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Final

Bio120- lecture review notes for final.docx

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Department
Biology
Course
BIOL 120
Professor
Simon Chuong
Semester
Winter

Description
16) Plant Hormones Bianca Rubino  Plant hormones direct growth and development  Stimulate and inhibit responses  Depends on concentration, location, and timing  A Key Component in plant communication  Some hormones act in the same tissues where they are produced  Can act by binding a protein thereby initiating a response cascade  Response can be positive or negative; developmental or growth responses integrate Plant Hormones  Auxins, Cytokinins, Gibberellins, Abscisic Acid, Ethylene, Brassinosteroids Auxins  First hormone discovered  Involved in cell elongation  Darwin and son, Francis, founded in 1880s  Studying phototropism  In 1913, Peter Boysen-Jensen  Signal can pass through permeable agar but not impermeable mica  In 1926, Fritz Went  Found an active substance accumulating in the agar that caused growth on coleoptiles w/ tips removed  Structural formula is that of indoleacetic acid (IAA)  There are some synthetic (more stable) and natural Auxins (IAA is most common)  IAA is a modified amino acid derived from tryptophan  Produced mainly in the Shoot Apical Meristem (SAM), young leaves, and developing fruits/seeds  All tissues are capable of producing low levels of IAA  Move by diffusion and polar transport  Polar transport in the shoot (basipetal) and in the root (acropetal)  Main route of polar auxin in stems/leaves is via parenchyma cells  In leaves, can be transported nonpolarly in the sieve tubes of phloem  In the root, auxin moves in the sieve tubes toward the tip and is then redirected to the epidermis and cortex where it is transported basipetally (toward the root-shoot junction)  Auxin is implicated in many aspects of plant development  Thought to define the root-shoot axis – established during embryogenesis  Short term affect = stimulates cell elongation  Important role in the differentiation of the vascular tissues  Promotes the joining of vascular leaf traces OR re-differentiation of lost vascular tissue  Activates H+ATPase in cell membrane to pump protons into cell wall  Decrease in wall pH activates enzyme expansins  Expansins cleave bonds between cellulose and hemicellulose weakens cell wall  AKA acid growth hypothesis  Promotes lateral root development  Stimulates the Pericycle in lateral root formation and the vascular and cork cambium in 2ndary growth  Involved in apical dominance (suppression of axillary bud growth)  Removal of shoot apex „releases‟ axillary buds  Strigolactones have been identified as the novel signals that act downstream of the auxin signaling to inhibit branching  Promotes fruit development  Parthenogenic fruit formation can be stimulated by the exogenous application of auxins  Developing seeds are a source of auxin without which promotes maturation of the ovary wall  Location of cells synthesizing auxin changes as the leaves mature reflecting a shift in cell maturation  Synthetic Auxins  Among the first herbicides developed (2,4-D)  (NAA) is commonly used to induce the formation of adventitious roots in cuttings and reduce fruit drop  Basis of Agent Orange (Vietnam war)  2,4-D and NAA are not as readily degraded as IAA by IAA-oxidase o Plants are treated with these to retain artificially high levels of these agents  Effects can be lethal  Agents are effective at selectively killing broad-leaf plants – used on grass Cytokinins  Discovered by Johannes van Overbeek, 1941  Its discovery laid the groundwork for in vitro methods for plant propagation  50‟s – Miller, Skoog and coworkers identified a purine compound – Kinetin  named the group of growth regulators cytokinins bc of involvement with cytokinesis  Zeatin – isolated from maize kernels, most active of the naturally occurring cytokinins  Usually a modified form of adenine  Synthesized actively dividing tissues such as seeds/fruits/leaves/roots  Transported through xylem  Decay leaf senescence  Direct amino acids to locations of higher cytokinin concentration  Promote cell division in the shoot apical meristem  The ratio of Auxins: Cytokinins regulates the production of roots and shoots in tissue cultures  Application of cytokinins to axillary buds induces cell division and promotes branching  Auxin : cytokinin ratio  Cytokinins alone have little effect  Auxin alone promotes cell elongation  Low ratio –give rise to roots in tissue culture  Equal ratio –give rise to undifferentiated cells (callus)  High ratio –cause cells to divide and differentiate into shoot buds  Effects depend on type of plant tissue and plant species  Gottlieb Haberlandt – recognized the Totipotency of plant cells in 1905  Tissue culture method developed in the 60‟s by culturing cells/tissues/organs in artificial medium containing nutrients and hormones (auxin : cytokinins) 17) Plant Hormones Continued Bianca Rubino Gibberellins (GA)  Discovered in 1926 by Japanese Scientist Kurosawa  1934, Yabuta and Sumiki isolated and named it  plants contains 10 or more GAs  