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DEVELOPMENT AND HORMONE ACTION

The Transcriptional Repressor ARR1-SRDX Suppresses Pleiotropic Cytokinin Activities in Arabidopsis^1,[C],[W]

Institute of Biology/Applied Genetics, Free University of Berlin, D–14195 Berlin, Germany (A.H., E.R., W.G.B., M.R., T.S.); and Katholieke Universiteit Leuven, ESAT/SCD, B–3001 Leuven-Heverlee, Belgium (J.A.)

The signal transduction of the phytohormone cytokinin is mediated by a multistep histidine-to-aspartate phosphorelay system. One component of this system are B-type response regulators, transcription factors mediating at least part of the response to cytokinin. In planta functional analysis of this family is hampered by the high level of functional redundancy of its 11 members. We generated a dominant repressor version of the Arabidopsis (Arabidopsis thaliana) response regulator ARR1 (ARR1-SRDX) using chimeric repressor silencing technology in order to study the extent of the contribution of B-type response regulators to cytokinin activities. In a protoplast test system, ARR1-SRDX suppressed ARR6:β-glucuronidase reporter gene activation by different B-type ARRs. 35S:ARR1-SRDX transgenic Arabidopsis plants showed phenotypic changes reminiscent of plants with a reduced cytokinin status, such as a strongly reduced leaf size, an enhanced root system, and larger seeds. Several bioassays showed that 35S:ARR1-SRDX plants have an increased resistance toward cytokinin. The rapid induction of a large part of the cytokinin response genes was dampened. The transcript levels of more than 500 genes were more than 2.5-fold reduced in 35S:ARR1-SRDX transgenic seedlings, suggesting a broad function of B-type ARRs. Collectively, the suppression of pleiotropic cytokinin activities by a dominant repressor version of a B-type ARR indicates that this protein family is involved in mediating most, if not all, of the cytokinin activities in Arabidopsis. In addition, a role for B-type ARRs in mediating cross talk with other pathways is supported by the resistance of 35S:ARR1-SRDX seeds to phytochrome B-mediated inhibition of germination by far-red light. This study demonstrates the usefulness of chimeric repressor silencing technology to overcome redundancy in transcription factor families for functional studies.

² These authors contributed equally to the article.

³ Present address: Flanders Interuniversity Institute for Biotechnology, MicroArray Facility, B–3000 Leuven, Belgium.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Thomas Schmülling (tschmue{at}zedat.fu-berlin.de).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.115436

^* Corresponding author; e-mail tschmue{at}zedat.fu-berlin.de.

Received December 21, 2007; accepted May 16, 2008; published May 23, 2008.

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