title: Cellular mechanisms underlying growth asymmetry during stem gravitropism.
abstract: Plant stems respond to gravitropic stimulation with a rapid, local and reversible change in cell growth rate (elongation), generally on both the upper and lower sides of the stem. The cellular and biochemical mechanisms for this differential growth are reviewed. Considerable evidence implicates an asymmetry in wall pH in the growth response. The strengths and weaknesses of the wall "loosening enzyme" concept are reviewed and the possibility of expansin involvement in the bending response of stems is considered. Also discussed is the possibility that wall stiffening processes, e.g. phenolic coupling driven by oxidative bursts or altered orientation of newly deposited cellulose, might mediate the growth responses during gravitropism.
authors.affiliation: Department of Biology, Pennsylvania State University, University Park 16802, USA. dcosgrove@psu.edu
authors.forename: D J
authors.initials: DJ
authors.lastname: Cosgrove
mesh_terms: D002473:Cell Wall / Q000201:enzymology / Q000502:physiology; D016695:Glycosyltransferases / Q000378:metabolism; D018522:Gravitropism / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D018521:Plant Physiological Phenomena; D010940:Plant Proteins / Q000378:metabolism; D018547:Plant Stems / Q000166:cytology* / Q000254:growth & development / Q000502:physiology*
chemical_list: D010940:Plant Proteins; C095825:expansin protein, plant; D016695:Glycosyltransferases; C073693:xyloglucan endotransglycosylase