Production of phytohormones by root-associated saprophytic actinomycetes isolated from the actinorhizal plant Ochetophila trinervis

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 of this plant with multiple enzymatic activities were selected for plant growth experiments in pots: Streptomyces sp. (BCRU-MM40), Actinoplanes sp. (BCRU-ME3) and Micromonospora sp. (BCRU-MM18). For experiments, the symbiotic N-2-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 (GA(3)) 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 saprophytic rhizoactinomycetes were higher than that produced by the symbiotic Frankia strain. Zeatine biosynthesis was higher (mu g ml(-1)) than IAA and GA(3) (ng ml(-1)), 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' culture, some root hair deformation was observed. This is the first report on identification of IAA, GA(3) and Z in saprophytic actinomycetes and their potential role in plant-microbe interaction.