C, Flower morphology of 35S: ARR1-SRDX transgenic plants compared with the wild type. The graph shows the time (DAG) of flower bud opening in transgenic plants compared with the wild type. B, Quantitative analysis of the early-flowering phenotype of 35S: ARR1-SRDX transgenic plants. The plants were grown under long-day conditions and photographed at 19 DAG. A, Flower induction occurs earlier in 35S: ARR1-SRDX transgenic plants compared with wild-type plants. Reproductive development of 35S: ARR1-SRDX transgenic plants. This study demonstrates the usefulness of chimeric repressor silencing technology to overcome redundancy in transcription factor families for functional studies. 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. 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. 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. The rapid induction of a large part of the cytokinin response genes was dampened. Several bioassays showed that 35S:ARR1-SRDX plants have an increased resistance toward cytokinin. 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. In a protoplast test system, ARR1-SRDX suppressed ARR6:beta-glucuronidase reporter gene activation by different B-type ARRs. 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 planta functional analysis of this family is hampered by the high level of functional redundancy of its 11 members. One component of this system are B-type response regulators, transcription factors mediating at least part of the response to cytokinin. The signal transduction of the phytohormone cytokinin is mediated by a multistep histidine-to-aspartate phosphorelay system.
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