CONICET | Buscador de Institutos y Recursos Humanos

The aim of the present study was to evaluate phytohormone production by symbiotic and saprophytic actinomycetes isolated from the actinorhizal plant Ochetophila trinervis which had previously proved to stimulate nodulation by Frankia. Three saprophytic strains out of 122 isolated from the rhizosphere and rhizoplane of this plant with multiple enzymatic activities were selected for plant growth experiments: Streptomyces sp. (BCRU-MM40), Actinoplanes sp. (BCRU-ME3) and Micromonospora sp. (BCRU-MM18). For experiments, the symbiotic N2-fixing strain Frankia (BCU110501) isolated from nodules of the same actinorhizal plant was used. Phytohormone production was evaluated in supernatant of non-inoculated and inoculated culture media in exponential growth phase. Indole 3-acetic acid (IAA) and gibberellic acid (GA3) were analyzed by gas chromatography-mass spectrometry (GC-MS), while Zeatine (Z) production was determined by gas chromatography-flame ionization detector and high performance liquid chromatography (HPLC-fluorescent and UV). The levels of the three phytohormones produced by the rhizoactinomycetes was higher than that produced by the symbiotic Frankia strain. Zeatine biosynthesis was higher (µg/ml) than IAA and GA3 (ng/ml), and Micromonospora strain produced the highest levels of these phytohormones. Although O. trinervis has been shown to be intercellularly infected by Frankia without mediation of root hair deformation, when plants were co-inoculated with actinomycetes some root hair deformation was observed either with actinomycetes or the supernatant of actinomycetes culture. A similar deformation of root hair could be obtained thorugh root treatment with pure phythormones. This is the first report on identification of IAA, GA3 and Z in saprophytic actinomycetes and their potential role in plant-microbe interaction

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