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Regulation of root development in Arabidopsis thaliana by phytohormone-secreting epiphytic methylobacteria

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Abstract

In numerous experimental studies, seedlings of the model dicot Arabidopsis thaliana have been raised on sterile mineral salt agar. However, under natural conditions, no plant has ever grown in an environment without bacteria. Here, we document that germ-free (gnotobiotic) seedlings, raised on mineral salt agar without sucrose, develop very short root hairs. In the presence of a soil extract that contains naturally occurring microbes, root hair elongation is promoted; this effect can be mimicked by the addition of methylobacteria to germ-free seedlings. Using five different bacterial species (Methylobacterium mesophilicum, Methylobacterium extorquens, Methylobacterium oryzae, Methylobacterium podarium, and Methylobacterium radiotolerans), we show that, over 9 days of seedling development in a light-dark cycle, root development (hair elongation, length of the primary root, branching patterns) is regulated by these epiphytic microbes that occur in the rhizosphere of field-grown plants. In a sterile liquid culture test system, auxin (IAA) inhibited root growth with little effect on hair elongation and significantly stimulated hypocotyl enlargement. Cytokinins (trans-zeatin, kinetin) and ethylene (application of the precursor ACC) likewise exerted an inhibitory effect on root growth but, in contrast to IAA, drastically stimulated root hair elongation. Methylobacteria are phytosymbionts that produce/secrete cytokinins. We conclude that, under real-world conditions (soil), the provision of these phytohormones by methylobacteria (and other epiphytic microbes) regulates root development during seedling establishment.

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Acknowledgements

This work was supported by the Alexander von Humboldt Foundation (Bonn, Germany) (AvH Stanford 2013/2014 to UK). We thank Dr. Z.Y. Wang for the provision of plant material and consultation.

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  1. Institute of Biology, University of Kassel, Heinrich-Plett-Str. 40, 34132, Kassel, Germany

    Jana Klikno & Ulrich Kutschera

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  1. Jana Klikno
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  2. Ulrich Kutschera
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Correspondence to Ulrich Kutschera.

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Handling Editor: Peter Nick

Dedicated to the memory of Peter Sitte (1929–2015)

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Klikno, J., Kutschera, U. Regulation of root development in Arabidopsis thaliana by phytohormone-secreting epiphytic methylobacteria. Protoplasma 254, 1867–1877 (2017). https://doi.org/10.1007/s00709-016-1067-7

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