Over 136 GA‟s have been isolated- higher concentration in immature and germinating seeds – various amounts throughout the plant as well  Synthesized in apical meristems, young leaves and embryos  Promote cell elongation  Exogenous application can reverse dwarfism  Stimulate stem elongation via modification of wall extensibility by stimulating xyloglucan endotransglycosylase (XET)  Enhances expansin synthesis  Promotes a transverse arrangement of microtubules  Contribute to fruit formation  Plays a role in embryo growth and seed germination  Role in flowering of some plants Seed Dormancy  Seeds of many plants require a period of dormancy before they will germinate  Cold period (vernalization) must be experienced before seeds will germinate OR light is required to break dormancy  GAs can substitute in for both cold or light Germination  In barley seeds, GAs play an important role in mobilizing food reserves through the action of hydrolytic enzymes Fruit Growth  Important commercial application  When applied to developing branches of grapes, it promotes elongation of stem internodes and increase in grape size Abscisic Acid (ABA)  Discovered by Paul Wareing in 1949  Dormin and abscisin (founded by Frederick Addicott in the 60s) are chemically identical  Plays no direct role is abscission – it stimulates ethylene production  Synthesized in cells that contain plastids  Transported by xylem and phloem  Plays a major role in seed development  Stimulates production of seed storage proteins  Promotes seed dormancy and inhibits seed germination  Mutants (lacking ABA) fail to become dormant (viviparous)  Plays a role in root-to-shoot- signaling  Under water stress, roots increase ABA biosynthesis, release ABA into xylem so that the leaf stomatas are closed  Mutants have a wilty phenotype and must be grown under conditions of high humidity GA and ABA  Have antagonistic effects on  Seed germination  Floral transition  Fruit development  GA promotes these while ABA inhibits them Ethylene  Effects discovered before Auxins  “Illuminating gas” 1800s  leaks of this gas caused defoliation of shade trees along streets  in 1901, Dimitry Neljobov demonstrated that the gas, ethylene, was the active component of the illuminating gas  Synthesized from the amino acid, methionine, giving rise to ACC  ACC is converted into ethylene, CO2, and ammonium ion by enzymes on the tonoplast  Treatments that stimulate ethylene productions affect the formation of ACC  Promotes shedding (abscission) of leaves, flowers, fruits  Triggers the enzymes that cause cell walls to break down  Abscission is controlled by an interaction between auxin and ethylene  Auxin decreases the sensitivity of abscission zone cells to ethylene  Touch or physical stress induces ethylene production  Synthesized in all parts of plants  Meristematic and nodal regions tend to be most active  Increases during leaf abscission, flower senescence and fruit ripening  Produced in response to stress  Can be transported in intercellular air spaces, and outside the plant  Enables plants to adapt to underground obstacles by initiating a response known as triple response  Slowing of stem/root elongation; thickening of stem/root; curving to grow horizontally  Induces lateral response  Changes microtubule orientation – this leads to change in cellulose microfibril deposition  Some fruits have a large, rapid increase in ethylene production that precedes a sharp increase in cellular respiration (release of CO2)  AKA climacteric fruits (apples, bananas, tomatoes)  Nonclimacteric fruits (citrus, grapes, watermelon)  Fruit growers use ethylene to control fruit ripening Brassinosteroids  Newly discovered – act like Auxin  Bind to plasma membrane receptor proteins but don‟t enter the cell  Stimulate cell division and elongation in stems  Cause xylem differentiation  Promote pollen tube growth  Slow root growth  Enhance ethylene synthesis  Delay senescence  Mutants (lacking the hormone) look like auxin mutants Plant Hormones  Protein degradation, phosphorylation and RNA processing play important roles in hormone signaling  Auxin binds directly to a cellular factor involved in protein degradation  Ethylene, cytokinins and brassionsteroids bind to membrane receptors and mediate signaling through phosphorylation events  Brassionsteroids and GAs indirectly regulate protein stability Other Compounds  Polyamines and Jasmonic acid appear to have hormone-like activity Polyamines  Promote cell division, synthesis of DNA, RNA and proteins  Involved in root initiation, tuber formation, and are involved in embryo, flower, and fruit development  More abundant in plants than any other hormone!  Millimolar amounts are needed to produce a biological response Jasmonic Acid  Inhibits growth of seeds and is active in plant defense again pathogenic organisms  Stimulates formation of flower, fruit and seeds  Promotes accumulation of proteins during seed development  Methyl-jasmonate may serve as a volatile signal that stimulates defense responses in neighbouring plants (interplant comm)  Methyl-jasmonate and cis-jasmone se
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