Plant Hormones Flashcards
Hormones
--> Naturally occurring small molecules • Active at low concentrations (micro M) • Production and action may be in the same and/or different cells • Multiple effects, complex interactions • Six major classes: Auxin Cytokinins Ethylene Gibberellins Brassinosteroids Abscisic acid
six major classes of hormones
Auxin Cytokinins (similar to Tryptophan in structure) Ethylene (smallest structure) Gibberellins Brassinosteroids (largest structure) Abscisic acid
What processes do plant hormones control?
cell division, cell elongation, and cell differentiation
plant growth and development
- embryogenesis
- seed dormancy
(seeds are prevented from germination even under favorable conditions) - germination (seeds developing into plants)
- apical dominance (central stem is dominant over side stems)
- shoot/ leaf/ root development
- leaf abscission (plants shed their leaves)
- senescence
- transition to flowering
- fruit development
- fruit ripening
- stress responses
- defense against pathogens
- obstacle avoidance
- gravitropism (turning or growth movement of plant in response to gravity)
- phototropism (growth towards a light source)
Plant hormone actions are complex
- some hormones work together, some
work against each other
• end results depend on which hormones,
how much, where, when, what
Auxin
- first plant hormone discovered
1880 by Charles and Francis Darwin - “the power of movement in plants”
suggesting: signal emanating from the tip regulates movement
He noticed that if light is shone on a coleoptile (shoot tip) from one side the shoot bends (grows) toward the light. The ‘bending’ did not occur in the tip itself but in the elongating part just below it. Removing the tip or covering it with foil meant that the shoot could no longer ‘bend’ toward the light. Covering the elongating part of the shoot did not affect the response to light at all!
Darwin Concluded that: “Some influence is transmitted from the tip to the more basal regions of the shoot thereby regulating growth and inducing curvature”
Boysen-Jensen and Paal’s experiments in 1910s
- -> Boysen-Jensen cut the tips off coleoptiles and placed a thin piece of silver or mica between the coleoptile and the lower shoot. The result was that the shoot did not grow or curve toward the light.
- When he repeated the experiment using a block of gelatin / agar instead, the result was that the shoot grew and curved towards the light. Thus he concluded that the Darwin’s ‘influence’ was a water soluble chemical, capable of diffusing through the agar / gelatin from the tip where it was produced to the lower, elongating part of the shoot where it had its effect. This experiment proved that a signal must be transported from the coleoptile to the rest of the plant, as Darwin had originally surmised
–> Arpad Paal set forth to discover the chemical signal. He removed the tips of dark grown coleoptiles and placed them on one side of the cut surface. He observed that the coleoptiles then curved away from the side on which the tips were placed, even though the plants were still in the dark. This result suggested that a substance produced in the coleoptile is transported downward and this substance stimulates growth of the plant
Suggesting: The signal moves down from the tip to the shaded lower part of a plant.
auxin = increase growth
F. W. Went in 1926
Finally in 1926, Frits Went proved that a chemical signal is transported from the coleoptile to the rest of the plant. He removed coleoptiles from Avena sativa and placed them on agar blocks/gelatin. Tips were then discarded, and gelatin cut up into smaller blocks -> He then placed these blocks on top of the tip-less coleoptiles. When the agar block was centered on top the coleoptile grew straight. If the agar block was offset, resulting in an uneven distribution of the chemical on one side, the shoot would curve as though it was growing towards a light source. This proved that the response was due to a water soluble chemical that diffused from the tip of the plant down the dark / shaded side of the coleoptile causing it to curve towards the light.
–> This proved definitively that a chemical substance is produced in the coleoptile and then transported to the rest of the plant to elicit a specific cellular growth response. Thus, the research of the four scientists established the existence of the first plant hormone. Went named this plant hormone auxin (from the Greek word auxein meaning “to grow”)
suggesting: the substance is diffusible and active.
