Sakshi Education
Plant Hormones Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are small molecules derived from various metabolic pathways and are structurally dissimilar but are involved in regulating plant growth and mediating response to biotic and abiotic stresses. Plants differ from animals in many aspects with respect to hormone production and mechanisms of hormone signalling. Plants, unlike animals lack glands that produce and secrete hormones. Plants possess receptors which are diverse and differ from animal systems. The major mechanism of controlling the signaling cascade is through protein degradation. Hormones interact at multiple levels to regulate various growths and defense processes.
Major classes of Phytohormones:
Auxins were the first class of growth regulators to be discovered. Auxins promote cell enlargement, bud formation and root initiation. In combination with cytokinins, they enhance the production of other hormones that control the growth of stems, roots, flowers and fruits. They influence cell elongation by altering cell wall plasticity. The production of Auxins decreases when in the presence of light and increases in dark. Auxins stimulate cambium cells to multiply and cause differentiation of secondary xylem in stems. The action of Auxins establishes a mechanism called Apical Dominance, which inhibits the growth of buds at the bottom of the stems and promotes lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers. Cessation of Auxin production by the growing point of a plant leads to initiation of leaf abscission. Auxins regulate gene expression spatially facilitating seed specific protein synthesis responsible for seed setting in the flowers subsequent to pollination.
Auxins when present in high concentrations are toxic to plants in large which enable to be utilized as herbicides. Synthetic auxin herbicides such as 2,4-D and 2,4,5-T are employed for weed control. Auxins, especially 1- Naphthalene acetic acid (NAA) and Indole-3- Butryic acid (IBA) are commonly applied to stimulate root growth in plant cuttings. IBA is considered as the classical rooting hormone. The major naturally occurring auxin is IAA. IAA has been implicated in virtually every aspect of plant growth and development, as well as defense responses. IAA is generally synthesized from tryptophan via at least two pathways: the tryptamine (TAM) and indole-3-pyruvic acid (IPA) pathways but IAA can also be synthesized from indole, bypassing tryptophan.
Jasmonic Acid:
Jasmonic Acid (JA) and its volatile methyl ester (MeJA) are members of the hormonally active jasmonate family of compounds that are common throughout the plant kingdom. JA and MeJA have been discovered to play a key role in plant defence signalling to biotic stress, acting as the major signals that induce the expression of defensive proteinase inhibitors (PIs). Jasmonates belong to a family of fatty acid derivatives, collectively called oxylipins that are produced by oxidative metabolism of polyunsaturated fatty acids. JA is synthesized from linolenic acid (18:3) via the octadecanoid pathway. JA can be conjugated to isoleucine to form JA-Ile or converted to the volatile methyl-JA. The mechanism of Jasmonate synthesis and functioning in plants is pretty similar to that of arachidonic acid-derived signalling compounds (e.g. prostaglandins) in animals as species in both kingdoms employ lipoxygenase and cytochromes P450 to oxidize polyunsaturated fatty acid substrates.
JAs are widely considered to function as ‘master regulators’ of plant defence responses to biotic stress. The above fact is supported by evidences such as a rapid increase in endogenous jasmonate levels in response to wounding, pathogen infection, and other types of biotic stress. In addition, treatment of plants with exogenous JAs results in major re-programming of gene expression, including defence-related genes that are activated by herbivore and pathogen attack. Moreover, mutants that are defective in either the synthesis or perception of JAs are severely compromised in resistance to a wide range of plant invaders. Genetic alterations that cause constitutive activation of the jasmonate signalling pathway result in enhanced resistance to certain herbivores and pathogens. Several jasmonate-regulated compounds that confer resistance to herbivore attack have been identified. One of such compounds include Protein-based defences such as proteinase inhibitors (PIs), threonine deaminase and arginase that impair thr procurement of essential amino acids from the leaf diet in the lepidopteran midgut. JA-regulated secondary metabolites including alkaloids (e.g. nicotine) and glucosinolates exert toxic and antifeedant effects on leaf-eating invaders.
