PLANT DEVELOPMENT They are chemical compounds and found naturally in plants. They are also synthesised commercially and used in agricultural practices. They are known as plant hormones or phytohormones. They are derivatives of adenine (kinetin), carotenoids (ABA), terpenes (GA3) and indole compounds (auxins). Ethylene is a gaseous hormone They are present in a very low concentration and act as chemical signals between cells Environmental factors influence gene expression and hormone production Plant hormones may show different effects at different stages and at different concentrations Plant hormones act by signal transduction, i.e. an external signal is converted to internal signal and which in turn causes one or more cellular responses According to their actions, they can be classified into two categories: 1. Plant growth promoters, which induce cell division, elongation, differentiation and the formation of flowers, fruits and seeds, e.g. auxins, gibberellins, cytokinins 2. Plant growth inhibitors are linked to dormancy, abscission and various stress responses, e.g. Abscisic acid (ABA) 3. Ethylene, the gaseous hormone has inhibitory as well as growth-promoting effects Brassinosteroids also have been discovered to work as a phytohormone Different plant hormones may work antagonistically or complimentary (synergistically) to each other. There are many events that get affected by more than one phytohormones, such as apical dominance, dormancy, abscission, senescence, etc. Auxins Produced in root and shoot apices. It gets transported to various parts. The transport of auxin is polar or unidirectional. Natural auxins- IAA (Indole acetic acid) and IBA (Indole butyric acid), synthetic auxins- 2,4-D (2, 4-Dichlorophenoxyacetic acid), NAA (naphthalene acetic acid). Effects of auxins: Apical Dominance Induces cell differentiation in xylem Induce parthenocarpy, i.e. formation of seedless fruits, e.g. Tomatoes Promote flowering, e.g. Pineapples Delay abscission of young leaves and fruits, whereas, promote falling of older leaves and fruits Root initiation in stem cuttings for vegetative propagation 2, 4-D is widely used as herbicides to kill dicot weeds Gibberellins: Gibberellins are promotery PGR seen in more than 100 forms named as They are denoted as GA1, GA2, GA3 and so on. Gibberellic Acid is the most common one. It serves the following functions: 1. Cell elongation. 2. Early maturity. 3. Seed germination. 4. Breaking of dormancy. Cytokinins - Cytokinins have specific effects on cytokinesis and were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA. The most common forms are zeatin, kinetin, etc. They are mainly made in the roots. Some of the Functions: 1. Cell division and cell differentiation. 2. Overcome apical dominance. 3. Promote nutrient mobilization. 4. Essential for tissue culture. Ethylene - It is called a gaseous hormone that stimulates isodiametric or transverse growth; however, it retards the longitudinal one. It serves the following functions: 1. Inhibition of longitudinal Growth. 2. Senescence. 3. Promote apical dominance. 4. Fruit ripening. Abscisic Acid - It is also referred to as stress hormone or dormin. It works like a general plant growth inhibitor. Abscisic acid is formed at the terminal buds of the top of the plant or in the roots of the plants. It can serve the following functions: 1. Bud dormancy. 2. Induce Parthenocarpy. 3. Seed development and maturation. 4. Leaf senescence. Photoperiodism - Photoperiodism is the effect of photoperiods or day duration of light hours on the plant's growth and development, especially flowering. The flowering plants have been divided into the following categories based on photoperiodic response: 1. Short Day Plants - The flowers which need exposure to light for a period less than this critical duration before the flowering is initiated. For example - Xanthium, Sugarcane, Potato Rice. 2. Long Day Plants - The plant flowers when they need a long photoperiod of light, greater than the critical period. Example - Barley, Radish, Lettuce. 