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Phytohormon: Auxin a breif description and Biosynthesis
- 1. Rajasthan Agricultural Research Institute Growth and Development of Vegetable Crops VSC - 512 AUXIN Presented to: Dr. S.K. Bairwa Presented by: Aman Kumar Meena M.Sc. Vegetable Science
- 2. Introduction Plant Hormones: These are chemical substances that are produced by plant in low concentration, which regulates the physiological plant processes. Hormones usually moves within plant from site of production to the site of action. Phytohormone term was given by Thimann (1948) There are 2 major classes of phytohormones Phytohormones Growth Promotors Auxin Cytokinin Gibberellin Growth Inhibitors Ethylene Abscisic Acid
- 3. History of Auxin Auxin was the 1st hormone discovered. Charles Darwin 1st noticed the reaction of grass seedling toward the light if tip is covered and uncovered. Fritz W. 1st described auxins and their role in plant growth in 1926. Auxin is derived from Greek word to increase or grow
- 5. Biosynthesis of Auxin Naturally occurring Auxin is in the form of indole-3-acetic acid (IAA). The similarity in structures of Tryptophan amino acid and IAA indicated that tryptophan is the probable precursor. Major sources of auxin are apical shoot, pollen, embryo and developing buds. Indole acetic acid is generated by tryptophan amino acid by several pathways. Two major pathways Of IAA synthesis has been proposed in the pants Tryptophan dependent pathway. Tryptophan independent pathway (observed in Zea mays and Arabidopsis thaliana) Tryptophan Auxin
- 6. Tryptophan dependent biosynthesis IAA is structurally related to the amino acid tryptophan, and early studies on social biosynthesis suggest that tryptophan is the probable precursor. Tryptophan converts to IAA by several pathways: 1. The indole-3-acetamide (IAM) pathway. 2. The indole-3-pyruvic acid (IPA) pathway. 3. The indole-3-acetonitrile (IAN) pathway. 4. The tryptamine (TAM) pathway.
- 7. The indole-3-pyruvic acid (IPA) pathway It involves deamination if tryptophan, followed by decarboxylation reaction to form indole-3-acetaldehyde. Indol-3-acetaldehyde is then oxidized to IAA by IAA dehydrogenase enzyme.
- 8. The indole-3-acetamide (IAM) pathway This pathway uses indole-3-acetamide as an intermediate, this pathway is used by various pathogenic bacteria such as Pseudomonas savastanoi & Agrobacterium tumefaciens. This pathway involves the to enzymes, Tryptophan monooxygenase and indole-3- acetamide hydrolase.
- 9. The indole-3-acetonitrile (IAN) pathway In the indole-3-acetonitrile pathway, tryptophan if first converted to indole- 3-acetaldoxime and then to indole-3- acetonitrile. The enzymes that converts IAN to IAA is nitrilase. This pathway is important in 3 families: Brassicaceae, Poaceae & Musaceae.
- 10. The TAM pathway The tryptamine pathway is similar to the IPA pathway, except that the order of deamination and decarboxylation reaction is reversed and different enzymes are involved. In Lycopersicon (tomato) evidence of both IPA and TAM pathway have been found.
- 11. Physiological Effects of Auxin Cell Elongation Apical Dominance Root Initiation Prevention of Abscission Parthenocarpy Respiration Callus Formation Vascular Differentiation
- 12. The primary physiological effect of auxin in plants is to stimulate the elongation of cells in shoot. A very common example of this can be observed in phototropic curvatures where the unilateral light unequally distributes the auxin in the stem tip (i.e., More auxin on shaded side that on illuminated side). Many theories have been proposed to explain the mechanism of cell elongation probably : By reducing the wall pressure By increasing the permeability of cells to water By an increase in the wall synthesis By inducing the synthesis of RNA and Protein which turn lead to an increase in cell wall plasticity and extension. Cell Elongation
- 13. Apical Dominance Apical or terminal buds of many vascular plants are very active while the lateral buds remain inactive. Removal of apical buds promotes lateral buds to grow. Apical dominance is due to much higher auxin content in the apical buds than lateral buds. Skoog and Thimann (1934) first pointed out that the apical dominance might be under the control of auxin produced at the terminal bud and which is transported downward through the stem to the lateral bud and hinder their growth. (Experiment done on Broad Bean)
- 14. Root Initiation In contrast to the stem, the higher concentration of auxin inhibits the elongation of root but the number of lateral branch roots is considerably increased i.e. the higher conc. of auxin initiates more lateral branch roots. Application of IAA in lanolin paste to the cut end of a young stem results is an early and extensive rooting. This fact is of great practical importance and has been widely utilised to promote root formation in economically useful plants which are propagated by cuttings.
- 15. Prevention of Abscission Auxin prevents the abscission of young leaves and fruits by delaying the formation of abscission zone. When auxin level decline, a abscission layer forms at the base of fruit stalk or petiole. This abscission zone cuts off the water and nutrient supply, causing the leaf and fruit to fall to the ground. Planofix it is a PGR that Contains NAA, it is used to control the fruit drop by preventing the formation of abscission layer.
- 16. Parthenocary Auxin can induce the formation of parthenocarpic fruits. In nature also, this phenomenon is not uncommon and in such cases the concentration of auxins in the ovaries has been found to be higher than in the ovaries of plants which produce fruits only after fertilization. In the latter cases, the concentration of the auxin in ovaries increases after pollination and fertilization.
- 17. Respiration It has been established that the auxin stimulates respiration and there is a correlation between auxin induced growth and an increased respiration rate. According to French and Beevers (1953), the auxin may increase the rate of respiration indirectly though increased supply of ADP(Adenosine diphosphate) by rapidly utilizing the ATP in the expanding cells.
- 18. Callus Formation Besides cell elongation the auxin may also be active in cell division. In fact, in many tissue cultures where the callus growth is quite normal, the continued growth of such callus takes place only after the addition of auxin.
- 19. Vascular Difference Auxin induces vascular differentiation in plant. This has also been confirmed in tissue culture experiments and form studies with transgenic plants. Cytokinins are also known to participate in differentiation of vascular tissues and it is belived that vascular differentiation in plants is probably under the control of both auxin and cytokinins.