Different hormones can be sorted into different classes, depending on their chemical structures. Within each class of hormone, chemical structures can vary, but all members of the same class have similar physiological effects. Initial research into plant hormones identified five major classes:
abscisic acid,
auxins,
Gibberellins,
cytokinins and
ethylene. This list was later expanded, and
brassinosteroids,
jasmonates, salicylic acid, and
strigolactones are now also considered major plant hormones. Additionally there are several other compounds that serve functions similar to the major hormones, but their status as
bona fide hormones is still debated.
Abscisic acid Abscisic acid (also called ABA) is one of the most important plant growth inhibitors. It was discovered and researched under two different names,
dormin and
abscicin II, before its chemical properties were fully known. Once it was determined that the two compounds are the same, it was named abscisic acid. The name refers to the fact that it is found in high concentrations in newly
abscissed 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. In general, it acts as an inhibitory chemical compound that affects
bud growth, and seed and bud dormancy. It mediates changes within the
apical meristem, causing bud dormancy and the alteration of the last set of leaves into protective bud covers. Since it was found in freshly abscissed leaves, it was initially thought to play a role in the processes of natural leaf drop, but further research has disproven this. In plant species from temperate parts of the world, abscisic acid plays a role in leaf and seed dormancy by inhibiting growth, but, as it is dissipated from seeds or buds, growth begins. In other plants, as ABA levels decrease, growth then commences as
gibberellin levels increase. Without ABA, buds and seeds would start to grow during warm periods in winter and would be killed when it froze again. Since ABA dissipates slowly from the tissues and its effects take time to be offset by other plant hormones, there is a delay in physiological pathways that provides some protection from premature growth. Abscisic acid accumulates within seeds during fruit maturation, preventing seed germination within the fruit or before winter. Abscisic acid's effects are degraded within plant tissues during cold temperatures or by its removal by water washing in and out of the tissues, releasing the seeds and buds from dormancy. ABA exists in all parts of the plant, and its concentration within any tissue seems to mediate its effects and function as a hormone; its degradation, or more properly
catabolism, within the plant affects metabolic reactions and cellular growth and production of other hormones. Plants start life as a seed with high ABA levels. Just before the
seed germinates, ABA levels decrease; during germination and early growth of the seedling, ABA levels decrease even more. As plants begin to produce shoots with fully functional leaves, ABA levels begin to increase again, slowing down cellular growth in more "mature" areas of the plant. Stress from water or predation affects ABA production and catabolism rates, mediating another cascade of effects that trigger specific responses from targeted cells. Scientists are still piecing together the complex interactions and effects of this and other phytohormones. In plants under water stress, ABA plays a role in closing the
stomata. Soon after plants are water-stressed and the roots are deficient in water, a signal moves up to the leaves, causing the formation of ABA precursors there, which then move to the roots. The roots then release ABA, which is translocated to the foliage through the vascular system and modulates potassium and sodium uptake within the
guard cells, which then lose
turgidity, closing the stomata.
Auxins Auxins are compounds that positively influence cell enlargement, bud formation, and root initiation. They also promote the production of other hormones and, in conjunction with
cytokinins, control the growth of stems, roots, and fruits, and convert stems into flowers. Auxins were the first class of growth regulators discovered. A Dutch Biologist
Frits Warmolt Went first described auxins. They affect cell elongation by altering cell wall plasticity. They stimulate
cambium, a subtype of
meristem cells, to divide, and in stems cause
secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower down the stems in a phenomenon known as
apical dominance, and also to promote
lateral and
adventitious root development and growth. Leaf abscission is initiated by the growing point of a plant ceasing to produce auxins. Auxins in seeds regulate specific protein synthesis, as they develop within the flower after
pollination, causing the flower to develop a fruit to contain the developing seeds. In large concentrations, auxins are often toxic to plants; they are most toxic to
dicots and less so to
monocots. Because of this property,
synthetic auxin herbicides including
2,4-dichlorophenoxyacetic acid (2,4-D) and
2,4,5-trichlorophenoxyacetic acid (2,4,5-T) have been developed and used for
weed control by defoliation. Auxins, especially
1-naphthaleneacetic acid (NAA) and
indole-3-butyric acid (IBA), are also commonly applied to stimulate root growth when taking
cuttings of plants. The most common auxin found in plants is
indole-3-acetic acid (IAA).
