A bHLH Complex Activates Vascular Cell Division via Cytokinin Action in Root Apical Meristem

Abstract

Higher organisms possess mechanisms to maintain stem cells' proliferative and pluripotent states in stem cell niches [1]. Plants possess two types of stem cell niches in the root and shoot apical meristems, where regulatory interactions exist between stem cells and organizing cells. Recent studies provided new insights into the molecular mechanism of stem cell maintenance [2-4]. However, earlier and more essential developmental events such as the acquisition of stem cell proliferative activity are still unknown. In vascular tissues, procambial cells function as stem cells and differentiate into xylem, phloem, and procambium. Procambial cell proliferation starts at root apical meristem (RAM) postembryonically; therefore, procambial cell development in RAM is a good model for investigating the regulation of stem cell proliferation. LONESOME HIGHWAY (LHW) and TARGET OF MONOPTEROS5 (TMO5), as well as its homolog, TMO5-LIKE1 (T5L1), encode bHLH proteins that function as heterodimers (LHW-TMO5 and LHW-T5L1) in vascular tissue organization [5-7]. LHW-T5L1 promotes vascular-cell-specific proliferation in RAM [7]. Here, we demonstrate that LHW-T5L1 promotes expression of key cytokinin production genes, including LONELY GUY3 (LOG3) and LOG4, in xylem precursor cells, resulting in elevated cytokinin levels in the surrounding cells. LHW-T5L1 can also promote expression of AHP6, which suppresses cytokinin signaling and then maintains xylem precursor cells at a nondividing state. Our results indicate that LHW-T5L1 establishes xylem precursor cells as a signal center that promotes procambial-cell-specific proliferation through cytokinin response.

Building a plant: cell fate specification in the early Arabidopsis embryo

“…4). An independent study that focused on the LHW-T5L1 complex identified LOG3, LOG4 and APH6 as its direct targets (Ohashi-Ito et al, 2014). Here, TMO5 promoter-driven LOG3 expression was able to rescue the vascular pattern defect of the log3 log4 log7 triple mutant, confirming that local LHW-induced and TMO5/T5L1-induced active CK production is vital for the activation of vascular cell division and, hence, the establishment of vascular patterning.…”

Breeding maize for silage and biofuel production, an illustration of a step forward with the genome sequence

“…In Arabidopsis, the LONESOME HIGH-WAY (LHW) gene and its three homologs LHW-like 1-3 (LHL1-LHL3) encode atypical bHLH transcription factors, which were considered to play an essential role in vascular development [137,138]. Similarly, the poplar bHLH BEE3-like gene (Brassinosteroid Enhanced Expression 3), was considered to increase the proliferation of xylem cells, thus promoting plant initial thickening [139].…”

AspWood: High-Spatial-Resolution Transcriptome Profiles Reveal Uncharacterized Modularity of Wood Formation in Populus tremula

“…TMO5 and LHW function as heterodimers, and coaccumulation of their transcripts in xylem precursor cells in root meristem is necessary and sufficient to trigger periclinal cell divisions of the adjacent procambium cells (De Rybel et al, 2013). This non-cell-autonomous stimulation of cell division activity appears to act through local biosynthesis of cytokinin by the LONELY GUY3 (LOG3) and LOG4 (De Rybel et al, 2014;Ohashi-Ito et al, 2014), which acts as a mobile signal to promote periclinal cell divisions in the procambium. We found that although the closest homologs of the TMO5 and LHW genes (Supplemental Data Set 9) had complex expression patterns in the wood-forming zone of aspen, high expression of both genes intersected in the xylem expansion zone (sample cluster ii; Figure 4C).…”