Plant Growth Hormones PDF

Growth hormones  Plant growth and development are controlled by extremely low concentrations of chemical substances called plant growth substances, phytohormones or plant growth regulators  Auxins Gibberellins Cytokinins Abscissic acid Ethylene Auxins Charles Darwin and Fr...

Growth hormones  Plant growth and development are controlled by extremely low concentrations of chemical substances called plant growth substances, phytohormones or plant growth regulators  Auxins Gibberellins Cytokinins Abscissic acid Ethylene Auxins Charles Darwin and Francis Darwin – 1887 – canary grass (Phalaris canariensis) – no response in excised tips – The power of movements in plants  First successful isolation – 1926 – Dutch botanist – F W Went. Bioassay for Auxins – Detection of a plant growth substance using a biological test material is termed as bio-assay For Auxins , the Avena coleoptile test or the Avena curvature test is the bioassay test The Split Pea Stem Curvature Test This test was 1st reported by Went (1934) and like the avena curvature test, depends upon a differential growth response. A stem section of a pea seedling of a pure strain (example-Alaska) is slit longitudinally and floated on the test solution. At first a negative curvature (curve outwards) results because of the uptake of water by the inner cortical cells. The epidermal cells respond to auxin with considerable growth in length, but the cortical cells respond to auxin with considerable increase in width. Following incubation in higher concentration of auxins, a +ve curvature results. (i) Pea seeds are germinated and grown in the dark for eight days. (ii) The seedlings are exposed for 3 hours per day to red light to increase sensitivity to auxin. (iii) The stems are harvested, decapitated and a half inch long portion between the second and third internode is selected. (iv) The sections are soaked in distilled water for an hour to remove any endogenous auxin that might be present in the stem section. (v) The cut stem is next slit longitudinally 3 mm in length and placed in a petri-dish containing 25 ml of auxin solution. (vi) The curvature of the slit stem tip is read after an incubation period of 6 hours  Auxein means “Grow” First crystalline auxin was obtained from human urine by Kogl and others – 1934, chemically called Indole 3-acetic acid (IAA) Occurrence – highest concentration of auxins occurs in the meristematic regions and actively growing regions  Polar transport of auxins This is a unique feature of auxins – the hormone diffuses along the longitudinal axis of the plant, irrespective of the plants orientation Synthetic auxins Phenoxy acetic acid Naphthalene acetic acid Picolinic acid 2,4 – dichlorophenoxy acetic acid (2,4 –D) 2,4,5-T trichlorophenoxy acetic acid (2,4,5–T) IAA  Mode of action 1. Adsorption of auxins to a hormone – specific binding site ( auxin binding protein – ABP 1 ) 2. Acidification of the membrane 3. Changes in nucleic acid and proteins 4. Changes in plasticity of the wall requiring changes in protein matrix , cellulose matrix and hemicellulose matrix Physiological effects of Auxin Auxins affect a variety of response in plants. 1. Stem elongation. Application of exogenous auxins causes stimulation of coleoptile and stem growth. The effect varies according to the tissues. In general, the most marked stimulation are obtained in stems and coleoptiles of etiolated seedlings. The stimulation of elongation is also seen in cut stem segments. The stem segments of etiolated pea and soybean seedlings are often used as bio-assay materials. However, at high concentrations (say above 10-4 M), auxins, inhibit growth instead of causing stimulation. This inhibition is believed to be associated with ethylene accumulation at high auxin concentrations. 2. Coleoptile curvature effect. As described in the bio-assay section auxins cause bending of the coleoptile. Bending occurs because of the polar transport of auxins. This polarity is much more evident in coleoptile than in many stem tissues. The presence of auxins on only one side of the coleoptile increases cell division on that side and therefore the coleoptile bends on other side.  Apical dominance It has generally been observed that in most seedlings, the apical tip of the stem inhibits development of lateral buds within certain areas of the apex. If the apical tip is removed, the lateral buds develop. The inhibition of lateral bud formation by the presence of apex is called apical dominance. This inhibition is attributed to the presence of auxins in the apex. If auxin is applied to the cut stump of a seedling, then also growth of lateral buds is inhibited. Rooting The auxin, indole acetic acid (IAA) has been identified as a rooting hormone. Exogenous application of IAA induces rooting in several stem cuttings. It hastens rooting as well as increases the number of roots formed. In many trees, formation of adventitious roots can be induced by the treatment with auxins. As such, auxins have found commercial application as rooting hormone. Elongation of roots also increases with the application of exogenous auxins, although in most cases, it inhibits root growth in intact plants.  