Brassinosteroids Brassinosteroids (BRs) are a class of polyhydroxysteroids, the only example of steroid-based hormones in plants. Brassinosteroids control cell elongation and division,
gravitropism, resistance to stress, and
xylem differentiation. They inhibit root growth and leaf abscission.
Brassinolide was the first brassinosteroid to be identified and was isolated from extracts of rapeseed (
Brassica napus) pollen in 1979. Brassinosteroids are a class of steroidal phytohormones in plants that regulate numerous physiological processes. This plant hormone was identified by Mitchell et al. who extracted ingredients from Brassica pollen only to find that the extracted ingredients' main active component was
Brassinolide. This finding meant the discovery of a new class of plant hormones called Brassinosteroids. These hormones act very similarly to animal steroidal hormones by promoting growth and development. In plants these steroidal hormones play an important role in cell elongation via BR signaling. The brassinosteroids receptor brassinosteroid insensitive 1 (BRI1) is the main receptor for this signaling pathway. This BRI1 receptor was found by Clouse et al. who made the discovery by inhibiting BR and comparing it to the wildtype in Arabidopsis. The BRI1 mutant displayed several problems associated with growth and development such as
dwarfism, reduced cell elongation and other physical alterations. which leads to a signal cascade that further regulates cell elongation. This signal cascade however is not entirely understood at this time. What is believed to be happening is that BR binds to the BAK1 complex which leads to a
phosphorylation cascade. This phosphorylation cascade then causes BIN2 to be deactivated which causes the release of
transcription factors.
Cytokinins , a cytokinin
Cytokinins (CKs) are a group of chemicals that influence cell division and shoot formation. They also help delay
senescence of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. They were called kinins in the past when they were first isolated from
yeast cells. Cytokinins and auxins often work together, and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime. Cytokinins counter the apical dominance induced by auxins; in conjunction with ethylene, they promote abscission of leaves, flower parts, and fruits. Among the plant hormones, the three that are known to help with immunological interactions are ethylene (ET), salicylates (SA), and jasmonates (JA), however more research has gone into identifying the role that cytokinins play in this. Evidence suggests that cytokinins delay the interactions with pathogens, showing signs that they could induce resistance toward these pathogenic bacteria. Accordingly, there are higher CK levels in plants that have increased resistance to pathogens compared to those which are more susceptible. For example, pathogen resistance involving cytokinins was tested using the
Arabidopsis species by treating them with naturally occurring CK (trans-zeatin) to see their response to the bacteria
Pseudomonas syringa. Tobacco studies reveal that over expression of CK inducing IPT genes yields increased resistance whereas over expression of CK oxidase yields increased susceptibility to pathogen, namely
P. syringae. While there's not much of a relationship between this hormone and physical plant behavior, there are behavioral changes that go on inside the plant in response to it. Cytokinin defense effects can include the establishment and growth of microbes (delay leaf senescence), reconfiguration of secondary metabolism or even induce the production of new organs such as
galls or nodules. These organs and their corresponding processes are all used to protect the plants against biotic/abiotic factors.
Ethylene Unlike the other major plant hormones,
ethylene is a gas and a very simple organic compound, consisting of just six atoms. It forms through the breakdown of
methionine, an amino acid which is in all cells. Ethylene has very limited solubility in water and therefore does not accumulate within the cell, typically diffusing out of the cell and escaping the plant. Its effectiveness as a plant hormone is dependent on its rate of production versus 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, which leads to elevated amounts of ethylene, inhibiting
leaf expansion (see
hyponastic response). As the new shoot is exposed to light, reactions mediated by
phytochrome in the plant's cells produce a signal for ethylene production to decrease, allowing leaf expansion. Ethylene affects cell growth and cell shape; when a growing shoot or root hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell. The resulting thicker stem is stronger and less likely to buckle under pressure as it presses 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 stem's natural
geotropic response, which is to grow upright, allowing it to grow around an object. Studies seem to indicate that ethylene affects stem diameter and height: when stems of trees are subjected to wind, causing lateral stress, greater ethylene production occurs, resulting in thicker, sturdier tree trunks and branches. Ethylene also affects
fruit ripening. Normally, when the seeds are mature, ethylene production increases and builds up within 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. Ethylene diffusion out of plants is strongly inhibited underwater. This increases internal concentrations of the gas. In numerous aquatic and semi-aquatic species (e.g.
