NCED3 (also known as STO1/SIS7; locus AT3G14440) encodes a chloroplast-localized 9-cis-epoxycarotenoid dioxygenase that catalyzes the first committed and rate-limiting step of abscisic acid (ABA) biosynthesis. It is a non-heme iron (Fe2+)-dependent dioxygenase of the carotenoid cleavage oxygenase (RPE65/NCED) family that oxidatively cleaves the 11,12 (11',12') double bond of 9-cis-epoxycarotenoids such as 9-cis-violaxanthin and 9'-cis-neoxanthin, yielding the C15 product xanthoxin (2-cis,4-trans-xanthoxin) plus a C25 apo-aldehyde. Xanthoxin is then exported from the plastid and converted in the cytosol to ABA. The mature protein is synthesized with an N-terminal transit peptide, imported into the chloroplast, and accumulates in the stroma with a portion peripherally associated with the thylakoid membrane. NCED3 is the principal stress-inducible NCED isoform in vegetative tissues: its transcription is strongly upregulated by drought and high salinity, and the resulting rise in ABA drives stomatal closure, reduced transpiration, and expression of stress-responsive genes. Loss-of-function plants are ABA-deficient, fail to accumulate ABA after osmotic/water stress, and are hypersensitive to desiccation, while overexpression elevates ABA and improves drought tolerance. NCED3 is expressed in roots, leaves, stems, silique envelopes and seeds, with notable activity at organ-attachment points and abscission zones.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009570 chloroplast stroma | IEA GO_REF:0000044 | ACCEPT | Summary: NCED3 localizes to the chloroplast stroma, consistent with the experimentally determined location (also annotated by IDA below). This electronic annotation derives from the UniProt subcellular location vocabulary and is correct. Supporting Evidence: PMID:12834401 AtNCED2, AtNCED3, and AtNCED6 are found in both stroma and thylakoid membrane-bound compartments. |
| GO:0016702 oxidoreductase activity, acting on single donors with incorporation of molecular oxygen, incorporation of two atoms of oxygen | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: This InterPro-derived term correctly describes the dioxygenase chemistry of NCED3 (incorporation of two oxygen atoms during carotenoid cleavage), but it is a broad parent of the specific term GO:0045549 (9-cis-epoxycarotenoid dioxygenase activity) that is also annotated. It is accurate but less informative than the specific catalytic term. Reason: Correct dioxygenase classification from InterPro IPR004294, but subsumed by the more specific MF term GO:0045549, which represents the core molecular function. Supporting Evidence: PMID:11532178 9-cis-epoxycarotenoid dioxygenase (NCED) is thought to be a key enzyme in ABA biosynthesis. file:ARATH/NCED3/NCED3-deep-research-falcon.md NCED3 contains conserved iron-chelating histidines |
| GO:0045549 9-cis-epoxycarotenoid dioxygenase activity | IEA GO_REF:0000003 | ACCEPT | Summary: The exact catalytic activity (EC 1.13.11.51) mapped from the UniProt Enzyme Commission assignment. This is the core molecular function of NCED3 and is independently supported by direct experimental evidence (IDA, PMID:11532178). Supporting Evidence: PMID:11532178 9-cis-epoxycarotenoid dioxygenase (NCED) is thought to be a key enzyme in ABA biosynthesis. file:ARATH/NCED3/NCED3-deep-research-falcon.md it catalyzes the xanthoxin-producing cleavage of 9-cis epoxycarotenoids in plastids |
| GO:0009688 abscisic acid biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: NCED3 catalyzes the first committed, rate-limiting step of ABA biosynthesis. This UniPathway-derived involved_in annotation is correct and captures the core biological process of the gene. Supporting Evidence: PMID:11532178 9-cis-epoxycarotenoid dioxygenase (NCED) is thought to be a key enzyme in ABA biosynthesis. file:ARATH/NCED3/NCED3-deep-research-falcon.md rate-limiting enzyme for stress-induced ABA synthesis |
