Identification and quantification of auxins in culture media of Azospirillum and Klebsiella and their effect on rice roots
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The production of auxins [indoleacetic acid (IAA) and related compounds] by Azospirillum brasilense and Klebsiella pneumoniae in culture media supplemented with tryptophan was investigated. Extracts from the supernatants of cultures incubated for different periods of time were partitioned into two fractions and each fraction was analysed separately by using HPLC. The chromatography was performed by using different optimized mobile phases and corresponding standards. Both N-fixing strains produced high quantities of extracellular IAA and tryptophol. The production of both compounds increased during the stationary phase of the culture. However, very low levels of indoleacetaldehyde and indolepyruvate produced by K. pneumoniae in the exponential phase of the culture, were only recorded when the samples were analysed immediately after the extraction. In addition, the effect of different concentrations of filter-sterilized culture supernatants of both strains on the development of rice roots grown in hydroponic culture medium was studied. Addition of the optimum concentrations (6–8%) of bacterial supernatants to such hydroponic cultures increased root elongation, root surface area, root dry matter and development of lateral roots and root hairs compared to untreated roots. On the other hand, the addition of high concentrations of the supernatant (more than 10%) strongly inhibited root elongation, lateral root development, and caused root outgrowths, i.e. round nodule-like tumours.
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