Plant Growth Regulators: Auxins and Gibberellins

Plant Growth Regulators

• Plant growth regulators include Indole 3'-acetic acid (an auxin), Gibberellin GA1, Abscisic acid and Ethylene.
• Another growth regulator is Cytokinin, such as Zeatin.

Auxin Function

• Auxin governs the formation of the main axis of polarity during early embryogenesis.
• The shoot meristem is at the top, and the root meristem is at the opposite pole.
• One effect of IAA is to enhance the elongation growth of cells.
• Auxin generally influences cell division and differentiation.

Gibberellins

• Gibberellins stimulate shoot elongation, especially in the internodes of stems.
• They induce rosette plants to shoot up for the formation of flowers and regulate flowering (e.g., spinach or lettuce).
• Gibberellins terminate seed dormancy, probably by softening the seed coat.
• They facilitate seed germination by expressing genes for necessary enzymes (e.g., amylases).
• These regulators have a number of functions, such as the preformation of fruits and the stimulation of their growth.

Cytokinins

• Cytokinins enhance plant growth by stimulating cell division.
• They increase the sprouting of lateral buds and are antagonists of the auxin IAA.
• Cytokinins retard senescence and counteract the phytohormone ethylene.
• Butterfly larvae (e.g., Stigmella, which invade beech trees) use this principle for their nutrition.

Abscisic Acid (ABA)

• Abscisic acid (ABA) includes substances that cause the abscission of leaves and fruits.
• It is responsible for the induction of dormancy of seeds and buds.
• ABA prevents vivipary.
• ABA has a major function in maintaining the water balance of plants, since it induces with nitric oxide (NO).

Ethylene

• Ethylene is involved in the induction of senescence and causes degradation of leaf material.
• Ethylene induces defense reactions after infection by fungi or when plants are wounded by feeding animals.

Hormone

• A hormone excites or gets things moving, a molecule released in one body part but regulates activity of cells in other parts.
• Hormones enter interstitial fluid, then the bloodstream, which delivers them.
• Neurotransmitters and hormones exert effects by binding to receptors on target cells.
• Norepinephrine acts as both a neurotransmitter (by sympathetic postganglionic neurons) and as a hormone (by chromaffin cells of the adrenal medullae).

Hormone Receptor

• A hormone receptor is a molecule that binds to a specific hormone, and receptors are a wide family of proteins.
• Hormones affect only specific target cells, influencing them by chemically binding to receptors.
• Only target cells for a given hormone have receptors, e.g., thyroid-stimulating hormone (TSH) binds to thyroid cells but not ovarian cells.

Exocrine Glands

• Exocrine glands secrete products into ducts to body cavities, organ lumens, or the body's outer surface.
• These include sudoriferous (sweat), sebaceous (oil), mucous, and digestive glands.

Endocrine Glands

• Endocrine glands secrete products (hormones) into the interstitial fluid surrounding the secretory cells rather than into ducts.

Autocrine

• In autocrine signaling, a cell signals to itself, releasing a ligand that binds to receptors on its surface.
• Autocrine signaling plays an important role in many processes and is important in cancer, playing a key role in metastasis.
• Cytokines are an example.

Paracrine

• Cells that are near one another communicate through the release of chemical messengers (ligands that can diffuse through the space between the cells).
• Cells communicate over relatively short distances.
• Allows cells to locally coordinate activities with their neighbors.
• Synaptic signaling and reproductive system development are examples.

Endocrine

• Cells that need to transmit signals over long distances often use the circulatory system.
• Signals are produced by specialized cells and released into the bloodstream, carrying them to target cells in distant parts of the body.
• Signals are known as hormones.

Chemical Classes of Hormones

• Chemically, hormones are divided into two broad classes: those soluble in lipids and those soluble in water.
• This chemical classification is also useful functionally since the two classes exert their effects differently.

