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'Plant hormones' (also known as 'plant growth regulators' ('PGRs') and 'phytohormones') are chemicals that regulate a plant's growth. According to a standard definition, plant hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in target cells at other locations.
Plant growth regulators or plant hormones are not nutrients, but chemicals that influence the development, differentiation and growth of cells and tissues. Most are naturally produced within plants. They are vital to plant growth; effecting flowering to seed development, seed dormancy to seed germination and seedling growth effecting which tissues grow up and which grow downward, leaf formation and stem growth, fruit development to ripening, finely to leaf drop and even plant death.
The concentration of hormones required for plant responses are very low (10-6 to 10-5 mol/L), due to these low levels it has been very difficult to study plant hormones and only since the late 1970's have scientist been able to start piecing together there affects and relationships to plant physiology. 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 synthesis of plant hormones is more diffuse and not always localized, but unlike animals which have a two circulatory systems that move fluids around the body actively, plants lack a rapid circulatory system with a heart to pump fluids and thus utilize hormone that are simple chemicals that can more easily be moved around the plant. These chemicals are very often produced very locally with in the plant body and those that are transported from one part of the plant to another utilize four types of movements; slow diffusion of ions and molecules, cytoplasmic streaming with in cells, sieve tubes and xylem that moves water and mineral solutes.
Plant hormones effect gene expression and transcription levels, cellular division and growth.
It is generally accepted that there are five major classes of plant hormones, some of these groups are made-up of many different chemicals that can vary from one plant to the next in structure, each chemical is grouped together into one of these classes because of there structural similarities and on there effects on plant physiology. Other PGRs exist naturally that are not easily grouped into these classes including chemicals that inhibit plant growth or interrupt the physiological processes with in plants. Each class has positive as well inhibitory functions and they most often work in tandem with each other, with varying ratios of one or more inter-playing to affect growth regulation.[1]
The five major classes are:
Abscisic acid also called ABA, was once called 'dormin' and 'abscicin II', because of its affects on plants, before its chemical properties were fully know. This chemical was named abscicic acid because it was found in high concentrations in newly abscissied or freshly fallen leaves. This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. Typically it is an inhibitory chemical compound that effects bud growth, seed and bud dormancy, though it also plays a role in the promotion of growth too. Since it was found in freshly adscissed leaves it was though it played a role in the processes of natural leaf drop but further research has disproved that idea. In plant species from temperate parts of the world it plays a role in leaf and seed
dormancy by inhibiting growth, as it is dissipated from the seed or buds, growth begins. In other plants as ABA levels decrease - gibberellin levels increase and growth then commences. It accumulates within seeds when its levels increase during fruit maturation, preventing seed germination within the fruit or seed germination before winter. Abscisic acid is degraded with in plant tissues during cold temperatures or by water washing in out of the tissues, releasing the seeds and buds from dormancy. In plants that are water stressed, it plays a role in closing the stomata, with in a short period of time after stress conditions are realized, it is produced with in the plants when the roots are deficient in water causing a Ph change with in the plants that moves up to the leaves causing ABA production which regulates the potassium or sodium uptake with in the guard cells which then loss turgidity, closing the stomata.[2][3]
Auxins are compounds that positively influence cell enlargement, bud formation and root initiation. They promote the production of other hormones. They were the first class of growth regulators discovered. They effect cell elongation by altering cell wall plasticity. Auxins decrease in light and increase were its dark. They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower on the stems, effecting a process called apical dominance, they also promote lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers. When auxin are no longer produced by the growing point of a plants this initiates leaf abscission. Seed produce auxins as they are growing with in the flower, causing the flower to develop a fruit to hold the seeds. Auxins are toxic to plants in large concentrations, they are most toxic to dicots but not monocots and because of this synthetic auxin herbicides include 2-4-D and 2-4-5-T have been developed and used for weed control. The most common auxin found in plants is indoleacetic acid or IAA.
Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation, they were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence or the aging of tissues, are responsible for mediating auxin transport threw out the plant, affect internodal length and leaf growth. They have a highly synergistic effect in concert with auxins and the ratios of these two groups of plant hormones effect most major growth periods during a plants lifetime. Cytokinins counter the apical dominance induced by auxins.
