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
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)
receptor: Transport inhibitor response 1 (TIR1)
TIR1 as an auxin receptor:
• TIR1 binds to auxin and the binding is enhanced by auxin.
• Mutations in TIR1 AFFECT auxin RESPONSE but NOT auxin BIOSYNTHESIS and TRANSPORT
• Mutant phenotypes CANNOT be rescued by auxin treatment
Additional information on TIR1:
• TIR1 is a component in the E3 ubiquitin protein ligase complex.
TIR1 receptor mutation
affects: auxin response
does not affect: auxin biosynthesis, auxin transport
- mutants cannot be rescued by auxin treatment
Transcriptional control of auxin response
- AUX/IAA proteins:
- —> Negatively acting transcription factors
- —> 29 AUX/IAA genes in Arabidopsis
- ARFs: Auxin Response Factors
- —> Positively acting transcription factors
- —> 22 ARF genes in Arabidopsis
- AUX/IAA proteins bind ARFs and prevent them from activating expression of auxin response genes.
- Auxin leads to ubiquitin-mediated destruction of AUX/IAA proteins
Auxin signal transduction
low auxin: E3 ubiquitin ligase complex, no auxin response genes (not expressed)
high auxin: auxin patch, 26s proteasome, auxin response genes expressed
auxin mutants transport can be rescued by auxin treatments because of auxin patch
Summary of the auxin mutants
Auxin mutants in biosynthesis:
- defects in auxin biosynthesis: YES
- defects in auxin transport: NO
- defects in auxin signaling: NO
- rescued by auxin treatments: YES
Auxin mutants in transport:
- defects in auxin biosynthesis: NO
- defects in auxin transport: YES
- defects in auxin signaling: NO
- rescued by auxin treatments: YES (auxin patch)
Auxin mutants in signaling:
- defects in auxin biosynthesis: NO
- defects in auxin transport: NO
- defects in auxin signaling: YES
- rescued by auxin treatments: NO
How to explain “the power of movement in plants
–> when light hits, auxin is synthesized in the tip. and then, auxin is transported in the shaded side. through signaling, those cells receive signals to grow. shaded side has receptors to induce signaling, cell elongation then occurs.
–> when you cut the tip, it shows that there is no change in growth of the plant (there is lack of auxin)
–> even if there is an opaque cap on tip, plant needs stimulus to produce auxin
Cytokinin (CK)
- promote cell division and cell differentiation
- another class of plant hormone
Discovery of cytokinins
1913: Gottlieb Haberlandt showed that a substance isolated from plant phloem material could stimulate plant cell division in culture.
1940s: Various extracts from plant tissues, such as bean pod, carrot ovules, coconut endosperm had the same ability.
1955: Carlos Miller and Folke Skoog isolated a substance from herring sperm DNA that also stimulated cell division, and called it kinetin, for cytokinesis.
1961-1963: Miller and Letham isolate the first naturally occurring plant cytokinin from corn, and called it zeatin (for Zea Maize).
Function of cytokinins
—> Cell division and cell differentiation
• Release buds from apical dominance
• Delay leaf senescence
• Stimulate leaf expansion
• Affect shoot/root differentiation in tissue
culture
Apical dominance
Apical dominance is the phenomenon whereby the main, central stem of the plant is dominant over (i.e., grows more strongly than) other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets.
–> The growing apical bud (top/tip of a growing stem) inhibits the growth of lateral buds (points along the stem where lateral branches form).
What determines the shape of Christmas trees? auxin mechanism
–> Typically, the end of a shoot contains an apical bud, which is the location where shoot growth occurs. The apical bud produces an auxin (IAA) that inhibits growth of the lateral buds further down on the stem towards the axillary bud.
—> AUXIN PROMOTES APICAL DOMINANCE
Cytokinins release apical dominance
- they regulate axillary bud growth and apical dominance
ex: macadamia seedlings, PBA (a CK)
Cytokinins delay leaf senescence
Cytokinins delay aging in plants
example: wheat leaves
cytokinins biosynthesis
Ipt: catalyze the first step in cytokinin biosynthesis
—-> Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins.
figure: plant A is healthy with huge leaves, plant B is wilted with dried leaves
Of these two plants, which one overexpressing IPT?
Plant A, because overexpression of IPT means high levels of cytokinins, and cytokinins stimulate leaf expansion
Auxin: cytokinin ratio regulates morphogenesis in cultured tissue
Morphogenesis - biological process that causes an organism to develop its shape
ex: tobacco tissue culture
results show that: more cytokinin = more shoot growth
Cytokinins Physiology
- Cytokinins homeostasis
• de novo synthesis
• Transport
• Conjugation
• Catabolism
----> Oxidation
----> Convert to other deactivated forms
- Cytokinins signal transduction
Cytokinin signal transduction
- Signaling molecule (ligand: Cytokinins)
- Receptor (3 related histidine kinases)
- Activation of receptor target(s) & second messengers
- Transcription factors (ARR)
- Genes regulated by transcription factors
- Plant responses
signal perception (1, 2) —-> transduction (3, 4) —-> signal responses (5, 6)
Ethylene
- ripening hormone
- promotes leaf abscission
- smallest in structure
- naturally occurring hydrocarbon gas
- opposite of auxin and cytokinin
- negative effect in plants
Discovery of Ethylene
1886: Dimitry Nikolayevich Neljubow
Etiolated (dark grown) pea seedlings grew normally outside but differently in his lab (the triple response)
The triple response (S.T.C.)