Lipid-Soluble Hormones

• Lipid-soluble hormones include steroid hormones, thyroid hormones, and nitric oxide.
• Steroid hormones are derived from cholesterol and are unique because of different chemical groups attached at sites.
• Thyroid hormones (T3 and T4) are synthesized by attaching iodine to the amino acid tyrosine. -The presence of two benzene rings within a T3 or T4 molecule makes these molecules very lipid-soluble
  • Nitric oxide (NO) is both a hormone and a neurotransmitter and its synthesis is catalyzed by the enzyme nitric oxide synthase.

Water-Soluble Hormones

• Water-soluble hormones include amine hormones, peptide and protein hormones, and eicosanoid hormones.
• Amine hormones are synthesized by decarboxylating and modifying amino acids and are called amines because they retain an amino group (-NH3+).
  • The catecholamines—epinephrine, norepinephrine, and dopamine-are synthesized by modifying the amino acid tyrosine.
  • Histamine is synthesized from the amino acid histidine by mast cells and platelets.
  • Serotonin and melatonin are derived from tryptophan.
• Peptide and protein hormones are amino acid polymers.
  • The smaller peptide hormones consist of chains of 3 to 49 amino acids; the larger protein hormones include 50 to 200 amino acids.
  • Examples of peptide hormones are antidiuretic hormone and oxytocin; protein hormones include growth hormone and insulin.
  • Several of the protein hormones, such as thyroid-stimulating hormone, have attached carbohydrate groups and thus are glycoprotein hormones.
• Eicosanoid hormones are derived from arachidonic acid, a 20-carbon fatty acid.
  • The two major types of eicosanoids are prostaglandins (PGs) and leukotrienes (LTs).
• Eicosanoids are important local hormones, and they may act as circulating hormones as well.

Thyroid Gland

• The thyroid gland is the only endocrine gland that stores its secretory product in large quantities-normally about a 100-day supply.
• Synthesis and secretion of T3 and T₄ occurs as follows:
• Thyroid follicular cells actively trap iodide ions (I¯) by actively transporting them from the blood into the cytosol.
  • As a result, the thyroid gland normally contains most of the iodide in the body.
• While the follicular cells are trapping I, they are also synthesizing thyroglobulin (TGB) a large glycoprotein that is produced in the rough endoplasmic reticulum and packaged into secretory vesicles.
  • The vesicles then undergo exocytosis, which releases TGB into the lumen of the follicle.
• Some of the amino acids in TGB are tyrosines that will become iodinated. -Negatively charged iodide ions cannot bind to tyrosine until they undergo oxidation (removal of electrons) to iodine: I→ Iº.
• As the iodide ions are being oxidized, they pass through the membrane into the lumen of the follicle.
• As iodine atoms (Iº) form, they react with tyrosines that are part of thyroglobulin molecules.
  • Binding of one iodine atom yields monoiodotyrosine (T1), and a second iodination produces diiodotyrosine (T2).
  • The TGB with attached iodine atoms, a sticky material that accumulates and is stored in the lumen of the thyroid follicle, is termed colloid. -Two T2 molecules join to form T4, or one T1 and one T2 join to form T3.
• Droplets of colloid reenter follicular cells by pinocytosis and merge with lysosomes. Digestive enzymes break down TGB, cleaving off molecules of T3 and T4.
• Because T3 and T4 are lipid-soluble, they diffuse through the plasma membrane into interstitial fluid and then into the blood.
• T4 normally is secreted in greater quantity than T3, but T3 is several times more potent.
  • Once T4 enters a body cell, most of it is converted to T3 by removal of one iodine.
• More than 99% of both the T3 and the T4 combine with transport proteins in the blood, mainly thyroxine-binding globulin (TBG).

Chemistry of Thyroxine

• Thyroid cellular mechanisms for iodine transport, thyroxine and triiodothyronine formation, and thyroxine and triiodothyronine release into the blood
  • DIT, diiodotyrosine; ER, endoplasmic reticulum; I−, iodide ion
  • I2, iodine; MIT, monoiodotyrosine; NIS, sodium-iodide symporter; RT3, reverse triiodothyronine
  • T3, triiodothyronine; T4, thyroxine; TG, thyroglobulin.