3. Day Neutral Plants - These plants can blossom throughout the year - for example - Wild Kidney, Bean. CHEMICAL COORDINATION AND INTEGRATION Human Endocrine System The location of the endocrine glands is in different parts of the human body. Different endocrine glands are the pituitary, pineal, thyroid, parathyroid, thymus, adrenal, and gonads. The kidney, liver, gastrointestinal tract also produces some hormones. The Hypothalamus The location of the hypothalamus is at the base of the forebrain. The group of secretory cells present in the hypothalamus is known as nuclei that produce hormones. This regulates the synthesis and secretion of pituitary hormones are regulated by the hormones secreted by the hypothalamus. The hormones produced from the hypothalamus are of two types(i) the releasing hormones (ii) the inhibiting hormones. For example, the gonadotropin releasing hormone stimulates the release of gonadotropins from the pituitary. These hormones reach the pituitary gland with the assistance of the portal circulatory system. The Pituitary Gland (Image will be uploaded soon) The location of the pituitary gland is in the cavity known as sella turcica which is attached to the hypothalamus via a stalk. It has two divisions as adenohypophysis or anterior pituitary and neurohypophysis or posterior pituitary. Hormones such as adrenocorticotropic hormone, growth hormone, thyroid-stimulating hormone melanocyte-stimulating hormone, prolactin, folliclestimulating hormone, and luteinizing hormone are secreted by the adenohypophysis. The release of Vasopressin and oxytocin are by neurohypophysis. (Image will be uploaded soon) (Image will be uploaded soon) Hormone Function Growth hormone Regulates general body growth such as an increase in the length of the bones, controls fat, protein, and carbohydrate metabolism, etc. Over secretion of Growth hormone results in gigantism whereas low secretion is associated with dwarfism. Adrenocorticotropic hormone Controls the hormones secreted by the adrenal cortex Thyroid-stimulating hormone Controls the hormones secreted by thyroid glands. Follicle-stimulating hormone The hormone controls the maturation of Graafian follicles in females and spermatogenesis in males. Luteinizing hormone Ovulation in females is promoted and the secretion of testosterone in males is stimulated. Prolactin Milk production in mammary glands Oxytocin Is milk ejecting hormone and also functions during childbirth. Vasopressin Also known as antidiuretic hormone (ADH) helps in the reabsorption of water in the distal convoluted tubules. The Pineal Gland The location of the pineal gland is on the dorsal side of the forebrain. The pineal gland secretes the melatonin hormone. The melatonin hormone regulates 24 hours rhythm in the body like body temperature, sleep-wake cycle etc. Along with the sleep-wake cycle, melatonin also controls: (i) metabolism (ii) pigmentation (iii) menstrual cycle (iv) defense capability Thyroid Gland (Image will be uploaded soon) Thyroid glands are located on either side of and anterior to the trachea. An isthmus connects the bilobed gland. The lobules of the thyroid gland are made up of follicles and thyroid follicles bound together by connective tissue are called stromal tissues. Thyroid follicles are composed of follicular cells that produce two hormones- tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3). A vital element required for thyroid hormone synthesis is Iodine. Deficiency of iodine will lead to hypothyroidism, called Goitre. During pregnancy, hypothyroidism affects the growth of the baby, its mental condition also gets affected. There will be a low intelligence quotient, abnormal skin, deaf. For adult women, there will be an irregular menstrual cycle in hypothyroidism. Another condition is when there is an increase in the secretion of thyroid hormone, it will be called hyperthyroidism. A form of hyperthyroidism is Exophthalmic goiter which is an enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate, and weight loss also referred to as Graves’ disease. Parathyroid Gland (Image will be uploaded soon) Normally, there are four parathyroid glands in the human being, one superior and one inferior located immediately behind each upper and lower pole of the thyroid. A peptide hormone is secreted by parathyroid called parathyroid hormone. It has the following effects: (i) Maintenance of proper calcium and phosphate level in blood. (ii) Role in bone formation. (iii) Excretion of calcium and phosphate. (iv) Effect on intestinal absorption of calcium, phosphate, and vitamin D. Thymus (Image will be uploaded soon) The location of the thymus is in between the lungs behind the sternum. An important role is played by them in the development of the immune system. The hormone secreted by this gland is called