PT4

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005315 phosphate transmembrane transporter activity	IEA GO_REF:0000002	MODIFY	Summary: InterPro (Phos_permease, IPR004738) mapping to phosphate transmembrane transporter activity. MtPT4 is a Pht1-family phosphate transporter whose activity was shown by complementation of yeast Pi-transport mutants, with low (~0.5 mM) apparent affinity and an acidic pH optimum consistent with H+-coupled uptake. Reason: The term is correct but generic. Pht1 transporters are H+/Pi symporters; UniProt records the H+-coupled reaction (RHEA:29939) from the original characterization, and PT4 is described as a secondary active transporter energized by the periarbuscular H+-ATPase gradient. The more informative child GO:0015317 phosphate:proton symporter activity is therefore proposed. No existing annotation carries an experimental molecular function term, so this MODIFY supplies the core MF. Proposed replacements: phosphate:proton symporter activity Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate. PMID:19692536 H + -ATPases, which create proton gradients necessary for secondary active transporters such as MtPT4, are induced in AM roots and have been localized to the periarbuscular membrane file:MEDTR/PT4/PT4-deep-research-falcon.md It is a transporter, not an enzyme that chemically converts phosphate.
GO:0005886 plasma membrane	IBA GO_REF:0000033	MODIFY	Summary: IBA (PANTHER PTN008392514) placing the family in the plasma membrane. Pht1 transporters generally act at the root plasma membrane, but MtPT4 is a lineage-specific (Pht1 subfamily I, AM-specific) member that is targeted exclusively to the branch domain of the periarbuscular membrane and is explicitly absent from the cortical-cell plasma membrane. Reason: The family-level node placement is sound for most Pht1 members, but there is target-specific evidence of divergence in localization: immunolocalization and native-promoter MtPT4-GFP show that MtPT4 is not in the plasma membrane and is restricted to the periarbuscular membrane. GO represents the periarbuscular membrane (GO:0085042) as a distinct host-derived membrane rather than as part of the plasma membrane, so the more accurate location is the periarbuscular membrane. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN008392514 · PTN008392514 SUPPORTS SOURCE BUT NOT TARGET Plasma membrane is appropriate for the broad MFS/SLC22/Pht1 node, but the AM-specific Pht1 subfamily I member MtPT4 is targeted to the periarbuscular membrane and excluded from the cortical-cell plasma membrane (immunolocalization and MtPT4-GFP). Proposed replacements: periarbuscular membrane Supporting Evidence: PMID:19692536 As reported previously, MtPT4 is not located in the plasma membrane, and the MtPT4-GFP marker is consistent with this and did not label the plasma membrane. PMID:19692536 the MtPT4-GFP marker is localized exclusively in the periarbuscular membrane in the region around the hyphal branches, and there is no signal arising from the membrane around the arbuscule trunk or from the plasma membrane of the cell PMID:12368495 immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane
GO:0005886 plasma membrane	IEA GO_REF:0000044	MODIFY	Summary: UniProt subcellular-location mapping (Cell membrane, SL-0039) to plasma membrane. The UniProt entry itself notes that the protein is present on the periarbuscular membrane in arbuscule-containing cells. Reason: MtPT4 is specifically excluded from the cortical-cell plasma membrane and resides in the periarbuscular membrane (branch domain). The periarbuscular membrane is continuous with, but compositionally distinct from, the plasma membrane and has its own GO term, which should replace plasma membrane. Proposed replacements: periarbuscular membrane Supporting Evidence: PMID:19692536 As reported previously, MtPT4 is not located in the plasma membrane, and the MtPT4-GFP marker is consistent with this and did not label the plasma membrane. PMID:12368495 immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane
GO:0006817 phosphate ion transport	IDA PMID:12368495 A phosphate transporter from Medicago truncatula involved in...	ACCEPT	Summary: IDA from the original MtPT4 characterization: yeast phosphate-transport mutant complementation and kinetic estimates demonstrate that MtPT4 transports phosphate. Reason: Direct functional evidence of phosphate transport by MtPT4; this is the core process of the protein (more precisely phosphate ion transmembrane transport across the periarbuscular membrane). Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate. PMID:12368495 The transport properties and spatial expression patterns of MtPT4 are consistent with a role in the acquisition of phosphate released by the fungus in the AM symbiosis.
