Surprisingly, besides quantitative changes of the root system and a defect of vascular development in the arr1 arr10 arr12 triple mutant ( Mason et al., 2005 Yokoyama et al., 2007), no strong morphological alterations were described. However, different cytokinin response assays showed increasing cytokinin resistance for higher order mutants ( Mason et al., 2005 Yokoyama et al., 2007). Loss-of-function mutants of single B-type ARR genes ( ARR1, ARR2, ARR10, ARR11, and ARR18) caused no strong phenotypic alterations. The analysis of B-type ARR mutants has revealed their involvement in cytokinin signaling but also a high degree of functional redundancy ( Sakai et al., 2001 Horák et al., 2003 Hass et al., 2004 Mason et al., 2005). Reverse transcription-PCR, microarray, and promoter- GUS fusion experiments have demonstrated that the members of the B-type ARR family have large and overlapping expression domains ( Mason et al., 2004 Tajima et al., 2004 Zimmermann et al., 2004 summarized by Heyl et al., 2006). B-type ARRs regulate the transcription of their target genes in response to cytokinin treatment ( Sakai et al., 2000 Hwang and Sheen, 2001 Lohrmann et al., 2001 Imamura et al., 2003). Several B-type ARRs have been shown to bind to the same or very similar sequence motifs ( Sakai et al., 2000 Hosoda et al., 2002 Imamura et al., 2003). The DNA-binding domains share a high degree of sequence conservation, in particular in the nine amino acids that were identified in ARR10 to be most likely in direct contact with the DNA ( Hosoda et al., 2002). They possess, in addition to the response regulator domain, a MYB-class DNA-binding domain ( Riechmann et al., 2000). A negative feedback on the cytokinin signaling pathway was shown to be mediated by members of this protein class ( Hwang and Sheen, 2001 To et al., 2004).Īrabidopsis encodes 11 B-type ARRs. These transcription factors regulate the transcription of their target genes, one group of which codes for A-type response regulators. After transphosphorylation within the receptor, the phosphate group is transferred to His phosphotransfer proteins, which subsequently locate to the nucleus, where they activate B-type response regulators (ARRs) via phosphorylation. In the current model, which has been developed mainly in Arabidopsis ( Arabidopsis thaliana), the hormone is perceived by membrane-bound hybrid His-kinase receptors, which autophosphorylate upon binding of the hormone ligand. In recent years, significant progress has been made toward the understanding of how the cytokinin signal is perceived and transduced ( Heyl and Schmülling, 2003 Kakimoto, 2003 Mizuno, 2004 Heyl et al., 2006 Hwang and Sakakibara, 2006 Müller and Sheen, 2007). The plant hormone cytokinin is involved in many developmental processes and plays a critical role in numerous physiological responses to changes in the environment ( Mok and Mok, 2001). 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: β-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.
0 kommentar(er)
