Plant Hormones and Growth Regulators

PLANT HORMONES AND GROWTH REGULATORS

Plant hormones (phytohormones) are physiological inter-cellular messengers that control the complete plant lifecycle, including germination, rooting, growth, flowering, fruit ripening, foliage and death. In addition, plant hormones are secreted in response to environmental factors such as excess of nutrients, drought conditions, light, temperature and chemical or physical stress. So, levels of hormones will change over the lifespan of a plant and are dependent upon season and environment.

The term ‘plant growth factor’ is usually employed for plant hormones or substances of similar effect that are administered to plants. Growth factors are widely used in industrialized agriculture to improve productivity. The application of growth factors allows synchronization of plant development to occur. For instance, ripening fruits can be controlled by setting desired atmospheric ethylene levels. Using this method, fruits that are separated from their parent plant will still respond to growth factors; allowing commercial plants to be ripened in storage during and after transportation. This way the process of harvesting can be run much more efficiently and effectively. Other applica-tions include rooting of seedlings or the suppression of rooting with the simultaneous promotion of cell division as required by plant cell cultures. Just like with animal hormones, plant growth factors come in a wide variety, producing different and often antagonistic effects. In short, the right combination of hormones is vital to achieve the desired behavioural characteristics of cells and the produc-tive development of plants as a whole. The plant growth regulators are classified into synthetic and native. The synthetic regulators are also known as exogenous regulators and the native are called the endogenous,

Five major classes of plant hormones are mentioned: auxins, cytokinins, gibbereilins, abscisic acid and ethylene. However as research progresses, more active molecules are being found and new families of regulators are emerging; one example being polyamines (putrescine or spermidine). Plant growth regulators have made the way for plant tissue culture techniques, which were a real boon for mankind in obtaining therapeutically valuable secondary metabolites.

Auxins

The term auxin is derived from the Greek word auxein which means to grow. Generally compounds are considered as auxins if they are able to induce cell elongation in stems and otherwise resemble indoleacetic acid (the first auxin isolated) in physiological activity. Auxins usually affect other processes in addition to cell elongation of stem cells but this characteristic is considered critical of all auxins and thus ‘helps’ define the hormone.

Auxins were the first plant hormones discovered. Charles Darwin was among the first scientists to pool in plant hormone research. He described the effects of light on movement of canary grass coleoptiles in his book ‘The Power of Movement in Plants’ presented in 1880. The coleoptile is a specialized leaf originating from the first node which sheaths the epicotyl in the plants seedling stage protecting it until it emerges from the ground. When unidirectional light shines on the coleoptile, it bends in the direction of the light. If the tip of the coleoptile was covered with aluminium foil, bending would not occur towards the unidirectional light. However if the tip of the coleoptile was left uncovered but the portion just below the tip was covered, exposure to unidirectional light resulted in curvature toward the light. Darwin’s experiment suggested that the tip of the coleoptile was the tissue responsible for perceiving the light and producing some signal which was transported to the lower part of the coleoptile where the physiological response of bending occurred. When he cut off the tip of the coleoptile and exposed the rest of the coleoptile to unidirectional light curvature did not occur confirming the results of his experiment.

Salkowski (1885) discovered indole-3-acetic acid (IAA) in fermentation media. The isolation of the same product from plant tissues would not be found in plant tissues for almost 50 years. IAA is the major auxin involved in many of the physiological processes in plants. Fitting in 1907 put his efforts in studying signal transaction by making incisions on the light or dark side of the plant. He failed because the signal was capable of crossing or going around the incision, In 1913, modification was made in Fitting’s experiment by Boysen-Jensen, in that they inserted pieces of mica to block the transport of the signal and showed that transport of auxin toward the base occurs on the dark side of the plant as opposed to the side exposed to the unidirectional light. In 1918, Paal confirmed Boysen-Jensen’s results by cutting off coleoptile tips in the dark, exposing only the tips to the light, replacing the coleoptile tips on the plant but off centered to one side or the other. Results showed that whichever side was exposed to the coleoptile, curvature occurred toward the other side. Soding 1925, followed Paal’s idea and showed that if tips were cut off there was a reduction in growth but if they were cut off and then replaced growth continued to occur.

