NCED3 (AT3G14440, Q9LRR7) — curation notes

Identity and overview

• 9-cis-epoxycarotenoid dioxygenase NCED3, chloroplastic. EC 1.13.11.51. Also named STO1 (SALT TOLERANT 1) and SIS7.
• Carotenoid oxygenase family (InterPro IPR004294; Pfam PF03055 RPE65). 599 aa precursor with an N-terminal chloroplast transit peptide (1-40).
• Non-heme Fe(2+) dioxygenase; binds 1 Fe(2+) per subunit (His-coordinated residues 297, 346, 411, 585 by similarity to O24592) [UniProt Q9LRR7].

Molecular function / catalysis

• Catalyzes the oxidative cleavage of 9-cis-epoxycarotenoids at the 11,12 (11',12') double bond, producing xanthoxin (2-cis,4-trans-xanthoxin) + a C25 apo-aldehyde. This is the first committed, rate-limiting step of ABA biosynthesis from carotenoids.
• UniProt curates three Rhea reactions (RHEA:23328 generic; RHEA:16541 9-cis-violaxanthin; RHEA:19677 9'-cis-neoxanthin), all EC 1.13.11.51, with experimental evidence from PubMed:11532178 and PubMed:15466233.
• Catalytic activity demonstrated: PMID:11532178 and the gene "controls the level of endogenous ABA under drought-stressed conditions."
• Inhibited by abamine and abamineSG (NCED-specific inhibitors) [UniProt; PMID:15247398, PMID:16682205].

Biological process — ABA biosynthesis under stress

• AtNCED3 is the major stress-induced NCED in leaves; its expression is induced by drought and salt and controls endogenous ABA levels under stress PMID:11532178.
• Overexpression increases ABA and improves drought tolerance; antisense/disruption gives a drought-sensitive phenotype PMID:11532178.
• sto1/nced3 mutant: ABA-deficient, fails to accumulate ABA after hyperosmotic stress, salt-stress tolerant (enhanced germination on NaCl/sorbitol), hypersensitive to Li+ [PMID:15466233 "Mutant sto1 plants were unable to accumulate ABA following a hyperosmotic stress" / "enhanced germination on both ionic (NaCl) and nonionic (sorbitol) hyperosmotic media"]. Confirmed by complementation with WT NCED3 and ABA rescue.
• sis7/nced3 mutant is sugar-insensitive; NCED3 "is primarily required for ABA biosynthesis under drought conditions" PMID:18854047.
• Peanut AhNCED1 driven by the AtNCED3 promoter complements the nced3 mutant, restoring drought-induced ABA accumulation [PMID:16870153 abstract] — i.e., the IMP for GO:0006970 response to osmotic stress reflects NCED3's role in producing ABA needed for osmotic-stress responses.

Localization

• Plastid / chloroplast stroma, partially bound to the thylakoid membrane [UniProt "Plastid, chloroplast stroma ... Note=Partially bound to the thylakoid"].
• Experimental fractionation: AtNCED3 found in both stroma and thylakoid membrane-bound compartments PMID:12834401. All AtNCEDs are plastid-targeted.

Expression / tissue

• Expressed in roots, leaves, stems, silique envelopes, seeds; localized at organ-attachment points and abscission zones [UniProt tissue specificity]. NCED2/NCED3 account for root NCED activity; NCED3 is the major stress-induced leaf NCED PMID:12834401.

Curation reasoning summary

• Core MF: GO:0045549 9-cis-epoxycarotenoid dioxygenase activity (IDA, PMID:11532178) — ACCEPT as core.
• GO:0016702 (oxidoreductase, dioxygenase parent) IEA from InterPro is correct but a broad parent of GO:0045549 — KEEP_AS_NON_CORE (accurate but less informative than the specific term).
• GO:0010436 carotenoid dioxygenase activity (IBA, GO_Central) and GO:0016121 carotene catabolic process (IBA) appear in UniProt's GO block but not in the supplied GOA TSV; metal ion binding GO:0046872 (IEA UniProtKB-KW) likewise. Added as NEW where biologically warranted (Fe binding is a documented cofactor requirement).
• Stress-response BP terms (GO:0009414 water deprivation, GO:0006970 osmotic stress, GO:0042538 hyperosmotic salinity) are acts_upstream_of_or_within IMP — these are downstream physiological consequences mediated by ABA, not the direct enzymatic process. Kept but several marked non-core; ABA biosynthesis (GO:0009688) is the direct core process.

Deep research synthesis (Falcon / Edison Scientific report)

The Falcon deep-research report (NCED3-deep-research-falcon.md) corroborates and extends the existing review; no decisions were changed (no UNDECIDED actions were present). Key reinforcing points, all added as supported_by deep-research quotes on the relevant annotations:
- Core MF/BP: "it catalyzes the xanthoxin-producing cleavage of 9-cis epoxycarotenoids in plastids" and NCED3 is the "rate-limiting enzyme for stress-induced ABA synthesis" — strengthens GO:0045549 and GO:0009688 as core. Substrates are 9-cis-neoxanthin and 9-cis-violaxanthin; cleavage at the 11',12' double bond; xanthoxin is "the immediate ABA precursor exported to the cytosol for further conversion to ABA."
- Iron cofactor: "NCED3 contains conserved iron-chelating histidines (His-297, His-346, His-411, His-585)" — independent corroboration of the Fe(2+)-binding NEW annotation (GO:0046872, with proposed GO:0005506 iron ion binding) beyond the UniProt keyword.
- Localization: "NCED3 is plastid-localized via an N-terminal targeting sequence" and "Experimental chloroplast fractionation and immunoblotting support NCED3 presence in both thylakoid and stroma fractions" — supports GO:0009507/GO:0009570/GO:0009535. Kalladan et al. 2019 (PMID:30760635 — not yet cached) emphasizes sub-chloroplast partitioning (thylakoid vs stroma) as a post-translational regulatory mechanism, with a reduced-function Sha allele altering processing.
- Stress phenotypes: sto1/nced3 mutants "show a confirmed inability to accumulate ABA during osmotic/salt stress" and "drought/desiccation-related defects" — supports the non-core stress-response BP terms.
- No new GO IDs verifiable from GOA/UniProt were introduced beyond those already annotated; recent context (ABACUS2 biosensors, stomatal defense vs spider mites, crop-engineering overexpression) is downstream physiology and was not converted into new core annotations.
- Newly surfaced primary references not yet in the review: Kalladan et al. 2019 (Plant Physiol, doi:10.1104/pp.18.01185) and Ruggiero et al. 2004 (= PMID:15466233, already cited). Status kept DRAFT.