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Transcript
Control & Regulation Hormonal influences on growth The Pituitary Gland • The pituitary is an important source of hormones and is often referred to as the “master gland” because it can regulate other hormone producing organs. • This is a small gland and is connected to the underside of the brain by a stalk. Monday, May 22, 2017 G R Davidson 2 The Pituitary Gland • It is composed of the anterior (front) and the posterior lobe (back). • The anterior lobe is where the hormones are made. Monday, May 22, 2017 G R Davidson 3 The pituitary gland Hypothalamus Pituitary Monday, May 22, 2017 G R Davidson 4 The pituitary gland Monday, May 22, 2017 G R Davidson 5 Human Growth Hormone • One of the hormones produced by the pituitary is human growth hormone (GH, occasionally HGH in some texts). – It promotes growth by speeding up the transport of amino acids into soft tissues and bone cells. – This speeds up protein synthesis and therefore growth. Monday, May 22, 2017 G R Davidson 6 Human Growth Hormone • This speeds up protein synthesis • If somatotrophin is not produced in the correct quantities, a number of conditions may arise: – Not enough hormone during adolescence pituitary dwarf – Too much hormone during adolescence pituitary giant – Too much hormone during adulthood acromegaly Monday, May 22, 2017 G R Davidson 7 Thyroid Stimulating Hormone (TSH) • Thyroid Stimulating Hormone ( TSH) is also produced by the pituitary gland. • It travels in the blood stream to another hormone producing endocrine gland called the thyroid gland. Monday, May 22, 2017 G R Davidson 8 Thyroid Stimulating Hormone (TSH) • TSH controls the secretions of the thyroid gland. • The thyroid gland secretes an organic compound containing iodine called thyroxine. – The iodine in this compound comes from the diet. Monday, May 22, 2017 G R Davidson 9 Thyroid Stimulating Hormone (TSH) • Thyroxine controls the metabolic rate is therefore particularly important in growth. – Metabolism is the sum of the chemical reactions in our body. – The rate of metabolism must be kept within certain limits or ill health results. – The amount of thyroxine produced by the thyroid gland must be carefully controlled within an individual. Monday, May 22, 2017 G R Davidson 10 Thyroid Stimulating Hormone (TSH) • This is achieved by negative feedback. • The presence of an excess or lack of thyroxine sets in motion processes which correct its production. Monday, May 22, 2017 G R Davidson 11 Thyroid Stimulating Hormone (TSH) • If the thyroxine level becomes too low, the thyroid gland is stimulated to produce more. • The result is that the level of the chemical is kept reasonably constant although small fluctuations in the amount are inevitable – like trying to balance a see-saw. Monday, May 22, 2017 G R Davidson 12 Thyroid Stimulating Hormone Produces T.S.H. Pituitary Gland Thyroid Gland Produces Thyroxine Increased Metabolism Monday, May 22, 2017 G R Davidson 13 Negative Feedback Pituitary Gland Less Hormone Produced T.S.H. More Hormone Produced Thyroid Gland Thyroxine Overproduction Monday, May 22, 2017 Underproduction G R Davidson 14 Production of IAA • Plants do not produce hormones, they produce plant growth substances which affect their growth and development. • One type of plant growth substance is the auxins. • Indole-3-Acetic Acid ( IAA) is an auxin which stimulates plant growth by stimulating cell elongation. Monday, May 22, 2017 G R Davidson 15 Production of IAA • IAA is produced in root and shoot tips as well as leaf meristems and is transported to other parts of the plant. • Short distance transport of IAA is achieved by diffusion from cell to cell. • Long distance transport is by the phloem seive tubes. Monday, May 22, 2017 G R Davidson 16 The effect of IAA on cells and organs • We know that IAA causes elongation which results in shoot growth. – Very small quantities of IAA are required. – 0.01mg of IAA in a litre of water is enough to cause significant growth. Monday, May 22, 2017 G R Davidson 17 The effect of IAA on cells and organs • The usual way of applying IAA to a plant is to dissolve it in a lanolin paste. – The auxin dissolves better in lanolin than water – a paste is easier to apply to specific areas of a plant. Monday, May 22, 2017 G R Davidson 18 The effect of IAA on cells and organs • If a plant is left on a windowsill for any length of time, the stems and leaves curve towards the light. – Movement of auxin towards the shaded side makes it grow faster than the side nearest. – This unequal rate of growth causes the plant to bend towards the light. – This directional growth movement of the plant shoot to light is called phototropism. Monday, May 22, 2017 G R Davidson 19 IAA and Phototropism • Light causes an unequal distribution of IAA which result in growth causes the plant to bend towards the light. • This directional growth movement of the plant shoot to light is called phototropism. Monday, May 22, 2017 G R Davidson 20 IAA and Phototropism Region of auxin production Normal auxin diffusion Region of cell elongation uneven auxin diffusion Reduced cell elongation