JA plays a key role in defence against some fungal pathogens, including Pythium and Alternaria brassicicola. It has been that these pathogens kill mutants that cannot synthesize JA, while neighboring wild-type plants remain healthy. Application of exogenous Jasmonate substantially protects mutant plants, reducing the incidence of disease to a level close to that of wild-type controls, but JA treatment does not protect the Jasmonate-insensitive mutant coi1 from infection. Pythium species are ubiquitous in soil and root habitats worldwide, but most are considered to be minor pathogens. Thus, jasmonate is essential for plant defence against Pythium and because of the high exposure of plant roots to Pythium inoculum in soil, may well be fundamental to survival of plants in nature.
Many plant defence responses against insect attack occur both locally at the site of wounding and systemically in undamaged tissues of the plant. This phenomenon implies the existence of signals that are transported from the site of wounding to other parts of the plant. Biochemical and pharmacological experiments established a central role for jasmonates in regulating wound induced systemic PI expression. Grafts between wild type and jasmonate-insensitive mutant ( jai1) plants showed that Jasmonate perception is required for recognition of the transmissible signal in distal responding leaves. Conversely, experiments performed with JA biosynthetic mutants showed that JA synthesis is required at the site of wounding for the production of the systemic signal. The fact that JA biosynthetic enzymes are located in the companion cell–sieve element complex of the phloem, which efficiently transports organic acids such as JA supports the above said systemic resistance. Mutants that are deficient in JA synthesis, including fad3, fad7, fad8, dad1, aos and opr3 (=dde1) are male-sterile, as is the JA-perception mutant, coi 1. The JA synthesis mutants in Arabidopsis can be restored to fertility by exogenous JA. This means that the mutants are an ideal tool for genetic and genomic approaches to identifying JA-responsive genes that initiate pollen and stamen maturation. Investigation of the Arabidopsis JA- synthesis mutants identified three characteristics of the male-sterile phenotype floral organs develop normally within the closed bud, but the anther filaments donot elongate sufficiently to position the locules above the stigma at the time of flower opening; the anther locules don’t dehisce at the time of flower opening (although limited dehiscence occurs later); (3) even though pollen on mutant plants develops to the trinucleate stage, the pollen grains are predominantly (>97%) inviable. Application of JA to flower buds corrected all three of these defects, resulting in rates of pollen germination equivalent to wild type in vitro tests, and abundant seed set on treated plants. The ability of JA to restore fertility to mutant plants is extremely stage specific–only flower buds corresponding to stage12 in floral development responded to JA.
Gibberllins:
Gibberllins or GAs are naturally occurring compounds in plants and fungi. These compounds were first discovered in a fungus called Gibberella fujikurai which produced a chemical compound that produced abnormal growth in rice plants. Since their discovery, gibberellin (GA) have been applied mainly to control plant growth and to improve yield quantity and quality. Manipulation of gibberellin (GA) signalling has contributed to the ‘Green Revolution’. Impairment of allelles governing the GA signalling and biosynthesis yielded dwarfism, which have led to strong yield increases following the introduction of dwarfing wheat and rice.
DELLA proteins are GA-labile growth repressors that mainly, but not exclusively, repress transcription factors. GA inactivates DELLAs by targeting them for degradation by the ubiquitin–proteasome system. GA and DELLAs repress PIF and BZR1 transcription factors to control light regulated development. GA and DELLAs antagonistically control different steps in the signal transduction of the phytohormone jasmonic acid. GA and DELLAs modulate microtubule formation by modulating prefoldin activity. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA.
Gibberlins
Ethylene is a gaseous plant hormone that is formed from the breakdown of the amino acid – methionine. It profoundly regulates growth and development but also plays an important role in a wide range of processes, including fruit ripening, abscission, senescence and responses to biotic and abiotic stresses. Plants synthesize ethylene at a faster rate in rapidly growing and dividing cells, especially in darkness and in response to diverse developmental factors and environmental stimuli. Responses to ethylene occur through a conserved signalling pathway comprising of a unique combination of signalling components including receptors localised primarily at the endoplasmic reticulum membrane and subsequent signalling mechanism leads to changes in gene expression. Ethylene also exhibits complex interactions with the signalling pathways of a number of other plant signals.
