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Auxins: effects on bacteria and tumours

Background: Phytohormones auxines are a group of molecules involved in mediating a number of essential plant growth and developmental processes. These plant hormones are not only synthesized by higher plants, but also by lichens, mosses, fungi and bacteria. Furthermore, not only bacterial species that live in soil and/or interact with plants produce phytohormons. Some human-associated and pathogenic bacteria produce auxins, too. It was shown that indoleacetic acid (IAA), as the most widespread natural auxin present in most living organisms, can trigger alterations in the main metabolic pathways of bacterial, yeast and human cells. Methods: Auxins indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) were used to treat phytoplasmas and tumours. Phytoplasma-infected plants were maintained on Murashige and Skoog medium supplemented with IAA (11 or 57μ M) or IBA (2.5, 4.9, 9.8 or 19.7 μ M). Randomly chosen shoots from each treatment were tested for the presence of phytoplasmas by nested polymerase chain reaction amplifying highly conserved phytoplasmal 16S rRNA gene. Cytotstatic activity on tumour cells was assessed on 5 human cell lines, which are derived from 4 cancer types: MCF-7 (breast carcinoma), SW 620 (colon carcinoma), HCT 116 (colon carcinoma), MOLT-4 (acute lymphoblastic leukemia), H 460 (lung carcinoma). IAA and IBA were added to the cell culture medium in five, 10-fold dilutions (10-7 to 10-3 M) and incubated for 72 hours. The cell growth rate was evaluated by performing the MTT assay, which detects dehydrogenase activity in viable cells. Results: The mechanism of auxins effect on phytoplasmas (mycoplasma-like organisms, economically important plant pathogenic bacteria), involves changes in plant-host metabolism and gene expression. It was shown that phytoplasma-free plants can be obtained upon treatment of infected plants with IBA, while treatment with IAA induced recovery of plants, but the pathogen was still present in the plant tissue. IAA also inhibited the growth of tumor cells (IC50 ≈ 500 μ M), whereby the breast cancer cell line MCF-7 was the most sensitive cell line. Thus both auxines could be further evaluated as novel anticancer therapeutics. Conclusions: Auxins exhibit both antibacterial and antitumour activity.

auxins; phytoplasma; cytostatic; antibacterial; antitumour

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