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Phytohormones auxin & ethylene synthesis and effects
- 1. Phytohormones Auxin & Ethylene BIOSYNTHESIS & THEIR FUNCTIONS BY MAHAM ADNAN L1F16BSBC0027
- 2. PHYTOHORMONES Plant hormones are a group of naturally occurring, organic substances which influence physiological processes at low concentrations. The processes influenced consist mainly of growth, differentiation, development and stomatal movement are affected. The synthesis of plant hormones may be localized, but may also occur in a wide range of tissues, or cells. They may be transported and have their action at a distance.
- 3. CLASSIFICATION Phytohormones consist of five classes: 1. Auxins, 2. Abscisic acid, 3. Cytokinins, 4. Gibberellins, 5. Ethylene —as well as their precursors and synthesized analogs.
- 4. Phytohormone- Classification & Function
- 5. AUXIN: THE GROWTH HORMONE Auxin is indole-3-acetic acid (IAA). Defined as compounds with biological activities similar to those of IAA, including the ability to promote cell elongation in coleoptile and stem sections. A common feature of all active auxins is a molecular distance of about 0.5 nm between a fractional positive charge on the aromatic ring and a negatively charged carboxyl group.
- 6. DISCOVERY OF AUXIN 1. Charles Darwin In 1880, Charles Darwin and his son Francis performed experiments on coleoptiles, the sheaths enclosing young leaves in germinating grass seedlings. The experiment exposed the coleoptile to light from a unidirectional source, and observed that they bend towards the light. Darwin’s experiment Coleoptile seeding
- 7. 2. Peter Boysen-Jensen In 1913, Danish scientist Peter Boysen- Jensen demonstrated that the signal was not transfixed but mobile. 3. Frits Went In 1926, the Dutch botanist Frits Warmolt Went showed that a chemical messenger diffuses from coleoptile tips. Went's experiment identified how a growth promoting chemical causes a coleoptile to grow towards light.
- 8. SITE OF SYNTHESIS & TRANSPORT IAA is synthesized from indole primarily in leaf primordia and young leaves, and in developing seeds. IAA biosynthesis is associated with rapidly dividing and rapidly growing tissues, especially in shoots. Shoot apical meristems, young leaves, and developing fruits and seeds are the primary sites of IAA synthesis IAA transport is cell to cell, mainly in the vascular cambium and the procambial strands, but probably also in epidermal cells. Transport to the root probably also involves the phloem.
- 9. BIOSYNTHESIS Multiple Pathways Exist for the Biosynthesis of IAA. 1. The IPA (Indole-3-pyruvic Acid) pathway 2. The TAM (Tryptamine) pathway 3. The IAN (Indole-3-acetonitrile) pathway 4. The Bacteria Pathway
- 11. 1. The IPA pathway It involves a deamination reaction to form IPA, followed by a decarboxylation reaction to form indole-3-acetaldehyde (IAld). Indole-3-acetaldehyde is then oxidized to IAA by a specific dehydrogenase. 2. The TAM pathway The order of the deamination and decarboxylation reactions is reversed, and different enzymes are involved. Species that do not utilize the IPA pathway possess the TAM pathway. In at least one case (tomato), there is evidence for both the IPA and the TAM pathways
- 12. 3. The IAN pathway In the indole-3-acetonitrile (IAN) pathway, tryptophan is first converted to indole-3- acetaldoxime and then to indole-3-acetonitrile. The enzyme that converts IAN to IAA is called nitrilase. The IAN pathway may be important in only three plant families: the Brassicaceae (mustard family), Poaceae (grass family), and Musaceae (banana family). 4. The Bacteria Pathway It uses indole-3-acetamide (IAM) as an intermediate and is used by various pathogenic bacteria, such as Pseudomonas savastanoi and Agrobacterium tumefaciens. This pathway involves the two enzymes tryptophan monooxygenase and IAM hydrolase. The auxins produced by these bacteria often elicit morphological changes in their plant hosts.
- 13. EFFECTS 1. Cell enlargement 2. Cell division 3. Vascular tissue differentiation 4. Root initiation 5. Tropistic responses
- 15. 6. Apical Dominance 7. Leaf and fruit abscission 8. Fruit setting and growth 9. Fruit ripening 10. Flowering In several systems (e.g., root growth) auxin, particularly at high concentrations, is inhibitory.
- 16. ETHYLENE: THE GASEOUS HORMONE Ethylene (CH2=CH2) is synthesized from methionine in many tissues in response to stress, and is the fruit ripening hormone. It does not seem to be essential for normal mature vegetative growth, as ethylene-deficient transgenic plants grow normally. It is the only hydrocarbon with a pronounced effect on plants. Ethylene structure
- 17. PROPERTIES Ethylene is the simplest known olefin. Its molecular weight is 28, and it is lighter than air under physiological conditions. It is flammable and readily undergoes oxidation. Ethylene can be oxidized to ethylene oxide. In most plant tissues, ethylene can be completely oxidized to CO2. Ethylene is released easily from the tissue and diffuses in the gas phase through the intercellular spaces and outside the tissue.