In 1926, Fritz Went reported a plant growth substance, isolated by placing agar blocks under coleoptile tips for a period of time then removing them and placing them on decapitated Avena stems. After placement over the agar, the stems resumed growth. In 1928, again Went developed a method of quantifying this plant growth substance. His results suggested that the curvatures of stems were proportional to the amount of growth substance in the agar. This test was called the avena curvature test. Much of our current knowledge of auxin was obtained from its applications. It was Went’s work, which had a great influence in stimulating plant growth substance research. He is often credited with dubbing the term auxin but it was actually Kogl and Haagen-Smit who purified the compound auxentriolic acid (auxin A) from human urine in 1931. Later Kogl isolated other compounds from urine which were similar in structure and function to auxin A. One of which was indole-3 acetic acid (IAA) initially discovered by Salkowski in 1885. In 1954 a committee of plant physiologists was set up to characterize the group auxins.

Indole acetic acid (IAA) is the principle natural auxin and other natural auxins are indole-3-acetonitrile (IAN), phenyl acetic acid and 4-chloroindole-3-acetic acid. The exogenous or synthetic auxins are indole-3-butyric acid (IBA), α-napthyl acetic acid (NAA), 2-napthyloxyacetic acid (NOA), 1-napthyl acetamide (NAD), 5-carboxymeth-yl-N, N-dimethyl dithiocarbamate, 2,4-dichlorophenoxy acetic acid (2,4-D), etc.

Production and occurrence

Produced in shoot and root meristematic tissue, in young leaves, mature root cells and small amounts in mature leaves. Transported throughout the plant parts and the production of IAA will be more in day time. It is released by all cells when they are experiencing conditions which would normally cause a shoot meristematic cell to produce auxin. Ethylene has direct or indirect action over to enhance the synthesis auxin.

IAA is chemically similar to the amino acid tryptophan which is generally accepted to be the molecule from which IAA is derived. Three mechanisms have been suggested to explain this conversion:

·  Tryptophan is converted to indolepyruvic acid through a transamination reaction. Indolepyruvic acid is then converted to indoleacetaldehyde by a decarboxylation reaction. The final step involves oxidation of indoleac-etaldehyde resulting in indoteacetic acid.

·  Tryptophan undergoes decarboxylation resulting in tryptamine. Tryptamine is then oxidized and deaminated to produce indoleacetaldehyde. This molecule is further oxidized to produce indoleacetic acid.

·  IAA can be produced via a tryptophan-independent mechanism. This mechanism is poorly understood, but has been proven using tip (-) mutants. Other experiments have shown that, in some plants, this mechanism is actually the preferred mechanism of IAA biosynthesis.

                          Pathways of IAA biosynthesis

The enzymes responsible for the biosynthesis of IAA are most active in young tissues such as shoot apical meristems and growing leaves and fruits. These are the same tissues where the highest concentrations of IAA are found. One way plants can control the amount of IAA present in tissues at a particular time is by controlling the biosynthesis of the hormone. Another control mechanism involves the production of conjugates which are, in simple terms, mol-ecules which resemble the hormone but are inactive. The formation of conjugates may be a mechanism of storing and transporting the active hormone. Conjugates can be formed from IAA via hydrolase enzymes. Conjugates can be rapidly activated by environmental stimuli signaling a quick hormonal response. Degradation of auxin is the final method of controlling auxin levels. This process also has two proposed mechanisms outlined below:

The oxidation of IAA by oxygen resulting in the loss of the carboxyl group and 3-methyleneoxindole as the major breakdown product. IAA oxidase is the enzyme which catal-yses this activity. Conjugates of IAA and synthetic auxins such as 2,4-D can not be destroyed by this activity.

