Ethylene (=) is an unsaturated

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

gas (

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

) acting naturally as a

plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...

. It is the simplest alkene gas and is the first gas known to act as hormone. It acts at trace levels throughout the life of the plant by stimulating or regulating the

ripening Ripening is a process in fruits that causes them to become more palatable. In general, fruit becomes sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does not make th ...

fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particu ...

, the opening of

flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...

s, the

abscission Abscission () is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed. In zoology, abscission is the intentional shedding of a body part, such as the shedding of a claw, husk, or the autotomy of a ...

(or shedding) of

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves. This escape response is particularly important in

rice Rice is the seed of the grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima ''Oryza glaberrima'', commonly known as African rice, is one of the two domesticated rice species. It was first domesticated and grown i ...

farming. Commercial fruit-ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening () has been seen to produce CO₂ levels of 10% in 24 hours.

History of ethylene in plant biology

Ethylene has been used since the ancient Egyptians, who would gash figs in order to stimulate ripening (wounding stimulates ethylene production by plant tissues). The ancient Chinese would burn

incense Incense is aromatic biotic material that releases fragrant smoke when burnt. The term is used for either the material or the aroma. Incense is used for aesthetic reasons, religious worship, aromatherapy, meditation, and ceremony. It may also b ...

in closed rooms to enhance the ripening of pears. In 1864, it was discovered that gas leaks from street lights led to stunting of growth, twisting of plants, and abnormal thickening of stems. In 1874 it was discovered that smoke caused pineapple fields to bloom. Smoke contains ethylene, and once this was realized the smoke was replaced with

ethephon Ethephon is a plant growth regulator. Mechanism of action Upon metabolism by the plant, it is converted into ethylene, a potent regulator of plant growth and ripeness. It is also a butyrylcholinesterase inhibitor. Uses in various crops Etheph ...

or naphthalene acetic acid, which induce ethylene production. The scientific knowledge of ethylene as a factor in plant physiology started in the late 19th century. In 1896, Russian botanist Dimitry Neljubow studied the response pea to illuminating gas to which they showed movement. He discovered ethylene as the active component in the light source that stimulated pea behaviour. He reported his discovery in 1901. Sarah Doubt also showed in 1917 that ethylene from illuminating gas stimulated

. Farmers in Florida would commonly get their crops to ripen in sheds by lighting

kerosene lamps A kerosene lamp (also known as a paraffin lamp in some countries) is a type of lighting device that uses kerosene as a fuel. Kerosene lamps have a wick or mantle as light source, protected by a glass chimney or globe; lamps may be used on a t ...

, which was originally thought to induce ripening from the heat. In 1924, Frank E. Denny discovered that it was the molecule ethylene emitted by the kerosene lamps that induced the ripening. Reporting in the '' Botanical Gazette'', he wrote:

Ethylene was very effective in bringing about the desired result, concentrations as low as one part (by volume) of ethylene in one million parts of air being sufficient to cause green lemons to turn yellow in about six to ten days... Furthermore, coloring with either ethylene or gas from the kerosene stoves caused the loss of the "buttons" ( calyx, receptacle, and a portion of the peduncle)... Yellowing of the ethylene treated fruit became visible about the third or fourth day, and full yellow color was developed in six to ten days. Untreated fruit remained green during the same period of time.

The same year, Denny published the experimental details separately, and also experimentally showed that use of ethylene was more advantageous than that of kerosene. In 1934, British biologist Richard Gane discovered that the chemical constituent in ripe bananas could cause ripening of green bananas, as well as faster growth of pea. He showed that the same growth effect could be induced by ethylene. Reporting in ''

Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...

'' that ripe fruit (in this case

Worcester Pearmain 'Worcester Pearmain' is an early season English cultivar of domesticated apple, that was developed in Worcester, England, by a Mr. Hale of Swanpool in 1874.y the appleis very small—perhaps of the order of 1 cubic centimetre during the whole life-history of the fruit; and the cause of its prodigious biological activity in such small concentration is a problem for further research. Its production by apple ceases or is very much reduced in the absence of oxygen. He subsequently showed that ethylene was produced by other fruits as well, and that obtained from apple could induce

seed germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fern ...

and plant growth in different vegetables (but not in cereals). His conclusions were not universally accepted by other scientists. They became more convincing when William Crocker, Alfred Hitchcock, and Percy Zimmerman reported in 1935 that ethylene acts similar to

auxins Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essenti ...

in causing plant growth and senescence of vegetative tissues. Thus, establishing that ethylene is a plant hormone.

