Chapter 23

One molecule! Named on assumption of role in abscission

of leaves and other tissues.

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Primary functions Prevention of precocious germination Initiation and maintenance of seed dormancy Stomatal control Protection of cells in seeds from desiccation

Other functions may include: Induction of storage proteins in seeds Heterophylly Initiation of secondary roots Flowering and senescence

Notice what’s missing …..

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ABA is synthesized from a -carotene, typically in mature (water stressed) leaves. Begins in the chloroplast Completed in the cytosol

Highly mobile Accumulate in sink tissues

Roots and seeds.

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Based upon the expression of genes encoding enzymes involved in ABA synthesis, there is also evidence that ABA is synthesized in: Guard cells

Water stress, senescent leaves, and cotyledons Phloem companion cells Xylem parenchyma

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3 steps1. Cell divisions and tissue differentiation –

embryogenesis and endosperm proliferation

2. Cell divisions cease; storage compounds accumulate

3. Seeds dehydrate → quiescent/dormant Quiescent → germination upon rehydration Dormant → additional treatment required

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Regulates embryo maturation and seed germination Peaks during embryo maturation ABA imposes dormancy on the embryo

Prevents vivipary & precocious germination ABA also induces seed desiccation

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Cultured embryos before dormancy will germinate (i.e., precocious germination) – ABA inhibits the process

Vivipary – preharvest sprouting Happens in corn if it matures in wet weather Lack of ABA triggers (unless deficient in GA!)

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ABA allows for dessication tolerance Normally dessication damages membranes and

cellular contents Triggers synthesis of proteins and lipids

Only one of several signals controlling expression of these genes

Maintains mature embryo in dormant state

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Primary Dormancy – seeds released from the plant in a dormant state

Secondary Dormancy – seeds released from the plant in a nondormant state become dormant if germination conditions

unfavorable

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Temporal delay in germination process Additional time for seed dispersal Maximizes embryo survival by preventing

germination in unfavorable circumstances Coat-imposed dormancy (e.g., quiescent) Embryo dormancy (e.g., dormant) Role of cotyledons (peach, hazel, ash …)

See http://5e.plantphys.net/article.php?ch=23&id=8

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Prevention of water uptake. Mechanical constraint. Interference with gas exchange. Retention of inhibitors. Inhibitor production.

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Presence of inhibitors – ABA Absence of promoters – GA

Maintenance – ABA biosynthesis Loss – decrease in ABA/GA ratio

ABA also inhibits GA-induced synthesis of enzymes breaking down storage reserves

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External Triggers After ripening – moisture content drops Chilling (0-10°C)/ stratification

Single vs double dormancy Light

Simple exposure Specific photoperiod All light-requiring seeds exhibit seed coat dormancy

→ removal of seed coat allows germination in absence of light.

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Promotes root growth; inhibits shoot growth Wild type – normal ABA levels Viviparous -- ABA deficient

Ample water shoot growth greater in wild type Root growth slightly greater

Low water potential Shoot growth greater in viviparous mutant Root growth much greater in wild type

Inhibits ethylene production!

Increase in root/shoot ratio at low water potential

And …..

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Mediates response to water stress Accumulates in water-stressed leaves Inhibits stomatal opening/triggers stomatal closure

Exact mechanism difficult to discern – ABA ubiquitous Stomates close before ABA detected

Current line of thought ….. Inhibition of electron

transport lowers stroma pH Increase in apoplast pH pH gradient triggers ABA

release into apoplast

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Mediates response to water stress Accumulates in water-stressed leaves Stomates close before ABA detected

ABA concentration in apoplast triggers initial closure Increased synthesis prolongs effect

Closure not always rely on deficit within leaves

Response to soil desiccation before loss of turgor pressure

Some sort of feed-forward loop – closes stomates before water potential changes

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Additional roles for ABA include: Inhibition of lateral/secondary root development A possible role in the control of flower formation. Promotion of leaf senescence in absence of

ethylene Accumulates in dormant buds

Levels drop after exposure to cold temps

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