title: 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.
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan. Electronic address: kyoko@bs.s.u-tokyo.ac.jp.
authors.forename: Kyoko
authors.initials: K
authors.lastname: Ohashi-Ito
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.
authors.forename: Maria
authors.initials: M
authors.lastname: Saegusa
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.
authors.forename: Kuninori
authors.initials: K
authors.lastname: Iwamoto
authors.affiliation: Center for Frontier Research, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540, Japan; Department of Genetics, Graduate University for Advanced Studies (SOKENDAI), 1111 Yata, Mishima, Shizuoka 411-8540, Japan; Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Oda
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan.
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Katayama
authors.affiliation: Plant Productivity Systems Research Group, RIKEN Center for Sustainable Resource Science, 1-7-22 Suehiro, Tsurumi, Yokohama 230-0045, Japan.
authors.forename: Mikiko
authors.initials: M
authors.lastname: Kojima
authors.affiliation: Plant Productivity Systems Research Group, RIKEN Center for Sustainable Resource Science, 1-7-22 Suehiro, Tsurumi, Yokohama 230-0045, Japan.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Sakakibara
authors.affiliation: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan. Electronic address: fukuda@bs.s.u-tokyo.ac.jp.
authors.forename: Hiroo
authors.initials: H
authors.lastname: Fukuda
mesh_terms: D017360:Arabidopsis / Q000235:genetics* / Q000254:growth & development* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics* / Q000378:metabolism; D002455:Cell Division; D003583:Cytokinins / Q000235:genetics / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D018506:Gene Expression Regulation, Plant; D018519:Meristem / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D008969:Molecular Sequence Data; D020411:Oligonucleotide Array Sequence Analysis; D052585:Phloem / Q000166:cytology / Q000254:growth & development; D018517:Plant Roots / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D017422:Sequence Analysis, DNA; D013234:Stem Cells / Q000378:metabolism; D052584:Xylem / Q000166:cytology / Q000254:growth & development
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D003583:Cytokinins