Control & Regulation

Control & Regulation Hormonal influences on growth Mr G R Davidson

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. G R Davidson

The Pituitary Gland • It is composed of the anterior (front) and the posterior lobe (back). • The anterior lobe is where the hormones are made. G R Davidson

Hypothalamus Pituitary The pituitary gland G R Davidson

The pituitary gland G R Davidson

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. G R Davidson

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 G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

Produces T.S.H. Pituitary Gland Thyroid Gland Produces Thyroxine Increased Metabolism Thyroid Stimulating Hormone G R Davidson

Pituitary Gland T.S.H. Less Hormone Produced More Hormone Produced Thyroid Gland Thyroxine Overproduction Underproduction Negative Feedback G R Davidson

Production ofIAA • 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. G R Davidson

Production ofIAA • 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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

IAAand 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. G R Davidson

Region of auxin production Normal auxin diffusion uneven auxin diffusion increased cell elongation Reduced cell elongation Region of cell elongation Oat coleoptile IAAand Phototropism G R Davidson

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. G R Davidson

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. G R Davidson

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 . G R Davidson

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. G R Davidson

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. G R Davidson

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 G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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 . G R Davidson

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. G R Davidson

Effect of Gibberellic Acid on Dwarf seedlings • Pea plants can either be tall or dwarf depending on which alleles are inherited. • Threepossible theories could explain dwarfism:- • A growth substance is not produced in sufficient quantities by dwarf varieties. • A growth substance is prevented from acting. • A growth inhibitor is preventing the growth of the dwarf. G R Davidson

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 gibberellicacid is the most common. • They are transported around the plant in the phloem and xylem tissue. G R Davidson

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. G R Davidson

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. G R Davidson

The internode where G.A. was applied has elongated 4 days later Lanolin paste with a minute amount of G.A. added here Effect of Gibberellic Acid G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

Testa -amylase produced Aleurone Layer Endosperm GA produced Embryo The effect of gibberellic acid on α-Amylase induction G R Davidson

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 . G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

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. G R Davidson

Summary ofGibberellins • Gibberellins: • Cause internode elongation. • Break dwarfism. • Break dormancy in buds and monocotyledonous seeds. G R Davidson

Control & Regulation