Editing Plant hormone (section)

===Ethylene===
{{Main|Ethylene as a plant hormone}}
[[File:Ethene-2D-flat.png|155x155px|Ethylene|thumb]]
Unlike the other major plant hormones, [[ethylene]] is a gas and a very simple organic compound, consisting of just six atoms. It forms through the breakdown of [[methionine]], an amino acid which is in all cells. Ethylene has very limited solubility in water and therefore does not accumulate within the cell, typically diffusing out of the cell and escaping the plant. Its effectiveness as a plant hormone is dependent on its rate of production versus its rate of escaping into the atmosphere. Ethylene is produced at a faster rate in rapidly growing and dividing cells, especially in darkness. New growth and newly germinated seedlings produce more ethylene than can escape the plant, which leads to elevated amounts of ethylene, inhibiting [[leaf expansion]] (see [[hyponastic response]]).

As the new shoot is exposed to light, reactions mediated by [[phytochrome]] in the plant's cells produce a signal for ethylene production to decrease, allowing leaf expansion. Ethylene affects cell growth and cell shape; when a growing shoot or root hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell. The resulting thicker stem is stronger and less likely to buckle under pressure as it presses against the object impeding its path to the surface. If the shoot does not reach the surface and the ethylene stimulus becomes prolonged, it affects the stem's natural [[geotropic]] response, which is to grow upright, allowing it to grow around an object. Studies seem to indicate that ethylene affects stem diameter and height: when stems of trees are subjected to wind, causing lateral stress, greater ethylene production occurs, resulting in thicker, sturdier tree trunks and branches.

Ethylene also affects fruit ripening. Normally, when the seeds are mature, ethylene production increases and builds up within the fruit, resulting in a [[climacteric (botany)|climacteric]] event just before seed dispersal. The nuclear protein Ethylene Insensitive2 (EIN2) is regulated by ethylene production, and, in turn, regulates other hormones including ABA and stress hormones.<ref>{{cite journal | vauthors = Wang Y, Liu C, Li K, Sun F, Hu H, Li X, Zhao Y, Han C, Zhang W, Duan Y, Liu M, Li X | display-authors = 6 | title = Arabidopsis EIN2 modulates stress response through abscisic acid response pathway | journal = Plant Molecular Biology | volume = 64 | issue = 6 | pages = 633–44 | date = August 2007 | pmid = 17533512 | doi = 10.1007/s11103-007-9182-7 | s2cid = 42139177 }}</ref> Ethylene diffusion out of plants is strongly inhibited underwater. This increases internal concentrations of the gas. In numerous aquatic and semi-aquatic species (e.g. ''Callitriche platycarpus'', rice, and ''[[Rumex palustris]]''), the accumulated ethylene strongly stimulates upward elongation. This response is an important mechanism for the adaptive escape from submergence that avoids asphyxiation by returning the shoot and leaves to contact with the air whilst allowing the release of entrapped ethylene.<ref>{{cite journal| vauthors = Jackson MB |date=1985|title=Ethylene and Responses of Plants to Soil Waterlogging and Submergence|journal=Annual Review of Plant Physiology|volume=36|issue=1|pages=145–174|doi=10.1146/annurev.pp.36.060185.001045|issn=0066-4294}}</ref><ref>{{cite journal | vauthors = Jackson MB | title = Ethylene-promoted elongation: an adaptation to submergence stress | journal = Annals of Botany | volume = 101 | issue = 2 | pages = 229–48 | date = January 2008 | pmid = 17956854 | pmc = 2711016 | doi = 10.1093/aob/mcm237 }}</ref><ref>{{cite journal | vauthors = Jackson MB, Ram PC | title = Physiological and molecular basis of susceptibility and tolerance of rice plants to complete submergence | journal = Annals of Botany | volume = 91 Spec No | issue = 2 | pages = 227–41 | date = January 2003 | pmid = 12509343 | pmc = 4244997 | doi = 10.1093/aob/mcf242 }}</ref><ref>{{cite journal | vauthors = Voesenek LA, Benschop JJ, Bou J, Cox MC, Groeneveld HW, Millenaar FF, Vreeburg RA, Peeters AJ | display-authors = 6 | title = Interactions between plant hormones regulate submergence-induced shoot elongation in the flooding-tolerant dicot Rumex palustris | journal = Annals of Botany | volume = 91 Spec No | issue = 2 | pages = 205–11 | date = January 2003 | pmid = 12509341 | pmc = 4244986 | doi = 10.1093/aob/mcf116 }}</ref> At least one species (''[[Potamogeton pectinatus]]'')<ref>{{cite journal | vauthors = Summers JE, Voesenek L, Blom C, Lewis MJ, Jackson MB | title = Potamogeton pectinatus Is Constitutively Incapable of Synthesizing Ethylene and Lacks 1-Aminocyclopropane-1-Carboxylic Acid Oxidase | journal = Plant Physiology | volume = 111 | issue = 3 | pages = 901–908 | date = July 1996 | pmid = 12226336 | pmc = 157909 | doi = 10.1104/pp.111.3.901 }}</ref> has been found to be incapable of making ethylene while retaining a conventional morphology. This suggests ethylene is a true regulator rather than being a requirement for building a plant's basic body plan.