C-2 of the heterocyclic ring may be oxidized resulting in oxindole-3-acetic acid. C-3 may be oxidized in addition to C-2 resulting in dioxindole-3-acetic acid. The mechanisms by which biosynthesis and degradation of auxin molecules occur are important to future agricultural applications. Information regarding auxin metabolism will most likely lead to genetic and chemical manipulation of endogenous hormone levels resulting in desirable growth and differ-entiation of important plant species.

Functions of auxin

·  Stimulates cell elongation.

·  The auxin supply from the apical bud suppresses growth of lateral buds. Apical dominance is the inhibiting influ-ence of the shoot apex on the growth of axillary buds. Removal of the apical bud results in growth of the axillary buds. Replacing the apical bud with a lanolin paste containing IAA restores the apical dominance. The mechanism involves another hormone - ethylene. Auxin (IAA) causes lateral buds to make ethylene, which inhibits growth of the lateral buds.

·  Differentiation of vascular tissue (xylem and phloem) is stimulated by IAA.

·  Auxin stimulates root initiation on stem cuttings and lateral root development in tissue culture (adventitious rooting).

·  Auxin mediates the tropistic response of bending in response to gravity and light (this is how auxin was first discovered).

·  Auxin has various effects on leaf and fruit abscission, fruit set, development, and ripening, and flowering, depending on the circumstances.

Cytokinins

Cytokinins are compounds with a structure resembling adenine which promote cell division and have other similar functions to kinetin. They also regulate the pattern and frequency of organ production as well as position and shape. They have an inhibitory effect on senescence. Kinetin was the first cytokinin identified and so named because of the compounds ability to promote cytokinesis (cell division). Though it is a natural compound, it is not made in plants, and is therefore usually considered a ‘synthetic’ cytokinin. The common naturally occurring cytokinin in plants today is called zeatin which was isolated from corn.

Cytokinin have been found in almost all higher plants as well as mosses, fungi, bacteria, and also in many prokary-otes and eukaryotes. There are more than 200 natural and synthetic cytokinins identified. Cytokinin concentrations are more in meristematic regions and areas of continuous growth potential such as roots, young leaves, developing fruits, and seeds.

Haberlandt (1913) and Jablonski and Skoog (1954) identified that a compound found in vascular tissues had the ability to stimulate cell division. In 1941, Johannes van Overbeek discovered that the milky endosperm from coconut and other various species of plants also had this ability. The first cytokinin was isolated from herring sperm in 1955 by Miller and his associates. This compound was named kinetin because of its ability to promote cytokine-sis (cell division). The first naturally occurring cytokinin was isolated from corn in 1961 by Miller and it was later called zeatin. Since that time, many more naturally occur-ring cytokinins have been isolated and the compound was common to all plant species in one form or another.

The naturally occurring cytokinins are zeatin, N⁶ dim-ethyl amino purine, isopentanyl aminopurine. The syn-thetic cytokinins are kineatin, adenine, 6-benzyl adenine benzimidazole and N, N’-diphenyl urea.

Production and occurrence

Produced in root and shoot meristematic tissue, in mature shoot cells and in mature roots in small amounts. If is rapidly transported in xylem stream. Peak production occurs in day time and their activity is reduced in plants suffering drought. It is directly or indirectly induced by high levels of Gibberlic acid.

Cytokinin is generally found in meristematic regions and growing tissues. They are believed to be synthesized in the roots and translocated via the xylem to shoots. Cytokinin biosynthesis happens through the biochemical modification of adenine. They are synthesized by following pathway.

A product of the mevalonate pathway called isopentyl pyrophosphate is isomerized. This isomer can then react with adenosine monophosphate with the aid of an enzyme called isopentenyl AMP synthase. The result is isopentenyl adenosine-5’-phosphate (isopentenyl AMP). This product can then be converted to isopentenyl adenosine by removal of the phosphate by a phosphatase and further converted to isopentenyl adenine by removal of the ribose group. Isopentenyl adenine can be converted to the three major forms of naturally occurring cytokinins.