| GO:0009688 abscisic acid biosynthetic process | IMP PMID:18854047 Identification, cloning and characterization of sis7 and sis... | ACCEPT | Summary: The sis7/nced3 mutant is ABA-deficient and sugar-insensitive, confirming NCED3's role in ABA biosynthesis. This is the core biological process. The acts_upstream_of_or_within qualifier is appropriate for a mutant-phenotype (IMP) annotation. Supporting Evidence: PMID:18854047 Mutations in the SIS7/NCED3/STO1 gene, which is primarily required for ABA biosynthesis under drought conditions, confer a sugar-insensitive phenotype, indicating that a constitutive role in ABA biosynthesis is not necessary to confer sugar insensitivity. file:ARATH/NCED3/NCED3-deep-research-falcon.md loss-of-function compromises ABA accumulation during stress with downstream impacts on water relations and stress survival |
| GO:0009507 chloroplast | ISM GO_REF:0000122 | KEEP AS NON CORE | Summary: NCED3 is a chloroplast-targeted protein (it carries an N-terminal chloroplast transit peptide). This sequence-model prediction is correct but is a broad parent of the experimentally supported chloroplast stroma / thylakoid membrane locations. Reason: Accurate but less specific than the IDA-supported chloroplast stroma and thylakoid membrane annotations. Supporting Evidence: PMID:12834401 Although all five AtNCEDs are targeted to plastids, they differ in binding activity of the thylakoid membrane. file:ARATH/NCED3/NCED3-deep-research-falcon.md NCED3 is plastid-localized via an N-terminal targeting sequence (stroma targeting domain reported for NCED proteins). |
| GO:0006970 response to osmotic stress | IMP PMID:16870153 Regulation of ABA level and water-stress tolerance of Arabid... | KEEP AS NON CORE | Summary: NCED3 produces the ABA required for responses to osmotic/water stress; the nced3 mutant fails to accumulate ABA during drought and is complemented by an ectopically expressed NCED. This is a downstream physiological process mediated by NCED3-derived ABA rather than the direct enzymatic function, so it is retained as non-core. The acts_upstream_of_or_within relation is appropriate. Reason: Genuine ABA-mediated involvement in osmotic-stress response, but indirect (downstream of the core ABA-biosynthetic activity). Supporting Evidence: PMID:16870153 Ectopic expression of AhNCED1 gene in 129B08/nced3 mutant Arabidopsis (with impaired AtNCED3 gene involved in ABA biosynthesis under water stress) driven by the AtNCED3 promoter restores its ability to accumulate ABA during drought stress, and reverts its hypersensitivity to nonionic osmotic stress and soil drought. file:ARATH/NCED3/NCED3-deep-research-falcon.md Mutants show a confirmed inability to |
| GO:0045549 9-cis-epoxycarotenoid dioxygenase activity | IDA PMID:11532178 Regulation of drought tolerance by gene manipulation of 9-ci... | ACCEPT | Summary: Direct experimental evidence that NCED3 is a 9-cis-epoxycarotenoid dioxygenase controlling ABA levels. This is the core molecular function and is supported by UniProt's three curated Rhea reactions (EC 1.13.11.51). Supporting Evidence: PMID:11532178 the expression of an NCED gene of Arabidopsis, AtNCED3, is induced by drought stress and controls the level of endogenous ABA under drought-stressed conditions. file:ARATH/NCED3/NCED3-deep-research-falcon.md it catalyzes the xanthoxin-producing cleavage of 9-cis epoxycarotenoids in plastids |
