glick 1996; Vande Broek et al. 2000). The indole pyruvate (IPyA) route, which is believed to be the main pathway for IAA synthesis in plants, has recently been reported in bacteria (Koga et al. 1991; Costacurta et al. 1994; Zimmer et al. 1994, 1998; Brandl and Lindow 1996).
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