CHL1 (NPF6.3 / NRT1.1; At1g12110, UniProt Q05085) is the founding member of the plant NRT1/PTR (NPF) nitrate transporter family and the first cloned plant nitrate transporter. It is a plasma-membrane, proton-coupled nitrate symporter belonging to the major facilitator superfamily (MFS), expressed most strongly in roots (epidermal and vascular cells) as well as guard cells. CHL1 is a dual-affinity transporter showing biphasic kinetics, with a high-affinity phase (Km ~50 uM) and a low-affinity phase (Km ~4 mM). The affinity mode is switched by phosphorylation of Thr101 by the CBL1/CBL9-CIPK23 kinase module: phosphorylation favors high-affinity transport while dephosphorylation favors low-affinity transport. Beyond transport, CHL1 acts as a nitrate sensor ("transceptor") that initiates the primary nitrate response and regulates expression of other nitrate transport genes such as NRT2.1; its sensing function is separable from its uptake activity. CHL1 also facilitates basipetal auxin transport, linking local nitrate availability to lateral root development, and contributes to stomatal opening and to root-to-shoot nitrate translocation. The gene was originally identified through chlorate-resistance screens (chlorate is a toxic nitrate analog), giving rise to the CHL1 name.
Definition: A signaling cascade initiated by perception of extracellular nitrate by a nitrate sensor/transceptor (such as CHL1/NRT1.1), leading to CBL/CIPK-mediated phosphorylation events and downstream transcriptional reprogramming (primary nitrate response).
Justification: No precise GO term currently captures the transceptor-initiated nitrate signaling pathway distinct from the generic "response to nitrate" (GO:0010167). The falcon synthesis describes nitrate-induced Ca2+ signals perceived by CBL1/CBL9, activating CIPK23, which phosphorylates NRT1.1 at Thr101 and shifts apparent affinity from Km ~4 mM to Km ~40 uM.
Parent term: response to nitrate
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006857 oligopeptide transport | IEA GO_REF:0000002 | REMOVE | Summary: Oligopeptide transport is inferred from membership in the proton-dependent oligopeptide transporter (POT/PTR) family (InterPro IPR018456). The NPF/NRT1-PTR family is evolutionarily related to peptide transporters, but CHL1/NRT1.1 specifically transports nitrate, not peptides. Reason: Family-level (InterPro) over-annotation. CHL1/NRT1.1 recognizes specifically nitrate and chlorate but NOT the di-peptide Ala-Ala (UniProt FUNCTION), so peptide transport is not a function of this protein. The PTR family name reflects ancestry, not CHL1 substrate specificity. Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md Across sources, nitrate (NO3−) is the core substrate, and NPF6.3 contributes substantially to nitrate uptake |
| GO:0016020 membrane | IEA GO_REF:0000120 | MODIFY | Summary: Generic membrane localization from UniProt/InterPro. CHL1 is a multi-pass membrane protein. Reason: "Membrane" is correct but uninformative; the experimentally established and functionally relevant location is the plasma membrane (GO:0005886), which is already annotated (IDA). Generalize/replace with the more specific term. Proposed replacements: plasma membrane Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md localizes to the **plasma membrane** of **root epidermal** and **vascular** cells, with additional localization reported in **guard cells** |
| GO:0022857 transmembrane transporter activity | IEA GO_REF:0000002 | MODIFY | Summary: Generic transmembrane transporter activity inferred from InterPro domains. CHL1's actual transported substrate is nitrate. Reason: Too general. The specific, experimentally established molecular function is nitrate transmembrane transporter activity (GO:0015112), already annotated (IMP). Replace the generic parent with the specific child. Proposed replacements: nitrate transmembrane transporter activity Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md NPF6.3/CHL1/NRT1.1 is widely regarded as a **dual-affinity** nitrate transporter with **biphasic kinetics**, enabling transport across a wide external nitrate range. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | MODIFY | Summary: Generic transmembrane transport process inferred from InterPro. CHL1's specific process is nitrate transmembrane transport. Reason: Too general. The specific, experimentally established process is nitrate transmembrane transport (GO:0015706), already annotated (IMP). Replace the generic parent with the specific child. Proposed replacements: nitrate transmembrane transport Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md Across sources, nitrate (NO3−) is the core substrate, and NPF6.3 contributes substantially to nitrate uptake |