GO:0006817 phosphate ion transport	IEA GO_REF:0000120	ACCEPT	Summary: Combined automated (ARBA + InterPro Phos_permease) annotation to phosphate ion transport. Reason: Consistent with the experimentally demonstrated phosphate transport activity of MtPT4 and with its Pht1 family membership. Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate.
GO:0006817 phosphate ion transport	IMP PMID:12368495 A phosphate transporter from Medicago truncatula involved in...	ACCEPT	Summary: IMP attributed to the original characterization (complementation of yeast phosphate-transport mutants). In planta genetic evidence (Javot et al. 2007) shows that mtpt4 mutants fail to acquire Pi delivered by the AM fungus. Reason: Phosphate transport is the core function of MtPT4. Loss of PT4 abolishes symbiotic Pi transport, independently confirming the curator's annotation. Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate. PMID:17242358 MtPT4, a Medicago truncatula member of subfamily I, is essential for the acquisition of P(i) delivered by the AM fungus. PMID:25841038 In Medicago truncatula, a Pi transporter, PT4, is required for symbiotic Pi transport, and in pt4, symbiotic Pi transport fails, arbuscules degenerate prematurely, and the symbiosis is not maintained.
GO:0009610 response to symbiotic fungus	IEA GO_REF:0000117	KEEP AS NON CORE	Summary: ARBA machine-learning annotation to response to symbiotic fungus, reflecting AM-specific induction of PT4 expression. Reason: PT4 transcription is strongly and specifically induced in arbuscule-containing cells, so the response annotation is factually grounded in expression data. However this is an expression readout; PT4 does not mediate the response, and its core role is phosphate transport within the AM association. Kept as non-core. Supporting Evidence: PMID:12368495 MtPT4 is expressed only in mycorrhizal roots, and the MtPT4 promoter directs expression exclusively in cells containing arbuscules.
GO:0009610 response to symbiotic fungus	IEP PMID:12368495 A phosphate transporter from Medicago truncatula involved in...	KEEP AS NON CORE	Summary: IEP: MtPT4 is expressed only in mycorrhizal roots and its promoter is active exclusively in arbuscule-containing cortical cells. Reason: Accurate expression-based annotation; PT4 is a classic marker of AM colonization. It describes induction of the gene rather than an activity of the protein, so it is non-core relative to symbiotic phosphate transport. Supporting Evidence: PMID:12368495 MtPT4 is expressed only in mycorrhizal roots, and the MtPT4 promoter directs expression exclusively in cells containing arbuscules.
GO:0009610 response to symbiotic fungus	IEP PMID:20687807 Transcription of two blue copper-binding protein isogenes is...	KEEP AS NON CORE	Summary: IEP from a study of blue copper-binding protein genes in which MtPT4 transcript levels were used as a reference marker of the functional AM association and correlated (weakly) with mycorrhizal development. Reason: The paper measured MtPT4 expression during colonization, so the expression-based annotation is supported, but MtPT4 was only a reference gene in that work. Non-core, expression-only. Supporting Evidence: PMID:20687807 MtPT4, selected as a reference gene of the functional plant-fungus association, showed a weaker correlation to mycorrhizal development.
GO:0009610 response to symbiotic fungus	IEP PMID:25841038 Suppression of Arbuscule Degeneration in Medicago truncatula...	KEEP AS NON CORE	Summary: IEP: PT4 expression in arbuscule-containing cortical cells during AM colonization, reported in the PT4/PT8/AMT2;3 study. Reason: Supported expression-based annotation; induction by the fungus is not the protein's function. Non-core. Supporting Evidence: PMID:25841038 In Medicago truncatula, a Pi transporter, PT4, is required for symbiotic Pi transport, and in pt4, symbiotic Pi transport fails, arbuscules degenerate prematurely, and the symbiosis is not maintained.
