Editing Botany (section)

=== Plant hormones ===
[[File:Phototropism Diagram.svg|thumb|upright=1.25|alt=A diagram of the mechanism of phototropism in oat coleoptiles| '''1''' An oat [[coleoptile]] with the sun overhead. [[Auxin]] (pink) is evenly distributed in its tip. <br />'''2''' With the sun at an angle and only shining on one side of the shoot, auxin moves to the opposite side and stimulates [[cell elongation]] there. <br />'''3''' and '''4''' Extra growth on that side causes the shoot to [[phototropism|bend towards the sun]].{{sfn|Mauseth|2012|p=351}}]]
{{Further|Plant hormone|Phytochrome}}

Plants are not passive, but respond to [[signal transduction|external signals]] such as light, touch, and injury by moving or growing towards or away from the stimulus, as appropriate. Tangible evidence of touch sensitivity is the almost instantaneous collapse of leaflets of ''[[Mimosa pudica]]'', the insect traps of [[Venus flytrap]] and [[bladderwort]]s, and the pollinia of orchids.{{sfn|Darwin|1880|pp = 129–200}}

The hypothesis that plant growth and development is coordinated by [[plant hormone]]s or plant growth regulators first emerged in the late 19th century. Darwin experimented on the movements of plant shoots and roots towards [[heliotropism|light]]{{sfn|Darwin|1880|pp = 449–492}} and [[geotropism|gravity]], and concluded "It is hardly an exaggeration to say that the tip of the radicle . . acts like the brain of one of the lower animals . . directing the several movements".{{sfn|Darwin|1880|p = 573}} About the same time, the role of [[auxin]]s (from the Greek {{transliteration|grc|auxein}}, to grow) in control of plant growth was first outlined by the Dutch scientist [[Frits Went]].{{sfn|Plant Hormones|2013}} The first known auxin, [[indole-3-acetic acid]] (IAA), which promotes cell growth, was only isolated from plants about 50 years later.{{sfn|Went|Thimann|1937|pp = 110–112}} This compound mediates the tropic responses of shoots and roots towards light and gravity.{{sfn|Mauseth|2003|pp = 411–412}} The finding in 1939 that plant [[callus (cell biology)|callus]] could be maintained in culture containing IAA, followed by the observation in 1947 that it could be induced to form roots and shoots by controlling the concentration of growth hormones were key steps in the development of plant biotechnology and genetic modification.{{sfn|Sussex|2008|pp = 1189–1198}}

[[File:Venus Fly Trap Eating Compilation Scott's Revenge On The Caterpillars.ogv|thumb|upright=1.25|alt=a video compilation of Venus's fly trap catching insects|Venus's fly trap, ''Dionaea muscipula'', showing the touch-sensitive insect trap in action]]
[[Cytokinin]]s are a class of plant hormones named for their control of cell division (especially [[cytokinesis]]). The natural cytokinin [[zeatin]] was discovered in corn, ''[[Zea mays]]'', and is a derivative of the [[purine]] [[adenine]]. Zeatin is produced in roots and transported to shoots in the xylem where it promotes cell division, bud development, and the greening of chloroplasts.{{sfn|Campbell|Reece|Urry|Cain|2008|pp = 827–830}}{{sfn|Mauseth|2003|pp = 411–413}} The [[gibberelins]], such as [[gibberelic acid]] are [[diterpene]]s synthesised from [[Acetyl-CoA carboxylase|acetyl CoA]] via the [[mevalonate pathway]]. They are involved in the promotion of germination and dormancy-breaking in seeds, in regulation of plant height by controlling stem elongation and the control of flowering.{{sfn|Taiz|Zeiger|2002|pp = 461–492}} [[Abscisic acid]] (ABA) occurs in all land plants except liverworts, and is synthesised from [[carotenoid]]s in the chloroplasts and other plastids. It inhibits cell division, promotes seed maturation, and dormancy, and promotes stomatal closure. It was so named because it was originally thought to control [[abscission]].{{sfn|Taiz|Zeiger|2002|pp = 519–538}} [[Ethylene#Ethylene as a plant hormone|Ethylene]] is a gaseous hormone that is produced in all higher plant tissues from [[methionine]]. It is now known to be the hormone that stimulates or regulates fruit ripening and abscission,{{sfn|Lin|Zhong|Grierson|2009|pp = 331–336}}{{sfn|Taiz|Zeiger|2002|pp = 539–558}} and it, or the synthetic growth regulator [[ethephon]] which is rapidly metabolised to produce ethylene, are used on industrial scale to promote ripening of cotton, [[pineapple]]s and other [[climacteric (botany)|climacteric]] crops.

Another class of [[phytohormone]]s is the [[jasmonate]]s, first isolated from the oil of ''[[Jasminum grandiflorum]]''{{sfn|Demole|Lederer|Mercier|1962|pp = 675–685}} which regulates wound responses in plants by unblocking the expression of genes required in the [[systemic acquired resistance]] response to pathogen attack.{{sfn|Chini|Fonseca|Fernández|Adie|2007|pp = 666–671}}

In addition to being the primary energy source for plants, light functions as a signalling device, providing information to the plant, such as how much sunlight the plant receives each day. This can result in adaptive changes in a process known as [[photomorphogenesis]]. [[Phytochrome]]s are the [[Photoreceptor protein|photoreceptors]] in a plant that are sensitive to light.{{sfn|Roux|1984|pp = 25–29}}