PLANT HORMONES OR PHYTOHORMONES
Introduction to Phyto-hormones:
Plant hormones are a group of naturally occurring, organic substances which influence physiological processes at low concentrations. The processes influenced consist mainly of growth, differentiation and development, though other processes, such as stomatal movement, may also be affected. Plant hormones have also been referred to as ‘phytohormones’ though this term is infrequently used.
Went and Thimann in 1937 define a hormone as a substance which is transferred from one part of an organism to another. Its original use in plant physiology was derived from the mammalian concept of a hormone. This involves a localized site of synthesis, transport in the bloodstream to a target tissue, and the control of a physiological response in the target tissue via the concentration of the hormone. Auxin, the first-identified plant hormone, produces a growth response at a distance from its site of synthesis, and thus fits the definition of a transported chemical messenger. However this was before the full range of what we now consider plant hormones was known. It is now clear that plant hormones do not fulfill the requirements of a hormone in the mammalian sense. The synthesis of plant hormones may be localized (as occurs for animal hormones), but it may also occur in a wide range of tissues, or cells within tissues. While they may be transported and have their action at a distance this is not always the case. At one extreme we find the transport of cytokinins from roots to leaves where they prevent senescence and maintain metabolic activity, while at the other extreme the production of the gas ethylene may bring about changes within the same tissue, or within the same cell, where it is synthesized. Thus, transport is not an essential property of a plant hormone.
The term ‘hormone’ was first used in medicine about 100 years ago for a stimulatory factor, though it has come to mean a transported chemical message. The word in fact comes from the Greek, where its meaning is ‘to stimulate’ or ‘to set in motion’. Thus the origin of word itself does not require the notion of transport per se, and the above definition of a plant hormone is much closer to the meaning of the Greek origin of the word than is the current meaning of hormone used in the context of animal physiology.
Plant hormones are a unique set of compounds, with unique metabolism and properties that form the subject of this book. Their only universal characteristics are that they are natural compounds in plants with an ability to affect physiological processes at concentrations far below those where either nutrients or vitamins would affect these processes.
Classes
Depending on their chemical structures, different hormones can be sorted into different classes. Within each class of hormone the exact structures vary, but they have similar physiological effects. Initial research into plant hormones identified five major classes: abscisic acid, auxin, cytokinins, ethylene and gibberellins. This list was later expanded and brassinosteroids, jasmonates, salicylic acid and strigolactones are now considered as major plant hormones. Additionally there are also several other compounds that fulfill a similar function to the major hormones, but their status as bonefide hormones is still debated.
1. Abscisic acid (ABA)
Abscisic acid is a single compound with the following formula:
The first name given was "abscisin II" because it was thought to control the abscission of cotton bolls. At almost the same time another group named it "dormin" for a purported role in bud dormancy. By compromise the name abscisic acid was coined. It now appears to have little role in either abscission (which is regulated by ethylene) or bud dormancy, but we are stuck with this name. As a result of the original association with abscission and dormancy, ABA has become thought of as an inhibitor. While exogenous applications can inhibit growth in the plant, ABA appears to act as much as a promoter, such as in the promotion of storage protein synthesis in seeds, as an inhibitor, and a more open attitude towards its overall role in plant development is warranted. One of the main functions is the regulation of stomatal closure.
Site of synthesis of ABA
ABA is synthesized from glyceraldehyde-3-phosphate via isopentenyl diphosphate and carotenoids (in roots and mature leaves, particularly in response to water stress. Seeds are also rich in ABA which may be imported from the leaves or synthesized in situ.
Transport of ABA
ABA is exported from roots in the xylem and from leaves in the phloem. There is some evidence that ABA may circulate to the roots in the phloem and then return to the shoots in the xylem.
Mode of Action
ABA is a plant hormone that regulates seed development, dormancy and germination, stomata closure, and responses to drought stress, and its core signaling pathway has recently been identified. In Arabidopsis, this pathway involves families of ABA receptors (PYR1/RCAR11, PYL1/RCAR12, PYL2/RCAR14, PYL3/RCAR13, PYL8/RCAR3, PYL9/RCAR1), protein phosphatase 2Cs (ABI1, ABI2, HAB1, HAB2, PP2CA/AHG3), and SNRK2 protein kinases (SnRK2.6/OST1/SRK2E, SNRK2.2/SRK2D and SnRK2.3/SRK2IA).