Cambial activity. Stimulation of cambial activity in trees by exogenous application of auxins was first reported in 1936 by Soding. Now it has been recorded in several other studies. Flowering The exogenous application of auxins increases formation of female flowers and ovary wall growth leading to parthenocarpy in cucurbits, grapes, etc. This effect however, seems to be indirect. Burg and Burg (1966) have reported that auxins induce ethylene production which may be the cause of parthenocarpy. Abscission. C.D. Larue (1936) demonstrated for the first time that many synthetic auxins inhibited the abscission of coleus leaves. Since then, control of abscission by IAA has been demonstrated in various other plants as well. In some cases, IAA induces abscission, specially in the aged tissues. This is believed to be due to auxin induced synthesis of ethylene, as ethylene is known to promote abscission. Plant growth movements. Because of their polar transport and effect on cellular growth, auxins are known to be involved in phototropic and geotropic plant movements. When a growing plant is illuminated by a unilateral light, it responds by bending towards light. According to Cholodony-Went theory (1927, 1928), there is a higher concentration of auxin on the shaded side than on the light side of a unilaterally illuminated shoot. This unequal distribution of auxins could be the result of light induced inactivation of auxins on light side, light induced lateral transport of auxin or inhibition of basipetal transport of auxin. The result of unequal distribution of auxins, is unequal growth of shoot, more on dark side than on light side, resulting in a curvature.  Shortening of internodes – apple and pear – α-naphthelene acetic acid Stem elongation Preventing lodging – α naphthyl-acetamide treatment Root initiation - NAA, IBA Apical dominance Prevention of abscission layer – dilute soln. of 2,4-D, IAA, NAA  Flower initiation – Auxins generally inhibit flowering , but in pineapple NAA is found to promote flowering. Lettuce – auxins are used to delay flowering Parthenocarpy Eradication of weed – Roots are extremely sensitive to auxins. Increased amount of auxins over stimulate the activity and distort roots 2,4-D & 2,4,5-T – mixture – Agent orange  Cell division – callus formation is initiated by auxins. Can be used practically during grafting to strengthen the union between stock and scion Commonly also used in tissue culture Cambial activity – stimulation of cambial activity is observed by exogenous allocation of auxins (Soding, 1936). Xylem differentiation is more pronounced Plant movements Physiological effects/exogenous applications 1. Stem elongation –stimulates stem and coleoptile growth 2. Coleoptile curvature effect – polarity in coleoptiles 3. Apical dominance 4. Rooting – increases number of roots formed (IAA), induces adventitious root formation 5. Cambial activity – first reported in 1936 by Soding. 6. Flowering – increases formation of female flowers and ovary wall growth leading to parthenocarpy 7. Abscission – 1st studied in 1936 by C D Larue in coleus leaves. Sometimes induces abscission as well b’coz of ethylene synthesis. 8. Induces plant growth movements Gibberellins Bakanae disease of Rice ( foolish seeding seedling) Gibberella fujikuroi also known as Fusarium moniliforme. Finally isolated in 1939 by T. Yobuta and T. Hayashi from the filtrates of the fungus - called gibberellin A, later known as gibberillic acid. Bio-assay Increased growth in dwarf seedlings Dwarf strains of maize and pea – due to negligible amounts of endogenous gibberellins. When applied to dwarf maize seedlings in concentrations as small as 1 nanogram, marked increase in the length is noticed Barley endosperm test  Barley endosperm test – barley aleurone layers are incubated with GA for 24 hrs. GA stimulates amylase activity which causes hydrolysis of starch to glucose. Increased glucose concentration shows the presence of GA in the test solution  Biosynthesis of gibberellins occurs in the root apex , elongating shoots, young leaves and mainly seeds and fruits. Gibberellins do not get transported polar, like Auxin. They move in several different ways like amino acids etc. Simple diffusion, through xylem, phloem, They mainly promote shoot growth by accelerating rates of cell elongation in the sub apical meristem where young internodes are developing  Physiological effects/exogenous applications 1. Induction of bolting ( premature flowering) – stem elongation 2. Promotion of plant growth – stem elongation, even genetic dwarfness. Most plants grow tall in dark than in light. 3. Promotion of seed germination and bud growth – enhances cell elongation in radicle. Overcomes dormancy in buds.  