LECTURE 17 – PLANT HORMONES PART TWO Plant Hormones: GIBBERELLINS • Discovered by Japanese scientists in 1926, when studying a disease of rice caused by fungal pathogen, Gibberella fujikuroi, that resulted in spindly, palecoloured and sickly feelings “foolish seedlings” • In 1934, it was isolated and named gibberellins (GA) • Most plants contain 10 or more Gas • To date over 136 different gibberellins have been isolated • GAs are present in various amounts in all parts of the plant but are in highest concentrations in immature and germinating seeds **GAs are synthesized in apical meristems, young leaves and embryos (like auxins). They are one of several types of hormones that promote stem elongation. • Exogeneous application of GAs can reverse dwarfism • GAs play a role in both embryo growth and seed germination • GAs play a role in flowering in some plants • GAs contribute to fruit formation GAs promote Stem Elongation • Application of GA stimulates stem elongation in dwarf plants • GA promotes stem elongation via modification of wall extensibility by stimulating xyloglucan endotransglycosylase (XET) • GA enhances expansin synthesis • GA promotes a transverse arrangement of microtubules Gibberellins and Seed Dormancy • The seeds of many plants require a period of dormancy before they will germinate • In some plants a cold period must be experienced before seeds will germinate (known as stratification) while others in light is required to break dormancy • GAs can substitute for either vernalization or lightinduction of germination Gibberellins and Germination – In barley seeds, GAs play an important role in mobilizing food reserves through the action of hydrolytic enzymes (aamylase, hydrolases)
Gibberellins and Fruit Growth – have an important commercial application. When applied to developing bunches of grapes, GAs promote elongation of stem internodes and increase grape size (more room to grow and larger fruit). Plant Hormones: ABSCISIC ACID • Discovered by Paul Wareing in 1949 who found that buds contained a growth inhibitory substance which he called “dormin” • In 1960s, Frederick Addicott found that cotton leaves and fruit contained a substance capable of accelerating abscission which he called “abscising” • Dormin and abscising were later shown to be chemically indentical
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Today it is known as abscisic acid or ABA
Abscisic Acid (ABA) – it is known that ABA plays no direct role in abscission (ABA stimulates ethylene production) • Synthesized in cells that contain plastids (chloroplasts or amyloplasts)
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Transported by phloem and xylem ABA plays a major role in seed development ABA stimulates the production of seed storage proteins It promotes seed dormancy and inhibits seed germination
Mutants that lack ABA or are insensitive to ABA fail to stay dormant (examples, mangroves and viviparous mutants of maize) **ABA plays a role in roottoshoot signaling Under water stress conditions (drought, salt, freezing) roots increase ABA biosynthesis, releasing it into xylem and causing closure of leaf stomata • Mutants incapable of synthesizing ABA have a wilty phenotype and must be grown under conditions of high humidity GA and ABA have antagonistic effects in several important processes 1. Seed germination 2. Floral transition 3. Fruit Development **GA promotes these processes while ABA inhibits them Plant Hormones: ETHYLENE • Effects discovered before auxin
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History goes back to “illuminating gas” that was used in the 1800s to light lamps on city streets Leaks of illuminating gas caused defoliation of shade trees along streets In 1901, Dimitry Neljobov demonstrated that the gas, ethylene, was the active component in illuminating gas
Ethylene – is synthesized from the amino acid, methionine, giving rise to ACC(1aminocyclopropane1 carboxylic acid) • ACC is converted to ethylene, CO2, and ammonium ion by enzymes on the tonoplast
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The formation of ACC is the step that is affected by those treatments that stimulate ethylene production
Ethylene and Abscission – promotes shedding of leaves, flowers and fruits in many plant species (which was originally believed to be caused by ABA)
• Ethylene triggers the enzymes to cause cell wall to break down • In many plants, however, abscission is controlled by an interaction between auxin and ethylene • Auxin seems to decrease the sensitivity of abscission zone cells to ethylene Ethylene and Touch
Ethylene – synthesis in all plant parts (ripening of fruits, senescing leaves) • Meristematic and nodal regions tend to be most active
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Increases during leaf abscission, flower senescence and fruit ripening
Also produced in response to stress: wounding, flooding, chilling, disease, temperature or drought stress. • Can be transported in intracellular air spaces, and outside the plant **It enables plants to adapt to underground obstacles by initiating a response known as the triple response 1. Slowing of stem or root elongation 2. Thickening of the stem or root 3. Curving to grow horizontally
Ethylene: cell expansion – induces lateral cell expansion by changing microtubule (MT) orientation (from a transverse orientation to a vertical orientation) • Shift in MTs lead to a change in cellulose microfibril deposition Ethylene and Fruit Ripening • Ripening involves a number of changes such as degradation of chlorophyll, softening of flesh by enzymatic digestion of middle lamella and synthesis of sugars from starches, organic acids or oils
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Some fruits have a large, rapid increase in ethylene production that precedes a sharp increase in cellular respiration (release of CO2) • Such fruits are known as climacteric fruits because of this peak in respiration (apples, avocados, bananas, mangoes, peaches, tomatoes) nonclimacteric fruits (cherries, citrus) • Fruit growers use ethylene to control fruit ripening **Ethylene regulates fruit ripening Plant Hormones: BRASSINOSTEROIDS • Newly discovered group of hormones that act like auxin
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They were first discovered in the genus Brassica and hence were named Brassinosteroids These compounds bind to plasma membrane receptor protein but do not enter the cell Brassinosteroids stimulate cell division and elongation in stems, cause xylem differentiation, promote pollen tube growth, slow root growth, enhance ethylene synthesis and delay senescence Mutants defective in brassinsteroid pathways look like auxin mutants
Plant Hormones – protein degredation, phosphorylation and RNA processing play important roles in hormone signaling
Plant Hormones: OTHER COMPOUNDS