Abscisic acid (ABA):

A Seed maturation & antistress signal

Dr.Yuwalee Unpaprom1

สรีรวิทยาของพืชประยกุต ์ชว310

S-Abcisic acid

(S-ABA)

Hormones in PlantSource: Santner et al., 2009 2

Abscisic Acid (ABA)

Physiologists suspected that the

phenomena of seed & bud

dormancy were caused by inhibitory

compounds (dormin).

Winter dormancy

& chilling in

woody plants

Discovery that dormin was

chemically identical to a

substance that promotes the

abscission of cotton fruits, abscissin II

Renamed abscisic acid (ABA)

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Occurrence, Chemical Structure, and Measurement of ABA

ABA is a ubiquitous plant hormone in vascular plants.

It has been detected in

mosses but appears to be absent in liverworts.

ABA has been detected in

living tissues from the root

cap to the apical bud.

It is synthesized in almost all

cells that contain chloroplasts or amyloplasts.

Chloroplasts in plant cells Potato amyloplasts

The chemical structure of ABA

determines its physiological activity

ABA is a 15-C compound that

resembles the terminal portion of

some carotenoid molecules.

Nearly all the naturally

occurring ABA is in the cis form

ABA also has an asymmetric C

atom at position 1´ in the ring,

resulting in the S and R(or+ and -)

enantiomers.

The S and R forms cannot be interconverted in the plant tissue.

ABA is assayed by biological, physical, and chemical methods

A variety of bioassays have

been used for ABA

inhibition of germination

inhibition of GA-induced α-

amylase synthesis

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+ABA GA

Coleoptile growth inhibition.

Biosynthesis, Metabolism, and Transport of ABA

ABA is synthesized from a

carotenoid intermediate

Biosynthesis takes place in chloroplasts and other plastids.

The pathway begins with

isopentenyl diphosphate (IPP) and leads to the

synthesis of the C40 xanthophyll violaxanthin.

Violaxanthin

Xanthoxal (C15)

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Maize mutants (viviparous; vp) that are blocked at

other steps in the carotenoid pathway; exhibit vivipary

Synthesis of 9-cis-epoxycarotenoid

dioxygenase (NCED) is rapidly

induced by water stress.

Upon rewatering, the ABA

level declines to normal in the same amount of time.

ABA is translocated in vascular tissue ; xylem, phloem

As water stress begins, some of

the ABA carried by the xylem

stream may be synthesized in roots

that are indirect contract with the drying soil.

The major control of ABA

distribution among plant cell

compartments follows the “anion trap” concept.

ABA closes stomata in response

to water stress

Elucidation of the roles of ABA in

freezing, salt, and water stress led to

the characterization of ABA as

a stress hormone.

Biosynthesis of ABA is very

effective in causing stomatal closure.

ABA promotes seed storage reserve

accumulation and desiccation

tolerance

During mid – to late

embryogenesis, when seed ABA

levels are highest, seeds

accumulate storage compounds

that will support seedling growth

at germination.

As maturing seeds

begin to lose water,

specific mRNA

encoding so-called late-

embrogenesis-abundant

(LEA) proteins thought

to be involved in

desiccation tolerance

accumulate in embryos.

ABA closes stomata in response

to water stress

Elucidation of the roles of ABA in

freezing, salt, and water stress led to

the characterization of ABA as a stress hormone.

ABA promotes leaf senescence

independently of ethylene

ABA is involved in

leaf senescence, it

might indirectly

increase ethylene

formation & stimulate abscission.

Leaf segments senesce faster in darkness

than in light, and turn yellow as a result of chlorophyll breakdown.