Ethylene biosynthesis in plants

Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds. Ethylene production is regulated by a variety of developmental and environmental factors. During the life of the plant, ethylene production is induced during certain stages of growth such as germination,

of fruits,

of leaves, and senescence of flowers. Ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators. The pathway for ethylene biosynthesis is named the Yang cycle after the scientist

Shang Fa Yang Shang Fa Yang (; November 10, 1932 – February 12, 2007) was a Taiwanese-American plant scientist and a professor at the University of California, Davis. He was awarded the Wolf Prize in Agriculture and elected a member of the US National Academy ...

who made key contributions to elucidating this pathway. Ethylene is

biosynthesized Biosynthesis is a multi-step, enzyme- catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules ...

from the amino acid

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...

to ''S''-adenosyl-L-methionine (SAM, also called Adomet) by the enzyme Met adenosyltransferase. SAM is then converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS). The activity of ACS determines the rate of ethylene production, therefore regulation of this

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

is key for the ethylene biosynthesis. The final step requires oxygen and involves the action of the enzyme ACC-oxidase (ACO), formerly known as the ethylene forming enzyme (EFE). Ethylene biosynthesis can be induced by endogenous or exogenous ethylene. ACC synthesis increases with high levels of auxins, especially

indole acetic acid Indole-3-acetic acid (IAA, 3-IAA) is the most common naturally occurring plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, con ...

(IAA) and

cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...

Ethylene perception in plants

Ethylene is perceived by a family of five transmembrane protein dimers such as the ETR1 protein in ''Arabidopsis''. The

genes In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

encoding In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...

ethylene receptors have been cloned in the reference plant '' Arabidopsis thaliana'' and many other plants. Ethylene receptors are encoded by multiple genes in plant

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

s. Dominant

missense In genetics, a missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution. Substitution of protein from DNA mutations Missense mu ...

mutations in any of the gene family, which comprises five receptors in ''Arabidopsis'' and at least six in tomato, can confer insensitivity to ethylene. Loss-of-function mutations in multiple members of the ethylene-receptor family result in a plant that exhibits constitutive ethylene responses. DNA sequences for ethylene receptors have also been identified in many other plant species and an ethylene binding protein has even been identified in Cyanobacteria.

Ethylene response to salt stress

A large portion of the soil has been affected by over salinity and it has been known to limit the growth of many plants. Globally, the total area of saline soil was 397,000,000 ha and in continents like Africa, it makes up 2 percent of the soil. The amount of soil salinization has reached 19.5% of the irrigated land and 2.1% of the dry-land agriculture around the world. Soil salinization affects the plants using osmotic potential by net solute accumulation. The osmotic pressure in the plant is what maintains water uptake and cell turgor to help with stomatal function and other cellular mechanisms. Over generations, many plant genes have adapted, allowing plants’ phenotypes to change and built distinct mechanisms to counter salinity effects. The plant hormone ethylene is a combatant for salinity in most plants. Ethylene is known for regulating plant growth and development and adapted to stress conditions through a complex

signal transduction pathway Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellular ...

. Central membrane proteins in plants, such as ETO2, ERS1 and EIN2, are used for ethylene signaling in many plant growth processes. ETO2, Ethylene overproducer 2, is a protein that, when mutated, will gain a function to continually produce ethylene even when there is no stress condition, causing the plant to grow short and stumpy. ERS1, Ethylene response sensor 1, is activated when ethylene is present in the signaling pathway and when mutated, it loses a function and cannot bind to ethylene. This means a response is never activated and the plant will not be able to cope with the abiotic stress. EIN2, Ethylene insensitive 2, is a protein that activates the pathway and when there is a mutation here the EIN2 will block ethylene stimulation and an ethylene response gene will not be activated. Mutations in these proteins can lead to heightened salt sensitivity and limit plant growth. The effects of salinity have been studied on ''Arabidopsis'' plants that have mutated ERS1 and EIN4 proteins. These proteins are used for ethylene signaling again certain stress conditions, such as salt and the ethylene precursor ACC is allowing suppress of any sensitivity to the salt stress. Mutations in these pathways can cause lack of ethylene signaling, causing stunt in plant growth and development.