Callitriche platycarpus, rice, and
Rumex palustris), the accumulated ethylene strongly stimulates upward elongation. This response is an important mechanism for the adaptive escape from submergence that avoids asphyxiation by returning the shoot and leaves to contact with the air whilst allowing the release of entrapped ethylene. At least one species (
Potamogeton pectinatus) has been found to be incapable of making ethylene while retaining a conventional morphology. This suggests ethylene is a true regulator rather than being a requirement for building a plant's basic body plan.
Gibberellins Gibberellins (GAs) include a large range of chemicals that are produced naturally within plants and by fungi. They were first discovered when Japanese researchers, including Eiichi Kurosawa, noticed a chemical produced by a fungus called
Gibberella fujikuroi that produced abnormal growth in rice plants. It was later discovered that GAs are also produced by the plants themselves and control multiple aspects of development across the life cycle. The synthesis of GA is strongly upregulated in seeds at germination and its presence is required for germination to occur. In seedlings and adults, GAs strongly promote cell elongation. GAs also promote the transition between vegetative and reproductive growth and are also required for pollen function during fertilization. Gibberellins breaks the dormancy (in active stage) in seeds and buds and helps increasing the height of the plant. It helps in the growth of the stem.
Jasmonates Jasmonates (JAs) are lipid-based hormones that were originally isolated from
jasmine oil. JAs are especially important in the plant response to attack from
herbivores and
necrotrophic pathogens. The most active JA in plants is
jasmonic acid. Jasmonic acid can be further
metabolized into
methyl jasmonate (MeJA), which is a
volatile organic compound. This unusual property means that MeJA can act as an airborne signal to communicate herbivore attack to other distant leaves within one plant and even as a signal to neighboring plants. In addition to their role in defense, JAs are also believed to play roles in seed germination, the storage of protein in seeds, and root growth.
Jasmonic acid methyl ester (JAME) has been shown to regulate genetic expression in plants. They act in signalling pathways in response to herbivory, and upregulate expression of defense genes.
Jasmonyl-isoleucine (JA-Ile) accumulates in response to herbivory, which causes an upregulation in defense gene expression by freeing up transcription factors. Studies have shown that there is significant crosstalk between defense pathways.
Salicylic acid Salicylic acid (SA) is a hormone with a structure related to
benzoic acid and
phenol. It was originally isolated from an extract of
white willow bark (
Salix alba) and is of great interest to human medicine, as it is the precursor of the painkiller
aspirin. In plants, SA plays a critical role in the defense against biotrophic pathogens. In a similar manner to JA, SA can also become
methylated. Like MeJA,
methyl salicylate is volatile and can act as a long-distance signal to neighboring plants to warn of pathogen attack. In addition to its role in defense, SA is also involved in the response of plants to abiotic stress, particularly from drought, extreme temperatures, heavy metals, and osmotic stress. Salicylic acid (SA) serves as a key hormone in plant innate immunity, including resistance in both local and systemic tissue upon biotic attacks, hypersensitive responses, and cell death. Some of the SA influences on plants include seed germination, cell growth, respiration, stomatal closure, senescence-associated gene expression, responses to abiotic and biotic stresses, basal thermo tolerance and fruit yield. A possible role of salicylic acid in signaling disease resistance was first demonstrated by injecting leaves of resistant tobacco with SA. The result was that injecting SA stimulated pathogenesis related (PR) protein accumulation and enhanced resistance to tobacco mosaic virus (TMV) infection. Exposure to pathogens causes a cascade of reactions in the plant cells. SA biosynthesis is increased via isochorismate synthase (ICS) and phenylalanine ammonia-lyase (PAL) pathway in plastids. It was observed that during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. Therefore with increased internal concentration of SA, plants were able to build resistant barriers for pathogens and other adverse environmental conditions.