Induction of flowering – induces flowering in plants which are photoperiod sensitive and cold requiring species. Generally shows less effect on short day plants, egs. where flowering is induced- Bryophyllum sp., Xanthium sp. , Raphanus sativus etc. Prevention of senescence – R A Fletcher and D J Osborne (1965) – first to demonstrate the exogenous application of GA can prevent senescence. Prevents rind disorders during storage  Mobilization of food and minerals in seeds- stimulates hydrolysis of stored macromolecules and their transport to embryonic axis. Also induces parthenocarpy – more effective than auxins Breaking dormancy – seen effectively in potato tubers Commercial application Often sprayed in vineyards to increase grapes size. GA3 is used to produce seedless grape varieties – Thompson’s seedless grapes Improvement in size , colour and quality of apples, pears etc. has been done by spraying gibberellins, Used to increase (alpha) α - amylase activity in barley seeds in malt production in beer industry. Causes elongation of internodes in sugar cane without causing loss of sugar content Cytokinins  Discovery Haberlandt in 1921 found that vascular tissues in various plants contained an unknown diffusible factor which stimulated cell division Coconut Milk factor (CMF) - J. Van Overbeek in 1941 discovered that coconut milk (endosperm) also has the ability to stimulate cell division in very young Datura embryos, which did not contain this factor. Folke Skoog, F C Steward et al., identified a chemical factor which induced cell division in tissue culture – supplemented with adenine Jablonski and Skoog in 1954 discovered compounds in vascular tissues which promoted cell division  Partial breakdown product of yeast or herring sperm DNA also stimulated cell division and callus formation – Miller and Skoog (1945) The active compound was named ‘kinetin’ which was the first cytokinin to be named. Kinetin – 6 furfurylamine purine Most cytokinins are purines or purine derivatives ‘Zeatin’ (discovered in corn) is the most Bioassay Plant callus in tissue culture – tobacco pith cultures Lettuce seed germination test Expansion of cotyledons – Radish, cucumber, lettuce – specific for cytokinins Prevention of Xanthium and barley leaf senescence – measurement of chlorophyll content after 48 hrs Cytokinins - Physiological effects 1. Promotion of cell division and organ formation – promoting cell division is the major function of cytokinin. Cytokinins also initiate bud formation on intact tissues. Auxins inhibit this action. Hence a balance between auxins and cytokinins is needed for differentiation and growth. Relieves apical dominance Cks promote bud primordia in root segments of Convolvulus arvensis 2. Promotion of seed germination- application of ck can promote germination and even break dormancy in some seeds. Seeds germinate better in dark than light. 3. Expansion of cotyledons and leaves – induced expansion of cotyledons by causing cell size increase. 4. Delays senescence: induces translocation of nutrients from adjoining leaves and delays senescence – Richmond Lang Effect From vegetative parts to floral parts  Promotes chloroplast development Tobacco callus tissue developed in the dark contains etioplasts with no grana and lamella. Appln of ck promotes lamellar development. Effect on over all plant growth Counteraction to apical dominance Breaking dormancy Mechanism of action Increased nucleic acid and protein synthesis Increased enzyme activity. Biosynthesis Synthesized in the roots and transported to the shoots via xylem Synthetic cytokinins – BAP – Benzyl Amino Purine 6 – Furfuryl aminopurine  Diffusion Osmosis – exosmosis, endosmosis Imbibition Plasmolysis , deplasmolysis Water potential Osmotic potential Pressure potential Matrix potential Free energy Semi permeable membrane Abscissic acid – powerful inhibitor of seed and bud germination. First isolated from cotton fruits by W C Lui and H R Cairn 1957. Abscisin – stimulated abscission in cotton petioles Inhibitor of apical growth in birch leaves – dormin More important in causing dormancy and stomatal closure than in abscission Synthesized in cells containing chloroplasts or amyloplasts Translocated through phloem and xylem Bio-assay Inhibition of seed germination Elongation of seedlings – 1mM – 50% inhibition Inhibition of alpha-amylase synthesis – excised barley aleurone layers Biosynthesis During desiccation of vegetative tissues Green fruits in the beginning of winter For closure if stomata For overcoming stress For causing dormancy ABA is synthesized partially by the mevalonic pathway in chloroplasts. Carotenoids present within chloroplasts breakdown to produce ABA Physiological Effect 1.Inhibition of seed germination Exogenous supply of AbA inhibits germination of most non-dormant seeds. For causing an inhibition, AbA must be continuously present. As soon as it is removed by washing of the seeds, the germination can take place. Endogenous AbA also inhibits germination. The AbA content of dormant seeds is high and it decreases during germination.  