- 18. SITE OF SYNTHESIS AND TRANSPORT Ethylene is synthesized by most tissues in response to stress. It is synthesized in tissues undergoing senescence or ripening. Meristematic regions and nodal regions are the most active in ethylene biosynthesis. Ethylene moves by diffusion from its site of synthesis. 1-aminocyclopropane-1-carboxylic acid (ACC) can be transported and may account for ethylene effects at a distance from the causal stimulus.
- 19. BIOSYNTHESIS
- 20. BIOSYNTHESIS Plant tissues convert l- [14C]methionine to [14C]ethylene, and that the ethylene is derived from carbons 3 and 4 of methionine. The CH3—S group of methionine is recycled via the Yang cycle. Without this recycling, the amount of reduced sulfur present would limit the available methionine and the synthesis of ethylene. S-adenosylmethionine (AdoMet), which is synthesized from methionine and ATP, is an intermediate in the ethylene biosynthetic pathway, and the immediate precursor of ethylene is 1-aminocyclopropane-1-carboxylic acid (ACC). ACC is the immediate precursor of ethylene in higher plants and that oxygen is required for the conversion. ACC synthase, the enzyme that catalyzes the conversion of AdoMet to ACC , has been characterized in many types of tissues of various plants.
- 21. ACC synthase is an unstable, cytosolic enzyme. Its level is regulated by environmental and internal factors, such as wounding, drought stress, flooding, and auxin. ACC oxidase catalyzes the last step in ethylene biosynthesis: the conversion of ACC to ethylene. In tissues that show high rates of ethylene production, such as ripening fruit, ACC oxidase activity can be the rate-limiting step in ethylene biosynthesis. ACC oxidase might require Fe2+ and ascorbate for activity. Ethylene biosynthesis is stimulated by several factors, including developmental state, environmental conditions, other plant hormones, and physical and chemical injury.
- 23. EFFECTS The effects of ethylene are : Maintenance of the apical hook in seedlings. Stimulation of numerous defense responses in response to injury or disease. Release from dormancy. Fruit ripening
- 24. Shoot and root growth and differentiation. Leaf and fruit abscission. Flower opening. Flower and leaf senescence.
- 27. THANK YOU!
Editor's Notes
- A variety of other signaling molecules that play roles in resistance to pathogens and defense against herbivores have also been identified, including jasmonic acid, salicylic acid, and the polypeptide systemin
- Structure of three natural auxins. Indole-3-acetic acid (IAA) occurs in all plants, but other related compounds in plants have auxin activity. Peas, for example, contain 4-chloroindole-3-acetic acid. Mustards and corn contain indole-3-butyric acid (IBA).
- The apex (tip) of the shoot contains the apical meristem within the apical bud. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth (analogous to stem cells in animals). Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant.
- Cell enlargement - auxin stimulates cell enlargement and stem growth Cell division - auxin stimulates cell division in the cambium and, in combination with cytokinin, in tissue culture Vascular tissue differentiation - auxin stimulates differentiation of phloem and xylem Root initiation - auxin stimulates root initiation on stem cuttings, and also the development of branch roots and the differentiation of roots in tissue culture Tropistic responses - auxin mediates the tropistic (bending) response of shoots and roots to gravity and light Leaf and fruit abscission - auxin may inhibit or promote (via ethylene) leaf and fruit abscission depending on the timing and position of the source Fruit setting and growth - auxin induces these processes in some fruit Fruit ripening - auxin delays ripening Flowering - auxin promotes flowering in Bromeliads In several systems (e.g., root growth) auxin, particularly at high concentrations, is inhibitory.
- Aminoethoxy-vinylglycine (AVG) and aminooxyacetic acid (AOA) block the conversion of AdoMet to ACC (see Figure 22.1). AVG and AOA are known to inhibit enzymes that use the cofactor pyridoxal phosphate. The cobalt ion (Co2+) is also an inhibitor of the ethylene biosynthetic pathway, blocking the conversion of ACC to ethylene by ACC oxidase, the last step in ethylene biosynthesis. Carbon dioxide at high concentrations (in the range of 5 to 10%) also inhibits many effects of ethylene, such as the induction of fruit ripening, although CO2 is less efficient than Ag+.
- Etiolated dicot seedlings are usually characterized by a pronounced hook located just behind the shoot apex . This hook shape facilitates penetration of the seedling through the soil, protecting the tender apical meristem. Although ethylene inhibits flowering in many species, it induces flowering in pineapple and its relatives, and it is used commercially in pineapple for synchronization of fruit set. Flowering of other species, such as mango, is also initiated by ethylene. senescence is a genetically programmed developmental process that affects all tissues of the plant. Enhanced ethylene production is associated with chlorophyll loss and color fading, which are characteristic features of leaf and flower senescence. ethylene production generally increases in response to pathogen attack in both compatible (i.e., pathogenic) and noncompatible (nonpathogenic) interactions. The shedding of leaves, fruits, flowers, and other plant organs is termed abscission. Abscission takes place in specific layers of cells, called abscission layers, which become morphologically and biochemically differentiated during organ development. 3. Seeds that fail to germinate under normal conditions (water, oxygen, temperature suitable for growth) are said to be dormant. Ethylene has the ability to break dormancy and initiate germination in certain seeds, such as cereals. In addition to its effect on dormancy, ethylene increases the rate of seed germination of several species.