PLANT HORMONES 101 Greene, D. [email protected] University of Massachusetts, Stockbridge School of Agriculture, Amherst, MA KEYWORDS Plant growth regulators, Plant hormones, auxins, gibberellins, cytokinins, abscisic acid, ethylene, directors of plant growth and development, naturally occurring compounds. ABSTRACT The growth and development of a plant is directed and guided by hormones produced by the plant. Plant hormones generally do not participate directly in the growth of a plant but they do send the signals that lead to gene expression and gene activation, enzyme activation and/or synthesis that ultimately results in developmental changes. There are five universally recognized major classes of hormones: auxins, gibberellins, cytokinins, abscisic acid and ethylene. There are two hormones that have a less dominant role in the regulation of plant growth and development: brassinosteroids and the jasmonates. Major Hormones Auxins: This group of hormones was the first to be discovered in the 1930s. Indole-3- acetic acid is the primary endogenous auxin found in most plants. While it is limited in the number of field uses it is probably has the greatest influence on how trees are grown and managed. Auxins do promote cell elongation. Apical dominance. Auxins are produced in the apical buds of shoots and diffuse downward inhibiting the growth and development of bud below. If the apical bud is removed or pruned off then inhibition of growth is lifted and lateral buds start to develop. These buds develop until they become dominate and reestablish the apical dominance. Growth of lateral buds are allowed due to the buildup of sugars and the presence of cytokinins. Leaf abscission. Auxins produced in the leaf diffuse downward and prevent abscission. As long as sufficient auxin is reaching the abscission zone abscission will not occur. Phototropism and geotropism. Plants tend to bend and grow toward light and response to gravity because of the movement of auxin resulting in asymmetric distribution of auxin. Auxin mediated ethylene production. Gibberellins (GA): This is a group of hormones that is largely responsible for stem elongation and growth of shoots. A major goal in tree fruit production is to find cultural and chemical means to reduce and counteract the growth-stimulating effects GAs have on plants. There are over 135 known naturally occurring GAs. Each plant species has just a few GAs that are particular to that plant. GA4 and GA7 are dominant GAs in apple. Growth promotion. Limiting GA production in apple is often a goal in pome fruit production. Flower bud formation. GAs are known to be natural inhibitors of flower development in pome fruit. It is largely accepted that GAs produced in the seeds play a major role in inhibiting return bloom. GA7 is particularly inhibitory in apple. Elongation of fruit. GAs as well as cytokinins are known to influence the shape (elongation) of apples. Cytokinins: This is a group of hormones that participates in many physiological processes but their effects are less dramatic than observed for other hormones. They are cell division factors and they help counteract stress in the plant and inhibit senescence. Cytokinins interact with auxins in the regulation of apical dominance; auxins reinforcing apical dominance and cytokinins help overcoming it. Abscisic Acid: While abscisic acid does promote abscission it is far more dominant and plays a more important role in other physiological events. Stomatal Movement and Transpiration. ABA plays a dominant role in regulating stomatal movement which in turn largely determines the rate of transpiration and water loss from a plant. Stress and Senescence. ABA is produced when plants are placed under stress. This may lead to abscission and accelerate senescence. Ethylene: Because ethylene is a gas its acknowledgement as an important plant hormone was ignored for many years. Advance fruit ripening. When fruit ripen they give off ethylene. The start of significant ethylene production (or CO2 evolution) generally signals the physiological marker that indicates that fruit are ripe. This ethylene then speeds the ripening process. Controlling ethylene production and action is key to regulating ripening, fruit quality and storage potential. Promote Abscission. Exogenous application of ethylene or ethylene production in the plant in response to stress will promote abscission. Controlling heat, water and cold stress will alleviate premature leaf abscission. Auxin induced ethylene production. Elevated rates of auxin increase ethylene production in plants. Some responses attributed to auxins may be cause by ethylene such as epinasty. Less Known Plant Hormones Brassinosteroids are required for vegetative growth and pollen tube growth, promote cell division and cell expansion and are required for plant morphological development. Jasmonates are best known for inducing plant defenses against injury due to insect diseases of mechanical injury. 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