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Hormones 101 Duane W. Greene University of Massachusetts What are Plant Hormones? • Plant hormones are organic compounds, produced by the plant that In low concentration (10-3 M) regulate or direct growth and development in a plant. Plant Growth Regulators • Plant growth regulators are very similar to plant hormones except PGRs may or may not be produced by the plant. Many are synthetic or not universally found in all plants. • Some plant hormones are also considered PGRs (GA4+7, GA3 , ABA, ACC). Plant Hormones • PGRs act as signal givers or directors of growth and development. • In general, they are not directly involved in the processes that they direct. • One could look at PGRs as the first domino in a series of event that ultimately results in a physiological response. • Another way of looking at this that they are the trigger that initiates a physiological action. Plant hormones signal to initiate a response which is then passed on and results in a response. PGR Response Send signals that lead to gene expression, gene activation and activation and/or synthesis of enzymes Major Classes of Hormones • • • • • • • Auxins Gibberellins Cytokinins Abscisic Acid Ethylene (Brassinosteroids) (Jasmonates) Auxins • This is the first hormone that was isolated and identified- 1930’s. • Indole-3-acetic acid is the primary endogenous auxin. • While limited in the number of field applications, auxin probably has the greatest influence on how you grow and manage your trees. Auxins control or influence: • • • • • • • • • Apical Dominance Phototropism Geotropism Abscission: leaves, fruit and flowers Cell elongation Cambial activity Root formation Increase flowering Auxin induced ethylene production Apical Dominance Apical Dominance Leaf Abscission Fruit Abscission Auxins present in the king fruit suppress the auxins coming from the lateral fruit. This reduction on auxin triggers the production of enzymes that weaken and destroy the abscission zone. We suspect that the reduction in auxin is carbohydrate (CHO) mediated. The greater the CHO deficit the more abscission. Phototropism Geotropism Geotropism Gibberellins (GA) • Stem Elongation • Stimulate seed germination • Flowering – Promotion – Inhibition • Stimulate pollen tube growth • Elongates fruit • Improves fruit finish Stem Elongation • Generally vigorous vegetative growth is undesirable for tree fruit. • Inhibitors of GA biosynthesis are used to retard growth and make trees more efficient, productive and carry higher quality fruit. Inhibition of Flowering • GA inhibits flower bud formation in tree fruit. • Seeds in developing fruit are rich sources of GA. • Since trees generally set more fruit than is desirable, attempts are made to cause a large number of fruit containing seeds to abscise early in the season so that flower bud formation for the following year will not be inhibited or at least inhibited minimally. • Inhibition may occur early. Elongation of Fruit Cytokinins • • • • • • • Cell division and cell growth Deferral of senescence Apical dominance, promote lateral bud break Cell differentiation Nutrient mobilization Morphogenesis Fruit elongation Cell Division • Cytokinins were discovered in the late 1940s because they caused cell division. • The cytokinins were given their name because they did promote cytokinesis (cell division). Cytokinins inhibit scenesence • Cytokinins delay and defer senescence by regulating enzymes that encourage cytokinin synthesis and inhibit their degradation. Apical Dominance • Cytokinins interact with auxin to regulate apical dominance. • Sugars are also invovled. Abscisic Acid (ABA) • BA was discovered in large part because it caused leaf abscission in cotton. • However, ABA only plays a secondary role in abscission. Both auxins and ethylene are more prominent hormones in regulation of abscission. Abscisic Acid • • • • • Controls stomatal movement Inhibition of vegetative growth Promotes abscission Involved in seed and seed dormancy Produced in stressed plants Stomatal Movement & Transpiration • ABA plays a pivotal role in the regulation of stomatal movement water loss. • In response to water stress ABA increase in the leaves which results in cations moving out of the guard cells esp. K+. • The lower osmotic pressure in the guard cell results in osmotic loss of water causing the stomates to close. Ethylene • • • • • Advances fruit ripening Promotes senescence Enhances flowering in pome fruit Causes epinasty Frequently produced as result of auxin application • Auxin induced ethylene production Ethylene the Hormone • Ethylene was ignored as a legitimate plant hormone for many years. • Scientist could not understand how a small gas molecule could be a hormone • It was not until ethylene could be detected a very low rate with a gas chromatograph that its importance as recognized. Ripening of Fruit • When fruit ripen they give off large amounts of ethylene. • Efforts are made either inhibit ethylene production (ReTain) or block its response (SmartFresh, Harvista). • Ethylene management is a key to extend storage life. Ethylene Promotes Abscission • Ethylene along with auxins are key hormones that regulate leaf abscission. • Ethylene production is increased in stressed plants. • This in turn results in a decrease in auxin content which can then lead to abscission Auxin Induced Ethylene Production • Auxins stimulate ethylene production. Frequently, responses in a plant that appear ethylene-like are actually triggered by auxin. • Examples: – Epinasty of leaves following NAA application – Leaf curling following 2,4-D application Jasmonates • Produced in response to wounding as a natural defense compound. • The can be growth retardants and retaerd root and coleoptile growth • Promote senescence. • Most common are methyl jasmonate and jasmonic acid. Brassinosteroids • Steroids that promote and are necessary for normal stem elongation. • Plants that lack this hormone are dwarfed and this can be reversed by providing brassinosteriods. Summary • Plant hormones are the organic compound produced by the plant that regulate the growth and development of the plant. • They control all aspect of growth and development of aa plant. • They should be considered signal givers that control the growth in an ordered sequenced series of events.