- -> Isolated the substance and called it auxin (greek for to increase or to grow)
What processes do auxins control?
cell elongation, division, and differentiation
- embryogenesis
- seed dormancy
(seeds are prevented from germination even under favorable conditions)
- germination (seeds developing into plants)
- apical dominance (central stem is dominant over side stems)
- shoot/ leaf/ root development
- leaf abscission (plants shed their leaves)
- senescence
- transition to flowering
- fruit development
- fruit ripening
- stress responses
- defense against pathogens
- obstacle avoidance
- gravitropism (turning or growth movement of plant in response to gravity)
- phototropism (growth towards a light source)
auxin physiology
auxin homeostasis, auxin signal transduction
auxin homeostasis
- de novo synthesis (increases auxin levels)
- transport (reduce auxin levels)
- conjugation (reduce)
- catabolism (reduce)
auxin biosynthesis
- mainly synthesized in meristems and young dividing tissues
- By tryptophan-dependent and tryptophan- independent pathways
- ex: Indole-3-acetic acid (IAA)
the tryptophan-DEPENDENT pathway
evidence:
- isotope feeding (measure if auxins are labeled)
- mutant analysis (mutate certain enzymes and observe if auxin levels changed, YUCCA gene)
converting tryptophan to IAA
YUCCA genes
- can affect pathway from tryptophan to conversion to IAA (auxin)
- play a role in tryptophan biosynthesis
• YUCCA genes encode flavin monooxygenase, catalyzing a key step in auxin biosynthesis.
• YUCCAs belong to a multiple gene family.
Mutation in YUCCA
- Mutation in a single YUCCA gene does not affect development.
- Combinational mutations in the YUCCA genes result in dwarf but more branched plants.
- The dwarf phenotypes can be rescued by auxin treatment.
- Overexpression of YUCCAs leads to auxin overproduction.
–> knock-out experiment: remove it
–> reconstruction: over-
expression
–> YUCCA genes play key role in tryptophan biosynthesis
However, plants lacking tryptophan synthase (producing tryptophan) still make IAA! Why?
a tryptophan-independent pathway also contributes to auxin biosynthesis
Why are auxin-deficient mutants not available yet?
too many pathways, not easy to knock out
Auxin transport
- passive flow via phloem : LONG distance (nonpolar) –> from source to sink (from phloem to any part of the plant)
- directional transport via carrier proteins: SHORT distance (polar)
Directional transport via carrier proteins
Polar transport
Influx carriers: AUX1 protein
Efflux carriers: PIN proteins
How do you test that auxin polar transport is important for plant development?
knockout/ disrupt it/ block it
or over-express it
Disrupt auxin polar transport
with:
- chemical inhibitors
- mutants in carrier proteins
the pin1 mutant:
• lacks lateral organs.
• Rescued by exogenous auxin application.
Auxin signal transduction
- Signaling molecule ligand (Auxin)
- Receptor (TIR1)
- Activation of receptor target(s) & second messengers
- Transcription factors (Aux/IAA, ARF)
- Genes regulated by transcription factors
- Plant responses
signal perception (1, 2) —-> transduction (3, 4) —-> signal responses (5, 6)
Why protein degradation?
Central dogma: DNA --> RNA --> Protein • Eliminate damaged proteins • Recycle essential amino acids • Maintain the stoichiometric levels of enzymes • Control the key regulatory proteins
How important is proteolysis for all living organisms?
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids.
– important in digestion,
types of protein degradation
- Proteases: enzymes consisting of a single protein or a small number of proteins that have proteolysis activity, breaks down proteins into amino acids, digestion
- Proteosomes: a large protein complex acts in an ATP-dependent manner for protein degradation
• Ubiquitin-mediated protein degradation
- requires proteosomes, ubiquitin, E1, E2,
E3 proteins
—> The addition of a chain of multiple copies of ubiquitin (UB) targets a protein for destruction by the intracellular protease known as the 26S proteasome, a large complex that breaks down proteins to their constituent amino acids for reuse.
Ubiquitin-mediated protein degradation
- Ubiquitin: small conserved protein, recyclable, mark to- be degraded proteins
- E1, E2, and E3: enzymes for Ubiquitin conjugation
- Proteosome to degrade ubiquitinated proteins
