Study Guides (248,234)
Canada (121,419)
Biology (1,099)
BIOL 120 (21)

Bio120- lecture review notes for final.docx

14 Pages
Unlock Document

BIOL 120
Simon Chuong

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
More Less

Related notes for BIOL 120

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.