Environmental and biological triggers of ethylene

Environmental cues such as flooding, drought, chilling, wounding, and pathogen attack can induce ethylene formation in plants. In flooding, roots suffer from lack of oxygen, or anoxia, which leads to the synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC). ACC is transported upwards in the plant and then oxidized in leaves. The ethylene produced causes

nastic movements Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor. Decrease in turgor pressure causes shrinkage while ...

(epinasty) of the leaves, perhaps helping the plant to lose less water in compensation for an increase in resistance to water transport through oxygen-deficient roots .

Corolla senescence

The corolla of a plant refers to its set of petals. Corolla development in plants is broken into phases from

anthesis Anthesis is the period during which a flower is fully open and functional. It may also refer to the onset of that period. The onset of anthesis is spectacular in some species. In ''Banksia'' species, for example, anthesis involves the extension ...

to corolla wilting. The development of the corolla is directed in part by ethylene, though its concentration is highest when the plant is fertilized and no longer requires the production or maintenance of structures and compounds that attract pollinators. The role of ethylene in the developmental cycle is as a hormonal director of senescence in corolla tissue. This is evident as ethylene production and emission are maximized in developmental phases post-pollination, until corolla wilting. Ethylene-directed senescence of corolla tissue can be observed as color change in the corolla or the wilting/ death of corolla tissue. At the chemical level, ethylene mediates the reduction in the amount of fragrance volatiles produced. Fragrance volatiles act mostly by attracting pollinators. Ethylene's role in this developmental scenario is to move the plant away from a state of attracting pollinators, so it also aids in decreasing the production of these volatiles. Ethylene production in corolla tissue does not directly cause the senescence of corolla tissue, but acts by releasing secondary products that are consistent with tissue ageing. While the mechanism of ethylene-mediated senescence are unclear, its role as a senescence-directing hormone can be confirmed by ethylene-sensitive petunia response to ethylene knockdown. Knockdown of ethylene biosynthesis genes was consistent with increased corolla longevity; inversely, up-regulation of ethylene biosynthesis gene transcription factors were consistent with a more rapid senescence of the corolla.

List of plant responses to ethylene

* Seedling triple response, thickening and shortening of

hypocotyl The hypocotyl (short for "hypocotyledonous stem", meaning "below seed leaf") is the stem of a germinating seedling, found below the cotyledons (seed leaves) and above the radicle ( root). Eudicots As the plant embryo grows at germination, it se ...

with pronounced apical hook. * Stimulation of ''Arabidopsis'' hypocotyl elongation * In

pollination Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds ...

, when the pollen reaches the stigma, the precursor of the ethylene,

ACC ACC most often refers to: * Atlantic Coast Conference, an NCAA Division I collegiate athletic conference located in the US *American College of Cardiology, A US-based nonprofit medical association that bestows credentials upon cardiovascular spec ...

, is secreted to the petal, the ACC releases ethylene with ACC oxidase. * Stimulates leaf senescence * Controls root growth inhibition in compacted soils * Stimulates senescence of mature

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

cells in preparation for plant use * Induces leaf

* Induces seed germination * Induces

root hair Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root ...

growth — increasing the efficiency of water and mineral absorption * Induces the growth of

adventitious root Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plan ...

s during flooding * Stimulates survival under low-oxygen conditions ( hypoxia) in submerged plant tissues * Controls adaptive

Translation (biology) In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire proces ...

dynamics during plant submergence * Stimulates epinasty — leaf petiole grows out, leaf hangs down and curls into itself * Stimulates

fruit ripening Ripening is a process in fruits that causes them to become more palatable. In general, fruit becomes sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does not make the ...