The core ABA signaling pathway is shown in the figure to the left. In un stimulated cells, the ABA receptor (R) is an unliganded dimer (monomer shown) in the cytosol and nucleus, and the SNRK2 protein kinase (K) is bound to a protein phosphatase 2C (P) in a complex (K-P) in which the kinase is dephosphorylated and inactivated by the phosphatase. When the ABA level rises, ABA binds to the ABA receptor, and this creates a binding surface for PP2C. The activated receptor (R.ABA) dissociates to monomers which bind to protein phosphatase 2C (R.ABA-P). The sequestration of the protein phosphatase frees the protein kinase to be activated by auto phosphorylation or phosphorylation by another protein kinase. Activation of SnRK2.6/OST1/SRK2E leads to phosphorylation of: 1) ion channels SLAC1 and KAT1 in guard cells and stomatal closure; 2) transcription factor ABI5 in seeds/seedlings and dormancy/growth arrest; or 3) phosphorylation of transcription factor AREB/ABF in vegetative tissue and stress tolerance and growth regulation.
Effects of ABA
ü Stomatal closure-water shortage brings about an increase in ABA which leads to stomatal closure.
ü ABA inhibits shoot growth (but has less effect on, or may promote, root growth). This may represent a response to water stress.
ü ABA induces storage protein synthesis in seeds.
ü ABA counteracts the effect of gibberellin on α-amylase synthesis in germinating cereal grains.
ü ABA affects the induction and maintenance of some aspects of dormancy in seeds It does not, however, appear to be the controlling factor in ‘true dormancy’ or ‘rest,’ which is dormancy that needs to be broken by low temperature or light.
ü Increase in ABA in response to wounding induces gene transcription, notably for proteinase inhibitors, so it may be involved in defense against insect attack.
2. Auxins
Indole-3-acetic acid (IAA) is the main auxin in most plants.
Compounds which serve as IAA precursors may also have auxin activity (e.g., indoleacetaldehyde). Some plants contain other compounds that display weak auxin activity (e.g., phenylacetic acid). IAA may also be present as various conjugates such as indoleacetyl aspartate (Chapter B1)). 4-chloro-IAA has also been reported in several species though it is not clear to what extent the endogenous auxin activity in plants can be accounted for by 4-Cl-IAA. Several synthetic auxins are also used in commercial applications.
Site of biosynthesis of Auxins
IAA is synthesized from tryptophan or indole primarily in leaf primordia and young leaves, and in developing seeds.
Transport of Auxins
IAA transport is cell to cell mainly in the vascular cambium and the procambial strands, but probably also in epidermal cells. Transport to the root probably also involves the phloem.
Mode of Action
Auxin is a pivotal plant hormone that controls many aspects of plant growth and development. It binds to a small family of intracellular receptors, of which the best characteristics is TIR1 (Transport Inhibitor Response 1). It mediates physiological effects through transcriptional regulation, when TIR1 binds auxin, the complex then binds to AUX/IAA (auxin/Indole 3-acetic acids) proteins. In an absence of auxin, Auxin/IAAs directly inetract with ARFs (Auxin Response Factors) and inhibit their transcriptional activity. In the presence of auxin, AUX/IAAs are rapidly removed, abrogating their inhibition of ARFs and the expressions of auxin response genes.
Apart from functioning as an auxin receptor, TIR1 is a F-box protein. It acts as a component of the SCF, a E3 ubiquitin ligase that transfers ubiquitin molecules to the Aux/IAAA proteins, targeting them for proteolysis by the 26S proteasome. In this way auxin stimulates Aux/IAA-TIR1 binding and ubiquitination and degradation of AUX/IAAs. But, how the TIR1 protein senses and becomes activated by auxin remains unclear.
Auxin also acts through its interaction with another receptor protein, Auxin Binding Protien-1 (ABP1). It is located in the lumen of ER as well as plasma membrane. ABP1 is associated with auxin response at plasma membrane, including activation of proton pump and cell wall acidification and contributes to auxin regulated gene expression. At this point we don’t know the signalling downstream of ABP1 is transduced.
Effects of Auxins
ü Cell enlargement-auxin stimulates cell enlargement and stem growth.
ü Cell division-auxin stimulates cell division in the cambium and, in combination with cytokinin, in tissue culture.
ü Vascular tissue differentiation-auxin stimulates differentiation of phloem and xylem.
ü Root initiation-auxin stimulates root initiation on stem cuttings, and also the development of branch roots and the differentiation of roots in tissue culture.
