Chapter 37 - Plant Hormones

Chapter 37 - Plant Hormones - Chapter 37 T ropisms I)...

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Chapter 37 Tropisms I) Tropisms a. A plant responds to an external stimulus by directional growth b. Can be negative or positive depending if it grows towards (positive) or away (negative) from the stimulus c. Phototropism i. Directional growth caused by light ii. Most exhibit positive phototropism iii. Triggered by blue light (wavelength < 500 nm) iv. Photoreceptors 1. A light sensitive substance that absorbs light 2. Phototropins a. Light activated Kinases i. Enzymes that transfer phosphate groups b. Yellow pigments that absorb blue light and triggers phototropic response c. Phototropins become phosphorylated with blue light 3. Gravitropism a. Growth in response to the direction of gravity b. Involves the hormone auxin 4. Thigmotropism a. Growth in response to a mechanical stimulus (e.g. contact with solid object)
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II) Plant Hormones and Development a. Plant Hormones Background i. An organic compound that acts as a chemical signal cliciting a variety of responses that regulate growth and development ii. Affects of hormones, in plants, are usually felt in areas where the hormones are produced iii. Usually very small and simple molecules iv. List of Hormones 1. Table 37-1 Page 791 b. Spreading Plant Hormones i. Plant hormones act by signal transduction ii. Signal Transduction 1. Receptor converts an extracellular signal to a intracellular signal 2. Causes some change in the cell iii. Many plant hormones bind to enzyme-linked receptors located on the plasma membrane of plant cell iv. Mechanism of Action for Hormone Auxin 1. Receptor for auxin (TIR1: transport inhibitor response 1) binds with auxin a. TIR1 is an F-box Protein i. Short sequence of amino acids found in molecules that catalyzes the addition of ubiquitin tags to proteins targeted for destruction ii. Evolved early in eukaryotic cells
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2. Attachment of receptor and auxin catalyzes the attachment of the molecule ubiquitin to repressor proteins that inhibit certain genes 3. Ubiquitin attaches with protein molecules that will be destroyed 4. Genes that were repressed by those repressor proteins are now activated, resulting in changes in cell growth and development c. Auxins Promote Cell Elongation i. Found by Darwin and his son Francis in the 1870’s in experiments involved with positive phototropism 1. Page 793 for experiment details ii. Purification and elucidation of the structure were done by Kenneth Thimann iii. Auxin 1. Group of natural and artificial plant hormones 2. Indoleacetic Acid (IAA) is most important a. Movement of auxin in the plant is said to be polar, or unidirectional 3. Auxin moves downward along the shoot, from the site
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Chapter 37 - Plant Hormones - Chapter 37 T ropisms I)...

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