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Role of cytokinin in the regulation of root gravitropism

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Abstract

The models explaining root gravitropism propose that the growth response of plants to gravity is regulated by asymmetric distribution of auxin (indole-3-acetic acid, IAA). Since cytokinin has a negative regulatory role in root growth, we suspected that it might function as an inhibitor of tropic root elongation during gravity response. Therefore, we examined the free-bioactive-cytokinin-dependent ARR5::GUS expression pattern in root tips of transformants of Arabidopsis thaliana (L.) Heynh., visualized high cytokinin concentrations in the root cap with specific monoclonal antibodies, and complemented the analyses by external application of cytokinin. Our findings show that mainly the statocytes of the cap produce cytokinin, which may contribute to the regulation of root gravitropism. The homogenous symmetric expression of the cytokinin-responsive promoter in vertical root caps rapidly changed within less than 30 min of gravistimulation into an asymmetrical activation pattern, visualized as a lateral, distinctly stained, concentrated spot on the new lower root side of the cap cells. This asymmetric cytokinin distribution obviously caused initiation of a downward curvature near the root apex during the early rapid phase of gravity response, by inhibiting elongation at the lower side and promoting growth at the upper side of the distal elongation zone closely behind the root cap. Exogenous cytokinin applied to vertical roots induced root bending towards the application site, confirming the suspected inhibitory effect of cytokinin in root gravitropism. Our results suggest that the early root graviresponse is controlled by cytokinin. We conclude that both cytokinin and auxin are key hormones that regulate root gravitropism.

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Fig. 1a–j

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Abbreviations

ARR5::GUS :

Cytokinin-activated promoter sequence of a response regulator fused to β-glucuronidase

BA :

N6-Benzylaminopurine

CEZ :

Central elongation zone

DEZ :

Distal elongation zone

IAA :

Indole-3-acetic acid

IPT :

Isopentenyltransferase

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Acknowledgements

We thank Thomas Schmülling (Freie Universität Berlin, Germany) and Joseph J. Kieber (University of North Carolina, Chapel Hill, USA) for kindly providing seeds of ARR5::GUS transformants of Arabidopsis, and Elmar Weiler (Universität Bochum, Germany) for the generous gift of cytokinin antibodies. This work was supported by the Deutsche Forschungsgemeinschaft to C.I.U.

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Author notes

  1. Erez Aloni

    Present address: Department of Philosophy, The Hebrew University, 91905, Jerusalem, Israel

Authors and Affiliations

  1. Department of Plant Sciences, Tel Aviv University, 69978, Tel Aviv, Israel

    Roni Aloni & Erez Aloni

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

    Markus Langhans & Cornelia I. Ullrich

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  1. Roni Aloni
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  2. Markus Langhans
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  3. Erez Aloni
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  4. Cornelia I. Ullrich
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Correspondence to Roni Aloni.

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Aloni, R., Langhans, M., Aloni, E. et al. Role of cytokinin in the regulation of root gravitropism. Planta 220, 177–182 (2004). https://doi.org/10.1007/s00425-004-1381-8

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