ü Tropistic responses-auxin mediates the tropistic (bending) response of shoots and roots to gravity and light.
ü Apical dominance-the auxin supply from the apical bud represses the growth of lateral buds).
ü Leaf senescence-auxin delays leaf senescence.
ü Leaf and fruit abscission-auxin may inhibit or promote (via ethylene) leaf and fruit abscission depending on the timing and position of the source.
ü Fruit setting and growth-auxin induces these processes in some fruit.
ü Assimilate partitioning-assimilate movement is enhanced towards an auxin source possibly by an effect on phloem transport.
ü Fruit ripening-auxin delays ripening.
ü Flowering-auxin promotes flowering in Bromeliads.
ü Growth of flower parts-stimulated by auxin.
ü Promotes femaleness in dioeciously flowers.
3. Gibberellins (GAs)
The gibberellins (GAs) are a family of compounds based on the ent- gibberellane structure; over which 125 members exist. While the most widely available compound is GA3 or gibberellic acid, which is a fungal product, the most important GA in plants is GA1, which is the GA primarily responsible for stem elongation. Many of the other GAs are precursors of the growth-active GA1.
Site of biosynthesis of gibberellins
GAs are synthesized from glyceraldehyde-3-phosphate, via isopentenyl diphosphate, in young tissues of the shoot and developing seed. Their biosynthesis starts in the chloroplast and subsequently involves membrane and cytoplasmic steps.
Transport of gibberellins
Some GAs are probably transported in the phloem and xylem. However the transport of the main bioactive polar GA1 seems restricted.
Mode of Action
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These proteins have a conserved DELLA (Asp-Gu-Leu-Leu-Ala) sequence at their N terminal end that is critical for their function. Like auxins, gibberellins bind to an intra-cellular receptor protein. It binds with the soluble gibberellin receptor termed GA intensive protein dwarf1 (GID1). Gibberellin binding to GID1 triggers its interaction with DELLA proteins. This interaction stimulates binding of the DELLA proteins to SCF (Skp1/Cullin/F-box), a ubiquitin ligases, leading to poly ubiquitination and degradation of the DELLA protein by the 26S proteasome. While this relatively simple GA-signalling cascade involves two major players, a receptor and a DELLA protein.
Effects
ü Stem growth-GA1 causes hyper elongation of stems by stimulating both cell division and cell elongation This produces tall, as opposed to dwarf, plants.
ü Bolting in long day plants-GAs cause stem elongation in response to long days.
ü Induction of seed germination-GAs can cause seed germination in some seeds that normally require cold (stratification) or light to induce germination.
ü Enzyme production during germination- GA stimulates the production of numerous enzymes, notably α-amylase, in germinating cereal grains.
ü Fruit setting and growth-This can be induced by exogenous applications in some fruit (e.g., grapes). The endogenous role is uncertain.
ü Induction of maleness in dioeciously flowers.
4. Cytokinins
CKs are adenine derivatives characterized by an ability to induce cell division in tissue culture (in the presence of auxin). The most common cytokinin base in plants is zeatin. Cytokinins also occur as ribosides and ribotides.
Sites of biosynthesis of cytokinins
CK biosynthesis is through the biochemical modification of adenine (Chapter B3). It occurs in root tips and developing seeds.
Transport of cytokinins
CK transport is via the xylem from roots to shoots.
Mode of action
Cytokinins induce cell division, cell expansion in cotyledon, chloroplast and etioplast development, suppression of epical dominance and senescence and differentiation of in virto cultured cells. However, very little is known about the mechanism of cytokinin at the molecular level. Cytokinins are perceived and transduced by a two component signaling system which is the most common form of signaling pathway that responds to extra cellular events in a bacteria and plants. The canonical two component system in a bacteria consists of sensor that is autophosphorylating histidine kinase and a response regulator, which transfers the phosphate from sensor kinase to a conserved aspartate within itself.
The sensor histidine kinase is located in the membrane. It can be activated by binding a ligand that is in the extra cellular medium. Activation causes the kinase to auto phosphorylates. The reaction transfers the phosphate from ATP on to a histidine residue in a kinase domain. The sensor interacts with the effector protein i-e. response regulator. The response regulator has two domains- conserved receiver domain and effector domain. The receiver domain catalyses transfer of the phosphate group from the histidine on the sensor to an aspartic acid residue in its own domain. This activates effector domain. The usual end target of the two component pathway is the regulation of gene transcription.
