title: Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism.
abstract: Gravity profoundly influences plant growth and development. Plants respond to changes in orientation by using gravitropic responses to modify their growth. Cholodny and Went hypothesized over 80 years ago that plants bend in response to a gravity stimulus by generating a lateral gradient of a growth regulator at an organ's apex, later found to be auxin. Auxin regulates root growth by targeting Aux/IAA repressor proteins for degradation. We used an Aux/IAA-based reporter, domain II (DII)-VENUS, in conjunction with a mathematical model to quantify auxin redistribution following a gravity stimulus. Our multidisciplinary approach revealed that auxin is rapidly redistributed to the lower side of the root within minutes of a 90° gravity stimulus. Unexpectedly, auxin asymmetry was rapidly lost as bending root tips reached an angle of 40° to the horizontal. We hypothesize roots use a "tipping point" mechanism that operates to reverse the asymmetric auxin flow at the midpoint of root bending. These mechanistic insights illustrate the scientific value of developing quantitative reporters such as DII-VENUS in conjunction with parameterized mathematical models to provide high-resolution kinetics of hormone redistribution.
authors.affiliation: Centre for Plant Integrative Biology,  University of Nottingham, Sutton Bonington LE12 5RD,  United Kingdom.
authors.forename: Leah R
authors.initials: LR
authors.lastname: Band
authors.forename: Darren M
authors.initials: DM
authors.lastname: Wells
authors.forename: Antoine
authors.initials: A
authors.lastname: Larrieu
authors.forename: Jianyong
authors.initials: J
authors.lastname: Sun
authors.forename: Alistair M
authors.initials: AM
authors.lastname: Middleton
authors.forename: Andrew P
authors.initials: AP
authors.lastname: French
authors.forename: Géraldine
authors.initials: G
authors.lastname: Brunoud
authors.forename: Ethel Mendocilla
authors.initials: EM
authors.lastname: Sato
authors.forename: Michael H
authors.initials: MH
authors.lastname: Wilson
authors.forename: Benjamin
authors.initials: B
authors.lastname: Péret
authors.forename: Marina
authors.initials: M
authors.lastname: Oliva
authors.forename: Ranjan
authors.initials: R
authors.lastname: Swarup
authors.forename: Ilkka
authors.initials: I
authors.lastname: Sairanen
authors.forename: Geraint
authors.initials: G
authors.lastname: Parry
authors.forename: Karin
authors.initials: K
authors.lastname: Ljung
authors.forename: Tom
authors.initials: T
authors.lastname: Beeckman
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Garibaldi
authors.forename: Mark
authors.initials: M
authors.lastname: Estelle
authors.forename: Markus R
authors.initials: MR
authors.lastname: Owen
authors.forename: Kris
authors.initials: K
authors.lastname: Vissenberg
authors.forename: T Charlie
authors.initials: TC
authors.lastname: Hodgman
authors.forename: Tony P
authors.initials: TP
authors.lastname: Pridmore
authors.forename: John R
authors.initials: JR
authors.lastname: King
authors.forename: Teva
authors.initials: T
authors.lastname: Vernoux
authors.forename: Malcolm J
authors.initials: MJ
authors.lastname: Bennett
mesh_terms: D017360:Arabidopsis / Q000254:growth & development / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004777:Environment; D018522:Gravitropism / Q000502:physiology; D007210:Indoleacetic Acids / Q000378:metabolism*; D007700:Kinetics; D008954:Models, Biological; D008962:Models, Theoretical; D018521:Plant Physiological Phenomena; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism* / Q000502:physiology; D015398:Signal Transduction; D049490:Systems Biology / Q000379:methods; D013997:Time Factors
chemical_list: D007210:Indoleacetic Acids