increased cell elongation Oat coleoptile Monday, May 22, 2017 G R Davidson 21 The effect of IAA on cells and organs • It is useful to plants as it ensures that they grow towards the light that they need for photosynthesis. – Growth of a shoot towards light is referred to as a positive tropism. – Since the shoot tip is responsible for detecting light, if it is removed, the plant ceases to grow and therefore to respond to light. Monday, May 22, 2017 G R Davidson 22 The effect of IAA on cells and organs • This ensures that the roots grow downwards. • These patterns of unequal growth are explained in terms of IAA concentrations. Monday, May 22, 2017 G R Davidson 23 The effect of IAA on cells and organs • Plants also show a directional growth movement to gravity. • This is called geotropism. • However, in roots, the IAA seems to have the opposite inhibitory effect to its promotion effect in shoots . Monday, May 22, 2017 G R Davidson 24 The effect of IAA on cells and organs • It is thought that an inhibitor slows down cell elongation, causing the root to curve downwards. • The inhibitor is thought not to be an auxin. • Since the root is growing downwards in the direction of gravity, it is said to exhibit positive geotropism. Monday, May 22, 2017 G R Davidson 25 Role of IAA in apical dominance • IAA can exert effects on other aspects of plant growth and development other than elongation. – If a house plant or hedge is allowed to grow unchecked, it becomes very spindly. – If, however, the top is trimmed, it grows much bushier. Monday, May 22, 2017 G R Davidson 26 Role of IAA in apical dominance • This is because the plants exhibit apical dominance, i.e. growth of the terminal bud inhibits the growth of the side buds. – When the apical bud is removed, side shoots can then develop. – This is the theory behind pruning. – When an apical bud is removed auxin in lanolin paste will suppress growth of side shoots Monday, May 22, 2017 G R Davidson 27 Role of IAA in leaf abscission • Abscission is the organised shedding of part of a plant, e.g. a leaf, an unfertilised flower or fruit. – At the base of the organ, in a region called the abscission zone, a layer of cells begins to break down to form the abscission layer. – When this layer breaks down completely, the organ drops off the plant. Monday, May 22, 2017 G R Davidson 28 Role of IAA in leaf abscission • Deciduous trees and shrubs often lose leaves in the winter which helps protect the plant against water shortages. • Before leaf fall, the IAA concentration drops, causing the abscission layer to be formed. Monday, May 22, 2017 G R Davidson 29 Role of IAA in leaf abscission • The walls of the cells in this layer become weakened ands the leaf eventually drops off. • During Spring and Summer, this is prevented by high concentrations of auxin which prevent the abscission layer forming. Monday, May 22, 2017 G R Davidson 30 Role of IAA in fruit formation • High levels of auxin are necessary for “fruit set” i.e. the retention of the ovary which becomes the fruit after fertilisation. • The ovary and the ripe fertilised seeds continue to produce auxins which stimulates fruit growth but delays ripening . Monday, May 22, 2017 G R Davidson 31 Role of IAA in fruit formation • Fruits can be developed without fertilisation by a process called parthenocarpy. – Parthenocarpy occurs naturally in some fruits, such as bananas, pineapple and some varieties of grapes. – Parthenocarpy can be artificially induced by adding auxins to some fruits such as tomatoes and peppers. Monday, May 22, 2017 G R Davidson 32 Effect of Gibberellic Acid on Dwarf seedlings • Pea plants can either be tall or dwarf depending on which alleles are inherited. • Three possible theories could explain dwarfism:1. A growth substance is not produced in sufficient quantities by dwarf varieties. 2. A growth substance is prevented from acting. 3. A growth inhibitor is preventing the growth of the dwarf. Monday, May 22, 2017 G R Davidson 33 Effect of Gibberellic Acid on Dwarf seedlings • The growth substance is a member of a second group of plant growth substances known as gibberellins. – They stimulate cell division and elongation. – There are more than fifty known naturally occurring gibberellins, of which gibberellic acid is the most common. – They are transported around the plant in the phloem and xylem tissue. Monday, May 22, 2017 G R Davidson 34 Effect of Gibberellic Acid on Dwarf seedlings • Gibberellins are responsible for an occurrence known as bolting. – This involves the very fast growth of the internodes of stems and flowering in plants that normally require cold or long days before they bloom, – e.g. if dwarf cabbage plants are sprayed with a solution containing gibberellic acid they can grow stems about 4m high and produce flowers in their first year of growth rather than the second. Monday, May 22, 2017 G R Davidson 35 Effect of Gibberellic Acid on Dwarf seedlings • It would seem to be the case, therefore that theory number 1 is correct. – If dwarfism was due to a block in the functioning of gibberellic acid then the seedling would be unable to use any gibberellic acid applied to