Several possibilities have been suggested for the inhibition of seed germination (a) The inhibition is possibly because of inhibition of enzymes involved in germination process. The hormone inhibits protease and amylase and GA induced amylase in barley aleurone layers. It also inhibits the synthesis of carboxypeptidase C, which is considered as a specific enzyme for germination. (b) The hormone also appears to be inhibiting water uptake by germinating mustard seeds. (c) M. Coccuci and N. Negrini (1989) from their studies with radish seed germination have concluded that AbA inhibits seed germination by inhibiting some activity depending on calcium-calmodulin system. Inhibition of seedling growth AbA inhibits growth of the seedlings in some cases. In Glycine max for example, 1 mM AbA inhibits seedling growth by about 50% in 48 h In some cases, similar growth inhibition is observed in almost 2h. Auxin stimulated elongation growth in coleoptile is also inhibited by AbA. It has been suggested that AbA inhibits seedling growth by decreasing the water potential. Inhibition of bud growth Exogenous AbA induces bud dormancy in woody plants. As in dormant seeds, AbA content in dormant buds is high and it decreases by treatments which lead to the breaking of dormancy. The hormone inhibits lateral growth the bud, as has been reported in tomato.  Plants in which bud growth inhibited by far-red treatment, also have high AbA content. Further, cultivars with high apical dominance contain high AbA levels. It appears that this it level of AbA is maintained by the IAA content of the tissue, as lAA Is been invariably implicated in apical dominance. Stomatal closing The most significant and best known effect of AbA is its control of Stomata Exogenous application of AbA to epidermal strips causes stomatal closure. In fact, factors such as water stress, chilling cic. which induce somatal closure, also increase AbA content of guard cells. The response of AbA on stomatal closing is very fast and occurs within a few minutes of AbA application. AbA inhibits K* uptake into guard cells and has been shown to inhibit both K* uptake and proton release in a variety of tissues. Geotropism Appreciable amounts of AbA have been detected in maize root tips. The accumulation of AbA in the tip appears to require light and gravity. AbA produced in the cap seems to be translocated basipetally and to stimulate a positive geotropic response. Further, the exogenously applied AbA induces positive geotropism although it inhibits root growth. Senescence and abscission Numerous studies indicate that AbA is an endogenous factor involved in senescence and abscision of leaves and other plant organs. Exogenous application of AbA induces primary yellowing in leaf tissues in a variety of species ranging from deciduous trees to herbaceous plants. In many studies, it has been found that a non- volatile substance is released during senescence of leaves, which accelerates abscission. D.J. Osborne has called this substance as senescence factor (S.F.). Many scientists have suggested that this senescence factor is AbA.  Regulates dormancy in buds and seeds by inhibiting growth process. (GA) Senescence and abscission in leaves Seed germination inhibition ( can be reversed by cytokinin) Anti – gibberellin Regulates gene expression Wilting leaves - Stomatal closure ( inhibits k+ ion exchange) Positive geotropic response Drought and Moisture stress Spray to cotton bolls before harvest Mode of action Competes with auxins, GA, CK for specific enzyme site – antagonistic Inhibits biosynthesis of other growth hormones Inhibit RNA and protein synthesis Stimulate production of hydrolytic enzymes Ethylene is a gaseous hormone Active in very low concentrations Dimitry 1901 – triple response – Stem elongation, Stimulation of radial swelling in stems, horizontal growth of stems H. H. Cousins – 1910 – oranges produced a gas that caused ripening in bananas Gane – 1934 – confirmed the volatile substance to be ethylene Bioassay – The triple response of etiolated pea plant – Niljubow ( 1901) Plant material enclosed in a small chamber containing an injection port. After incubation of weighed amounts of tissue for required time , samples of gas are withdrawn with a hypodermic syringe and tested on other plant. 1. Reduction in extension growth – 0.01ppm 1.0ppm – increase in horizontal growth and stem swelling 2. Acceleration of senescence – leaves enclosed in chamber , loss of chlorophyll is recorded 3. Induction of stem and leaf epinasty Physiological responses Seed germination – ethylene breaks dormancy and induces germination – lettuce, ground nut etc. Maximum growth is observed at 0.3 ppm Maximum germination is obtained at 40-50 ppm and max extension growth at 0.3 ppm. Inhibition of germination by ethylene has been recorded in maize and some weeds Growth inhibition and morphogenetic effects – inhibits elongation of stem roots and leaves but enhances radial growth. Reduction in growth may be due to inhibition of cell division.  Epinastic responses – causes bending of leaves Flowering inhibition and sex expression – inhibits flowering in most plants but promotes in mango and pine apple. Inhibition is related to photoperiod. Fruit ripening – Climacteric fruits – respiratory climacteric Senescence – degrades chlorophyll Abscission  Synthesized in large amounts in tissues undergoing senescence and in ripening fruits Known to break seed germination(40-50 ppm) Inhibits linear growth in dicots Adventitious root formation Formation of female flowers – Cucurbitaceae Commercial applications Used for synchronized flowering from centuries Ethrel or Etephon – 2 - chloroethyl phosphoric acid