thymosin. It participates in cellmediated immunity. It also participates in the production of antibodies. Adrenal gland (Image will be uploaded soon) It is a pair of glands located in the anterior part of the kidney. It is composed of two types of tissues- the outer adrenal cortex and the inner medulla. The adrenal medulla produces two hormones known as adrenaline and noradrenaline. Adrenaline is also known as an emergency hormone as it is secreted at the time of fear, fight, or flight. Adrenaline dilates the pupil, increases alertness, sweating, etc. (Image will be uploaded soon) The 3 layers of adrenal cortex are(i) zona reticularis (inner layer), (ii) zona fasciculata (middle layer), and (iii) zona glomerulosa (outer layer). Hormones of the adrenal cortex are commonly known as corticoids. Corticoids which participate in carbohydrate metabolism are referred to as glucocorticoids. They stimulate proteolysis, lipolysis, and gluconeogenesis. The glucocorticoid includes cortisol, cortisone in which cortisol accounts for 95% of glucocorticoid activity which helps in anti-inflammatory reactions and suppresses the immune system. Corticoids that help in regulating the level of water and electrolyte balance are referred to as mineralocorticoids. Aldosterone is the main hormone of mineralocorticoids. It helps in the reabsorption of sodium ions and water and the excretion of potassium and phosphate ions. Pancreas The pancreas is considered to be a dual gland, i.e., it is endocrine as well as exocrine in function. The endocrine part of the pancreas consists of islets of Langerhans that are mainly made up of two types of cells- alpha cells and beta cells. Alpha cells secrete hormones known as glucagon whereas beta cells secrete insulin. Glucagon maintains normal blood glucose. It increases blood glucose levels. It also promotes gluconeogenesis. So, glucagon is a hyperglycemic hormone. (Image will be uploaded soon) Insulin also regulates glucose levels in our bodies. It enhances cellular glucose uptake thus decreases blood glucose levels. So, it is known as a hypoglycemic hormone. Prolonged hyperglycemia causes diabetes mellitus. Testes The location of testes is in the scrotal sacs present outside the abdominal cavity. They are the primary sex organ as well as endocrine in function. The testis includes seminiferous tubules and interstitial cells. Interstitial cells or Leydig cells secrete hormones known as testosterone. This hormone regulates spermatogenesis, the development of beards and mustaches, as well as the maturation of male accessory sex organs. (Image will be uploaded soon) Ovary The paired ovaries lie in the lower pelvic region of the abdominal cavity. It is one of the primary sex organs. Ovaries are mainly concerned with the production of ova or eggs and they also secrete two hormones, known as estrogen and progesterone. Estrogen Helps in: (i) development of accessory sex organs. (ii) development of mammary glands. (iii) menstrual cycle. (iv) ovulation. (v) water balance, by increasing water content and thickness of the skin. Progesterone is responsible for changes during pregnancy hence, called pregnancy hormones. Progesterone Helps in: (i) enhancing the blood supply of the wall of the uterus. (ii) placentation. (iii) development of breast during pregnancy. (iv) helps in premenstrual changes in the uterus. (v) also helps in the promotion of retention of water and sodium salts by affecting kidney functions. Hormones of Kidney, Gastrointestinal Tract, and Kidney Heart wall secretes a hormone referred to as atrial natriuretic factor (ANF) which decreases the blood pressure. ANF is released when blood pressure increases. The hormone secreted by kidney cells is called erythropoietin that helps in the promotion of RBC formation. Gastrin, Secretin, Cholecystokinin, and Gastric inhibitory peptides are secreted by the gastrointestinal tract. Mechanism of Hormone Action The function of different hormones is to control and regulate activity levels of the target tissue. The binding of hormones to hormone-receptor produces their effects on target tissues. The hormone-receptor complex formed will result in certain biochemical changes within the target tissue. There will be the interaction of hormones with the membrane receptors as well as intracellular receptors. Steroid receptors are generally intracellular in nature whereas peptide hormones generally bind to membrane receptors.
