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Growth Hormones

Ethylene and Abscisic Acid Ethylene and Abscisic Acid

Plant Physiology 751Plant Physiology 751

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Ethylene response

Pineapple flowering www.pbase.com

http://www.youtube.com/watch?v=KCUceQulHdw

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Ethylene Biosynthesis

Some physiological effects of ethyleneTriple response

Arabidopsis

Epinasty (downward bending of leaves)

Inhibition of flower senescence

STS = Silver thiosulfate

Promotion of root hair formation

Air

Air

C2H4

C2H4

Lettuce

TomatoPea

Air C2H4 Air C2H4

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Ethylene and fruit ripening

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Role of Auxin and Ethylene during leaf abscission formation and senescence

Ethylene Response Factor 1

Transcription factors are degraded by F box proteins in the absence of ethylene

Ethylene signal transduction mechanism

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The pathway summary:

The RAN1 protein is essential for assembling the Cu cofactor with the receptor for Ethylene binding.

In the absence of Ethylene, the receptor activates the downstream kinase CTR1 that in turn inhibits the response/gene expression.

On the contrary in the presence of Ethylene, the receptor is “inhibited” so is the CTR1 kinase. The response is now “on”.

The kinase cascade in Ethylene response serve as a negative regulator of the gene expression.

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Ethylene: Things to Remember

• Major hormone regulating fruit ripening• Two types of fruit based on amount of ethylene produced and respiration

(definitions and example plant names mentioned in HANDOUT)– Climacteric fruits– Non-climacteric fruits

• Induce flowering in Pineapple• Promotes root hair growth• Induces triple response (inhibition and swelling of hypocotyl, inhibition of

root elongation and exaggeration of the curvature of the apical hook• S-Adenosyl Methionine derived from Methionine is the precursor for its

biosynthesis• ACC-Synthase and ACC-Oxidase are the two rate limiting enzymes in its

pathway• The Ethylene response factors contributes submergence tolerance in rice

and Tomato• Induce leaf senescence and abscission tissue formation

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Abscisic Acid (ABA)

isopentenyl PP (C5)

farnesyl PP (C15)

geranylgeranyl PP (C20)

phytoene

zeaxanthin

violaxanthin

9-cis neoxanthin

C40

xanthoxal

ABA aldehyde

vp2, vp5, vp7, vp9corn mutants

aba1

aba2

vp14

flacca, sitiens: tomatodroopy: potatoaba3: Atnar2a: barley

Abscisic acid biosynthesisa sesquiterpenoid

ABA

ABA pool size is regulated by both biosynthesis and inactivation

Developmental and physiological effects of ABA

• seed maturation• seed dormancy (ABA/GA ratio)• stomatal closure• promotes root growth/inhibits shoot growth at low water potentials• promotes leaf senescence (but not abscission directly – ethylene)• accumulates in dormant buds of perennials

synthesis

ABA’s principle role is in maintaining water balance and osmotic stress tolerance

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Roles of ABA in plants• Under Abiotic stress

conditions such as drought/water stress it promotes root growth and suppresses shoot growth

• Plays role in closing stomata in response to water stress

• Role in senescence and Abscission layer formation

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ABA: Things to Remember• Regulates seed dormancy• Seed dormancy is is of two types namely the primary and secondary

(see handout)• Vivipary in seeds• ABA/GA ratio controls seed dormancy• Induces Abscission tissue formation leading to senescence and

shattering of leaf and fruits.• Similar to Gibberellic Acid the precursor molecule for Biosynthesis is

Gerany-Geranyl-pyrophosphate (GGPP)• Under Abiotic stress conditions such as drought/water stress it

promotes root growth and suppresses shoot growth• Inhibits flowering by interacting with Flowering time control gene (FCA)

in Arabidopsis