| Category | Finding | Key evidence | Source (with DOI URL and year) |
|---|---|---|---|
| Protein identity / family | Arabidopsis thaliana NPF6.3, also called CHL1/NRT1.1 (At1g12110; UniProt Q05085), is an NPF/NRT1/PTR-family major facilitator superfamily transporter functioning as a plasma-membrane nitrate transceptor. | Reviews and primary literature consistently identify CHL1/NPF6.3 as the first cloned plant nitrate transporter, a member of the NPF (NRT1/PTR) family, with both transport and sensing functions. Structural summaries place it in the proton-coupled NPF clade. (pqac-00000003, pqac-00000004, pqac-00000001) | Ho et al., *Cell* (2009), https://doi.org/10.1016/j.cell.2009.07.004; Sun & Zheng, *Front. Physiol.* (2015), https://doi.org/10.3389/fphys.2015.00386; Nedelyaeva et al., *Int. J. Mol. Sci.* (2024), https://doi.org/10.3390/ijms252413648 |
| Primary substrate | The primary substrate is nitrate (NO3−), transported by proton-coupled symport. | AtNPF6.3 shows nitrate transport in heterologous systems and in planta, with proposed 2 H+/1 NO3− stoichiometry and essential proton/nitrate-binding features including the ExxER motif and His356. (pqac-00000001, pqac-00000000, pqac-00000004) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648; Sun & Zheng (2015), https://doi.org/10.3389/fphys.2015.00386 |
| Chlorate relationship | CHL1 was originally identified through chlorate-related phenotypes; chlorate is a nitrate analog linked to the gene’s historical naming. | The CHL1 name derives from chlorate resistance, reflecting recognition of chlorate as a structural analog of nitrate in genetic studies, although the strongest direct functional evidence centers on nitrate transport. (pqac-00000002) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648 |
| Chloride competition | AtNPF6.3 can mediate chloride transport under low-nitrate conditions, and chloride uptake is inhibited by nitrate, indicating competition between Cl− and NO3−. | Oocyte and review evidence indicate chloride-transport activity when nitrate is scarce; nitrate competitively suppresses chloride uptake, and no bound chloride was seen crystallographically. (pqac-00000007) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648 |
| Auxin-related function | NPF6.3 is also linked to auxin transport/signaling, helping connect nitrate availability to root developmental responses. | Recent reviews summarize AtNPF6.3 as having auxin-transport-related activity/signaling consequences; under low nitrate it promotes auxin removal from lateral root primordia, restraining lateral root growth, whereas higher nitrate relieves this effect. (pqac-00000000, pqac-00000010) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648; Aluko et al., *Front. Plant Sci.* (2023), https://doi.org/10.3389/fpls.2023.1074839 |
| Dual-affinity kinetics | NPF6.3 is a dual-affinity nitrate transporter with biphasic kinetics. | Reported kinetic values are ~40–80 µM (or ~50 µM) for the high-affinity mode and ~4 mM (or ~5 mM) for the low-affinity mode. (pqac-00000002, pqac-00000004, pqac-00000003) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648; Sun & Zheng (2015), https://doi.org/10.3389/fphys.2015.00386; Ho et al. (2009), https://doi.org/10.1016/j.cell.2009.07.004 |
| Regulatory switch | Thr101 is the key affinity-mode switch: phosphorylation favors high-affinity transport, dephosphorylation favors low-affinity transport. | Thr101 phosphomimic and non-phosphorylatable mutants convert the transporter to monophasic high- or low-affinity behavior; phosphorylation increases conformational flexibility and can increase nitrate uptake rate. (pqac-00000001, pqac-00000004, pqac-00000005, pqac-00000015) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648; Sun & Zheng (2015), https://doi.org/10.3389/fphys.2015.00386; Ho et al. (2009), https://doi.org/10.1016/j.cell.2009.07.004 |
| Upstream kinase module | CBL1/CBL9-CIPK23 phosphorylates NPF6.3 at Thr101 in nitrate/Ca2+ signaling. | Nitrate-triggered Ca2+ signals activate CBL1/9 and CIPK23; CIPK23 directly phosphorylates Thr101 and is central to switching NPF6.3 between affinity states and nitrate signaling outputs. (pqac-00000003, pqac-00000006, pqac-00000001) | Ho et al. (2009), https://doi.org/10.1016/j.cell.2009.07.004; Jia et al., *Int. J. Mol. Sci.* (2023), https://doi.org/10.3390/ijms241914406; Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648 |
