title: Transcriptional and Hormonal Regulation of Gravitropism of Woody Stems in Populus.
abstract: Angiosperm trees reorient their woody stems by asymmetrically producing a specialized xylem tissue, tension wood, which exerts a strong contractile force resulting in negative gravitropism of the stem. Here, we show, in Populus trees, that initial gravity perception and response occurs in specialized cells through sedimentation of starch-filled amyloplasts and relocalization of the auxin transport protein, PIN3. Gibberellic acid treatment stimulates the rate of tension wood formation and gravibending and enhances tissue-specific expression of an auxin-responsive reporter. Gravibending, maturation of contractile fibers, and gibberellic acid (GA) stimulation of tension wood formation are all sensitive to transcript levels of the Class I KNOX homeodomain transcription factor-encoding gene ARBORKNOX2 (ARK2). We generated genome-wide transcriptomes for trees in which gene expression was perturbed by gravistimulation, GA treatment, and modulation of ARK2 expression. These data were employed in computational analyses to model the transcriptional networks underlying wood formation, including identification and dissection of gene coexpression modules associated with wood phenotypes, GA response, and ARK2 binding to genes within modules. We propose a model for gravitropism in the woody stem in which the peripheral location of PIN3-expressing cells relative to the cambium results in auxin transport toward the cambium in the top of the stem, triggering tension wood formation, while transport away from the cambium in the bottom of the stem triggers opposite wood formation.
authors.affiliation: US Forest Service, Pacific Southwest Research Station, Davis, California 95618.
authors.forename: Suzanne
authors.initials: S
authors.lastname: Gerttula
authors.affiliation: US Forest Service, Pacific Southwest Research Station, Davis, California 95618.
authors.forename: Matthew
authors.initials: M
authors.lastname: Zinkgraf
authors.affiliation: Wake Forest University, Winston-Salem, North Carolina 27106.
authors.forename: Gloria K
authors.identifier: http://orcid.org/0000-0002-0377-4517
authors.initials: GK
authors.lastname: Muday
authors.affiliation: Wake Forest University, Winston-Salem, North Carolina 27106.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Lewis
authors.affiliation: Division of Glycoscience, School of Biotechnology, Royal Institute of Technology, AlbaNova University Centre, S-106 91 Stockholm, Sweden Biophysics Division, Petersburg Nuclear Physics Institute, National Research Center Kurchatov Institute, Gatchina 188300, Russia.
authors.forename: Farid M
authors.identifier: http://orcid.org/0000-0002-7660-4172
authors.initials: FM
authors.lastname: Ibatullin
authors.affiliation: Division of Glycoscience, School of Biotechnology, Royal Institute of Technology, AlbaNova University Centre, S-106 91 Stockholm, Sweden Michael Smith Laboratories and Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada.
authors.forename: Harry
authors.initials: H
authors.lastname: Brumer
authors.affiliation: Department of Wood Science, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada.
authors.forename: Foster
authors.initials: F
authors.lastname: Hart
authors.affiliation: Department of Wood Science, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada.
authors.forename: Shawn D
authors.identifier: http://orcid.org/0000-0002-0175-554X
authors.initials: SD
authors.lastname: Mansfield
authors.affiliation: Department of Computer Science, University of California, Davis, California 95618.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Filkov
authors.affiliation: US Forest Service, Pacific Southwest Research Station, Davis, California 95618 Department of Plant Biology, University of California, Davis, California 95618 agroover@fs.fed.us.
authors.forename: Andrew
authors.identifier: http://orcid.org/0000-0002-6686-5774
authors.initials: A
authors.lastname: Groover
mesh_terms: D058506:Cambium / Q000166:cytology / Q000235:genetics / Q000502:physiology; D020869:Gene Expression Profiling; D005875:Gibberellins / Q000378:metabolism; D018522:Gravitropism / Q000235:genetics*; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D007210:Indoleacetic Acids / Q000378:metabolism; D009928:Organ Specificity; D010937:Plant Growth Regulators / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018547:Plant Stems / Q000166:cytology / Q000235:genetics / Q000502:physiology; D030821:Plants, Genetically Modified; D018087:Plastids / Q000235:genetics / Q000502:physiology; D032107:Populus / Q000166:cytology / Q000235:genetics* / Q000502:physiology; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014197:Trees; D014934:Wood / Q000166:cytology / Q000235:genetics / Q000502:physiology; D052584:Xylem / Q000235:genetics / Q000502:physiology
chemical_list: D005875:Gibberellins; D018398:Homeodomain Proteins; D007210:Indoleacetic Acids; D010937:Plant Growth Regulators; D010940:Plant Proteins; D014157:Transcription Factors; C007842:gibberellic acid