Ethylene is a gas that forms from the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water and does not accumulate with in the cell but is diffused out of the cell and escapes out of the plant. it's effectiveness as a plant hormone is dependent on its rate of production verse 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, the elevated amounts of ethylene inhibit leaf expansion. As the new shoot is exposed to light, reactions by photochrome in the plants cells produce a signal for ethylene production to decrease allowing leaf expansion. Ethylene effects cell growth and cell shape; when a growing shoot hits an obstacle while under ground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell, the resulting thicker stem can exert more pressure 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 stems natural geotropic response which is to grow upright, allowing it to grow around an object. Studies seem to indicate that ethylene effects stem diameter and height, when stems of trees are subjected to wind causing lateral stress, greater ethylene production occurs resulting in thicker more sturdy tree trunks and branches. Ethylene effects fruit riping, normally when the seeds are mature, ethylene production increases and builds-up with in the fruit resulting in a 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.[4]
Gibberellins or GAs play a major role in seed germination, effecting enzyme production that mobilizes food production that new cells need for growth. This is done by effecting chromosomal transcription. They were first discovered when Japanese researches noticed a chemical produced by a fungus called ''Giberella fujikoroi'' that produced abnormal growth in rice plants. The gibberellins include a large range of chemicals that are produced natural with in plants and by fungi. GAs produce bolting of rosette forming plants, affecting internodal length and flowering, cellular division, and in seeds growth after germination. In seedlings a layer of cells called the aleurone layer wraps around the endosperm tissue, as a seed germinates the seedling produces GA that is transported to the aleurone layer which then produces enzymes that break down stored food reserves with in the endosperm that are utilized by the growing seedling. Gibberellins also revers the inhibition of shoot growth, and dormancy induced by ABA.[5]
Other identified plant growth regulators include:
#brassinolides are plant steroids chemically similar to animal steroid hormones. First isolated from pollen of the mustard family and extensively studied in Arabadopsis. They promote cell elongation and cell division, differentiation of xylem tissues, and inhibit leaf abscission.[6] Plants found deficient in brassinolides suffer from dwarfism.
#salicylic acid in some plants activates genes that assist in the defense against pathogenic invaders.
#jasmonates are produced from fatty acids and seem to promote the production of defense proteins that are used to fend off invading organisms. They are believed to also have a role in seed germination, the storage of protein in seeds and seem to effect root growth.
#signalling peptides
#Systemin
★ Hormone
★ Another quality guide
★ Simple plant hormone table with location of synthesis and effects of application - this is the format used in the descriptions at the ends of the Wikipedia articles on individual plant hormones.
★ Hormonal Regulation of Gene Expression and Development - Detailed intro including genetic information.
1. Rost, Thomas L., and T. Elliot Weier. 1979. ''Botany: a brief introduction to plant biology''. New York: Wiley. Pages 155-170. ISBN 0-471-02114-8
2. ''Decreased root hydraulic conductivity reduces leaf water potential, initiates stomatal closure and slows leaf expansion in flooded plants of castor oil (Ricinus communis) despite diminished delivery of ABA from the roots to shoots in xylem sap''. Else M.A.1; Coupland D.2; Dutton L.3; Jackson M.B.3 Physiologia Plantarum, Volume 111, Number 1, January 2001 , pp. 46-54(9) Blackwell Publishing
3. ''Reactive oxygen species and nitric oxide are involved in ABA inhibition of stomatal opening.''
YAN, JIUPIANG; TSUICHIHARA, NOBUE; ETOH, TAKEOMI; IWAI, SUMIO
Plant, Cell & Environment, Volume 30, Number 10, October 2007 , pp. 1320-1325(6) Blackwell Publishing
4. ''Arabidopsis EIN2 modulates stress response through abscisic acid response pathway.'' Authors: Wang, Youning; Liu, Chuang; Li, Kexue; Sun, Feifei; Hu, Haizhou; Li, Xia1; Zhao, Yankun; Han, Chunyu; Zhang, Wensheng; Duan, Yunfeng; Liu, Mengyu. Source: Plant Molecular Biology, Volume 64, Number 6, August 2007 , pp. 633-644(12) Springer
5. ''A comparative study of the effects of abscisic acid and methyl jasmonate on seedling growth of rice'' Authors: Tsai F-Y.1; Lin C.C.1; Kao C.H.1 'Plant Growth Regulation', Volume 21, Number 1, January 1997 , pp. 37-42(6) Springer
6. http://www.ingentaconnect.com/content/klu/plan/1998/00000037/00000005/00163157
Plant growth regulators or plant hormones are not nutrients, but chemicals that influence the development, differentiation and growth of cells and tissues. Most are naturally produced within plants. They are vital to plant growth; effecting flowering to seed development, seed dormancy to seed germination and seedling growth effecting which tissues grow up and which grow downward, leaf formation and stem growth, fruit development to ripening, finely to leaf drop and even plant death.