Strigolactones , a
strigolactone|155px
Strigolactones (SLs) were originally discovered through studies of the germination of the parasitic weed
Striga lutea. It was found that the germination of
Striga species was stimulated by the presence of a compound
exuded by the roots of its host plant. It was later shown that SLs that are exuded into the soil also promote the growth of
symbiotic arbuscular mycorrhizal (AM) fungi. More recently, another role of SLs was identified in the inhibition of shoot branching. This discovery of the role of SLs in shoot branching led to a dramatic increase in the interest in these hormones, and it has since been shown that SLs play important roles in
leaf senescence,
phosphate starvation response, salt tolerance, and light signalling.
Other known hormones Other identified plant growth regulators include: •
Plant peptide hormones – encompasses all small secreted peptides that are involved in cell-to-cell signaling. These small peptide hormones play crucial roles in plant growth and development, including defense mechanisms, the control of cell division and expansion, and pollen self-incompatibility. The small
peptide CLE25 is known to act as a long-distance signal to communicate water stress sensed in the roots to the stomata in the leaves. •
Polyamines – are strongly basic molecules with low molecular weight that have been found in all organisms studied thus far. They are essential for plant growth and development and affect the process of mitosis and meiosis. In plants, polyamines have been linked to the control of
senescence and
programmed cell death. •
Nitric oxide (NO) – serves as signal in hormonal and defense responses (e.g. stomatal closure, root development, germination, nitrogen fixation, cell death, stress response). NO can be produced by a yet undefined NO synthase, a special type of nitrite reductase, nitrate reductase, mitochondrial cytochrome c oxidase or non enzymatic processes and regulate plant cell organelle functions (e.g. ATP synthesis in chloroplasts and mitochondria). •
Karrikins – are not plant hormones as they are not produced by plants themselves but are rather found in the smoke of burning plant material. Karrikins can promote seed germination in many species. The finding that plants which lack the receptor of karrikin receptor show several developmental
phenotypes (enhanced biomass accumulation and increased sensitivity to drought) have led some to speculate on the existence of an as yet unidentified karrikin-like endogenous hormone in plants. The cellular karrikin signalling pathway shares many components with the strigolactone signalling pathway. •
Triacontanol – a fatty alcohol that acts as a growth stimulant, especially initiating new basal breaks in the
rose family. It is found in
alfalfa (lucerne), bee's wax, and some waxy leaf cuticles.
Seed dormancy Plant hormones affect seed germination and dormancy by acting on different parts of the seed. Embryo dormancy is characterized by a high ABA:GA ratio, whereas the seed has high abscisic acid sensitivity and low GA sensitivity. In order to release the seed from this type of dormancy and initiate seed germination, an alteration in hormone biosynthesis and degradation toward a low ABA/GA ratio, along with a decrease in ABA sensitivity and an increase in GA sensitivity, must occur. ABA controls embryo dormancy, and GA embryo germination. Seed coat dormancy involves the mechanical restriction of the seed coat. This, along with a low embryo growth potential, effectively produces seed dormancy. GA releases this dormancy by increasing the embryo growth potential, and/or weakening the seed coat so the radical of the seedling can break through the seed coat. Different types of seed coats can be made up of living or dead cells, and both types can be influenced by hormones; those composed of living cells are acted upon after seed formation, whereas the seed coats composed of dead cells can be influenced by hormones during the formation of the seed coat. ABA affects testa or seed coat growth characteristics, including thickness, and effects the GA-mediated embryo growth potential. These conditions and effects occur during the formation of the seed, often in response to environmental conditions. Hormones also mediate endosperm dormancy: Endosperm in most seeds is composed of living tissue that can actively respond to hormones generated by the embryo. The endosperm often acts as a barrier to seed germination, playing a part in seed coat dormancy or in the germination process. Living cells respond to and also affect the ABA:GA ratio, and mediate cellular sensitivity; GA thus increases the embryo growth potential and can promote endosperm weakening. GA also affects both ABA-independent and ABA-inhibiting processes within the endosperm. ==Use in horticulture==