Physiological Function of Thyroxin

• Hormones increase transcription of large numbers of genes and have effect on growth.
• Increase active transport of ions through cell membranes.
• Stimulates carbohydrate and fat metabolism.
• Increased requirement for vitamins, a basal metabolic rate, and blood flow and cardiac output.
• Decreased body weight.

Chemistry of ADH & Oxytocin

• Vasopressin: Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-GlyNH2
• Oxytocin: Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-GlyNH2

Physiological Function of ADH

• Reabsorption of water conserves body water by decreasing urine volume.
• Decreases water loss through perspiration and raises blood pressure by constricting arterioles.

Functions of Oxytocin

• Stimulates contraction of smooth muscle cells of the uterus during childbirth.
• Stimulates contraction of myoepithelial cells in mammary glands to cause milk ejection.

Mode of Action of Steroid Hormones

• A free lipid-soluble hormone molecule diffuses from the blood, through interstitial fluid, and through the lipid bilayer of the plasma membrane into a cell.
• If the cell is a target cell, the hormone binds to and activates receptors located within the cytosol or nucleus.
  • The activated receptor-hormone complex then alters gene expression, turning specific genes of the nuclear DNA on or off.
• As the DNA is transcribed, new messenger RNA (mRNA) forms, leaves the nucleus, and enters the cytosol.
  • There, it directs synthesis of a new protein, often an enzyme, on the ribosomes.
• New proteins alter the cell's activity and cause the responses typical of that hormone.

Action of Water-Soluble Hormones

• Because amine, peptide, protein, and eicosanoid hormones are not lipid-soluble, they diffuse through the lipid bilayer of plasma membranes and bind to receptors inside target cells.
• Water-soluble hormones bind to receptors that protrude from the target-cell surface. Receptors are integral transmembrane proteins in plasma membrane.
• When a water-soluble hormone binds to its receptor at the outer surface of the plasma membrane, it acts as the first messenger.
  • The first messenger(hormone) causes production of a second messenger inside the cell, where specific hormone-stimulated responses take place.
• Cyclic adenosine monophosphate, also known as cyclic AMP is one common second messenger (cAMP).
• The action of a typical water-soluble hormone occurs as follows:
• A water-soluble hormone diffuses from the blood through interstitial fluid and then binds to its receptor at the exterior surface of a target cell's plasma membrane.
• The hormone-receptor complex activates a membrane protein called a G protein, which activates adenylyl cyclase. -Adenylyl cyclase converts ATP into cyclic AMP. Because the enzyme's active site is on the inner surface of the plasma membrane, this reaction occurs in the cytosol of the cell.
• Cyclic AMP activates one or more protein kinases, which may be free in the cytosol or bound to the plasma membrane.
  • A protein kinase is an enzyme that phosphorylates (adds a phosphate group to) other cellular proteins.
• Activated protein kinases phosphorylate one or more cellular proteins and phosphorylation activates some and inactivates others.
• Phosphorylated proteins in turn cause reactions that produce physiological responses.
  • Different protein kinases exist within different target cells to trigger synthesis or promote protein synthesis.
• Phosphodiesterase inactivates cAMP and the cell's response is turned off unless new hormone molecules bind to theirs receptor by the plasma membrane.

Physiological Functions of Hormones

• Follicle-stimulating hormone (FSH) initiates development of oocytes and induces ovarian secretion of estrogens in females, and stimulates testes to produce sperm in males.
• Luteinizing hormone (LH) stimulates secretion of estrogens and progesterone, ovulation, and formation of corpus luteum in females, and stimulates testes to produce testosterone in males.
• Glucocorticoids from zona fasciculata cells cause adrenocorticotropic hormone (ACTH) release and ACTH stimulates release:
• Corticotropin-releasing hormone (CRH) promotes ACTH secretion in response to stress and low blood levels of glucocorticoids.
• Increases protein breakdown , stimulate gluconeogenesis and lipolysis, provide resistance to stress, dampen inflammation, depress immune responses, except in liver.
• Estrogens and progesterone (female sex hormones) regulate the female reproductive cycle
• gonadotropic hormones together with of anterior pituitary, regulate female reproductive cycle, maintain pregnancy,prepare mammary glands for lactation, and promote development and maintenance of female secondary sex characteristics.