Other pathways or slight alterations of this one probably lead to the other forms. Degradation of cytokinins occurs largely due to the enzyme cytokinin oxidase. This enzyme removes the side chain and releases adenine. Derivatives can also be made but the difficulties are with pathways, which are more complex and poorly understood.

Functions of cytokinin

·  Stimulate cell division (cytokinesis).

·  Stimulate morphogenesis (shoot initiation/bud forma-tion) in tissue culture.

·  Stimulate the growth of lateral (or adventitious) buds-release of apical dominance.

·  Stimulate leaf expansion resulting from cell enlarge-ment.

·  May enhance stomatal opening in some species (Figure 6.2).

·  Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

·  Stimulate the dark-germination of light-dependent seeds.

·  Delays senescence.

·  Promotes some stages of root development.

                       Effect of cytokinin on stomatal opening

Ethylene

Ethylene has been used in practice since the ancient times, where people would use gas figs in order to stimulate ripen-ing, burn incense in closed rooms to enhance the ripening of pears. It was in 1864, that leaks of gas from street lights showed stunting of growth, twisting of plants, and abnormal thickening of stems. In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene. Doubt 1917, discovered that ethylene stimulated abscission. In 1932 it was demonstrated that the ethylene evolved from stored apple inhibited the growth of potato shoots enclosed with them. In 1934 Gane reported that plants synthesize ethylene. In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as inhibition of vegetative tissues. Ethylene is now known to have many other functions as well.

Production and occurrence

Production is directly induced by high levels of Auxin, root flooding and drought. It is found in germinating seeds and produced in nodes of stems, tissues of ripening fruits, response to shoot environmental, pest, or disease stress and in senescent leaves and flowers. Light minimizes the production of ethylene. It is released by all cells when they are experiencing conditions which would normally cause a mature shoot cell to produce ethylene.

Ethylene is produced in all higher plants and is produced from methionine in essentially all tissues. Production of ethylene varies with the type of tissue, the plant species, and also the stage of development. The mechanism by which ethylene is produced from methionine is a three step process. ATP is an essential component in the synthesis of ethylene from methionine. ATP and water are added to methionine resulting in loss of the three phosphates and S-adenosyl methionine (SAM). 1-amino-cyclopropane-l-carboxylic acid synthase (ACC-synthase) facilitates the production of ACC from SAM. Oxygen is then needed in order to oxidize ACC and produce ethylene. This reaction is catalysed by an oxidative enzyme called ethylene forming enzyme. The control of ethylene production has received considerable study. Study of ethylene has focused around the synthesis promoting effects of auxin, wounding, and drought as well as aspects of fruit-ripening. ACC synthase is the rate limiting step for ethylene production and it is this enzyme that is manipulated in biotechnology to delay fruit ripening in the ‘flavor saver’ tomatoes.

Functions of ethylene

·  Production stimulated during ripening, flooding, stress, senescence, mechanical damage, infection.

·  Regulator of cell death programs in plants (apoptosis). Stimulates the release of dormancy.

·  Stimulates shoot and root growth and differentiation (triple response).

·  Regulates ripening of climacteric fruits.

·  May have a role in adventitious root formation. Stimulates leaf and fruit abscission.

·  Flowering in most plants is inhibited by ethylene. Mangos, pineapples and some ornamentals are stimu-lated by ethylene.

·  Induction of femaleness in dioecious flowers. Stimulates flower opening.

·  Stimulates flower and leaf senescence.

Gibberellins

Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. All gibberellins are derived from the ent-gibberellane skeleton. The gibberellins are named GA₁. GA_n in order of discovery. Gibberellic acid was the first gibberellin to be structurally characterized as GA₃. There are currently 136 GAs identified from plants, fungi and bacteria.