New growth and newly-germinated seedlings produce huge amounts of ethylene resulting in elevated levels, inhibiting leaf expansion. A newly emerged shoot when exposed to light results in decrease of ethylene levels and thereby allows leaf expansion.
Ethylene affects cell growth and cell shape for instance when a growing shoot is impeded while underground, 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 affects stem diameter and height: When stems of trees are subjected to wind, causing lateral stress, greater ethylene production occurs, resulting in thicker and sturdier tree trunks and branches. Ethylene 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.
Cytokinins:
Cytokinins are a class of plant growth regulators that play an a significant role in many physiological and developmental processes in the plant, such as regulation of cell division, shoot formation, leaf senescence, nutrient uptake and pathogen resistance.The first cytokinin discovered was Kinetin (6-furfurylaminopurine) a degradation product of DNA is a derivative of adenine. In spite of its high cytokinin activity the substance kinetin has never been found in plants. The first naturally occurring cytokinin in plants was isolated from immature maize (Zea mays) kernels in 1963 and was named therefore zeatin.The ratio between auxin and cytokinin in culture media regulate the proliferation and differentiation of plant tissues. For instance, under a high auxin/cytokinin ratio undifferentiated plant cells give rise to root primordia, whereas a low auxin/cytokinin ratio favors formation of shoots. When concentration of both the hormones in high plant cells proliferate heavily without organ differentiation. This antagonistic mechanism is widely used to generate plants out of cultured plant cells by applying changing hormone ratios.
Cytokinins play an important role in the controlled ageing process, called senescence, of annual plants and of single individual leaves on perennial plants. The senescence Cytokinins influence vascular morphogenesis and are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth..
Cytokinins have a highly-synergistic effect in concert with auxins 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; they in conjunction with ethylene promote abscission of leaves, flower parts and fruits
Abscisic Acid:
Abscisic Acid (ABA) was discovered and researched under two different names before its chemical properties were fully known. It was called dormin and abscicin II. The name "Abscisic Acid" was given because it was found in high concentrations in newly-abscissed or freshly-fallen leaves.
a) Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis).
b) Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots.
c) Induces seeds to synthesize storage proteins.
d) Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase.
e) Has some effect on induction and maintenance of dormancy.
f) Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.
Seed dormancy:
Plant hormones affect seed germination and dormancy by affecting different parts of the seed. Embryo dormancy is characterized by a high ABA/GA ratio, whereas the seed has a high ABA sensitivity and low GA sensitivity. To release the seed from this type of dormancy and initiate seed germination, an alteration in hormone biosynthesis and degradation towards a low ABA/GA ratio, along with a decrease in ABA sensitivity and an increase in GA sensitivity needs to 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 while 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.
Salicylic acid:
Salicylic acid (SA) plays a major role in plant defense response. SA is synthesized from chorismate via isochorismate. Endogenous levels of SA increase drastically upon encountering a wide range of pathogens not only at the site of infection but also in distant tissues playing a central role in systemic acquired resistance (SAR). SAR is a defense pathway that provides systemic protection to a broad range of pathogens. The exact molecular mechanism of SA perception and its downstream signalling is yet to be deciphered completely. But the key proteins involved in the response have been identified such as NONEXPRESSER OF PR genes (NPR1). When SA levels increase in response to infection by a pathogen, NPR1 is translocated into the nucleus where it promotes the transcription of a large family of PATHOGENESIS RELATED (PR) genes which have antimicrobial activity.
Other identified plant growth regulators include:
-The End-
Major classes of Phytohormones:
- Auxins
- Jasmonic acid
- Gibberllins
- Ehylene
- Cytokinins
- Abscisic acid
- Salicylic acid
Auxins were the first class of growth regulators to be discovered. Auxins promote cell enlargement, bud formation and root initiation. In combination with cytokinins, they enhance the production of other hormones that control the growth of stems, roots, flowers and fruits. They influence cell elongation by altering cell wall plasticity. The production of Auxins decreases when in the presence of light and increases in dark. Auxins stimulate cambium cells to multiply and cause differentiation of secondary xylem in stems. The action of Auxins establishes a mechanism called Apical Dominance, which inhibits the growth of buds at the bottom of the stems and promotes lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers. Cessation of Auxin production by the growing point of a plant leads to initiation of leaf abscission. Auxins regulate gene expression spatially facilitating seed specific protein synthesis responsible for seed setting in the flowers subsequent to pollination.
