Plant Growth Controls: Hormones, Auxins, and Gibberellins

CONTROLS OF PLANT GROWTH  OBJECTIVES 1. To list the functions of the following hormones: auxins, gibberellins, cytokinins, absisic acid and ethene. 2. To describe a general mechanism of action for plant hormones. 3. To list some of the economic uses of plant hormones. ...

CONTROLS OF PLANT GROWTH  OBJECTIVES 1. To list the functions of the following hormones: auxins, gibberellins, cytokinins, absisic acid and ethene. 2. To describe a general mechanism of action for plant hormones. 3. To list some of the economic uses of plant hormones.  INTRODUCTION 1. From the time a plant seed germinates, a number of hormones influence growth and development. 2. Hormones are signaling molecules between cells. o One cell type produces and then secretes a particular kind of hormone, which stimulates or inhibits gene activity in other cell types that take up molecules of the hormone. o The changes in gene activity have predictable effects, as when they trigger the mitotic cell divisions and other processes that make stems grow longer. 3. Plant hormones help adjust patterns of growth and development in response to environmental rhythms, including seasonal shifts in day length and temperature. o In addition, they help adjust the patterns of response to environmental circumstances in which an individual plant finds itself, such as the amount of sunlight or shade, moisture and so on at a given site. 4. Commonly, two or more kinds of plant hormones must interact with one another to bring about specific effects on growth and development. The known plant hormones are : Auxins - IAA Gibberellins - GA3 Cytokinins - Zeatin Abscisic acid Ethylene  AUXINS 1. Indoleacetic acid (IAA) is the most common auxin in nature. 2. Certain synthetic auxins are used as herbicides. 3. Sites of production – dividing cells in apical and leaf meristems, exported from shoot tip. 4. Promote stem lengthening. 5. Play a role in responses to gravity and light. 6. E.g. indoleacetic acid; dichlorophenoxyacetic acid, 2,4-D (synthetic). 7. Parts affected & effects: a. Shoot tip & stem - decreased cell division, increased elongation. b. Lateral buds - growth inhibited (apical dominance), promotes secondary growth. c. Vascular cambium - promotes secondary growth. d. Leaf - inhibits leaf abscission. e. Fruit - promotes development, inhibits fruit abscission. f. Seed - no effect on dormancy g. Wounds - cell differentiation, growth of adventitious roots. Site of production. Shoot apical meristem, young leaves, seeds Principal actions. Stem elongation. Apical dominance. Root initiation. Fruit development  GIBBERELIN (gibberelic acid) 1. Is one of the premier plant hormones. 2. More than 80 forms have been isolated from plants, as well as from fungi. 3. In nature, gibberellin: a. helps seeds and buds break dormancy. b. makes stems lengthen. c. influences flowering. 4. Sites of production - chloroplasts, seed embryo, root tip, exported from young leaves. 5. Parts affected - meristems, stem, fruits, seed. 6. Effects - increased cell division, increased elongation, promotes development, initiates germination. 7. Applied by growers to enhance stem length, control ripening. 8. Gibberella fujikuroi - parasitic fungus – causes a damaging disease of rice – infected seedling grow abnormally tall and their stems are weakened). Site of production. Young leaves, Shoot apical meristems, embryo in seed. Principal actions. Seed germination. Stem elongation. Flowering. Fruit development  CYTOKININS Most abundant in root and shoot meristems and in maturing fruits. In mature plants, produced in roots and transported to shoots. Used to artificially extend the shelf life of cut flowers; delays leaf death. Sites of production o Dividing cells in roots, seeds, fruits, exported from roots. Effects of cytokinins o Meristems – increased cell division o Stem – increased elongation o Fruit – promotes development o Seed – initiates germination Principal actions o Cell division o Delay of leaf senescence o Inhibition of apical dominance o Flower development o Embryo development o Seed germination  ABSCISIC ACID (ABA) Causes the suspension of growth; promotes dormancy of buds and seeds. Used to induce dormancy in plants to be shipped. Also plays a role in drought response. Sites of production o Leaves – ageing leaves, stems, fruits, seeds. Parts affected & effects o Root – decreased cell elongation o Apical & lateral buds – induces dormancy o Leaf – promotes leaf abscission o Fruit – promotes fruit abscission o Stoma – promotes closure in drought conditions  ETHYLENE oUnlike other plant hormones, ethylene is a gas oSites of production – most plant organs – stem nodes, ripening fruit, damaged or senescing tissue oParts affected – fruit oEffects – promotes ripening oFunctions - induces aging responses, used to ripen General mechanism of action of plant hormones Figure. 9: General mechanism of action of plant hormones o Plant and animal hormones are similar in their basic mechanism of action. Basic mechanism of action plant hormone: 1. Bind to specific receptor proteins of the plasma membrane / in the target cells. 2. Each receptor has a 3D shape that binds only with one kind of hormone molecule (S1). 3. This binding activates a G protein in the plasma membrane (S2). 4. The G protein triggers the production of a second messenger, an intracellular signaling molecule that affects the function of the cell. 5. Ions such as Ca2+ serve as second messengers in many plant cells. 6. Cyclic AMP an important second messenger in a variety of prokaryotes and eukaryotes, may also be a signaling molecule for at least one plant hormones (auxin). 7. Once the concentration of Ca2+ / other second messenger increases in the cell, it may bind to proteins and activate or inactivate certain enzymes – protein kinase (S3). 8. The enzymes can phosphorylate proteins, which alter cell activity in some way, such as altered membrane permeability (S4) and/or altered gene expression (S4) then altered transcription and/or translation. Table 1: Plant hormones and signaling molecules Table 2: Overview of plant signaling molecules and their effects.  GLOSSARY Dormancy – A temporary state of reduced physiological activity / state when a plant stops growing under conditions that seem (to us) suitable for growth in which its metabolic activities idle. Hormones - signaling molecules secreted by some cells that travel to target cells, where they stimulate or inhibit gene activity. Coleoptile - a thin sheath that keeps some primary shoots from shedding as growth pushes them up though soil. DISCUSSION QUESTIONS 1. Describe the plant hormones and signaling molecules (source and mode of transport). Also their stimulatory and inhibitory effects. 2. Describe the functions of the following plants hormones: auxins, gibberellins, cytokinins, absisic acid and ethene. 3. Describe a general mechanism of action for plant hormones. 4. Describe some of the economic uses of plant hormones. Good Luck