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General Biology II - Plant Hormones - Lecture Notes | BIOL 1120, Papers of Biology

Material Type: Paper; Professor: Reeder; Class: General Biology II; Subject: Biology; University: Motlow State Community College; Term: Unknown 1989;

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BIOL 1120 REEDER
Plant Hormones
I. CHEMICAL MESSENGERS
A. Plant Hormone Characteristics
1. defined: an organic compound that acts as a highly specific chemical signal between cells
2. are effective in extremely small amounts
3. each hormone elicits different responses
4. effects of different plant hormones overlap (synergistic effect)
a. apical dominance: auxins, ethylene, cytokinins, abscisic acid
b. fruit development: auxins, gibberellins, cytokinins, ethylene, abscisic acid
c. seed germination: abscisic acid, gibberellins
5. may stimulate a certain response at one concentration and inhibit that same response at a
different concentration
6. ten plant hormones and hormone-like signaling molecules:
a. hormones: auxins, gibberellins, cytokinins, ethylene, abscisic acid
b. signaling molecules: brassinolide, salicylic acid, systemin, oligosaccharins, jasmonic
acid
7. similar in basic mechanism of action to animal hormones
a. bind to specific receptor proteins on the plasma membrane or in target cells
1) each three-dimensionally shaped receptor binds with only one kind of hormone
molecule
b. sometimes the binding triggers the production of a second messenger which is an
intracellular signaling molecule that affects the cell function
1) ions such as Ca2+ or cyclic AMP (cAMP)
2) the second messenger’s increase may bind to proteins and activate or inactivate
certain enzymes
3) activation or inactivation may lead to altered membrane permeability and/or altered
gene expression (transcription and/or translation)
II. THE MAJOR PLANT HORMONES
A. Auxins
1. Produced in the shoot apical meristem, young leaves, and seeds
a. naturally occurring form is called indoleacetic acid
2. Principal actions in stem elongation, apical dominance, root initiation, and fruit development
3. Plays a major role in apical dominance by the suppression of lateral bud growth as it diffuses
downward from the terminal bud’s apical meristem.
a. This is dependent on how active the growing tissue is as an auxin excess is growth
inhibiting and will starve the laterals of essential nutrients.
b. Auxin influences cells around the lateral buds to produce the hormone ethylene which
inhibits the lateral bud growth; abscisic acid accumulates during aging to suppress buds,
also.
c. The hormone, cytokinin, may also play a part in apical dominance through its mitosis-
promoting action whereas dormant laterals may contain insufficient amounts;
antagonistic to auxin: promotes growth of lateral buds while auxin inhibits.
d. When the terminal bud is removed, the lateral buds grow resulting in a bushy plant; the
number of flowers on an individual plant may be increased also in this way.
4. Stem elongation role:
a. Low concentrations of auxin promote cell wall plasticity; therefore it must be broken
down rapidly to prevent its accumulation
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BIOL 1120 REEDER

Plant Hormones

I. CHEMICAL MESSENGERS

A. Plant Hormone Characteristics

  1. defined: an organic compound that acts as a highly specific chemical signal between cells
  2. are effective in extremely small amounts
  3. each hormone elicits different responses
  4. effects of different plant hormones overlap (synergistic effect) a. apical dominance: auxins, ethylene, cytokinins, abscisic acid b. fruit development: auxins, gibberellins, cytokinins, ethylene, abscisic acid c. seed germination: abscisic acid, gibberellins
  5. may stimulate a certain response at one concentration and inhibit that same response at a different concentration
  6. ten plant hormones and hormone-like signaling molecules: a. hormones: auxins, gibberellins, cytokinins, ethylene, abscisic acid b. signaling molecules: brassinolide, salicylic acid, systemin, oligosaccharins, jasmonic acid
  7. similar in basic mechanism of action to animal hormones a. bind to specific receptor proteins on the plasma membrane or in target cells 1) each three-dimensionally shaped receptor binds with only one kind of hormone molecule b. sometimes the binding triggers the production of a second messenger which is an intracellular signaling molecule that affects the cell function 1) ions such as Ca2+^ or cyclic AMP (cAMP) 2) the second messenger’s increase may bind to proteins and activate or inactivate certain enzymes 3) activation or inactivation may lead to altered membrane permeability and/or altered gene expression (transcription and/or translation)