They are a group of diterpenoid acids that functions as plant growth regulators influencing a range of developmental processes in higher plants including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction and leaf and fruit senescence. The origin of research into gibberellins can be traced to Japanese plant pathologists who were investigating the causes of the ‘bakanae’ (foolish seedling) disease which seriously lowered the yield of rice crops in Japan, Taiwan and throughout the Asian countries. Symptoms of the disease are pale yellow, elongated seedlings with slender leaves and stunted roots. Severely diseased plants die whereas plants with slight symptoms survive but produce poorly developed grain, or none at all.

Bakanae is now easily prevented by treatment of seeds with fungicides prior to sowing. In 1898 Shotaro Hori demonstrated that the symptoms were induced by infection with a fungus belonging to the genus Fusarium, probably Fusarium heterosporium Necs.

In 1912, Sawada suggested that the elongation in rice-seedlings infected with bakanae fungus might be due to a stimulus derived from fungal hyphae.

Subsequently, Eiichi Kurosawa (1926) found that culture filtrates from dried rice seedlings caused marked elongation in rice and other sub-tropical grasses. He concluded that bakanae fungus secretes a chemical that stimulates shoot elongation, inhibits chlorophyll formation and suppresses root growth.

Although there has been controversy among plant pathologists over the nomenclature of bakanae fungus, in the 1930s, the imperfect stage of the fungus was named Fusarium moniliforme (Sheldon) and the perfect stage, was named as Gibberella fujikuroi (Saw.) Wr. by H.W. Wol-lenweber. The terms ‘Fujikuroi’ and ‘Saw’ in Gibberella fujikuroi (Saw.) Wr. were derived from the names of two distinguished Japanese plant pathologists, Yosaburo Fujikuro and Kenkichi Sawada.

In 1934, Yabuta isolated a crystalline compound from the fungal culture filtrate that inhibited growth of rice seedlings at all concentrations tested. The structure of the inhibitor was found to be 5-n-butylpicolinic acid or fusaric acid. The formation of fusaric acid in culture filtrates was suppressed by changing the composition of the culture medium. As a result, a noncrystalline solid was obtained from the culture filtrate that stimulated the growth of rice seedlings. This compound was named gibberellin by Yabuta.

In 1938, Yabuta and his associate Yusuke Sumiki finally succeeded in crystallizing a pale yellow solid to yield gib-berellin A and gibberellin B (The names were subsequently interchanged in 1941 and the original gibberellin A was found to be inactive.) Determination of the structure of the active gibberellin was hampered by a shortage of pure crystalline sample. In the United States, the first research on gibberellins began after the Second World War. In 1950, John E. Mitchell reported optimal fermentation procedures for the fungus, as well as the effects of fungal extracts on the growth of bean (Vicia faba) seedlings. In Northern USDA Regional Research Laboratories in Peoria, large scale fermentations were carried out with the purpose of producing pure gibberellin A for agricultural uses but initial fermentations were found to be inactive. Further researches were carried out by Sumiki in 1951, Stodola et al., 1955, Curtis and Cross, 1954 regarding gibberellins and finally the gibberllic acid was determined by its chemical and physical properties.

In 1955, members of Sumuki group, succeeded in sepa-rating the methyl ester of gibberellin A into three compo-nents, from which corresponding free acids were obtained and named gibberellins Al, A2, and A3. Gibberellin A3 was found to be identical to gibberellic acid. In 1957, Takahashi et al. isolated a new gibberellin named gibberellin A4 as a minor component from the culture filtrate.

In the mid 1950s, evidence that gibberellins were naturally occurring substances in higher plants began to appear in the literature. Margaret Radley in the UK demonstrated the presence of gibberellin-like substances in higher plants. In the United States, Bernard Phinney et al were the first to report gibberellin-like substance in maize. This was followed by the isolation of crystalline gibberellin Al, A5, A6 and A8 from runner bean (Phaseotus multiflorus). After 10 years the number of gibberellins reported in the literature isolated from fungal and plant origins rapidly increased. In 1968, J. MacMillan and N. Takahashi concluded that all gibberellins should be assigned numbers as gibberellin A1-x, irrespec-tive of their origin. Over the past 20 years using modern analytical techniques many more gibberellins have been identified. At the present time the number of gibberellins identified is 126.