it. – If it were an inhibitor then an application of gibberellic acid would have no further effect. Monday, May 22, 2017 G R Davidson 36 Effect of Gibberellic Acid 4 days later The internode where G.A. was applied has elongated Lanolin paste with a minute amount of G.A. added here Monday, May 22, 2017 G R Davidson 37 The effect of gibberellic acid on dormancy • In many countries, there are periods when water supply, temperature or light are not favourable for active plant growth. – During this period, many plants become inactive or dormant. – Some woody plants remain active above ground and their tissue is protected by bark or the scales of winter buds. Monday, May 22, 2017 G R Davidson 38 The effect of gibberellic acid on dormancy • During the Spring, new leaves and flowers emerge from these buds. • Breaking bud dormancy is associated with increasing gibberellic acid levels. Monday, May 22, 2017 G R Davidson 39 The effect of gibberellic acid on dormancy • Bud dormancy is associated with increasing levels of growth inhibitor. • If there is a high level of inhibitor, the buds remain dormant, but if the gibberellic acid level increases, dormancy is broken. Monday, May 22, 2017 G R Davidson 40 The effect of gibberellic acid on α-Amylase induction Testa -amylase produced Aleurone Layer Endosperm GA produced Embryo Monday, May 22, 2017 G R Davidson 41 The effect of gibberellic acid on α-Amylase induction • Many plants produce seeds at the end of the growing season. • The seeds remain dormant and will only germinate and produce new plants when conditions are favourable. – Seeds routinely remain dormant for periods of years, some can remain dormant for a 1000 years or more . Monday, May 22, 2017 G R Davidson 42 The effect of gibberellic acid on α-Amylase induction • Gibberellic acid plays a role in breaking seed dormancy in monocots . – Stored starch reserves in seeds must be broken to sugar before the embryo can use it as a food supply and grow. – Starch is broken down to sugar by the enzyme α -Amylase. – Gibberellic acid plays an important role in stimulating the production of this enzyme. Monday, May 22, 2017 G R Davidson 43 The effect of gibberellic acid on α-Amylase induction • In a soaked barley seed, the hormone gibberellin is produced by the embryo and then passed on to the aleurone layer. • It stimulates the aleurone layer at gene level to produce α -Amylase which digests the starch of the endosperm to maltose which is required for growth. Monday, May 22, 2017 G R Davidson 44 Application of Herbicides • They are particularly important in agriculture where unwanted plants can overcrowd cultivated plants and use up the nutrients intended for the crops. – Application of auxins causes bolting in broad leaved weeds but has no effect on cereals. Monday, May 22, 2017 G R Davidson 45 Application of Herbicides • They are also used in gardens to produce weed-free lawns and borders. • Synthetic auxins have now been manufactured by chemists as selective weed-killers. • They are produced cheaply and are usually very effective. Monday, May 22, 2017 G R Davidson 46 Application of Herbicides • Herbicides are substances used as selective weed killers. • Because plants can’t produce enzymes to break them down, they have long lasting effects. – These synthetic auxins stimulate bolting to such extremes that the plant exhausts its food reserves and dies of starvation. Monday, May 22, 2017 G R Davidson 47 Application of Herbicides • These synthetic auxins stimulate a plant’s growth rate to such extremes that the plant exhausts its food reserves and dies of starvation. Monday, May 22, 2017 G R Davidson 48 Applications of rooting powder • When propagating plants from cuttings, it is important to establish a good root system as soon as possible so that water and minerals can be taken to all parts of the growing plant. • A rooting powder accelerates the development of a root system and often contains a synthetic auxin. Monday, May 22, 2017 G R Davidson 49 Summary of Gibberellins • Gibberellins: – Cause internode elongation. – Break dwarfism. – Break dormancy in buds and monocotyledonous seeds. Monday, May 22, 2017 G R Davidson 50 Other applications of plant growth substances • These properties can be used in a variety of ways to help us control the growth and development of plants. • Because auxins inhibit the growth of lateral buds, applying auxin to freshly dug potatoes prevents the eyes from sprouting and allows the potatoes to be stored for up to three years. Monday, May 22, 2017 G R Davidson 51 Other applications of plant growth substances • Spraying fruit trees with auxin prevents an abscission layer forming and fruit is retained on the tree until it is ready for harvest. • Anti-auxins are sprayed over cotton fields to cause the leaves to fall. • This makes the mechanical picking of the cotton easier. Monday, May 22, 2017 G R Davidson 52 Other applications of plant growth substances • Gibberellins are also widely used, e.g. they stimulate germination and are sprayed over barley in the malting process. • Gibberellins are also used to produce longer stems in celery. Monday, May 22, 2017 G R Davidson 53