| GO:0005515 protein binding | IPI PMID:19766570 CHL1 functions as a nitrate sensor in plants. | MODIFY | Summary: Protein binding (IPI) capturing the physical interaction between CHL1/NRT1.1 and the protein kinase CIPK23 (Q93VD3). CIPK23 phosphorylates CHL1 at Thr101 and is the kinase that controls the high/low-affinity switch and nitrate signaling. Reason: "protein binding" is uninformative on its own. The functionally meaningful relationship is that CHL1 is a substrate/partner of the CIPK23 kinase in the CBL-CIPK nitrate-signaling module. Replace with a kinase-binding term to capture the regulatory interaction. Proposed replacements: protein kinase binding Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md Nitrate-induced Ca2+ signals are perceived by **CBL1/CBL9**, activating **CIPK23**, which phosphorylates NRT1.1 at Thr101 and shifts apparent affinity from **Km ~4 mM** to **Km ~40 µM**. |
| GO:0005515 protein binding | IPI PMID:31431511 Phosphorylation-Mediated Dynamics of Nitrate Transceptor NRT... | REMOVE | Summary: Protein binding (IPI) from a high-throughput/interaction study reporting an interaction with AT2G45820 (a membrane-associated protein). This is a generic binding annotation with the interactant indicated only by WITH/FROM. Reason: Generic "protein binding" provides no functional information and the interactant (AT2G45820) does not correspond to a well-characterized CHL1 functional partner in the literature. Per curation guidelines, uninformative protein binding should not be retained; the meaningful regulatory interaction (CIPK23) is captured separately. |
| GO:0005886 plasma membrane | IDA PMID:31431511 Phosphorylation-Mediated Dynamics of Nitrate Transceptor NRT... | ACCEPT | Summary: CHL1/NRT1.1 is directly localized to the plasma membrane (IDA). As a multi-pass membrane protein expressed in root epidermal/vascular cells and guard cells, the plasma membrane is the functionally relevant location for nitrate uptake from the soil solution. Reason: Plasma-membrane localization is experimentally established (IDA, PMID:31431511) and is the correct, specific cellular component for this nutrient uptake transporter. Supporting Evidence: |
| GO:0010540 basipetal auxin transport | IMP PMID:31431511 Phosphorylation-Mediated Dynamics of Nitrate Transceptor NRT... | KEEP AS NON CORE | Summary: CHL1/NRT1.1 contributes to basipetal auxin transport, removing auxin from lateral root primordia under low nitrate and thereby restraining lateral root growth; higher nitrate represses this effect. Reason: Experimentally supported (PMID:20627075, PMID:31431511) and mechanistically important for nitrate-dependent root architecture, but a downstream developmental output rather than the core molecular function. Keep as non-core. Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md NRT1.1 facilitates basipetal auxin transport and negatively regulates auxin biosynthesis/transport-related genes (e.g., TAR2, LAX3), thereby removing auxin from lateral root primordia under low nitrate and inhibiting lateral root growth |
| GO:0048527 lateral root development | IMP PMID:31431511 Phosphorylation-Mediated Dynamics of Nitrate Transceptor NRT... | KEEP AS NON CORE | Summary: CHL1/NRT1.1 links local nitrate supply to lateral root development via nitrate signaling and auxin-related mechanisms. Reason: A genuine, experimentally supported developmental role downstream of nitrate sensing and auxin transport, but pleiotropic/developmental rather than the core transport-sensing function. Keep as non-core. Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md NPF6.3 links local nitrate supply to lateral root development through nitrate signaling and auxin-related mechanisms. |
| GO:0048573 photoperiodism, flowering | IMP PMID:30972097 NRT1.1 Regulates Nitrate Allocation and Cadmium Tolerance in... | UNDECIDED | Summary: A single IMP annotation links CHL1/NRT1.1 to photoperiodic flowering, consistent with nitrate-status effects on flowering time. Reason: The cited reference PMID:30972097 ("NRT1.1 Regulates Nitrate Allocation and Cadmium Tolerance in Arabidopsis", Jian et al., Front Plant Sci 2019) is entirely about cadmium tolerance and nitrate/vacuolar allocation; the word "flower"/"flowering" does not appear anywhere in the full text, and the only photoperiod mention is the standard "16-h photoperiod" growth condition. The cited paper therefore provides no evidence for photoperiodic flowering, so the GO:0048573 IMP annotation appears to be a TAIR curation error (likely a mis-attributed PMID). Marking UNDECIDED pending clarification rather than propagating the annotation; this should be reported to TAIR. |
| GO:0010167 response to nitrate | IMP PMID:17148611 The Arabidopsis NRT1.1 transporter participates in the signa... | ACCEPT | Summary: CHL1/NRT1.1 acts as a nitrate sensor ("transceptor") that initiates the primary nitrate response. Mutant analyses (chl1 alleles) separate uptake from signaling defects, showing that CHL1 controls nitrate-responsive gene expression independently of its uptake activity. Reason: The transceptor/nitrate-sensing function is a landmark, experimentally validated role (PMID:19766570; uptake-deficient but sensing-competent mutants). Strongly corroborated by the falcon synthesis. Supporting Evidence: PMID:19766570 CHL1 functions as a nitrate sensor in plants file:ARATH/CHL1/CHL1-deep-research-falcon.md A core concept in nitrate biology is that NPF6.3 acts as a **transceptor**—a transporter with receptor-like signaling capability—helping initiate the **primary nitrate response (PNR)** and modulating expression of other nitrate transport genes (e.g., **NRT2.1**) and developmental programs. file:ARATH/CHL1/CHL1-deep-research-falcon.md 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. |