| GO:0009414 response to water deprivation | IMP PMID:11532178 Regulation of drought tolerance by gene manipulation of 9-ci... | KEEP AS NON CORE | Summary: nced3 disruption gives a drought-sensitive phenotype and overexpression improves drought tolerance, demonstrating NCED3's role in the water-deprivation response. This is a downstream ABA-mediated physiological process rather than the direct catalytic function, so it is retained as non-core. Reason: Well-supported but indirect; the response to water deprivation is mediated by NCED3-derived ABA, downstream of the core enzymatic activity. Supporting Evidence: PMID:11532178 antisense suppression and disruption of AtNCED3 gave a drought-sensitive phenotype. file:ARATH/NCED3/NCED3-deep-research-falcon.md Mutants show drought/desiccation-related defects (e.g., increased water loss and soil desiccation sensitivity), consistent with impaired ABA-mediated water conservation. |
| GO:0009535 chloroplast thylakoid membrane | IDA PMID:12834401 Molecular characterization of the Arabidopsis 9-cis epoxycar... | ACCEPT | Summary: Subcellular fractionation showed AtNCED3 is partially bound to the thylakoid membrane in addition to the stroma. UniProt notes the protein is partially bound to the thylakoid. Direct experimental evidence supports this location. Supporting Evidence: PMID:12834401 AtNCED2, AtNCED3, and AtNCED6 are found in both stroma and thylakoid membrane-bound compartments. file:ARATH/NCED3/NCED3-deep-research-falcon.md Experimental chloroplast fractionation and immunoblotting support NCED3 presence in |
| GO:0009570 chloroplast stroma | IDA PMID:12834401 Molecular characterization of the Arabidopsis 9-cis epoxycar... | ACCEPT | Summary: Direct experimental evidence (subcellular fractionation) places AtNCED3 in the chloroplast stroma. This is the primary functional compartment where carotenoid cleavage occurs. Supporting Evidence: PMID:12834401 AtNCED2, AtNCED3, and AtNCED6 are found in both stroma and thylakoid membrane-bound compartments. file:ARATH/NCED3/NCED3-deep-research-falcon.md Experimental chloroplast fractionation and immunoblotting support NCED3 presence in |
| GO:0009688 abscisic acid biosynthetic process | ISS PMID:12834401 Molecular characterization of the Arabidopsis 9-cis epoxycar... | ACCEPT | Summary: Sequence/family-based assignment of the ABA-biosynthetic role, consistent with NCED3 being the major stress-induced NCED. This is the core biological process and is also supported by direct mutant phenotypes (IMP). Supporting Evidence: PMID:12834401 A key regulated step in abscisic acid (ABA) biosynthesis in plants is catalyzed by 9-cis epoxycarotenoid dioxygenase (NCED), which cleaves 9-cis xanthophylls to xanthoxin, a precursor of ABA. |
| GO:0042538 hyperosmotic salinity response | IMP PMID:15466233 Uncoupling the effects of abscisic acid on plant growth and ... | KEEP AS NON CORE | Summary: The sto1/nced3 mutant cannot accumulate ABA after hyperosmotic stress and shows altered salt/salinity responses (salt-tolerant germination but Li+ hypersensitivity). This is a downstream ABA-mediated physiological process, so it is retained as non-core rather than as the core function. Reason: Genuine mutant phenotype linking NCED3 to the hyperosmotic salinity response, but indirect (mediated by NCED3-derived ABA), downstream of the core ABA-biosynthetic enzymatic activity. Supporting Evidence: PMID:15466233 Mutant sto1 plants were unable to accumulate ABA following a hyperosmotic stress, although their basal ABA level was only moderately altered. file:ARATH/NCED3/NCED3-deep-research-falcon.md Mutants show a confirmed inability to |
| GO:0045549 9-cis-epoxycarotenoid dioxygenase activity | ISS PMID:12834401 Molecular characterization of the Arabidopsis 9-cis epoxycar... | ACCEPT | Summary: Sequence/family-based assignment of the 9-cis-epoxycarotenoid dioxygenase activity, the core molecular function, consistent with the direct (IDA) and EC-based annotations of the same term. Supporting Evidence: PMID:12834401 A key regulated step in abscisic acid (ABA) biosynthesis in plants is catalyzed by 9-cis epoxycarotenoid dioxygenase (NCED), which cleaves 9-cis xanthophylls to xanthoxin, a precursor of ABA. |