Auxins when present in high concentrations are toxic to plants in large which enable to be utilized as herbicides. Synthetic auxin herbicides such as 2,4-D and 2,4,5-T are employed for weed control. Auxins, especially 1- Naphthalene acetic acid (NAA) and Indole-3- Butryic acid (IBA) are commonly applied to stimulate root growth in plant cuttings. IBA is considered as the classical rooting hormone. The major naturally occurring auxin is IAA. IAA has been implicated in virtually every aspect of plant growth and development, as well as defense responses. IAA is generally synthesized from tryptophan via at least two pathways: the tryptamine (TAM) and indole-3-pyruvic acid (IPA) pathways but IAA can also be synthesized from indole, bypassing tryptophan.
Jasmonic Acid:
Jasmonic Acid (JA) and its volatile methyl ester (MeJA) are members of the hormonally active jasmonate family of compounds that are common throughout the plant kingdom. JA and MeJA have been discovered to play a key role in plant defence signalling to biotic stress, acting as the major signals that induce the expression of defensive proteinase inhibitors (PIs). Jasmonates belong to a family of fatty acid derivatives, collectively called oxylipins that are produced by oxidative metabolism of polyunsaturated fatty acids. JA is synthesized from linolenic acid (18:3) via the octadecanoid pathway. JA can be conjugated to isoleucine to form JA-Ile or converted to the volatile methyl-JA. The mechanism of Jasmonate synthesis and functioning in plants is pretty similar to that of arachidonic acid-derived signalling compounds (e.g. prostaglandins) in animals as species in both kingdoms employ lipoxygenase and cytochromes P450 to oxidize polyunsaturated fatty acid substrates.
JAs are widely considered to function as ‘master regulators’ of plant defence responses to biotic stress. The above fact is supported by evidences such as a rapid increase in endogenous jasmonate levels in response to wounding, pathogen infection, and other types of biotic stress. In addition, treatment of plants with exogenous JAs results in major re-programming of gene expression, including defence-related genes that are activated by herbivore and pathogen attack. Moreover, mutants that are defective in either the synthesis or perception of JAs are severely compromised in resistance to a wide range of plant invaders. Genetic alterations that cause constitutive activation of the jasmonate signalling pathway result in enhanced resistance to certain herbivores and pathogens. Several jasmonate-regulated compounds that confer resistance to herbivore attack have been identified. One of such compounds include Protein-based defences such as proteinase inhibitors (PIs), threonine deaminase and arginase that impair thr procurement of essential amino acids from the leaf diet in the lepidopteran midgut. JA-regulated secondary metabolites including alkaloids (e.g. nicotine) and glucosinolates exert toxic and antifeedant effects on leaf-eating invaders.
JA plays a key role in defence against some fungal pathogens, including Pythium and Alternaria brassicicola. It has been that these pathogens kill mutants that cannot synthesize JA, while neighboring wild-type plants remain healthy. Application of exogenous Jasmonate substantially protects mutant plants, reducing the incidence of disease to a level close to that of wild-type controls, but JA treatment does not protect the Jasmonate-insensitive mutant coi1 from infection. Pythium species are ubiquitous in soil and root habitats worldwide, but most are considered to be minor pathogens. Thus, jasmonate is essential for plant defence against Pythium and because of the high exposure of plant roots to Pythium inoculum in soil, may well be fundamental to survival of plants in nature.
