NCEDs

NCEDs, or 9-cis-epoxycarotenoid dioxygenases, are a family of plant enzymes that catalyze a rate-limiting step in the biosynthesis of the phytohormone abscisic acid (ABA). They cleave 9-cis-epoxycarotenoids, such as 9-cis-violaxanthin and 9′-cis-neoxanthin, to produce xanthoxin, which is subsequently converted to abscisic aldehyde and then to ABA. This reaction links carotenoid metabolism to hormonal control of stress responses and development.

NCEDs belong to the carotenoid cleavage dioxygenase (CCD) family and are non-heme iron–dependent dioxygenases located primarily in plastids, the site of carotenoid substrates. Across land plants, NCEDs are encoded by a small multi-gene family; several members can be expressed in leaves, roots, seeds, or fruits, depending on species and developmental stage.

Functionally, ABA synthesized via the NCED-catalyzed step accumulates in response to abiotic stresses such as drought, high salinity, and heat, helping to regulate stomatal closure, root growth, and seed dormancy. Consequently, NCED gene expression is commonly upregulated by environmental stresses and by developmental cues linked to seed maturation and dormancy.

Significance arises from the central role of NCEDs in ABA homeostasis. Altering NCED expression or activity can influence ABA levels and thereby affect plant drought tolerance, seed behavior, and stress signaling. As a result, NCEDs are frequently investigated in plant physiology and crop improvement research, with interest in modulating stress resilience and growth through targeted manipulation of ABA biosynthesis.