Editing Plant hormone (section)

==Characteristics==
[[File:Phyllody on Coneflower with aster yellows.jpg|thumb|[[Phyllody]] on a [[purple coneflower]] (''Echinacea purpurea''), a plant development abnormality where leaf-like structures replace [[flower]] organs. It can be caused by hormonal imbalance, among other reasons.]]
The word hormone is derived from Greek, meaning ''set in motion''. Plant hormones affect [[gene expression]] and [[gene transcription|transcription]] levels, cellular division, and growth. They are naturally produced within plants, though very similar chemicals are produced by fungi and bacteria that can also affect plant growth.<ref>{{cite book | vauthors = Srivastava LM | title = Plant growth and development: hormones and environment |publisher=Academic Press |year=2002 |page=140 |isbn=978-0-12-660570-9 }}</ref> A large number of related [[chemical compound]]s are [[Chemical synthesis|synthesize]]d by humans.  They are used to regulate the growth of [[Agriculture|cultivate]]d plants, [[weed]]s, and [[in vitro]]-grown plants and plant cells; these manmade compounds are called '''plant growth regulators''' ('''PGRs'''). Early in the study of plant hormones, "phytohormone" was the commonly used term, but its use is less widely applied now.

Plant hormones are not [[nutrient]]s, but [[chemical]]s that in small amounts promote and influence the growth,<ref>{{cite book |vauthors=Öpik H, Rolfe SA, Willis JA, Street HE |title=The physiology of flowering plants |url=https://books.google.com/books?id=atZq3w6pOvQC&pg=PA191 |year=2005 |publisher=Cambridge University Press |isbn=978-0-521-66251-2 |page=191 |edition=4th }}{{Dead link|date=October 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> development, and differentiation of cells and [[tissue (biology)|tissues]]. The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized. Plants lack glands to produce and store hormones, because, unlike animals—which have two circulatory systems ([[lymphatic]] and [[cardiovascular]]) —plants use more passive means to move chemicals around their bodies.  Plants utilize simple chemicals as hormones, which move more easily through their tissues. They are often produced and used on a local basis within the plant body.  Plant cells produce hormones that affect even different regions of the cell producing the hormone.

Hormones are transported within the plant by utilizing four types of movements.  For localized movement, [[cytoplasm]]ic streaming within cells and slow diffusion of [[ion]]s and [[molecule]]s between cells are utilized. Vascular tissues are used to move hormones from one part of the plant to another; these include [[sieve tube]]s or [[phloem]] that move [[sugar]]s from the leaves to the [[root]]s and flowers, and [[xylem]] that moves water and mineral solutes from the roots to the [[foliage]].

Not all plant cells respond to hormones, but those cells that do are programmed to respond at specific points in their growth cycle.  The greatest effects occur at specific stages during the cell's life, with diminished effects occurring before or after this period. Plants need hormones at very specific times during plant growth and at specific locations.  They also need to disengage the effects that hormones have when they are no longer needed. The production of hormones occurs very often at sites of active growth within the [[meristem]]s, before cells have fully differentiated. After production, they are sometimes moved to other parts of the plant, where they cause an immediate effect; or they can be stored in cells to be released later. Plants use different pathways to regulate internal hormone quantities and moderate their effects; they can regulate the amount of chemicals used to biosynthesize hormones.  They can store them in cells, inactivate them, or cannibalise already-formed hormones by [[Conjugation (biochemistry)|conjugating]] them with [[carbohydrate]]s, [[amino acid]]s, or [[peptide]]s. Plants can also break down hormones chemically, effectively destroying them. Plant hormones frequently regulate the concentrations of other plant hormones.<ref>{{cite journal | vauthors = Swarup R, Perry P, Hagenbeek D, Van Der Straeten D, Beemster GT, Sandberg G, Bhalerao R, Ljung K, Bennett MJ | display-authors = 6 | title = Ethylene upregulates auxin biosynthesis in Arabidopsis seedlings to enhance inhibition of root cell elongation | journal = The Plant Cell | volume = 19 | issue = 7 | pages = 2186–96 | date = July 2007 | pmid = 17630275 | pmc = 1955695 | doi = 10.1105/tpc.107.052100 }}</ref> Plants also move hormones around the plant diluting their concentrations.

The concentration of hormones required for plant responses are very low (10<sup>−6</sup> to 10<sup>−5</sup> [[mole (unit)|mol]]/[[liter|L]]). Because of these low concentrations, it has been very difficult to study plant hormones, and only since the late 1970s have scientists been able to start piecing together their effects and relationships to plant physiology.<ref>{{harvnb|Srivastava|2002|p=143}}</ref>  Much of the early work on plant hormones involved studying plants that were genetically deficient in one or involved the use of tissue-cultured plants grown ''[[in vitro]]'' that were subjected to differing ratios of hormones, and the resultant growth compared. The earliest scientific observation and study dates to the 1880s; the determination and observation of plant hormones and their identification was spread out over the next 70 years.

Synergism in plant hormones refers to the how of two or more hormones result in an effect that is more than the individual effects. For example, auxins and cytokinins often act in cooperation during cellular division and differentiation. Both hormones are key to cell cycle regulation, but when they come together, their synergistic interactions can enhance cell proliferation and organogenesis more effectively than either could in isolation.