Root-AssociatedEnterobacterandKlebsiellainPoa pratensis: Characterization of an Iron-Scavenging System and a Substance Stimulating Root Hair Production

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Abstract
Forty strains of Enterobacter agglomerans, E. aerogenes, E. cloacae, Klebsiella pneumoniae, and K. terrigena isolated from plants or humans were analyzed for iron-scavenging systems, plant growth-promoting effects on grasses, and production of auxins and related indole compounds. Enterochelin was produced by all isolates of Klebsiella, none of which produced aerobactin. None of the isolates of Enterobacter from plants produced enterochelin or aerobactin, whereas isolates from humans produced both siderophore types. Inoculation with each enterobacterial isolate significantly increased the number of root hairs of Poa pratensis, with no significant difference between bacteria from plants or humans. Cell free ethyl acetate extracts were tested on newly germinated roots of P. pratensis. Extracts obtained at pH 7.0 significantly increased the number of root hairs, whereas extracts obtained at pH 2.8 increased production of root hairs only in few plants. A bioactive compound causing increased production of root hairs was isolated and characterized from the culture supernatant of a strain of E. agglomerans. Gas chromatography-mass spectrometry (GC-MS) analysis of this compound proved that the bioactive substance was an auxin, indole-3-acetic acid. Thin-layer chromatographic analysis of the neutral extracts showed that the enterobacterial isolates produced at least 10 indole compounds from which eight were identified by GC-MS. Slight differences in spectra of indole compounds were observed between bacterial isolates from plants and humans, but indole-3-acetic acid was detected in 88% of the enterobacterial isolates.

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