Many plant defence responses against insect attack occur both locally at the site of wounding and systemically in undamaged tissues of the plant. This phenomenon implies the existence of signals that are transported from the site of wounding to other parts of the plant. Biochemical and pharmacological experiments established a central role for jasmonates in regulating wound induced systemic PI expression. Grafts between wild type and jasmonate-insensitive mutant ( jai1) plants showed that Jasmonate perception is required for recognition of the transmissible signal in distal responding leaves. Conversely, experiments performed with JA biosynthetic mutants showed that JA synthesis is required at the site of wounding for the production of the systemic signal. The fact that JA biosynthetic enzymes are located in the companion cell–sieve element complex of the phloem, which efficiently transports organic acids such as JA supports the above said systemic resistance. Mutants that are deficient in JA synthesis, including fad3, fad7, fad8, dad1, aos and opr3 (=dde1) are male-sterile, as is the JA-perception mutant, coi 1. The JA synthesis mutants in Arabidopsis can be restored to fertility by exogenous JA. This means that the mutants are an ideal tool for genetic and genomic approaches to identifying JA-responsive genes that initiate pollen and stamen maturation. Investigation of the Arabidopsis JA- synthesis mutants identified three characteristics of the male-sterile phenotype floral organs develop normally within the closed bud, but the anther filaments donot elongate sufficiently to position the locules above the stigma at the time of flower opening; the anther locules don’t dehisce at the time of flower opening (although limited dehiscence occurs later); (3) even though pollen on mutant plants develops to the trinucleate stage, the pollen grains are predominantly (>97%) inviable. Application of JA to flower buds corrected all three of these defects, resulting in rates of pollen germination equivalent to wild type in vitro tests, and abundant seed set on treated plants. The ability of JA to restore fertility to mutant plants is extremely stage specific–only flower buds corresponding to stage12 in floral development responded to JA.
Gibberllins:
Gibberllins or GAs are naturally occurring compounds in plants and fungi. These compounds were first discovered in a fungus called Gibberella fujikurai which produced a chemical compound that produced abnormal growth in rice plants. Since their discovery, gibberellin (GA) have been applied mainly to control plant growth and to improve yield quantity and quality. Manipulation of gibberellin (GA) signalling has contributed to the ‘Green Revolution’. Impairment of allelles governing the GA signalling and biosynthesis yielded dwarfism, which have led to strong yield increases following the introduction of dwarfing wheat and rice.
DELLA proteins are GA-labile growth repressors that mainly, but not exclusively, repress transcription factors. GA inactivates DELLAs by targeting them for degradation by the ubiquitin–proteasome system. GA and DELLAs repress PIF and BZR1 transcription factors to control light regulated development. GA and DELLAs antagonistically control different steps in the signal transduction of the phytohormone jasmonic acid. GA and DELLAs modulate microtubule formation by modulating prefoldin activity. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA.
Gibberlins
- Stimulate stem elongation by stimulating cell division and elongation.
- Stimulates bolting/flowering in response to long days.
- Breaks seed dormancy in some plants which require stratification or light to induce germination.
- Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves.
- Induces maleness in dioecious flowers (sex expression).
- Can cause parthenocarpic (seedless) fruit development.
- Can delay senescence in leaves and citrus fruits
Ethylene is a gaseous plant hormone that is formed from the breakdown of the amino acid – methionine. It profoundly regulates growth and development but also plays an important role in a wide range of processes, including fruit ripening, abscission, senescence and responses to biotic and abiotic stresses. Plants synthesize ethylene at a faster rate in rapidly growing and dividing cells, especially in darkness and in response to diverse developmental factors and environmental stimuli. Responses to ethylene occur through a conserved signalling pathway comprising of a unique combination of signalling components including receptors localised primarily at the endoplasmic reticulum membrane and subsequent signalling mechanism leads to changes in gene expression. Ethylene also exhibits complex interactions with the signalling pathways of a number of other plant signals.
New growth and newly-germinated seedlings produce huge amounts of ethylene resulting in elevated levels, inhibiting leaf expansion. A newly emerged shoot when exposed to light results in decrease of ethylene levels and thereby allows leaf expansion.
Ethylene affects cell growth and cell shape for instance when a growing shoot is impeded while underground, 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 affects stem diameter and height: When stems of trees are subjected to wind, causing lateral stress, greater ethylene production occurs, resulting in thicker and sturdier tree trunks and branches. Ethylene 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.