Production and occurrence

Produced in the roots, embryo and germinating seeds. The level of gibberellins goes up in the dark when sugar cannot be manufactured and will be reduced in the light. It is released in mature cells (particularly root) when they do not have enough sugar and oxygen to support both themselves and released by all cells when they are experiencing conditions which would normally cause a mature root cell to produce GA.

Gibberellins are diterpenes synthesized from acetyl CoA via the mevalonic acid pathway. They all have either 19 or 20 carbon units grouped into either four or five ring systems. The fifth ring is a lactone ring as shown in the structures above attached to ring A. Gibberellins are believed to be synthesized in young tissues of the shoot and also the developing seed. It is not clear whether young root tissues also produce gibberellins. There is also some evidence that leaves may also contain them. The gibberellins are formed through the pathway, three acetyl CoA molecules are oxi-dized by two NADPH molecules to produce three CoA molecules as a side product and mevalonic acid. Mevalonic acid is then Phosphorylated by ATP and decarboxylated to form isopentyl pyrophosphate. Four of these molecules form geranylgeranyl pyrophosphate which serves as the donor for all GA carbon atoms.

This compound is then converted to copalylpyrophos-phate which has 2 ring systems. Copalylpyrophosphate is then converted to kaurene which has 4-ring systems. Sub-sequent oxidations reveal kaurenol (alcohol form), kaurenal (aldehyde form), and kaurenoic acid respectively.

Kaurenoic acid is converted to the aldehyde form of GA12 by decarboxylation. GA12 is the first true gibberellane ring system with 20 carbons. From the aldehyde form of GA12 arise both 20 and 19 carbon gibberellins but there are many mechanisms by which these other compounds arise. During active growth, the plant will metabolize most gibberellins by hydroxylation to inactive conjugates quickly with, the exception of GA3. GA3 is degraded much slower which helps to explain why the symptoms initially associ-ated with the hormone in the disease bakanae are present. Inactive conjugates might be stored or translocated via the phloem and xylem before their release (activation) at the proper time and in the proper tissue.

Functions of gibberellins

·  Stimulates stem elongation by stimulating cell division and elongation. GA controls internode elongation in the mature regions of plants. Dwarf plants do not make enough active forms of GA.

·  Flowering in biennial plants is controlled by GA. Bien-nials grow one year as a rosette and after the winter, they bolt (rapid expansion of internodes and formation of flowers).

·  Breaks seed dormancy in some plants that require strati-fication or light to induce germination.

·  Stimulates α-amylase production in germinating cereal grains for mobilization of seed reserves.

·  Juvenility refers to the different stages that plants may exist in. GA may help determine whether a particular plant part is juvenile or adult.

·  Stimulates germination of pollen and growth of pollen tubes.

·  Induces maleness in dioecious flowers (sex expres-sion).

·  Can cause parthenocarpic (seedless) fruit development or increase the size of seedless fruit (grapes).

·  Can delay senescence in leaves and citrus fruits. May be involved in phytochrome responses.

Abscisic Acid

Natural growth inhibiting substances are present in plants and affect the normal physiological process of them. One such compound is abscisic acid, a single compound unlike the auxins, gibberellins, and cytokinins. It was called ‘absci-sin II’ originally because it was thought to play a major role in abscission of fruits. At about the same time another group was calling it ‘dormin’ because they thought it had a major role in bud dormancy. Though abscisic acid gener-ally is thought to play mostly inhibitory roles, it has many promoting functions as well.