* Induces a climacteric rise in

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

in some fruit which causes a release of additional ethylene. * Affects

gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general featu ...

* Stimulates

nutation Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...

* Inhibits stem growth and stimulates stem and cell broadening and lateral branch growth outside of seedling stage (see

Hyponastic response The hyponastic response is an upward bending of leaves or other plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and incl ...

) * Interference with auxin transport (with high auxin concentrations) * Inhibits shoot growth and stomatal closing except in some water plants or habitually submerged species such as rice, ''

Callitriche ''Callitriche'' is a genus of largely aquatic plants known as water-starwort. Previously, it was the only genus in the family Callitrichaceae. However, according to the APG II system this family is now included in the Plantaginaceae (plantain fa ...

'' (e.g., ''C. platycarpa''), and '' Rumex'', where the opposite occurs to achieve an adaptive escape from submergence. * Induces flowering in

pineapple The pineapple (''Ananas comosus'') is a tropical plant with an edible fruit; it is the most economically significant plant in the family Bromeliaceae. The pineapple is indigenous to South America, where it has been cultivated for many centuri ...

s * Inhibits short day induced flower initiation in ''Pharbitus nil'' and '' Chrysanthemum morifolium''

Commercial issues

Ethylene shortens the shelf life of many fruits by hastening

and floral senescence. Ethylene will shorten the shelf life of

cut flowers Cut flowers are flowers or flower buds (often with some stem and leaf) that have been cut from the plant bearing it. It is usually removed from the plant for decorative use. Typical uses are in vase displays, wreaths and garlands. Many garde ...

and potted plants by accelerating floral senescence and floral

. Flowers and plants which are subjected to stress during shipping, handling, or storage produce ethylene causing a significant reduction in floral display. Flowers affected by ethylene include

carnation ''Dianthus caryophyllus'' (), commonly known as the carnation or clove pink, is a species of ''Dianthus''. It is likely native to the Mediterranean region but its exact range is unknown due to extensive cultivation for the last 2,000 years.Med ...

, geranium,

petunia ''Petunia'' is genus of 20 species of flowering plants of South American origin. The popular flower of the same name derived its epithet from the French, which took the word ''petun'', meaning "tobacco," from a Tupi–Guarani language. A tende ...

rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...

, and many others. Ethylene can cause significant economic losses for florists, markets, suppliers, and growers. Researchers have developed several ways to inhibit ethylene, including inhibiting ethylene synthesis and inhibiting ethylene perception. Aminoethoxyvinylglycine (AVG), Aminooxyacetic acid (AOA), and silver salts are ethylene inhibitors. Inhibiting ethylene synthesis is less effective for reducing post-harvest losses since ethylene from other sources can still have an effect. By inhibiting ethylene perception, fruits, plants and flowers don't respond to ethylene produced endogenously or from exogenous sources. Inhibitors of ethylene perception include compounds that have a similar shape to ethylene, but do not elicit the ethylene response. One example of an ethylene perception inhibitor is 1-methylcyclopropene (1-MCP). Commercial growers of

bromeliads The Bromeliaceae (the bromeliads) are a family of monocot flowering plants of about 80 genera and 3700 known species, native mainly to the tropical Americas, with several species found in the American subtropics and one in tropical west Africa, ...

, including

plants, use ethylene to induce flowering. Plants can be induced to flower either by treatment with the gas in a chamber, or by placing a

banana peel A banana peel, called banana skin in British English, is the outer covering of the banana fruit. Banana peels are used as food for animals, an ingredient in cooking, in water purification, for manufacturing of several biochemical products as w ...

next to the plant in an enclosed area. Chrysanthemum flowering is delayed by ethylene gas, and growers have found that carbon dioxide 'burners' and the exhaust fumes from inefficient glasshouse heaters can raise the ethylene concentration to 0.05 ppmv, causing delay in flowering of commercial crops.

References

External links

International Chemical Safety Card 0475
{{GABAAR PAMs Aging-related substances in plants Monomers Plant hormones GABAA receptor positive allosteric modulators Gaseous signaling molecules Plant growth regulators