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Abstract Proper cell-type identity relies on highly coordinated regulation of gene expression. Regulatory elements such as enhancers can produce cell type-specific expression patterns, but the mechanisms underlying specificity are not well understood. We previously identified an enhancer region capable of driving specific expression in giant cells, which are large, highly endoreduplicated cells in the Arabidopsis thaliana sepal epidermis. In this study, we use the giant cell enhancer as a model to understand the regulatory logic that promotes cell type-specific expression. Our dissection of the enhancer revealed that giant cell specificity is mediated primarily through the combination of two activators and one repressor. HD-ZIP and TCP transcription factors are involved in the activation of expression throughout the epidermis. High expression of HD-ZIP transcription factor genes in giant cells promoted higher expression driven by the enhancer in giant cells. Dof transcription factors repressed the activity of the enhancer such that only giant cells maintained enhancer activity. Thus, our data are consistent with a conceptual model whereby cell type-specific expression emerges from the combined activities of three transcription factor families activating and repressing expression in epidermal cells.more » « less
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SUMMARY Transcriptional regulators of the general stress response (GSR) reprogram the expression of selected genes to transduce informational signals into cellular events, ultimately manifested in a plant's ability to cope with environmental challenges. Identification of the core GSR regulatory proteins will uncover the principal modules and their mode of action in the establishment of adaptive responses. To define the GSR regulatory components, we employed a yeast‐one‐hybrid assay to identify the protein(s) binding to the previously established functional GSR motif, termed the rapid stress response element (RSRE). This led to the isolation of octadecanoid‐responsive AP2/ERF‐domain transcription factor 47 (ORA47), a methyl jasmonate inducible protein. Subsequently, ORA47 transcriptional activity was confirmed using the RSRE‐driven luciferase (LUC) activity assay performed in the ORA47 loss‐ and gain‐of‐function lines introgressed into the 4xRSRE::Luc background. In addition, the prime contribution of CALMODULIN‐BINDING TRANSCRIPTIONAL ACTIVATOR3 (CAMTA3) protein in the induction of RSRE was reaffirmed by genetic studies. Moreover, exogenous application of methyl jasmonate led to enhanced levels of
ORA47 andCAMTA3 transcripts, as well as the induction of RSRE::LUC activity. Metabolic analyses illustrated the reciprocal functional inputs of ORA47 and CAMTA3 in increasing JA levels. Lastly, transient assays identified JASMONATE ZIM‐domain1 (JAZ1) as a repressor of RSRE::LUC activity. Collectively, the present study provides fresh insight into the initial features of the mechanism that transduces informational signals into adaptive responses. This mechanism involves the functional interplay between the JA biosynthesis/signaling cascade and the transcriptional reprogramming that potentiates GSR. Furthermore, these findings offer a window into the role of intraorganellar communication in the establishment of adaptive responses. -
Abstract ARGONAUTES are the central effector proteins of
RNA silencing which bind target transcripts in a smallRNA ‐guided manner.Arabidopsis thaliana has 10 (ARGONAUTE ) genes, with specialized roles inAGO RNA ‐directedDNA methylation, post‐transcriptional gene silencing, and antiviral defense. To better understand specialization among genes at the level of transcriptional regulation we tested a library of 1497 transcription factors for binding to the promoters ofAGO ,AGO 1 , andAGO 10 using yeast 1‐hybrid assays. A ranked list of candidateAGO 7DNA ‐bindingTF s revealed binding of the promoter by a number of proteins in two families: the miR156‐regulatedAGO 7SPL family and the miR319‐regulatedTCP family, both of which have roles in developmental timing and leaf morphology. Possible functions forSPL andTCP binding are unclear: we showed that these binding sites are not required for the polar expression pattern of , nor for the function ofAGO 7 in leaf shape. NormalAGO 7 transcription levels and function appear to depend instead on an adjacent 124‐bp region. Progress in understanding the structure of this promoter may aid efforts to understand how the conservedAGO 7AGO 7‐triggered pathway functions in timing and polarity.TAS 3