abscisic

Abscisic acid (ABA) is a plant hormone belonging to the terpenoid family. It is primarily produced in plastids of leaves, seeds, and roots and acts as a key regulator of development and stress responses throughout the plant. The name reflects its historic association with abscission, but ABA controls many processes beyond leaf drop, most notably seed dormancy and drought response.

Biosynthesis occurs via the carotenoid pathway in plastids. A rate-limiting step is performed by 9-cis-epoxycarotenoid dioxygenase (NCED), generating xanthoxin, which is converted to abscisic acid. ABA can be stored as inactive glucose esters and is degraded by ABA 8'-hydroxylases (CYP707A) to inactive forms, modulating hormone levels.

Signaling begins with a family of soluble receptors (PYR/PYL/RCAR). Upon ABA binding, these receptors inhibit PP2C phosphatases, releasing SnRK2 protein kinases to activate ABA-responsive transcription factors such as AREB/ABF. In guard cells, ABA signaling promotes ion efflux and stomatal closure, reducing water loss during drought.

Physiological roles include enforcing seed dormancy and inhibiting germination during unfavorable conditions, regulating seed maturation, and mediating stomatal closure in response to dehydration. ABA also modulates responses to osmotic and salinity stress and interacts with other hormones, such as gibberellins and ethylene, to coordinate growth and development.

ABA is transported by xylem and phloem, distributing signals from roots to shoots and within tissues. Exogenous ABA has been studied to improve drought tolerance and control germination timing in crops, but practical use is limited by stability, cost, and potential growth trade-offs.