In 1963, when Frederick Addicott and his associates were the one to identify abscisic acid. Two compounds were isolated and named as abscisin I and abscisin II. Abscisin II is presently called abscisic acid (ABA). At the same time Philip Wareing, who was studying bud dormancy in woody plants and Van Steveninck, who was studying abscission of flowers and fruits discovered the same compound.

Production and occurrence

ABA is a naturally occurring sesquiterpenoid (15-carbon) compound in plants, which is partially produced via the mevalonic pathway in chloroplasts and other plastids. Because it is synthesized partially in the chloroplasts, it makes sense that biosynthesis primarily occurs in the leaves. The production of ABA is by stresses such as water loss and freezing temperatures. The biosynthesis occurs indi-rectly through the production of carotenoids. Breakdown of these carotenoids occurs by the following mechanism: Violaxanthin (forty carbons) is isomerized and then splitted via an isomerase reaction followed by an oxidation reaction. One molecule of xanthonin is produced from one molecule of violaxanthonin and it is not clear what happens to the remaining byproducts. The one molecule of xanthonin produced is unstable and spontaneously changed to ABA aldehyde. Further oxidation results in ABA. Activation of the molecule can occur by two methods. In the first, method, an ABA-glucose ester can form by attachment of glucose to ABA. In the second method, oxidation of ABA can occur to form phaseic acid and dihyhdrophaseic acid. Both xylem and phloem tissues carries ABA. It can also be translocated through parenchyma cells. Unlike auxins, ABA is capable of moving both up and down the stem.

Functions of abscisic acid

·  The abscisic acid stimulates the closure of stomata (water stress brings about an increase in ABA synthesis) (Figure 6.3).

·  Involved in abscission of buds, leaves, petals, flowers, and fruits in many, if not all, instances, as well as in dehiscence of fruits.

·  Production is accentuated by stresses such as water loss and freezing temperatures.

·  Involved in bud dormancy.

·  Prolongs seed dormancy and delays germination (vivipary).

·  Inhibits elongation.

·  ABA is implicated in the control of elongation, lateral root development, and geotropism, as well as in water uptake and ion transport by roots.

·  ABA coming from the plastids promotes the metabolism of ripening.

·  Promotes senescence.

·  Can reverse the effects of growth stimulating hormones.

    Closure of stomata and water stress brings about an increase in ABA synthesis

Polyamines

Polyamines are unique as they are effective in relatively high concentrations. Typical concentrations range from 5 to 500 mg/L. Polyamines influence flowering and promote plant regeneration. Few examples are Spermine, Spermidine and Putrescine. They play a major role in basic genetic processes such as DNA synthesis and gene expression. Spermine and spermidine bind to the phosphate backbone of nucleic acids. The interaction is mostly based on electrostatic interactions between negatively charged phosphates of the nucleic acids and the positively charged ammonium groups of the polyamines.

Polyamines are responsible for cell migration, proliferation and differentiation in plants. They represent a group of plant growth hormones, but they also have an effect on skin, hair growth, female fertility, fat depots, pancreatic integrity and regenerative growth in mammals. In addition, spermine is an important reagent widely used to precipitate DNA in molecular biology protocols. Spermidine is a standard reagent in PCR applications.

Spermine and spermidine are derivatives of putrescine (1,4-diaminobutane) which is produced from L-ornithine by action of ODC (ornithine decarboxylase). L-ornithine is the product of L-arginine degradation by arginase. Spermidine is a triamine structure that is produced by spermidine synthase (SpdS) which catalyses monoalkylation of putrescine (1,4-diaminobutane) with decarboxylated S-adenosylmethionine (dcAdoMet) 3-aminopropyl donor. The formal alkylation of both amino groups of putrescine with the 3-aminopropyl donor yields the symmetrical tetraamine spermine.

Brassinosteroids

There are approximately 60 naturally occurring polyhydroxy steroids known as brassinosteroids (BRs). They are named after the first one identified, brassinolide, which was isolated from rape in 1979. They appear to be widely distributed in the plant kingdom.