| GO:0009414 response to water deprivation | IMP PMID:12509525 The nitrate transporter AtNRT1.1 (CHL1) functions in stomata... | KEEP AS NON CORE | Summary: chl1 mutants show reduced stomatal opening and reduced transpiration, conferring enhanced drought tolerance; thus CHL1 contributes to stomatal physiology and the response to water deprivation through its guard-cell expression. Reason: Experimentally supported (PMID:12509525) but a pleiotropic guard-cell role secondary to the core nitrate transport/sensing function. Keep as non-core. Supporting Evidence: PMID:12509525 The nitrate transporter AtNRT1.1 (CHL1) functions in stomatal opening and contributes to drought susceptibility in Arabidopsis |
| GO:0015112 nitrate transmembrane transporter activity | IMP PMID:9844028 The Arabidopsis CHL1 protein plays a major role in high-affi... | ACCEPT | Summary: Nitrate transmembrane transporter activity is the core molecular function of CHL1/NRT1.1, demonstrated by mutant analysis (chl1 alleles cause reduced nitrate uptake and chlorate resistance) and by heterologous expression in Xenopus oocytes, where CHL1 transports nitrate by proton-coupled symport over a wide concentration range (dual-affinity, biphasic kinetics; Km ~50 uM high-affinity and ~4 mM low-affinity phases). Reason: This is the well-established, experimentally validated core function of CHL1/NRT1.1, the first cloned plant nitrate transporter. Supported by decades of primary literature (PMID:8453665, PMID:10330471, PMID:24572362, PMID:24572366) and corroborated by the falcon deep research synthesis. Supporting Evidence: PMID:9844028 The Arabidopsis CHL1 protein plays a major role in high-affinity nitrate uptake file:ARATH/CHL1/CHL1-deep-research-falcon.md NPF6.3/CHL1/NRT1.1 is widely regarded as a **dual-affinity** nitrate transporter with **biphasic kinetics**, enabling transport across a wide external nitrate range. file:ARATH/CHL1/CHL1-deep-research-falcon.md NPF6.3 is described as a **proton-coupled nitrate symporter**. |
| GO:0015706 nitrate transmembrane transport | IMP PMID:9844028 The Arabidopsis CHL1 protein plays a major role in high-affi... | ACCEPT | Summary: CHL1/NRT1.1 mediates nitrate transmembrane transport, the biological-process counterpart of its nitrate transporter activity, contributing a large fraction of root nitrate uptake across a broad external nitrate range. Reason: Directly supported by mutant uptake assays and heterologous expression. The falcon synthesis estimates CHL1/NPF6.3 contributes roughly 10-80% of whole-plant nitrate uptake depending on nitrate concentration. Supporting Evidence: file:ARATH/CHL1/CHL1-deep-research-falcon.md AtNPF6.3 has been estimated to contribute roughly 10–80% of whole-plant nitrate uptake. |
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Download this section (compressed HTML)Q: Beyond Thr101, how do the CNGC15 Ca2+ channel complex and ABI2/ABA inputs quantitatively tune the balance between CHL1's transport and signaling outputs in vivo?
Q: What structural changes accompany the homodimer-to-monomer transition upon Thr101 phosphorylation, and how do they convert the transporter between high- and low-affinity kinetic modes?
Q: Should TAIR be contacted about the GO:0048573 (photoperiodism, flowering) IMP annotation? Its cited reference PMID:30972097 concerns nitrate allocation and cadmium tolerance and contains no flowering evidence, suggesting a mis-attributed PMID / database error.
Experiment: Compare transport-dead but sensing-competent mutants (e.g., Pro492 variants) with sensing-dead variants in planta, measuring both nitrate uptake (15N flux, electrophysiology in oocytes) and primary-nitrate-response gene induction (e.g., NRT2.1, NIA1) to dissect the two functions.
Hypothesis: CHL1's nitrate-sensing (transceptor) function is mechanistically separable from its transport activity and depends on distinct residues.
Type: structure-function and physiological analysis
Experiment: Use phospho-specific antibodies and phosphomimic/non-phosphorylatable Thr101 mutants across a nitrate concentration gradient, combined with cipk23 and cbl1 cbl9 mutants, to quantify how phosphorylation state correlates with apparent Km in vivo.
Hypothesis: The CBL1/CBL9-CIPK23 module phosphorylates Thr101 in a nitrate-concentration- dependent manner to switch transport affinity.
Type: biochemical and genetic analysis
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