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Soil bacteria and protozoa affect root branching via effects on the auxin and cytokinin balance in plants

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Abstract

Lateral roots are crucial for the plasticity of root responses to environmental conditions in soil. The bacterivorous microfauna has been shown to increase root branching and to foster auxin producing soil bacteria. However, information on modifications of plant internal auxin content by soil bacteria and bacterivores is missing. Therefore, the effects of a rhizosphere bacterial community and a common soil amoeba (Acanthamoeba castellanii) on root branching and on auxin (indole-3-acetic acid) metabolism in Lepidium sativum and Arabidopsis thaliana were investigated. In a first experimental series, bacteria increased conjugated auxin concentrations in L. sativum shoots, but did not alter free bioactive auxin content nor root branching. In contrast, in presence of soil bacteria plus amoebae free auxin concentrations in shoots and root branching increased, demonstrating that effects of bacteria on auxin metabolism in plants were strongly modified by the bacterivorous amoebae. In a second experiment, A. thaliana reporter plants for auxin (DR5) and cytokinin (ARR5) responded similarly with increased root branching in the presence of amoebae. Surprisingly, in reporter plants cytokinin but not auxin responses were detectable, accompanied by higher soil nitrate concentrations in the presence of amoebae. Likely, increased nitrate concentrations in the rhizosphere led to an accumulation of cytokinin and interactions with free auxin in plants and finally to increased root growth in the presence of amoebae. Altogether, the results show that mutual control mechanisms exist between plant hormone metabolism and microbial signalling, and that effects on hormonal concentrations of plants by free-living bacteria are strongly influenced by bacterial grazers like amoebae.

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Acknowledgements

The seeds of the DR5::GUS transgenic line, A. thaliana ecotype Columbia-0, were kindly provided by G. Hagen and T. J. Guilfoyle (University of North Carolina, Wilmington, USA); and seeds of ARR5::GUS transformants of A. thaliana ecotype Wassilewskija, constructed by J. J. Kieber (University of North Carolina, USA) were kindly provided by T. Schmülling (Freie Universität Berlin, Germany) and J. J Kieber. This study was part of the “Virtual Institute of Biotic Interactions” supported by the Helmholtz Association.

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Authors and Affiliations

  1. Darmstadt University of Technology, Institute of Zoology, Schnittspahnstr. 3, 64287, Darmstadt, Germany

    K. Krome, K. Rosenberg, K. Kreuzer, S. Scheu & M. Bonkowski

  2. Darmstadt University of Technology, Institute of Botany, Schnittspahnstr. 3, 64287, Darmstadt, Germany

    C. Dickler & C. Ullrich-Eberius

  3. Dresden University of Technology, Institute of Botany, Zellescher Weg 22, 01062, Dresden, Germany

    J. Ludwig-Müller

  4. Dept. Terrestrial Ecology, University of Cologne, Zoological Institute, Weyertal 119, 50931, Köln, Germany

    M. Bonkowski

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  1. K. Krome
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  2. K. Rosenberg
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  3. C. Dickler
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  4. K. Kreuzer
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  5. J. Ludwig-Müller
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  6. C. Ullrich-Eberius
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  7. S. Scheu
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  8. M. Bonkowski
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Correspondence to M. Bonkowski.

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Krome, K., Rosenberg, K., Dickler, C. et al. Soil bacteria and protozoa affect root branching via effects on the auxin and cytokinin balance in plants. Plant Soil 328, 191–201 (2010). https://doi.org/10.1007/s11104-009-0101-3

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