ABA (Abscisic Acid) plays an important role in plant response to water stress. In the table below, column X represents some of the important enzymes in ABA biosynthesis/degradation pathways, while column Y summarizes the major function of these enzymes.

Column X (Enzymes)	Column Y (Functions)
A. 9-cis-epoxycarotenoid dioxygenase	i. Involved in the oxidative pathway of ABA catabolism
B. cytochrome P450 monooxygenase (CYP707A3)	ii. Production of a sugar – conjugated form of ABA
C. ABA glucosyltransferase	iii. Xanthoxin production
D. β – glucosidase	iv. Releases ABA from its sugar – conjugated form

Choose the option showing the correct match between column X and column Y:

Correct option is B

ABA (Abscisic Acid) is a crucial plant hormone that regulates stress responses, particularly indrought conditions. It is synthesized and degraded through specific enzymatic pathways:

1. (A)9-cis-epoxycarotenoid dioxygenase→ Xanthoxin production (iii)

   • 9-cis-epoxycarotenoid dioxygenase (NCED)is akey enzyme in ABA biosynthesis.
   • It catalyzes the conversion of9-cis-epoxycarotenoids to xanthoxin, an important ABA precursor.
   • Correct match: A-iii

2. (B) Cytochrome P450 monooxygenase (CYP707A3) → Oxidative ABA catabolism (i)

   • CYP707A3 is amajor enzyme involved in ABA degradation.
   • Itoxidizes ABA, leading to itsinactivation.
   • Correct match: B-i

3. (C) ABA glucosyltransferase → Production of a sugar-conjugated form of ABA (ii)

   • ABA glucosyltransferase catalyzes theaddition of glucoseto ABA, formingABA-glucose conjugates(ABA-GE).
   • This is a storage/inactive form of ABA.
   • Correct match: C-ii

4. (D) β-glucosidase → Releases ABA from its sugar-conjugated form (iv)

   • β-glucosidase breaks downABA-glucose conjugates (ABA-GE)to releasefree ABA.
   • This reactivates ABA understress conditions.
   • Correct match: D-iv

Information Booster

1. ABA Biosynthesis Pathway:

   • 9-cis-epoxycarotenoid dioxygenase (NCED)is arate-limiting enzymein ABA biosynthesis.
   • Xanthoxin is converted intoABA aldehyde, which is then oxidized to form ABA.

2. ABA Degradation Pathway:

   • ABA is degraded primarily viaoxidation, catalyzed byCYP707A3, reducing ABA levels duringstress recovery.

3. ABA Conjugation & Release:

   • ABA can be stored in aninactiveform as ABA-glucose ester (ABA-GE), catalyzed byABA glucosyltransferase.
   • Under stress,β-glucosidasehydrolyzes ABA-GE,releasing free ABA, which reactivates the stress response.

4. Role of ABA in Drought Stress:

   • ABAinduces stomatal closuretoreduce water loss.
   • It triggersgene expressionthat enhances drought resistance.

5. CYP707A3 and ABA Homeostasis:

   • Mutations inCYP707A3lead tohigher ABA levels, making plants more drought-resistant butdelayed in stress recovery.

6. Commercial Applications of ABA:

   • ABA is used inagricultureto enhance drought resistance in crops.
   • Synthetic ABA analogs are being explored for improving plantwater-use efficiency.