Cytokinins:
Cytokinins are a class of plant growth regulators that play an a significant role in many physiological and developmental processes in the plant, such as regulation of cell division, shoot formation, leaf senescence, nutrient uptake and pathogen resistance.The first cytokinin discovered was Kinetin (6-furfurylaminopurine) a degradation product of DNA is a derivative of adenine. In spite of its high cytokinin activity the substance kinetin has never been found in plants. The first naturally occurring cytokinin in plants was isolated from immature maize (Zea mays) kernels in 1963 and was named therefore zeatin.The ratio between auxin and cytokinin in culture media regulate the proliferation and differentiation of plant tissues. For instance, under a high auxin/cytokinin ratio undifferentiated plant cells give rise to root primordia, whereas a low auxin/cytokinin ratio favors formation of shoots. When concentration of both the hormones in high plant cells proliferate heavily without organ differentiation. This antagonistic mechanism is widely used to generate plants out of cultured plant cells by applying changing hormone ratios.
Cytokinins play an important role in the controlled ageing process, called senescence, of annual plants and of single individual leaves on perennial plants. The senescence Cytokinins influence vascular morphogenesis and are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth..
Cytokinins have a highly-synergistic effect in concert with auxins 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; they in conjunction with ethylene promote abscission of leaves, flower parts and fruits
Abscisic Acid:
Abscisic Acid (ABA) was discovered and researched under two different names before its chemical properties were fully known. It was called dormin and abscicin II. The name "Abscisic Acid" was given because it was found in high concentrations in newly-abscissed or freshly-fallen leaves.
a) Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis).
b) Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots.
c) Induces seeds to synthesize storage proteins.
d) Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase.
e) Has some effect on induction and maintenance of dormancy.
f) Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.
Seed dormancy:
Plant hormones affect seed germination and dormancy by affecting different parts of the seed. Embryo dormancy is characterized by a high ABA/GA ratio, whereas the seed has a high ABA sensitivity and low GA sensitivity. To release the seed from this type of dormancy and initiate seed germination, an alteration in hormone biosynthesis and degradation towards a low ABA/GA ratio, along with a decrease in ABA sensitivity and an increase in GA sensitivity needs to 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 while 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.
Salicylic acid:
Salicylic acid (SA) plays a major role in plant defense response. SA is synthesized from chorismate via isochorismate. Endogenous levels of SA increase drastically upon encountering a wide range of pathogens not only at the site of infection but also in distant tissues playing a central role in systemic acquired resistance (SAR). SAR is a defense pathway that provides systemic protection to a broad range of pathogens. The exact molecular mechanism of SA perception and its downstream signalling is yet to be deciphered completely. But the key proteins involved in the response have been identified such as NONEXPRESSER OF PR genes (NPR1). When SA levels increase in response to infection by a pathogen, NPR1 is translocated into the nucleus where it promotes the transcription of a large family of PATHOGENESIS RELATED (PR) genes which have antimicrobial activity.
Other identified plant growth regulators include:
- Brassinolides - plant steroids chemically similar to animal steroid hormones. First isolated from pollen of the mustard family and extensively studied in Arabidopsis. They promote cell elongation and cell division, differentiation of xylem tissues, and inhibit leaf abscission. Plants found deficient in rassinolides suffer from dwarfism.
- Systemin - a polypeptide consisting of 18 amino acids, functions as a long-distance signal to activate chemical defenses against herbivores. Recently discovered plant hormone in solanaceae family
- Polyamines - strongly basic molecules of low molecular weight that have been found in all organisms studied thus far - essential for plant growth and development and affect the process of mitosis and meiosis. The polyamines are organic compounds having two or more primary amino groups - such as putrescine(derived from arginine), cadaverine(from lysine), spermidine(from putrescine), and spermine (from spermidine) - that are growth factors in both eucaryotic and procaryotic cells.
- Nitric oxide (NO) - has been found to serve as as signal in hormonal and defense responses.
-The End-
Published date : 31 May 2014 05:43PM
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