Ethylene:

The gaseous Hormone

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Coal gas was used for street illumination, trees defoliated

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Coal gas & air pollution affect

plant growth

Ethylene (C2H4) was active component of coal gas

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First indication that ethylene is a natural product of plant tissues:

Published by H. H. Cousins in

1910

Cousins reported; emanation

from oranges caused the premature ripening of bananas

However, oranges synthesized

little ethylene compared to

other fruits

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In 1934, R. Gane & others identified

ethylene chemically as natural product of

plant metabolism

For 25 yrs ethylene was not recognized

After GC was introduced in ethylene in 1959, plant growth regulator was recognized

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STRUCTURE, BIOSYNTHESIS & MEASUREMENT OF ETHYLENE

Ethylene can be produced

by almost all parts of higher

plants

Meristematic & nodal

regions are the most active in ethylene biosynthesis

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The Properties of Ethylene are Deceptively Simple

Molecular weight: 28

Lighter than air

It is flammable and readily

undergoes oxidation; can be oxidized to ethylene oxide

Ethylene oxideEthylene

[o]

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Ethylene oxide can be hydrolyzed to ethylene glycol

Ethylene glycol

odorless,

colorless,

syrupy,

sweet-tasting

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In plants, ethylene can be

oxidized to CO2

Ethylene oxideEthylene

[O] [O]HOOC COOH CO2

Oxalic acid

Ethylene is released easily from

tissues & diffuses through the

intercellular spaces

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Ethylene gas is easily lost from

tissue and may effect other

tissues or organ

Ethylene-trapping system are

used during storage fruits,

vegetable and flowers: KMnO4

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The production of ethylene in

plants is highest in senescing

tissues & ripening fruits

Active at very low conc.- less

than 1 ppm (1µL/L)

Internal ethylene conc. In a ripe

apple: 2500 µL/L

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Regulated Biosynthesis Determines

the Physiological Activity of Ethylene

Plant tissues convert methionine to

ethylene

Yang cycle

Precursor of ethylene:

1-aminocyclopropane-1-carboxylic

acid

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AOA: aminooxyacetic acid

AVG: aminotoethoxy-vinylglycine

SAM

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Fruit ripening: ACC and ethylene

increase

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Stress-induced ethylene production:

drought, flooding, chilling, expose

to ozone, wounding

Auxins-induced ethylene

production: to promote ethylene

synthesis by enhancing ACC

synthase

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Inhibitors of ethylene synthesis:

AVG, AOA, Co2+

Inhibitors of ethylene action: Silver

ions (Ag+), applied as silver nitrate

(AgNO3) or silver thiosulfate

(Ag(S2O3)23-

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Inhibitors of ethylene: CO2 at high

conc. (5-10%)

The volatile compound trans-

cyclooctene (but not cis-) is strong

competitive inhibitor of ethylene

binding (1-methylcyclopropene:

MCP)

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Ethylene Promotes the Ripening of

Some Fruits

Fruit ripening refers to changes in

fruit that make it ready to eat.

Such: softening, starch hydrolysis,

sugar accumulation, disappearance

of organic acids & phenolic

compound

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Climacteric fruits

Nonclimacteric fruits

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Breaks seed & bud dormancy in

some sp.

Induced flowering in the pineapple

family

Enhances the rate of leaf

senescence

Induces the formation of roots &

root hairs

Other Eeffects of Ethylene

Salicylic Acid:

A phenolic phytohormone

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Salicylic acid (SA): from Latin

salix, willow tree (salicin)

A phenolic phytohormone

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Roles in plant growth and

development

Photosynthesis, transpiration, ion

uptake and transport

Induces specific changes in leaf

anatomy and chloroplast structure

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SA is involved in endogenous

signaling, mediating in plant defense

against pathogens

inducing the production

of pathogenesis-related

proteins

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The signal can also move to nearby

plants by salicylic acid being

converted to the volatile ester, methyl

salicylate.

It is involved in the systemic

acquired resistance (SAR)

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Salicylic acid is biosynthesized from

the amino acid phenylalanine

Sodium salicylatephenylalanine Salicylic acid

Commercially prepared:

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Unripe fruits and vegetables are

natural sources of salicylic acid:

blackberries, blueberries,

cantaloupes, dates, raisins, kiwi

fruits, guavas, apricots, green

pepper, olives, tomatoes, radish

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Salicylic acid is known for its ability to

ease aches and pains and reduce fevers

a key ingredient in many skin-care

products