Auxin-dependent compositional change in Mediator in ARF7- and ARF19-mediated transcription
Saturday, 2016/06/11 | 02:19:46
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Jun Ito, Hidehiro Fukaki, Makoto Onoda, Lin Li, Chuanyou Li, Masao Tasaka, and Masahiko Furutani SignificanceMediator complex relays the information from transcription factors to RNA polymerase II. Our results show that Mediator transmits auxin-dependent transcription through its compositional change in lateral root formation. The AUXIN/INDOLE 3-ACETIC ACID 14 (IAA14) transcriptional repressor inhibits the transcriptional activity of its binding partners AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19 by interacting with the dissociable CDK8 kinase module (CKM) of Mediator, a putative blocker of RNA polymerase II recruitment. Auxin-induced degradation of IAA14 dissociates the CKM component but not other Mediator subunits from ARF7 binding to the upstream region of its target gene. We suggest that this compositional change in Mediator enables a quick switch of information transmission from ARFs to target gene expression in response to auxin. AbstractMediator is a multiprotein complex that integrates the signals from transcription factors binding to the promoter and transmits them to achieve gene transcription. The subunits of Mediator complex reside in four modules: the head, middle, tail, and dissociable CDK8 kinase module (CKM). The head, middle, and tail modules form the core Mediator complex, and the association of CKM can modify the function of Mediator in transcription. Here, we show genetic and biochemical evidence that CKM-associated Mediator transmits auxin-dependent transcriptional repression in lateral root (LR) formation. The AUXIN/INDOLE 3-ACETIC ACID 14 (Aux/IAA14) transcriptional repressor inhibits the transcriptional activity of its binding partners AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19 by making a complex with the CKM-associated Mediator. In addition, TOPLESS (TPL), a transcriptional corepressor, forms a bridge between IAA14 and the CKM component MED13 through the physical interaction. ChIP assays show that auxin induces the dissociation of MED13 but not the tail module component MED25 from the ARF7 binding region upstream of its target gene. These findings indicate that auxin-induced degradation of IAA14 changes the module composition of Mediator interacting with ARF7 and ARF19 in the upstream region of their target genes involved in LR formation. We suggest that this regulation leads to a quick switch of signal transmission from ARFs to target gene expression in response to auxin.
See http://www.pnas.org/content/113/23/6562.abstract.html?etoc PNAS June 7 2016; vol.113; no.23: 6562–6567
Fig. 1. The effect of mab2 on the SLR/IAA14-ARF7-ARF19 auxin signaling module in LR formation. (A) Fourteen-day-old seedlings of Columbia, mab2-4, slr-1, mab2-4 slr-1, arf7-1 arf19-1, mab2-4 arf7-1 arf19-1, and mab2-4 arf7-1 arf19-1 slr-1. (Scale bar: 1 mm.) (B) LR density (emerged LR number per primary root length) for Columbia (Col; n = 20), mab2-4 (n = 21), slr-1 (n = 20), mab2-4 slr-1 (n = 22), arf7-1 arf19-1 (n = 20), mab2-4 arf7-1 arf19-1 (n = 21), and mab2-4 arf7-1 arf19-1 slr-1 (n = 20). Seven-day-old seedlings were used in this analysis. Data are presented as means ± SD. A two-tailed Student’s t test was performed. NS, not significant. *Significant differences: P < 0.05. (C) Nomarski images of roots of WT, mab2-1, and pIAA14::mIAA14-GFP transgenic plants in WT and mab2-1 backgrounds at 8 d. The arrowheads indicate the cell wall. (Scale bar: 50 µm.) (D–G) Expression of pLBD16::β-glucuronidase (GUS) in the mature root region of Col (D), slr-1 (E), mab2-4 (F), and mab2-4 slr-1 (G) for 10-d-old seedlings. The arrowheads indicate the cell wall of LR primordia. (Scale bars: 50 µm.) (H) Expression of LBD16 in Col and mab2-4 expressing pIAA14::mIAA14-GFP. The roots at 7 d were harvested after auxin treatment (black) and without auxin treatment (white). The relative abundance of LBD16 mRNA to ACT8 mRNA was measured by quantitative RT-PCR. The value measured for Col without auxin treatment was set at one. Data represent means ± SD (n = 3 independent biological replicates). Different letters in each graph indicate the statistical differences (P < 0.05; Student’s t test). |
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