In the presence of ethylene, dark grown seedlings show (3 RESPONSES):
- Shoot and root shortening
- Shoot thickening
- Apical hook curvature
Ethylene physiology
- Ethylene homeostasis
• de novo synthesis: derived from methionine
• Precursor transport and distribute by diffusion
• Can be inactivated by oxidation - Ethylene signal transduction
Ethylene signal transduction
- Signaling molecule (ligand: ethylene)
- Receptor (5 related histidine kinases)
- Activation of receptor target(s) & second messengers (CTR1)
- Transcription factors (EIN3)
- Genes regulated by transcription factors
- Plant responses
signal perception (1, 2) —-> transduction (3, 4) —-> signal responses (5, 6)
Function of ethylene
• Promotes fruit ripening
• promotes leaf abscission (shedding of leaves)
• promotes senescence (aging)
• inhibits cell elongation in roots
and stems
- negative effect in plants (similar to abscisic acid)
Ethylene promote fruit ripening
Climacteric fruits:
- Respiratory rise before ripening (CO2)
- A spike of ethylene production
- Response to ethylene treatment
- ripen with ethylene
- -> one bad apple spoils the barrel
- -> applications: ripen fruits after harvest
Non-climacteric fruits: ripen without ethylene
Climacteric fruits
The climacteric is a stage of fruit ripening associated with increased ethylene production and a rise in cellular respiration.
Apples, bananas, melons, apricots, tomatoes (among others) are climacteric fruit.
Non-climacteric fruits
Citrus, grapes, strawberries are non-climacteric (they ripen without ethylene and respiration bursts)
climacteric fruits mutants: tomato never-ripe mutants
The never-ripe mutant has a defect in one of the tomato ethylene receptors.
never-ripe mutant: doesn’t ripe
wild type: ripens because of ethylene
Non-climacteric fruit ripening mechanism
Auxin promote fruit ripening in non-climacteric fruits (ex: strawberry)
Ethylene promotes leaf abscission
leaf abscission is detachment of leaves/ shedding of leaves
–> This action of ethylene is opposite to that of auxin.
wild-type: no leaves because of presence of ethylene
ethylene insensitive mutant: abundant leaves because of the absence of ethylene
Ethylene promotes senescence
senescence is biological aging/ deterioration of plant
Control: wild-type: ethylene stimulates the deterioration of plant - plant is wilted
Silver thiosulfate: an ethylene inhibitor: plants are healthy and fresh
GA, ABA, and BR
- Gibberellins (Gibberellic Acid)
- Abscisic Acid
- Brassinosteroids
Hormone homeostasis
- de novo synthesis
- Transport
- Conjugation
- Catabolism
Biosynthesis of GA, ABA, and BR
• Common precursor: geranyl pyrophosphate
( derived from common precursor, can polymerize)
• GA, ABA and BR are synthesized via the TERPENOID pathway.
• The building block of terpenoids are five- carbon unit called ISOPRENE
Function of GA (Gibberellic Acid/ Gibberellins)
- Promotes stem growth
- Breaks seed dormancy (state where seeds are not germinated)
- Promotes flowering
- Increases fruit size, set, and spacing
-> positive effect in plants (similar to auxin and cytokinins)
-GA plants: short, wilted, small
+GA plants: tall, healthy, large
GA promote stem growth
experiment: pea plants with different levels of GAs
–> as levels of GA increases, plants become taller
no GAs = ultradwarf
little GAs = dwarf
some GAS = tall
a lot of GAs = very tall and slender
commercial use/ application -> make wood taller, bamboos, etc.
Functions of ABA (Abscisic Acid)
• Promotes seed desiccation and dormancy • Delays flowering time • Promotes the closure of stomata in times of water stress - has both negative effects (similar to ethylene) and positive effect? ( somata closing)
Vivipary 14 mutant of maize:
- -> an ABA-deficient mutant (no ABA)
- –> precocious germinate on the cob
BR: brassinosteroids
- Originally identified from pollens of the rape plant (Brassica napus L.).
- Present in very low quantity: 227 kg pollen to make 4 mg BR (Grove et al. 1979)
- Structurally similar to some steroid hormones in animals
Functions of brassinosteroids
Cell division, elongation, and differentiation
• Promote leaf expansion
• Promote stem and pollen elongation
• Promote vascular tissue differentiation
• Promote yields for grains and fruit crops
• Promote resistance to drought and cold
–> positive effect to plants (similar to auxin, cytokinins, and gibberellins)
BR biosynthesis mutants: det2
- DET2 encodes a reductase that acts in the BR biosynthetic pathway.
- A det2 mutant (loss of DET2 function) produces a dwarf plant
Wildtype: normal plant
det2 mutant: dwarf plant
–> det2 mutant can be rescued by BR treatment and return to wildtype
BR signaling mutants: bri1
- BRI1 is the receptor of BRs
- A bri1 mutant is dwarf.
wildtype: normal, healthy plant
bri1 mutant: dwarf plant
–> bri1 mutant cannot be rescued by BR treatment because hormones are specific
Summary of plant hormones
• Plant hormones regulate growth and development.
—-> 6 classes: auxin +, cytokinins +, ethylene - , gibberellic acid +, abscisic acid - , brassinosteroids + )
• Each hormone has distinct functions.
• Hormone actions are complex, depending on interacting hormones, hormone concentration, plant tissue, developmental stage, plant type etc.
• Hormone homeostasis is regulated by de novo synthesis, conjugation, transport, and/or catabolism.
• Hormone signal transduction is controlled by signaling components such as receptors, signal intermediates, transcription factors, and responsive genes.