| Contents |
| Characteristics |
| Classes of plant hormones |
| Abscisic acid |
| Auxin |
| Cytokinin |
| Ethylene |
| Gibberellin |
| Other Known Hormones |
| See also |
| External links |
Characteristics
The concentration of hormones required for plant responses are very low (10-6 to 10-5 mol/L), due to these low levels it has been very difficult to study plant hormones and only since the late 1970's have scientist been able to start piecing together there affects and relationships to plant physiology. 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 synthesis of plant hormones is more diffuse and not always localized, but unlike animals which have a two circulatory systems that move fluids around the body actively, plants lack a rapid circulatory system with a heart to pump fluids and thus utilize hormone that are simple chemicals that can more easily be moved around the plant. These chemicals are very often produced very locally with in the plant body and those that are transported from one part of the plant to another utilize four types of movements; slow diffusion of ions and molecules, cytoplasmic streaming with in cells, sieve tubes and xylem that moves water and mineral solutes.
Plant hormones effect gene expression and transcription levels, cellular division and growth.
Classes of plant hormones
It is generally accepted that there are five major classes of plant hormones, some of these groups are made-up of many different chemicals that can vary from one plant to the next in structure, each chemical is grouped together into one of these classes because of there structural similarities and on there effects on plant physiology. Other PGRs exist naturally that are not easily grouped into these classes including chemicals that inhibit plant growth or interrupt the physiological processes with in plants. Each class has positive as well inhibitory functions and they most often work in tandem with each other, with varying ratios of one or more inter-playing to affect growth regulation.[1]
The five major classes are:
Abscisic acid
Abscisic acid also called ABA, was once called 'dormin' and 'abscicin II', because of its affects on plants, before its chemical properties were fully know. This chemical was named abscicic acid because it was found in high concentrations in newly abscissied or freshly fallen leaves. This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. Typically it is an inhibitory chemical compound that effects bud growth, seed and bud dormancy, though it also plays a role in the promotion of growth too. Since it was found in freshly adscissed leaves it was though it played a role in the processes of natural leaf drop but further research has disproved that idea. In plant species from temperate parts of the world it plays a role in leaf and seed
dormancy by inhibiting growth, as it is dissipated from the seed or buds, growth begins. In other plants as ABA levels decrease - gibberellin levels increase and growth then commences. It accumulates within seeds when its levels increase during fruit maturation, preventing seed germination within the fruit or seed germination before winter. Abscisic acid is degraded with in plant tissues during cold temperatures or by water washing in out of the tissues, releasing the seeds and buds from dormancy. In plants that are water stressed, it plays a role in closing the stomata, with in a short period of time after stress conditions are realized, it is produced with in the plants when the roots are deficient in water causing a Ph change with in the plants that moves up to the leaves causing ABA production which regulates the potassium or sodium uptake with in the guard cells which then loss turgidity, closing the stomata.[2][3]
Auxin
Auxins are compounds that positively influence cell enlargement, bud formation and root initiation. They promote the production of other hormones. They were the first class of growth regulators discovered. They effect cell elongation by altering cell wall plasticity. Auxins decrease in light and increase were its dark. They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower on the stems, effecting a process called apical dominance, they also promote lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers. When auxin are no longer produced by the growing point of a plants this initiates leaf abscission. Seed produce auxins as they are growing with in the flower, causing the flower to develop a fruit to hold the seeds. Auxins are toxic to plants in large concentrations, they are most toxic to dicots but not monocots and because of this synthetic auxin herbicides include 2-4-D and 2-4-5-T have been developed and used for weed control. The most common auxin found in plants is indoleacetic acid or IAA.
Cytokinin
Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation, they were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence or the aging of tissues, are responsible for mediating auxin transport threw out the plant, affect internodal length and leaf growth. They have a highly synergistic effect in concert with auxins and the ratios of these two groups of plant hormones effect most major growth periods during a plants lifetime. Cytokinins counter the apical dominance induced by auxins.
