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Published on May 23, 2008; 10.1104/pp.107.115436

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Received December 21, 2007
Accepted May 16, 2008

The Transcriptional Repressor ARR1-SRDX Suppresses Pleiotropic Cytokinin Activities in Arabidopsis

Alexander Heyl , Eswar Ramireddy , Wolfram G. Brenner , Michael Riefler , Joke Allemeersch , and Thomas Schmulling *

Institute of Biology/Applied Genetics, Free University of Berlin, Albrecht-Thaer-Weg 6, D-14195 Berlin, Germany; K.U.Leuven, ESAT/SCD, Kasteelpark Arenberg 10, B-3001 Leuven-Heverlee, Belgium

* Corresponding author; email: tschmue{at}zedat.fu-berlin.de.

The signal transduction of the phytohormone cytokinin is mediated by a multi-step His-to-Asp 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 eleven members. We generated a dominant repressor version of Arabidopsis response regulator ARR1 (ARR1-SRDX) using chimeric repressor silencing technology (CRES-T) 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:GUS 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 towards cytokinin. The rapid induction of a large part of cytokinin response genes was dampened. The transcript levels of more than 500 genes were >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 crosstalk with other pathways is supported by resistance of 35S:ARR1-SRDX seeds to phyB-mediated inhibition of germination by far-red light. This study demonstrates the usefulness of CRES-T to overcome redundancy in transcription factor families for functional studies.




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