| GO:0005506 iron ion binding | ISS | NEW | Summary: NCED3 is a non-heme iron dioxygenase that binds one Fe(2+) ion per subunit, coordinated by conserved histidine residues (UniProt binding sites 297, 346, 411, 585; by similarity to O24592). Iron binding is required for catalysis. This is not present in the supplied GOA TSV (UniProt records the generic metal ion binding keyword); it is added here as a NEW annotation using the precise term iron ion binding (GO:0005506). Reason: Fe(2+) cofactor binding is a documented, catalytically essential property of NCED3; the precise term iron ion binding (GO:0005506) is used directly rather than the generic metal ion binding keyword term. Supporting Evidence: file:ARATH/NCED3/NCED3-uniprot.txt Binds 1 Fe(2+) ion per subunit. file:ARATH/NCED3/NCED3-deep-research-falcon.md NCED3 contains conserved iron-chelating histidines |
| GO:0010436 carotenoid dioxygenase activity | IBA GO_REF:0000033 | NEW | Summary: Carotenoid dioxygenase activity is the family-level molecular function of the carotenoid cleavage oxygenase family to which NCED3 belongs, and is a direct parent of the specific GO:0045549 activity. UniProt records this as an IBA (GO_Central) annotation, though it is absent from the supplied GOA TSV; proposed here as a NEW annotation. Non-core because the specific term GO:0045549 better represents the function. Reason: Accurate family-level term subsumed by the specific 9-cis-epoxycarotenoid dioxygenase activity that is the core MF; included for completeness with the phylogenetic (IBA) annotation present in UniProt. Supporting Evidence: PMID:12834401 A key regulated step in abscisic acid (ABA) biosynthesis in plants is catalyzed by 9-cis epoxycarotenoid dioxygenase (NCED), which cleaves 9-cis xanthophylls to xanthoxin, a precursor of ABA. file:ARATH/NCED3/NCED3-deep-research-falcon.md NCED3 catalyzes carotenoid cleavage of |
| GO:0016124 xanthophyll catabolic process | IBA GO_REF:0000033 | NEW | Summary: NCED3 cleaves 9-cis-violaxanthin and 9'-cis-neoxanthin, which are xanthophylls (oxygenated carotenoids), not carotenes (hydrocarbon carotenoids). The family-level catabolic process is therefore correctly represented by GO:0016124 (xanthophyll catabolic process), defined as the breakdown of xanthophylls, oxygen-containing carotenoids - which exactly matches the 9-cis-epoxycarotenoid (xanthophyll) substrates NCED3 cleaves. An earlier draft annotated GO:0016121 (carotene catabolic process), but that term is the wrong substrate class (carotenes are hydrocarbon carotenoids) and has been corrected here. This remains a non-core, family-level catabolic process; the core process annotation for this gene is ABA biosynthesis (GO:0009688). Reason: Correct family-level catabolic process using the verified term GO:0016124 (xanthophyll catabolic process). This replaces a wrong-substrate draft term GO:0016121 (carotene catabolic process); NCED3 cleaves xanthophylls, not carotenes. The reviewer's suggestion GO:0016117 was checked against QuickGO and is actually 'carotenoid biosynthetic process' (biosynthetic, not catabolic), so it was not used. The core process for this gene remains ABA biosynthesis (GO:0009688). Supporting Evidence: PMID:12834401 which cleaves 9-cis xanthophylls to xanthoxin, a precursor of ABA. file:ARATH/NCED3/NCED3-deep-research-falcon.md the immediate ABA precursor exported to the cytosol for further conversion to ABA |
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Q: To what extent is the rate-limiting control of stress-induced ABA exerted at the level of NCED3 transcription versus post-translational regulation such as thylakoid binding?
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