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Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism

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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 degrees gravity stimulus. Unexpectedly, auxin asymmetry was rapidly lost as bending root tips reached an angle of 40 degrees 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.
Keywords
ARABIDOPSIS, TRANSPORT, GRAVITY, GENE-EXPRESSION, AUX/IAA PROTEINS, ENDOPLASMIC-RETICULUM, SEDIMENTATION KINETICS, BOX PROTEIN TIR1, GROWTH, CELLS

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Chicago
Band, Leah R, Darren M Wells, Antoine Larrieu, Jianyong Sun, Alistair M Middleton, Andrew P French, Géraldine Brunoud, et al. 2012. “Root Gravitropism Is Regulated by a Transient Lateral Auxin Gradient Controlled by a Tipping-point Mechanism.” Proceedings of the National Academy of Sciences of the United States of America 109 (12): 4668–4673.
APA
Band, L. R., Wells, D. M., Larrieu, A., Sun, J., Middleton, A. M., French, A. P., Brunoud, G., et al. (2012). Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 109(12), 4668–4673.
Vancouver
1.
Band LR, Wells DM, Larrieu A, Sun J, Middleton AM, French AP, et al. Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 2012;109(12):4668–73.
MLA
Band, Leah R, Darren M Wells, Antoine Larrieu, et al. “Root Gravitropism Is Regulated by a Transient Lateral Auxin Gradient Controlled by a Tipping-point Mechanism.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 109.12 (2012): 4668–4673. Print.
@article{2087419,
  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 degrees gravity stimulus. Unexpectedly, auxin asymmetry was rapidly lost as bending root tips reached an angle of 40 degrees to the horizontal. We hypothesize roots use a {\textacutedbl}tipping point{\textacutedbl} 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.},
  author       = {Band, Leah R and Wells, Darren M and Larrieu, Antoine and Sun, Jianyong and Middleton, Alistair M and French, Andrew P and Brunoud, G{\'e}raldine and Sato, Ethel Mendocilla and Wilson, Michael H and P{\'e}ret, Benjamin and Oliva, Marina and Swarup, Ranjan and Sairanen, Ilkka and Parry, Geraint and Ljung, Karin and Beeckman, Tom and Garibaldi, Jonathan M and Estelle, Mark and Owen, Markus R and Vissenberg, Kris and Hodgman, T Charlie and Pridmore, Tony P and King, John R and Vernoux, Teva and Bennett, Malcolm J},
  issn         = {0027-8424},
  journal      = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA},
  keyword      = {ARABIDOPSIS,TRANSPORT,GRAVITY,GENE-EXPRESSION,AUX/IAA PROTEINS,ENDOPLASMIC-RETICULUM,SEDIMENTATION KINETICS,BOX PROTEIN TIR1,GROWTH,CELLS},
  language     = {eng},
  number       = {12},
  pages        = {4668--4673},
  title        = {Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism},
  url          = {http://dx.doi.org/10.1073/pnas.1201498109},
  volume       = {109},
  year         = {2012},
}

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