Effects
ü Cell division-exogenous applications of CKs induce cell division in tissue culture in the presence of auxin. This also occurs endogenously in crown gall tumors on plants. The presence of CKs in tissues with actively dividing cells (e.g., fruits, shoot tips) indicates that CKs may naturally perform this function in the plant.
ü Morphogenesis-in tissue culture and crown gall CKs promote shoot initiation. In moss, CKs induce bud formation.
ü Growth of lateral buds-CK applications, or the increase in CK levels in transgenic plants with genes for enhanced CK synthesis, can cause the release of lateral buds from apical dominance.
ü Leaf expansion, resulting solely from cell enlargement. This is probably the mechanism by which the total leaf area is adjusted to compensate for the extent of root growth, as the amount of CKs reaching the shoot will reflect the extent of the root system. However this has not been observed in transgenic plants with genes for increased CK biosynthesis, possibly because of a common the lack of control in these systems.
ü CKs delay leaf senescence.
ü CKs may enhance stomatal opening in some species.
ü Chloroplast development-the application of CK leads to an accumulation of chlorophyll and promotes the conversion of etioplasts into chloroplasts.
5. Ethylene
The gas ethylene (C2H4) is synthesized from methionine (Chapter B4) in many tissues in response to stress, and is the fruit ripening hormone. It does not seem to be essential for normal mature vegetative growth, as ethylene deficient transgenic plants grow normally. However they cannot, as seedlings, penetrate the soil because they lack the stem thickening and apical hook responses to ethylene, and they are susceptible to diseases because they lack the ethylene-induced disease resistance responses. It is the only hydrocarbon with a pronounced effect on plants.
Sites of synthesis of Ethylene
Ethylene is synthesized by most tissues in response to stress. In particular, it is synthesized in tissues undergoing senescence or ripening.
Transport of Ethylene
Being a gas, ethylene moves by diffusion from its site of synthesis. A crucial intermediate in its production, 1-aminocyclopropane-1-carboxylic acid (ACC) can, however, be transported and may account for ethylene effects at a distance from the causal stimulus.
Mode of action
Ethylene binds with membrane bound receptors. Ethylene receptors are dimeric, multipass trans membrane proteins, with a copper containing ethylene-binding domain. They are grouped in a family of five membranes- ETR1 (Ethylene receptor 1), ETR2, ERS1 (Ethylene response sensor 1), ERS2 and EIN4 (Ethylene insensitive 4). The receptors are primarily localised to a ER and have homology to bacterial two component receptors.
These receptors are negative regulators. Binding of ethylene inactivates the receptor. Ethylene receptors have non- cytosolic domain, which contains a copper atom that binds ethylene and a cytosolic intracellular histidine kinase domain. The binding of ethylene inactivates ethylene receptors, inhibiting the kinase domain and the downstream signalling pathway emanating from it. In its unbound, active state, the receptor activates Raf-like serine/threonine kinase, CTR1 (Constitutive Response 1). The function of CTR1 in ethylene signalling depends on its serine/threonine kinase activity and the association of its N-terminal domain with the ethylene receptor. By an unknown signalling mechanism, active CTR1 stimulates the ubiquiltylation and degradation of nuclear gene regulatory protein called EIN3 which is required for transcription of ethylene responsive genes. The EIN protein gets its name from the finding that plants with inactivating mutations in the gene that encodes it are ethylene insensitive. Ethylene binding inactivates the receptors, altering their conformation so that they no longer bind to CTR1. As a result, CTR1 is inactivated, and the down signalling pathway emanating from it is blocked; the EIN3 protein is no longer ubiquitylated and degraded and can now activate the transcription of the large number of ethylene responsive genes.
Effects
ü The so called triple response, when, prior to soil emergence, dark grown seedlings display a decrease in tem elongation, a thickening of the stem and a transition to lateral growth as might occur during the encounter of a stone in the soil.
ü Maintenance of the apical hook in seedlings.
ü Stimulation of numerous defense responses in response to injury or disease. ! Release from dormancy.
ü Shoot and root growth and differentiation. ! Adventitious root formation. ! Leaf and fruit abscission.
ü Flower induction in some plants (2E: G8). ! Induction of femaleness in dioecious flowers (2E: 8).
ü Flower opening.
ü Flower and leaf senescence.
ü Fruit ripening.
References
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