Abscisic Acid

Question 1

What was ABA linked to

Answer 1

Abscission of fruits and dormancy

Question 2

ABA Biosynthesis

Answer 2

ABA is synthesized through the mevalonic acid/terpenoid pathway, originating from the plant pigment zeaxanthin.

It occurs in the plastids and cytoplasm, with steps defined using mutants. ABA synthesis is crucial for water stress responses and seed dormancy.

ABA is relatively stable but can isomerize into less active forms.
Mutants defective in ABA biosynthesis exhibit traits like wilting and premature seed germination (vivipary).

Question 3

ABA transport

Answer 3

ABA moves through both xylem and phloem, facilitating responses across plant tissues.

Levels are regulated via degradation, compartmentalization, and conjugation (e.g., ABA-GE storage form).

Question 4

ABA Biological Roles

Answer 4

Seed Dormancy and Germination:
ABA peaks during mid to late embryogenesis, promoting:
Desiccation tolerance via Late Embryogenesis Abundant (LEA) proteins.
Accumulation of storage reserves (proteins and starch).
During germination, ABA levels decline while gibberellins (GA) rise to promote growth.

Stress Response:
ABA is critical under drought conditions, triggering stomatal closure to reduce water loss.
It induces genes for:
Osmoprotectants (e.g., proline, glycine betaine)
Antioxidative enzymes (e.g., peroxidase, superoxide dismutase)

Stomatal Regulation:
ABA increases stomatal resistance by reducing turgor in guard cells, limiting water loss.
Controls ion channels in guard cells, promoting the efflux of potassium (K+) and anions, leading to water loss and stomatal closure.

Question 5

ABA Signaling Pathway

Answer 5

Core Components:
Receptors: PYR/PYL/RCAR proteins are intracellular receptors that bind ABA.
Phosphatases: PP2C (Protein Phosphatase 2C) inhibits downstream signaling in the absence of ABA.
Kinases: SnRK2 (Sucrose Non-Fermenting 1-Related Kinase 2) proteins are activated in the presence of ABA, triggering responses.

Mechanism:
1. In the Absence of ABA:
PP2Cs inhibit SnRK2 by dephosphorylation, preventing ABA-responsive gene expression and ion channel activity.
2. In the Presence of ABA:
ABA binds to PYR/PYL receptors, forming a complex with PP2Cs and inhibiting their activity.
SnRK2 kinases are released from inhibition, becoming active.
Active SnRK2 phosphorylates target proteins, including:
Ion Channels: Induce stomatal closure by activating outward K+ and anion channels.
Transcription Factors: Induce ABA-responsive genes.

Second Messengers:
Calcium (Ca²⁺): A key secondary messenger, its levels increase in response to ABA and regulate ion channels through calcium-dependent protein kinases (CDPKs).
Reactive Oxygen Species (ROS): Can amplify ABA signals.

Question 6

ABA Specialized Functions

Answer 6

Hydrotropic Root Growth:
ABA aids in directing root growth toward regions of higher water potential. It coordinates with calcium signaling to enhance asymmetric cell division and elongation.

Desiccation Tolerance:
ABA-induced osmotic adjustment helps plants survive extreme water loss by maintaining cell turgor and stabilizing proteins and membranes.

Question 7

ABA Unique features

Answer 7

ABA is the only major hormone in stress responses, making it vital for plant survival under adverse conditions.

Its signaling network involves intricate feedback loops and crosstalk with other hormones like gibberellins (GA) and cytokinins.