In the early 1980s USDA scientists showed that BR could increase yields of radishes, lettuce, beans, peppers and potatoes. However, subsequent results under field conditions were disappointing because inconsistent results were obtained. For this reason testing was phased out in the United States. More recently large-scale field trials in China and Japan over a six-year period have shown that 24-epibrassinolide, an alternative to brassinolide, increased the production of agronomic and horticultural crops (including wheat, corn, tobacco, watermelon, and cucumber). However, once again depending on cultural conditions, method of application, and other factors, the results sometimes were striking while other times they were marginal. Further improvements in the formulation, application method, timing, effects of environmental conditions, and other factors need to be investigated further in order to identify the reason for these variable results.

Brassinosteroids may be a new class of plant growth substances. They are widely distributed within the plant kingdom, they have an effect at extremely low concentrations, both in bioassays and whole plants, and they have a range of effects that are different from the other classes of plant substances. Finally, they can be applied to one part of the plant and transported to another location where, in very low amounts, they elicit a biological response.

Functions of brassinosteroids

·  Promote shoot elongation at low concentrations.

·  Strongly inhibit root growth and development. Promote ethylene biosynthesis and epinasty.

·  Interfere with ecdysteroids (moulting hormones) in insects.

·  Have had contradictory effects in tissue culture. 24-epi-brassinolide has been shown to mimic culture condition-ing factors and to be synergistic with these factors in promoting carrot cell growth. However, in transformed tobacco cells brassinosteroids in low concentrations significantly inhibited cell growth.

·  Enhance xylem differentiation.

·  Decrease fruit abortion and drop.

·  Enhance resistance to chilling, disease, herbicide, and salt stress.

·  Promotion of germination.

·  Promote changes in plasmalemma energization and transport, assimilate uptake.

·  Increase RNA and DNA polymerase activities and synthesis of RNA, DNA, and protein.

Salicylic Acid

Salicylic acid has been known to be present in some plant tissues for quite some time, but has only recently been recognized as a potential PGR. Salicylic acid is synthesized from the amino acid phenylalanine. SA is thought by some to be a new class of plant growth regulator. It is a chemically characterized compound, ubiquitously found in the plant kingdom and has an effect on many physiological processes in plants at low concentrations. Further molecular studies on SA signal transduction should yield insights into the mechanism of action of this important regulatory compound.

Functions of salicylic acid

·  Promotes flowering.

·  Stimulates thermogenesis in Arum flowers.

·  Stimulates plant pathogenesis protein production (systemic acquired resistance).

·  May enhance longevity of flowers.

·  May inhibit ethylene biosynthesis.

·  May inhibit seed germination.

Blocks the wound response. Reverses the effects of ABA.

Jasmonates

Jasmonates are represented by jasmonic acid and its methyl ester. They were first isolated from the jasmine plant in which the methyl ester is an important product in the perfume industry. Jasmonic acid is synthesized from lino-lenic acid, which is an important fatty acid. Jasmonic acid is considered by some to be a new class of plant growth regulator. It is a chemically characterized compound and has been identified in many plant species. It has physiological effects at very low concentrations and indirect evidence suggests that it is transported throughout the plant.

Functions of jasmonates

·        Inhibition of many processes such as seedling longitu-dinal growth, root length growth, mycorrhizial fungi growth, tissue culture growth, embryogenesis, seed germination, pollen germination, flower bud formation, carotenoid biosynthesis, chlorophyll formation, rubisco biosynthesis, and photosynthetic activities

·        Promotion of senescence, abscission, tuber formation, fruit ripening, pigment formation, tendril coiling, differentiation in plant tissue culture, adventitious root formation, breaking of seed dormancy, pollen germination, stomatal closure, microtubule disruption, chlorophyll degradation, respiration, ethylene biosynthesis, and protein synthesis

·        They play an important role in plant defense by inducing proteinase synthesis.