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The transcriptional regulatory structure of plant genomes remains poorly defined relative to animals. It is unclear how many cis -regulatory elements exist, where these elements lie relative to promoters, and how these features are conserved across plant species. We employed the assay for transposase-accessible chromatin (ATAC-seq) in four plant species ( Arabidopsis thaliana , Medicago truncatula , Solanum lycopersicum , and Oryza sativa ) to delineate open chromatin regions and transcription factor (TF) binding sites across each genome. Despite 10-fold variation in intergenic space among species, the majority of open chromatin regions lie within 3 kb upstream of a transcription start site in all species. We find a common set of four TFs that appear to regulate conserved gene sets in the root tips of all four species, suggesting that TF-gene networks are generally conserved. Comparative ATAC-seq profiling of Arabidopsis root hair and non-hair cell types revealed extensive similarity as well as many cell-type-specific differences. Analyzing TF binding sites in differentially accessible regions identified a MYB-driven regulatory module unique to the hair cell, which appears to control both cell fate regulators and abiotic stress responses. Our analyses revealed common regulatory principles among species and shed light on the mechanisms producing cell-type-specific transcriptomes during development., (© 2018 American Society of Plant Biologists. All rights reserved.)

The number of root cortex cell layers varies among plants, and many species have several cortical cell layers. We recently demonstrated that the two rice orthologs of the Arabidopsis SHR gene, OsSHR1 and OsSHR2, could complement the A. thaliana shr mutant. Moreover, OsSHR1 and OsSHR2 expression in A. thaliana roots induced the formation of extra root cortical cell layers. In this article, we demonstrate that the overexpression of AtSHR and OsSHR2 in rice roots leads to plants with wide and short roots that contain a high number of extra cortical cell layers. We hypothesize that SHR genes share a conserved function in the control of cortical cell layer division and the number of ground tissue cell layers in land plants., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Agrobacterium rhizogenes (or Rhizobium rhizogenes) is able to transform plant genomes and induce the production of hairy roots. We describe the use of A. rhizogenes in tomato (Solanum spp.) to rapidly assess gene expression and function. Gene expression of reporters is indistinguishable in plants transformed by Agrobacterium tumefaciens as compared with A. rhizogenes. A root cell type- and tissue-specific promoter resource has been generated for domesticated and wild tomato (Solanum lycopersicum and Solanum pennellii, respectively) using these approaches. Imaging of tomato roots using A. rhizogenes coupled with laser scanning confocal microscopy is facilitated by the use of a membrane-tagged protein fused to a red fluorescent protein marker present in binary vectors. Tomato-optimized isolation of nuclei tagged in specific cell types and translating ribosome affinity purification binary vectors were generated and used to monitor associated messenger RNA abundance or chromatin modification. Finally, transcriptional reporters, translational reporters, and clustered regularly interspaced short palindromic repeats-associated nuclease9 genome editing demonstrate that SHORT-ROOT and SCARECROW gene function is conserved between Arabidopsis (Arabidopsis thaliana) and tomato., (© 2014 American Society of Plant Biologists. All Rights Reserved.)

In plants, MADS-box transcription factors are key regulators of floral and fruit development, organ dehiscence and stress responses. Nevertheless, the functions of most of them are still unknown. In Arabidopsis thaliana, the AGL17-like clade of MADS-box transcription factors comprises four members. AGL17 is involved in floral induction, whereas AGL44/ANR1 is involved in the adaptive development of roots in response to nitrate. AGL21 is primarily expressed in the roots and AGL16 in the leaves, suggesting that these transcription factors may be involved in the control of vegetative development. In Oryza sativa, the AGL17-like clade comprises five members, OsMADS23, OsMADS25, OsMADS27, OsMADS57 and OsMADS61. In a first attempt to characterize their functions, we used promoter::Gus reporter gene fusions and RT-qPCR to study the expression patterns of these genes and their regulation by different external stimuli. The OsMADS23, OsMADS25, OsMADS27 and OsMADS57 promoters were active in the root's central cylinder. In addition, the OsMADS57 promoter was active in leaves, whereas the OsMADS61 promoter was only active in the leaf tips and the stem base. OsMADS25 and OsMADS27 transcripts accumulated in response to osmotic stress, whereas the expression levels of OsMADS25, OsMADS27 and OsMADS57 were slightly induced by nitrate. Each of these five genes was responsive to various hormonal treatments. These distinct expression patterns indicate that these five genes have specific and non-redundant functions that likely differs from those of their A. thaliana homologs., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Organization of tissues in Arabidopsis thaliana root is made of, from outside in, epidermis, cortex, middle cortex, endodermis, pericycle and vascular tissues. Cortex, middle cortex and endodermis form the ground tissue (GT) system. Functional and molecular characterization of GT patterning mutants' properties has greatly increased our understanding of fundamental processes of plant root development. These studies have demonstrated GT is an elegant model that can be used to study how different cell types and cell fates are specified. This review analyzes GT mutants to provide a detailed account of the molecular network that regulates GT formation in A. thaliana. The most recent results indicate an unexpectedly complex network of transcription factors, epigenetic and hormonal controls that play crucial roles in GT development. Major differences exist between GT formation in dicots and monocots, particularly in the model plant rice, opening the way for evo-devo of GT formation in angiosperm. In rice, adaptation to submergence relies on a multilayered cortex. Moreover, variation in the number of cortex cell layers is also observed between the five root types. A mechanism of control for cortical cell number should then exist in rice and it remains to be determined if any of the Arabidopsis thaliana identified GT network members are also involved in this process in rice. Alternatively, a totally different network may have been invented. However, first available results suggest functional conservation in rice of at least two transcription factors, SHORT ROOT (SHR) and SCARECROW (SCR), involved in ground tissue formation in Arabidopsis., (Copyright © 2012 Elsevier Inc. All rights reserved.)