II. THE MAJOR PLANT HORMONES A. Auxins

  1. Produced in the shoot apical meristem, young leaves, and seeds a. naturally occurring form is called indoleacetic acid
  2. Principal actions in stem elongation, apical dominance, root initiation, and fruit development
  3. Plays a major role in apical dominance by the suppression of lateral bud growth as it diffuses downward from the terminal bud’s apical meristem. a. This is dependent on how active the growing tissue is as an auxin excess is growth inhibiting and will starve the laterals of essential nutrients. b. Auxin influences cells around the lateral buds to produce the hormone ethylene which inhibits the lateral bud growth; abscisic acid accumulates during aging to suppress buds, also. c. The hormone, cytokinin, may also play a part in apical dominance through its mitosis- promoting action whereas dormant laterals may contain insufficient amounts; antagonistic to auxin: promotes growth of lateral buds while auxin inhibits. d. When the terminal bud is removed, the lateral buds grow resulting in a bushy plant; the number of flowers on an individual plant may be increased also in this way.
  4. Stem elongation role: a. Low concentrations of auxin promote cell wall plasticity; therefore it must be broken down rapidly to prevent its accumulation
  1. Enzyme indoleacetic acid oxidase is utilized by the plant to precisely control its growth b. A more plastic cell wall will stretch more during active cell growth
  1. Fruit development role: a. produced by developing seeds, the pollen grain, and the ovulary wall to stimulate fruit development b. when applied to flowers in which fertilization has not occurred (no seeds), the ovary enlarges and develops into a seedless fruit
  2. Roles in gravitropism and phototropisms of stems and roots a. Stimulation of cell elongation (not of mitosis): cell wall acidification 1) In a gravitropic stem response (negative): if a stem is placed horizontally, auxin, under gravity’s influence accumulates on the lower side bringing about a greater rate of growth on that side bending the stem upward; abscisic acid (inhibiting) on the upper stem’s side. 2) In the phototropic stem response (positive): more auxin is concentrated on the side away from the light 3) The stem responds to light by bending in the zone of elongation (on the plant side away from the light) so that the shoot grows toward the light 4) When the shoot has grown so that light falls uniformly, the shoot resumes growth straight in one direction upward; auxin is now uniformly distributed
  3. Roles of synthetic auxins: a. Utilized as weed killers in herbicides when applied in higher concentrations than those at which auxin normally occurs in plants; “grow to death” by disrupting normal hormonal balance 1) 2,4-D for broad-leaved dicots; 2,4,5-T for woody seedlings and weeds (contains dioxin, “Agent Orange” in Vietnam); for reasons unknown, does not harm monocots a. 2,4,5-T is no longer allowed in the U.S. b. used to counteract the effect of hormones that promote fruit droppings (pears, apples, etc.) so as to further ripen c. used to stimulate root development on stem cuttings for asexual propagation: naphthalene acetic acid B. Gibberellins
  4. Produced in young leaves, shoot apical meristems, and seed embryos a. are more than 110 naturally occurring gibberellins and no synthetic ones b. occurs in green and brown algae, vascular plants and fungi
  5. Principal actions include seed germination, stem elongation, flowering, and fruit development
  6. Stem elongation: promotes rapid growth by stimulating cells to divide as well as elongate a. elongation inducement differs from auxin b. induces flowering via bolting
  1. rapid elongation of a floral stalk when flowering is initiated
  1. Hastening of seed germination a. mobilizes food reserves (starch of the endosperm) enzymatically for the embryo in newly germinated grass seeds b. in cereal types and grasses
  2. Like auxin, affects fruit development
  3. Artificial applications: a. Induce flowering:
  1. can substitute for the low temperature that biennials require before flowering initiation b. Enhance fruit development:
  2. applied to several grape varieties to produce larger fruits

b. high levels of abscisic acid in seed tissues inhibit germination until it is washed out

  1. water is significant to stimulate seed germination: washes out abscisic acid c. gibberellins also involved: reverses effects of dormancy d. cytokinins also involved: breaking dormancy
  1. Positive geotrophic response in roots (inhibits growth where more concentrated) a. if the root grows straight down, abscisic acid is evenly concentrated on all root sides b. if root is horizontal, its concentration is higher on the lower root side, cells on upper side elongate bending the root downward c. this response is also dependent on amyloplasts because of their high density and their accumulation at the bottoms of the cells responding to gravity’s pull
  2. Artificial application to germinating seeds leads to dormancy F. Additional Classes of Molecules
  3. Brassinosteroids a. site of production unknown b. principal actions include light-mediated gene expression, cell division, stem elongation, flower development, and leaf senescence
  4. Salicylic acid a. produced in a plant wound (infection site) b. principal action is resistance to disease organisms
  5. Systemin a. produced in a plant wound (site of herbivore or pathogen attack) b. principal action is defense initiation against predators (herbivores) or disease organisms
  6. Oligosaccharins a. site of production unknown b. may function in normal cell growth and development and possibly defense responses to pathogens
  7. Jasmonates a. produced possibly in leaves and probably many other tissues b. principal action is defense initiation against predators or pathogens