Major Plant Hormones 1.Auxins 2.Cytokinins 3.Gibberelins 4.Ethylene 5.Abscisic acid

Transcription

1 Plant Hormones Lecture 9: Control Systems in Plants What is a Plant Hormone? Compound produced by one part of an organism that is translocated to other parts where it triggers a response in target cells and tissues. Functions of Plant Hormones Control plant growth and development by affecting division, elongation, and cell differentiation Effect depends on site of action, stage of plant growth and hormone concentration Major Plant Hormones 1.Auxins 2.Cytokinins 3.Gibberelins 4.Ethylene 5.Abscisic acid 1. Auxins (IAA) indoleacetic acid: natural auxin in plants Promotes elongation & secondary growth Apical meristem is the major site of auxin production Inhibits lateral growth Induces female floral parts & fruit Auxin Functions Stimulate cell elongation stimulate differentiation of phloem and xylem Stimulate root initiation on stem cuttings and lateral root development in tissue culture mediate the tropistic response of bending in response to gravity and light suppresses growth of lateral buds delay leaf senescence 1

2 More Auxin Functions Apical dominance: with apical bud (left), apical bud removed (right) can induce fruit setting and growth in some plants involved in assimilate movement toward auxin, possibly by an effect on phloem transport delay fruit ripening promote flowering in some species stimulate growth of flower parts stimulate the production of ethylene at high concentrations Evidence for the role of auxin in apical dominance High auxin concentration Low auxin concentration 9 Drawings depicting Coleus (Lamiaceae family) Cell elongation in response to auxin: the acid growth hypothesis Role of auxin in cell elongation 2

3 Evidence for the role of auxin in adventitious root formation With synthetic auxin Adventitious roots growing from stem tissue Without synthetic auxin Saintpaulia (Gesneriaceae family) 2. Cytokinins Found in a variety of plants and have many functions Synthesized in meristematic tissues in roots and transported to aboveground organs Regulate growth and development of tissue primarily by promoting cell division Involved in germination, shoot differentiation, leaf senescence Interacts with other plant hormones for some functions Cytokinins Function Synthesized in meristematic tissues in roots and transported to aboveground organs Regulates apical dominance and lateral root initiation Slows down senescence (plant aging) and chlorophyll degradation in aging leaves Involved in development of sex organs and male sterility Cytokinins Promote the synthesis or activation of cytokinesis proteins Also function as antiaging hormones Agrobacterium inserts genes that increase rate of cytokinin and auxin production Causes massive cell division Formation of crown gall tumor 16 Hormones can work with each other to regulate various aspects of plant biology. Auxin and cytokinin work together in regulating lateral bud formation the sprouting of lateral buds depends on the ratio of cytokinin to auxin Auxin and Cytokinins Plant tissue can form shoots, roots, or an undifferentiated mass depending on the relative amounts of auxin and cytokinin 18 3

4 3. Gibberellins Gibberellins Named after the fungus Gibberella fujikuroi which causes rice plants to grow very tall Gibberellins belong to a large class of over 100 naturally occurring plant hormones All are acidic and abbreviated GA Have important effects on stem elongation Enhanced if auxin present GA is used as a signal from the embryo that turns on transcription of genes encoding hydrolytic enzymes in the aleurone layer When GA binds to its receptor, it frees GAdependent transcription factors from a repressor These transcription factors can now directly affect gene expression FUNCTIONS OF GIBBERELINS Stimulate stem elongation by stimulating cell division and elongation. Adding gibberellins to certain dwarf mutants restores normal growth and development Stimulates flowering in response to long days. Breaks seed dormancy in some plants. 22 The effect of gibberellin treatment on seedless grapes 4. Ethylene Gaseous hormone High [auxin] induces release of ethylene Causes senescence (aging) Fruit ripening one bad apple does spoil the whole bunch Abscission loss of leaves on deciduous trees 4

5 Ethylene Universally produced by all plants Angiosperms, Gymnosperms, Ferns, Mosses, Liverworts Also found in some fungi, yeast and bacteria Important roles in: Abscission Germination Senescence Stress response to pathogens Ethylene and Fruit Ripening Helps fruits go through color change, softening of walls, conversion of starch to sugar Ethylene is produced in low amounts throughout plant life some climacteric plants have sudden peaks in ethylene synthesis which signals ripening changes Ethylene gas is sprayed on fruit crops to ripen at same time Ethylene and Stress Some stress situations trigger ethylene production exposure to heat/cold physical damage attack by fungal or bacterial pathogens flooding that limits oxygen Similar to Abscisic acid s stress response FUNCTIONS OF ETHYLENE Stimulates the release of dormancy. Stimulates shoot and root growth and differentiation (triple response) Stimulates leaf and fruit abscission. 5. Abscisic Acid (ABA) Stimulates flower opening. Stimulates flower and leaf senescence. Stimulates fruit ripening Found universally in plants and algae Many functions! Important roles in: plant development bud & seed dormancy Germination cell division leaf senescence Abscission cellular response to stress 5

6 Abscisic Acid ABA as a Stress Hormone Acts as a general inhibitor of growth and metabolism Inhibits growth in hypocotyls, epicotyls, leaves, coleoptiles Seed dormancy ABA promotes seed dormancy so plant seeds can withstand desiccation ABA increases with various environmental or biological plant stresses Excess heat, pests, excess salt and/or dehydration Wilted plants have high levels of ABA In a drought, ABA increases in some plants, causing the stomata to close, preventing water loss ABA can also produces osmolytes that protect cell membranes from dehydration Which hormones cause the following. Apical dominance from apical bud Abscission Stimulates growth of axillary buds Root growth Stimulates closing of stomata Causes fruit ripening Stimulates seeds to break dormancy and germinate Growth inhibitor Cell division and differentiation Cell elongation Seedless fruit Answers to Previous Slide: Auxin Ethylene Cytokinins Cytokinins Abscisic acid Ethylene Gibberellins Abscisic acid Auxins and cytokinins Auxins Auxins Which hormone is made at each location? Made in roots and transported upwards Found in meristems of apical buds and seed embryos Found in tissues of ripening fruit Leaves stems, roots and green fruit Answers to Previous Slide: Cytokinins Auxins Ethylene Abscisic Acid 6

7 The end!!, Questions.?? 7