Ethylene as a plant hormone

Ethylene (CH
2=CH
2) is an unsaturated hydrocarbon gas (alkene) acting naturally as a plant hormone.[1] It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the abscission (or shedding) of leaves[2] and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves.[3] This escape response is particularly important in rice farming.[4] 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 (20 °C; 68 °F) has been seen to produce CO2 levels of 10% in 24 hours.[5]

An ethylene signal transduction pathway. Ethylene permeates the cell membrane and binds to a receptor on the endoplasmic reticulum. The receptor releases the repressed EIN2. This then activates a signal transduction pathway which activates regulatory genes that eventually trigger an ethylene response. The activated DNA is transcribed into mRNA which is then translated into a functional enzyme that is used for ethylene biosynthesis.

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 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.[1] 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 or naphthalene acetic acid, which induce ethylene production.[6] In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene.[7] Sarah Doubt discovered that ethylene stimulated abscission in 1917.[8] Farmers in Florida would commonly get their crops to ripen in sheds by lighting kerosene lamps, 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.[9] It was not until 1934 that Gane reported that plants synthesize ethylene.[10] In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as senescence of vegetative tissues.[11]

Ethylene biosynthesis in plants
The Yang cycle

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, ripening of fruits, abscission 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.[12] The pathway for ethylene biosynthesis is named the Yang cycle after the scientist Shang Fa Yang who made key contributions to elucidating this pathway.

Ethylene is biosynthesized from the amino acid methionine 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 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 (IAA) and cytokinins.

Ethylene perception in plants

Ethylene is perceived by a family of five transmembrane protein dimers such as the ETR1 protein in Arabidopsis. The genes encoding ethylene receptors have been cloned in the reference plant Arabidopsis thaliana and many other plants. Ethylene receptors are encoded by multiple genes in plant genomes. Dominant missense mutations in any of the gene family, which comprises five receptors in Arabidopsis and at least six in tomato, can confer insensitivity to ethylene.[13] Loss-of-function mutations in multiple members of the ethylene-receptor family result in a plant that exhibits constitutive ethylene responses.[14] 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.[1]

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.[15] The amount of soil salinization has reached 19.5% of the irrigated land and 2.1% of the dry-land agriculture around the world.[16] 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.[16] 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. 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.[17] 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.[17] 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 (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 .[18]

Corolla senescence

The corolla of a plant refers to its set of petals. Corolla development in plants is broken into phases from anthesis 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.[19][20] 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.[19] 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.[19]

List of plant responses to ethylene
  • Seedling triple response, thickening and shortening of hypocotyl with pronounced apical hook.
  • Stimulation of Arabidopsis hypocotyl elongation [21]
  • In pollination, when the pollen reaches the stigma, the precursor of the ethylene, ACC, is secreted to the petal, the ACC releases ethylene with ACC oxidase.
  • Stimulates leaf senescence
  • Controls root growth inhibition in compacted soils [22]
  • Stimulates senescence of mature xylem cells in preparation for plant use
  • Induces leaf abscission[23]
  • Induces seed germination
  • Induces root hair growth — increasing the efficiency of water and mineral absorption
  • Induces the growth of adventitious roots during flooding
  • Stimulates survival under low-oxygen conditions (hypoxia) in submerged plant tissues [24][25][26][27]
  • Stimulates epinasty — leaf petiole grows out, leaf hangs down and curls into itself
  • Stimulates fruit ripening
  • Induces a climacteric rise in respiration in some fruit which causes a release of additional ethylene.
  • Affects gravitropism
  • Stimulates nutational bending
  • Inhibits stem growth and stimulates stem and cell broadening and lateral branch growth outside of seedling stage (see Hyponastic response)
  • 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 (e.g., C. platycarpa), and Rumex, where the opposite occurs to achieve an adaptive escape from submergence.
  • Induces flowering in pineapples
  • Inhibits short day induced flower initiation in Pharbitus nil[28] and Chrysanthemum morifolium[29]

Commercial issues

Ethylene shortens the shelf life of many fruits by hastening fruit ripening and floral senescence. Ethylene will shorten the shelf life of cut flowers and potted plants by accelerating floral senescence and floral abscission. 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, geranium, petunia, rose, and many others.[30]

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.[31][32] 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, including pineapple 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 next to the plant in an enclosed area.

Chrysanthemum flowering is delayed by ethylene gas,[33] 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.

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