Regulation of theArabidopsisroot vascular initial population byLONESOME HIGHWAY

Abstract

Complex organisms consist of a multitude of cell types arranged in a precise spatial relation to each other. Arabidopsis roots generally exhibit radial tissue organization; however, within a tissue layer, cells are not identical. Specific vascular cell types are arranged in diametrically opposed longitudinal files that maximize the distance between them and create a bilaterally symmetric (diarch) root. Mutations in the LONESOME HIGHWAY (LHW) gene eliminate bilateral symmetry and reduce the number of cells in the center of the root, resulting in roots with only single xylem and phloem poles. LHW does not appear to be required for the creation of any specific cell type, but coordinately controls the number of all vascular cell types by regulating the size of the pool of cells from which they arise. We cloned LHW and found that it encodes a protein with weak sequence similarity to basic helix-loophelix (bHLH)-domain proteins. LHW is a transcriptional activator in vitro. In plants, LHW is nuclear-localized and is expressed in the root meristems, where we hypothesize it acts independently of other known root-patterning genes to promote the production of stele cells, but might also indirectly feed into established regulatory networks for the maintenance of the root meristem.

Vascular Cambium Development

“…TMO5 interacts with LONESOME HIGHWAY (LHW/ bHLH156; At2g27230), another bHLH transcription factor, and together they promote periclinal divisions in the vascular tissue of the embryonic and postembryonic root. Absence of the TMO5 or LHW family members results in a dramatic reduction in periclinal divisions within the vasculature, and therefore in the number of the vascular cell files (Ohashi- Ito and Bergmann, 2007;De Rybel et al, 2013;Ohashi-Ito et al, 2013). Remarkably, 78% of Arabidopsis genes are expressed during the course of embryogenesis, and 55% in the mature embryo (Xiang et al, 2011).…”

Combined Large-Scale Phenotyping and Transcriptomics in Maize Reveals a Robust Growth Regulatory Network

“…This subnetwork was enriched for two functional categories, regulation of transcription (12 genes) and cell wall synthesis and degradation (three genes). The node with the most edges in this subnetwork was a cell wall-related gene, KOR (GRMZM2G099101), and was coexpressed with eight TFs: three bHLH TFs, GRMZM2G074438, GRMZM2G128807, and GRMZM2G158281, the latter showing orthology to Arabidopsis LONESOME HIGHWAY, which is required for establishing and maintaining the normal vascular cell number and pattern in roots (Ohashi-Ito and Bergmann, 2007); two SET domain TFs, GRMZM2G409224 and GRMZM2G318803, the latter being a homolog of Arabidopsis SUVR4; E2F/DP (GRMZM2G462623) and GRF (GRMZM2G099862), both having homologs in Arabidopsis with known roles in leaf development (De Veylder et al, 2002;Kim et al, 2003); and an AGO gene with homology to Arabidopsis AGO4 (GRMZM2G589579). Other genes coexpressed in this subnetwork with know annotations include the cell cycle gene CYCLIN A2 (GRMZM2G017081), involved in vein development in Arabidopsis (Donner and Scarpella, 2013), and BRASSINOSTEROID SIGNALING KINASE2 (GRMZM2G054634), affecting growth through its role in the initial steps of brassinosteroid signal transduction in Arabidopsis (Sreeramulu et al, 2013).…”

To branch or not to branch: the role of pre-patterning in lateral root formation

“…Because the root tissue examined for oscillating transcriptional profiles was specific to longitudinal regions but encompassed all root tissues, the tissue-specific nature of any oscillating transcripts was not captured (Moreno-Risueno et al, 2010). The necessity for vascular continuity between primary and lateral roots might be a crucial reason for coordination between vascular patterning and LR pre-patterning, and this is supported by additional molecular connections between vascular patterning and the development of LRP (Bonke et al, 2003;Ohashi-Ito and Bergmann, 2007;Parizot et al, 2008). However, the role of cell-to-cell signaling between protoxylem and XPP cells remains an intriguing question requiring further investigation.…”