Ethylene
Ethylene is a gas that forms from the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water and does not accumulate with in the cell but is diffused out of the cell and escapes out of the plant. it's effectiveness as a plant hormone is dependent on its rate of production verse 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, the elevated amounts of ethylene inhibit leaf expansion. As the new shoot is exposed to light, reactions by photochrome in the plants cells produce a signal for ethylene production to decrease allowing leaf expansion. Ethylene effects cell growth and cell shape; when a growing shoot hits an obstacle while under ground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell, the resulting thicker stem can exert more pressure 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 stems natural geotropic response which is to grow upright, allowing it to grow around an object. Studies seem to indicate that ethylene effects stem diameter and height, when stems of trees are subjected to wind causing lateral stress, greater ethylene production occurs resulting in thicker more sturdy tree trunks and branches. Ethylene effects fruit riping, normally when the seeds are mature, ethylene production increases and builds-up with in the fruit resulting in a 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.[4]
Gibberellin
Gibberellins or GAs play a major role in seed germination, effecting enzyme production that mobilizes food production that new cells need for growth. This is done by effecting chromosomal transcription. They were first discovered when Japanese researches noticed a chemical produced by a fungus called ''Giberella fujikoroi'' that produced abnormal growth in rice plants. The gibberellins include a large range of chemicals that are produced natural with in plants and by fungi. GAs produce bolting of rosette forming plants, affecting internodal length and flowering, cellular division, and in seeds growth after germination. In seedlings a layer of cells called the aleurone layer wraps around the endosperm tissue, as a seed germinates the seedling produces GA that is transported to the aleurone layer which then produces enzymes that break down stored food reserves with in the endosperm that are utilized by the growing seedling. Gibberellins also revers the inhibition of shoot growth, and dormancy induced by ABA.[5]
Other Known Hormones
Other identified plant growth regulators include:
#brassinolides are plant steroids chemically similar to animal steroid hormones. First isolated from pollen of the mustard family and extensively studied in Arabadopsis. They promote cell elongation and cell division, differentiation of xylem tissues, and inhibit leaf abscission.[6] Plants found deficient in brassinolides suffer from dwarfism.
#salicylic acid in some plants activates genes that assist in the defense against pathogenic invaders.
#jasmonates are produced from fatty acids and seem to promote the production of defense proteins that are used to fend off invading organisms. They are believed to also have a role in seed germination, the storage of protein in seeds and seem to effect root growth.
#signalling peptides
#Systemin
See also
★ Hormone
External links
★ Another quality guide
★ Simple plant hormone table with location of synthesis and effects of application - this is the format used in the descriptions at the ends of the Wikipedia articles on individual plant hormones.
★ Hormonal Regulation of Gene Expression and Development - Detailed intro including genetic information.
1. Rost, Thomas L., and T. Elliot Weier. 1979. ''Botany: a brief introduction to plant biology''. New York: Wiley. Pages 155-170. ISBN 0-471-02114-8
2. ''Decreased root hydraulic conductivity reduces leaf water potential, initiates stomatal closure and slows leaf expansion in flooded plants of castor oil (Ricinus communis) despite diminished delivery of ABA from the roots to shoots in xylem sap''. Else M.A.1; Coupland D.2; Dutton L.3; Jackson M.B.3 Physiologia Plantarum, Volume 111, Number 1, January 2001 , pp. 46-54(9) Blackwell Publishing
3. ''Reactive oxygen species and nitric oxide are involved in ABA inhibition of stomatal opening.''
YAN, JIUPIANG; TSUICHIHARA, NOBUE; ETOH, TAKEOMI; IWAI, SUMIO
Plant, Cell & Environment, Volume 30, Number 10, October 2007 , pp. 1320-1325(6) Blackwell Publishing
4. ''Arabidopsis EIN2 modulates stress response through abscisic acid response pathway.'' Authors: Wang, Youning; Liu, Chuang; Li, Kexue; Sun, Feifei; Hu, Haizhou; Li, Xia1; Zhao, Yankun; Han, Chunyu; Zhang, Wensheng; Duan, Yunfeng; Liu, Mengyu. Source: Plant Molecular Biology, Volume 64, Number 6, August 2007 , pp. 633-644(12) Springer
5. ''A comparative study of the effects of abscisic acid and methyl jasmonate on seedling growth of rice'' Authors: Tsai F-Y.1; Lin C.C.1; Kao C.H.1 'Plant Growth Regulation', Volume 21, Number 1, January 1997 , pp. 37-42(6) Springer
6. http://www.ingentaconnect.com/content/klu/plan/1998/00000037/00000005/00163157
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