| ABI2 / ABA regulation | ABI2 antagonizes the CBL1/9-CIPK23 pathway, while ABA inhibits ABI2, thereby favoring NPF6.3 phosphorylation. | Review evidence indicates ABI2 dephosphorylates pathway components to block NPF6.3 phosphorylation; ABA inhibits ABI2, integrating nitrate and ABA signaling. (pqac-00000006) | Jia et al. (2023), https://doi.org/10.3390/ijms241914406 |
| Additional signaling partners | NPF6.3 participates in early nitrate signaling with Ca2+ channels and other nitrate-response regulators. | NRT1.1 forms a complex with CNGC15, and changing nitrate weakens this interaction to enable nitrate-induced Ca2+ signaling; CIPK8 positively regulates low-affinity nitrate responses. (pqac-00000006, pqac-00000003) | Jia et al. (2023), https://doi.org/10.3390/ijms241914406; Ho et al. (2009), https://doi.org/10.1016/j.cell.2009.07.004 |
| Mechanistic residues / motifs | Key mechanistic features include the ExxER proton-binding motif, His356 in the nitrate-binding pocket, Thr101 near the dimer interface, the K164–E476 salt bridge, and Pro492 involved in transport regulation. | His356Ala abolishes nitrate transport; ExxER and H356 are required for proton-coupled transport; K164–E476 acts as a gating salt bridge; Pro492 is important for transport but dispensable for sensory function; Thr101 controls the affinity switch. (pqac-00000000, pqac-00000001) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648 |
| Oligomeric/structural mechanism | Unmodified NPF6.3 forms homodimers; phosphorylation-linked changes in dimer coupling/flexibility underlie affinity conversion. | Structural analyses show dimeric NPF6.3 with Thr101 close to the dimer interface; phosphorylation promotes higher flexibility and favors the high-affinity state. (pqac-00000001, pqac-00000004) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648; Sun & Zheng (2015), https://doi.org/10.3389/fphys.2015.00386 |
| Subcellular and tissue localization | NPF6.3 localizes to the plasma membrane, especially in root epidermal and vascular cells, and is also reported in guard cells. | Reviews summarize expression throughout the plant with strongest root expression; protein localization is reported in plasma membranes of root epidermis and vasculature, with additional guard-cell localization. (pqac-00000002, pqac-00000011) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648 |
| Physiological role: nitrate uptake | NPF6.3 contributes substantially to root nitrate uptake over a broad external nitrate range. | Depending on external nitrate concentration, AtNPF6.3 has been estimated to contribute roughly 10–80% of whole-plant nitrate uptake. (pqac-00000002) | Nedelyaeva et al. (2024), https://doi.org/10.3390/ijms252413648 |
| Physiological role: nitrate sensing / primary nitrate response | NPF6.3 is a bona fide nitrate sensor/transceptor required for normal primary nitrate responses. | Mutant analyses (e.g., chl1 alleles) separate uptake from signaling defects and show that CHL1 controls nitrate-responsive gene expression and signaling across high- and low-nitrate ranges. (pqac-00000003, pqac-00000005) | Ho et al. (2009), https://doi.org/10.1016/j.cell.2009.07.004 |
| Physiological role: regulation of NRT2.1 and nitrate-response genes | NPF6.3 regulates expression of other nitrate transport/signaling genes, including NRT2.1. | NRT1.1/NPF6.3 is described as regulating expression of NRT2.1 and other primary nitrate-response genes; altered Thr101 status changes these transcriptional outputs. (pqac-00000004, pqac-00000005) | Sun & Zheng (2015), https://doi.org/10.3389/fphys.2015.00386; Ho et al. (2009), https://doi.org/10.1016/j.cell.2009.07.004 |
| Physiological role: root architecture | NPF6.3 links local nitrate supply to lateral root development through nitrate signaling and auxin-related mechanisms. | Reviews describe NPF6.3-mediated repression of lateral root growth under low nitrate via auxin transport/removal, while nitrate-dependent changes in Thr101 signaling influence root development and Ca2+ signaling outputs. (pqac-00000010, pqac-00000006) | Aluko et al. (2023), https://doi.org/10.3389/fpls.2023.1074839; Jia et al. (2023), https://doi.org/10.3390/ijms241914406 |


*Table: This table summarizes experimentally supported functional annotation facts for Arabidopsis NPF6.3/CHL1/NRT1.1, including transport properties, regulation, mechanism, localization, and physiological roles. It is useful as a compact evidence map for validating gene function and pathway context.*