| Aspect | Key findings (1-3 bullet phrases) | Evidence type | Organism(s) | Primary sources with year and URL |
|---|---|---|---|---|
| identity/domains/structure | • Verified target is **maize ABP1 / auxin-binding protein 1** matching UniProt P13689 • Soluble ~22 kDa glycoprotein, predominantly ER-localized, with C-terminal **KDEL** retention motif • Fold is cupin/germin-like **β-jellyroll barrel** with a metal-binding auxin pocket; crystal structure solved at **1.9 Å** (pqac-00000002, pqac-00000003, pqac-00000006) | structure, biochem | **Zea mays** | Woo et al., 2002, *EMBO J.* https://doi.org/10.1093/emboj/cdf291; Klämbt, 1990, *Plant Mol Biol* https://doi.org/10.1007/BF00019401 |
| auxin binding | • Purified maize ABP1 binds synthetic auxin **1-NAA** with **KD ~1.5 × 10^-7 M** at pH 5.5 • One auxin molecule binds per monomer by Scatchard analysis • Auxin carboxylate coordinates a **Zn** ion in the binding pocket; IAA predicted to bind with lower affinity than 1-NAA (pqac-00000002, pqac-00000003, pqac-00000008) | structure, biochem | **Zea mays** | Woo et al., 2002, *EMBO J.* https://doi.org/10.1093/emboj/cdf291 |
| pH dependence | • Highest auxin-binding affinity reported around **pH 5.5** • Reviews summarize repeated binding of purified maize/tobacco ABP1 at low **apoplastic pH (~5–5.5)** • This supports a model in which physiologically relevant binding occurs outside the cell rather than in neutral/alkaline cytosol (pqac-00000003, pqac-00000001, pqac-00000000) | biochem, review | **Zea mays**, tobacco, broader plant systems | Woo et al., 2002, *EMBO J.* https://doi.org/10.1093/emboj/cdf291; Monzer & Friml, 2025, *npj Sci. Plants* https://doi.org/10.1038/s44383-025-00002-8; Klämbt, 1990, *Plant Mol Biol* https://doi.org/10.1007/BF00019401 |
| localization/trafficking | • Most ABP1 is retained in the **ER lumen** via KDEL/HDEL-type retention • A much smaller pool has been reported at the **outer face of the plasma membrane / apoplast** • Maize turnover study found long protein half-life and that the plasma-membrane-associated fraction is only a **tiny fraction** of total ABP1 (pqac-00000005, pqac-00000000, pqac-00000006) | localization, trafficking, review | **Zea mays**; supporting discussion from other plants | Oliver et al., 2004, *Planta* https://doi.org/10.1007/BF00196668; Klämbt, 1990, *Plant Mol Biol* https://doi.org/10.1007/BF00019401; Woo et al., 2002, *EMBO J.* https://doi.org/10.1093/emboj/cdf291 |
| proposed signaling role | • ABP1 has long been proposed as a **rapid/cell-surface auxin perception** component rather than a transcriptional receptor • Recent framework places ABP1 with **TMK1** in an extracellular auxin-sensing complex that drives ultrafast phosphorylation and affects **PIN trafficking/auxin canalization** • Functional outputs historically linked to ABP1 include proton pump activation, ion channel regulation, protoplast swelling, and cell expansion (pqac-00000001, pqac-00000004, pqac-00000013) | review, signaling synthesis, physiological interpretation | Maize evidence foundational; mechanistic model developed mainly in **Arabidopsis** and broader plant systems | Monzer & Friml, 2025, *npj Sci. Plants* https://doi.org/10.1038/s44383-025-00002-8; Zeng et al., 2024, *PNAS* https://doi.org/10.1073/pnas.2412493121 |
| maize-specific functional evidence | • Early maize/corn work localized ABP activity strongly to coleoptile outer epidermal cells and antibodies against ABP blocked auxin-dependent elongation • Patch-clamp work in maize protoplasts linked an auxin-binding protein to auxin-stimulated plasma-membrane currents • Transcriptome studies report **ABP1 expression changes** under maize developmental/stress contexts, but these are correlative rather than definitive functional proof (pqac-00000000, pqac-00000004) | physiological, localization, transcriptome | **Zea mays** | Klämbt, 1990, *Plant Mol Biol* https://doi.org/10.1007/BF00019401; Monzer & Friml, 2025, *npj Sci. Plants* https://doi.org/10.1038/s44383-025-00002-8 |
| controversies/limitations | • Major caution: extensive **Arabidopsis ABP1** literature does **not** directly resolve maize ABP1 function • Arabidopsis **abp1 null** reports lacking obvious phenotypes challenged ABP1 essentiality, creating a long-running controversy • Thus, maize ABP1 has strong **biochemical/structural** evidence for auxin binding, but its in planta primary signaling role remains less definitively established than its binding chemistry (pqac-00000001, pqac-00000004, pqac-00000011, pqac-00000012) | review, genetics controversy | **Zea mays** distinguished from **Arabidopsis** | Monzer & Friml, 2025, *npj Sci. Plants* https://doi.org/10.1038/s44383-025-00002-8; Yu et al., 2023, *bioRxiv* https://doi.org/10.1101/2022.11.28.518138 |


*Table: This table summarizes the strongest gathered evidence for maize ABP1 (UniProt P13689), separating direct maize biochemical/structural findings from broader ABP1-TMK signaling models and the Arabidopsis controversy. It is useful for identifying what is firmly established versus still debated.*