ABCG36 (PEN3 / PDR8 / At1g59870; UniProt Q9XIE2) — curation notes

Identity

Full-size PDR-type ABCG transporter (two NBD + TMD modules, reverse topology). Synonyms PEN3,
PDR8. Plasma-membrane resident; enriched at the outer lateral domain of root epidermal cells and
focally recruited to fungal penetration sites/papillae in leaves.

Molecular function (what it transports)

• Camalexin: direct ATP-dependent export PMID:37146609.
• IBA (auxin precursor) efflux PMID:19648296; redundant with ABCG37 PMID:20498067. Wild-type does not export IAA (deep research, Aryal 2019; file:ARATH/ABCG36/ABCG36-deep-research-falcon.md).
• Indole-glucosinolate (PEN2 pathway) products: PMID:26023163; 4MeOI3M shown as substrate by Matern 2019 (deep research; not cached).
• Cadmium: PMID:17355438. Species unresolved.
• Substrate preference switch: PMID:37146609
• Ca2+ sensor interactions (CaM7 etc.) PMID:26315018.

Localization

• PM PMID:16473969
• Cycles through TGN PMID:28434950
• ER only as biosynthetic transit (EAP3 needed for ER exit) PMID:29085068.
• Not vacuole PMID:19095898; NOT chloroplast (TAIR IDA negation). Mitochondrial/chloroplast/vacuolar proteomics hits are likely contaminants.

Biological roles

• Pre-invasive (nonhost) defense vs fungi: Blumeria, E. pisi, P. cucumerina, Colletotrichum, Phakopsora, Fusarium (PMIDs 16473969, 20605856, 26315018, 29085068, 37146609).
• Oomycetes and Pseudomonas (PMIDs 16473969, 24889055, 23815470) — same export mechanism, secondary.
• Recruitment to penetration sites triggered by PAMP perception (flg22, chitin) via PRRs PMID:23836668. ABCG36 is a downstream cargo, not a component of the PRR signaling pathway.
• pen3 SA hyperactivation, chlorosis and resistance to adapted E. cichoracearum are secondary PMID:16473969.
• Root IBA homeostasis: root hair, cotyledon expansion, lateral root phenotypes are indirect consequences of reduced IBA efflux (IBA->IAA conversion required) PMID:19648296.
• Drought/salt tolerance from overexpression (Na content) PMID:20088904 — indirect.
• Root exudation of thymidine (pdr8) and coumarins (mostly ABCG37) PMID:28623273 — exudation phenotypes, not metabolic processes.

Curation decisions summary

• Core: ATP-dependent export of indolic defense metabolites (camalexin, IGS products) at PM/penetration sites -> defense response to fungus; IBA (auxin) efflux at the root epidermis PM.
• protein binding (CaM/CML/CBL4/KIC IPIs) -> MODIFY to calmodulin binding.
• Over-annotations: development terms (cotyledon, root, root hair), coumarin/thymidine metabolic process, indole glucosinolate catabolic process (PEN2 catalyses), PAMP receptor signaling pathway, SAR, negative regulation of defense response, heat (RNA-seq), mRNA binding (RIC), organelle proteomics HDA.
• Mitochondrion ISM prediction removed (contradicted by experimental PM localization).
• PMID:16877699 (PDR9/ABCG37 paper) supports "export from cell" only via background mention of pdr8; function itself is correct for ABCG36, kept.
• Necessity vs participation (project Q3): ABCG36 does perform a step of the defense response (export of antimicrobial indolics), so defense response to fungus is retained as core; the more mechanistic activity is captured by the transporter MF (ABC-type indole transporter activity). No GO term exists for camalexin/glucosinolate-product export.