Editing Gravitropism (section)

== In roots ==

[[File:Gravitropism in Roots.gif|thumb|In the process of plant roots growing in the direction of gravity by gravitropism, high concentrations of auxin move towards the cells on the bottom side of the root. This suppresses growth on this side, while allowing cell elongation on the top of the root. As a consequence of this, curved growth occurs and the root is directed downwards.<ref name=":5" />|alt=[this image is incorrect! the high auxin is always on the opposite side of the tropic movement!]]Root growth occurs by  division of stem cells in the root [[Meristem#Indeterminate growth of meristems|meristem]] located in the tip of the root, and the subsequent asymmetric expansion of cells in a shoot-ward region to the tip known as the elongation zone.  Differential growth during tropisms mainly involves changes in cell expansion versus changes in cell division, although a role for cell division in tropic growth has not been formally ruled out.  Gravity is sensed in the root tip and this information must then be relayed to the elongation zone so as to maintain growth direction and mount effective growth responses to changes in orientation to and continue to grow its roots in the same direction as gravity.<ref name=":0">{{Cite journal|last1=Perrin|first1=Robyn M.|last2=Young |first2=Li-Sen|last3=Narayana Murthy|first3=U.M.|last4=Harrison |first4=Benjamin R.|last5=Wang|first5=Yan|last6=WILL|first6=Jessica L.|last7=Masson |first7=Patrick H.|date=2017-04-21|title=Gravity Signal Transduction in Primary Roots|journal=Annals of Botany|volume=96|issue=5|pages=737–743|doi=10.1093/aob/mci227|issn=0305-7364|pmc=4247041|pmid=16033778}}</ref>

Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the [[plant hormone]] [[auxin]] known as [[polar auxin transport]].<ref>{{Cite journal|last1=Swarup|first1=Ranjan|last2=Kramer|first2=Eric M.|last3=Perry|first3=Paula|last4=Knox|first4=Kirsten|last5=Leyser|first5=H. M. Ottoline|last6=Haseloff|first6=Jim|last7=Beemster|first7=Gerrit T. S.|last8=Bhalerao|first8=Rishikesh|last9=Bennett|first9=Malcolm J.|date=2005-11-01|title=Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal|journal=Nature Cell Biology|volume=7|issue=11|pages=1057–1065|doi=10.1038/ncb1316|issn=1465-7392|pmid=16244669|s2cid=22337800}}</ref>  This was described in the 1920s in the [[Cholodny-Went model]]. The model was independently proposed by the Ukrainian scientist [[N. Cholodny]] of the [[University of Kyiv]] in 1927 and by [[Frits Went]] of the [[California Institute of Technology]] in 1928, both based on work they had done in 1926.<ref>{{cite book |url=https://books.google.com/books?id=JItGq_zGANAC&pg=PA235 |page=235
 |title=Horticultural Reviews
 |first=Jules |last=Janick
 |publisher=John Wiley & Sons |year=2010 |isbn=978-0-470-65053-0}}</ref> Auxin exists in nearly every organ and tissue of a plant, but it  has been reoriented in the gravity field, can initiate differential growth resulting in root curvature.

Experiments show that auxin distribution is characterized by a fast movement of auxin to the lower side of the root in response to a gravity stimulus at a 90° degree angle or more. However, once the root tip reaches a 40° angle to the horizontal of the stimulus, auxin distribution quickly shifts to a more symmetrical arrangement. This behavior is described as a "tipping point" mechanism for auxin transport in response to a gravitational stimulus.<ref name=":5">{{cite journal|last1=Band|first1=L. R.|last2=Wells|first2=D. M.|last3=Larrieu|first3=A.|last4=Sun|first4=J.|last5=Middleton|first5=A. M.|last6=French|first6=A. P.|last7=Brunoud|first7=G.|last8=Sato|first8=E. M.|last9=Wilson|first9=M. H.|last10=Peret|first10=B.|last11=Oliva|first11=M.|last12=Swarup|first12=R.|last13=Sairanen|first13=I.|last14=Parry|first14=G.|last15=Ljung|first15=K.|last16=Beeckman|first16=T.|last17=Garibaldi|first17=J. M.|last18=Estelle|first18=M.|last19=Owen|first19=M. R.|last20=Vissenberg|first20=K.|last21=Hodgman|first21=T. C.|last22=Pridmore|first22=T. P.|last23=King|first23=J. R.|last24=Vernoux|first24=T.|last25=Bennett|first25=M. J.|title=Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism|journal=Proceedings of the National Academy of Sciences|date=5 March 2012|volume=109|issue=12|pages=4668–4673|doi=10.1073/pnas.1201498109|pmid=22393022|pmc=3311388|bibcode=2012PNAS..109.4668B|doi-access=free}}</ref>