GO:0009610 response to symbiotic fungus	IEP PMID:26511916 Hyphal Branching during Arbuscule Development Requires Reduc...	KEEP AS NON CORE	Summary: IEP from the RAM1 study: PT4 induction during AM colonization depends on the GRAS transcription factor RAM1, which controls expression of periarbuscular membrane proteins. Reason: Expression of PT4 in colonized roots (RAM1-dependent) supports the IEP annotation, but this reflects transcriptional regulation of PT4, not a PT4 activity. Non-core. Supporting Evidence: PMID:26511916 the precise transcriptional control essential to place proteins in the periarbuscular membrane is controlled, at least in part, by RAM1
GO:0010247 detection of phosphate ion	IMP PMID:26476189 The phosphate transporters LjPT4 and MtPT4 mediate early roo...	MARK AS OVER ANNOTATED	Summary: IMP: mtpt4 mutants lack the phosphate-starvation-induced increase in lateral root number and show impaired expression of phosphate-starvation-induced genes; the authors suggest PT4 as a component of root-tip P-sensing machinery. Reason: Detection of phosphate ion (GO:0010247) asserts that the gene product receives the Pi stimulus and converts it into a signal (a transceptor role). The cited work shows a mutant phenotype in Pi-status responses, which the authors themselves only 'suggest' reflects sensing; an equally parsimonious explanation is that reduced Pi uptake at the root tip alters downstream Pi-status signalling. No evidence that PT4 signals independently of transport is available in the abstract. The curator has seen the full text, so this is not removed, but the claim exceeds what the phenotype establishes. Supporting Evidence: PMID:26476189 Lotus PT4i plants and Medicago mtpt4 mutants did not show any differential response to phosphate levels, suggesting that PT4 genes affect early root branching. PMID:26476189 We suggest PT4 genes as novel components of the P-sensing machinery at the root tip level, independently of AM fungi.
GO:0010311 lateral root formation	IMP PMID:26476189 The phosphate transporters LjPT4 and MtPT4 mediate early roo...	KEEP AS NON CORE	Summary: IMP: mtpt4 mutants did not alter lateral root number in response to phosphate levels (loss of the Pi-starvation-induced lateral-root increase). Reason: The phenotype concerns the Pi-responsiveness of lateral root branching in non-mycorrhizal roots. It is a genuine but peripheral, indirect developmental effect relative to PT4's core role in symbiotic Pi uptake. Kept as non-core. Supporting Evidence: PMID:26476189 Lotus PT4i plants and Medicago mtpt4 mutants did not show any differential response to phosphate levels, suggesting that PT4 genes affect early root branching.
GO:0016020 membrane	IEA GO_REF:0000002	ACCEPT	Summary: InterPro-based membrane annotation for a 12-TM MFS phosphate transporter. Reason: Correct, if general; MtPT4 is an integral multi-pass membrane protein (periarbuscular membrane). Supporting Evidence: PMID:12368495 immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane
GO:0016036 cellular response to phosphate starvation	IEP PMID:26476189 The phosphate transporters LjPT4 and MtPT4 mediate early roo...	KEEP AS NON CORE	Summary: IEP: PT4 expression in root tips of non-mycorrhizal roots depends on phosphate level (induced under low Pi). Reason: Expression of PT4 in root tips responds to Pi status, supporting the IEP; this is a secondary, AM-independent aspect of PT4 biology and an expression readout. Non-core. Supporting Evidence: PMID:26476189 quantitative RT-PCR confirmed the expression of Lotus and Medicago truncatula PT4 in the tips of non-mycorrhizal roots
GO:0016036 cellular response to phosphate starvation	IMP PMID:26476189 The phosphate transporters LjPT4 and MtPT4 mediate early roo...	KEEP AS NON CORE	Summary: IMP: mtpt4 mutants lose the lateral-root response to Pi starvation and show impaired induction of phosphate-starvation-induced genes and MtTIR1. Reason: Mutant phenotype supports a role for PT4 in the Pi-starvation response of non-mycorrhizal roots, plausibly through its Pi transport at the root tip. This is a secondary, non-core role. Supporting Evidence: PMID:26476189 Phosphate starvation-induced genes and a key auxin receptor, MtTIR1, showed an impaired expression in mtpt4 plants. PMID:26476189 Lotus PT4i plants and Medicago mtpt4 mutants did not show any differential response to phosphate levels, suggesting that PT4 genes affect early root branching.
GO:0022857 transmembrane transporter activity	IBA GO_REF:0000033	ACCEPT	Summary: IBA (PANTHER PTN008392514) for transmembrane transporter activity across the SLC22/Pht1-related MFS clade. Reason: Correct at the family level and consistent with the experimentally shown phosphate transport of MtPT4. The specific activity (phosphate:proton symporter) is captured via the MODIFY of GO:0005315. Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate.
GO:0022857 transmembrane transporter activity	IEA GO_REF:0000002	ACCEPT	Summary: InterPro (MFS domain, sugar transporter-like) mapping to transmembrane transporter activity. Reason: Correct but general. Note the sugar transporter-like domain reflects MFS fold homology; MtPT4 is a phosphate, not sugar, transporter. The specific activity is given by phosphate:proton symporter activity. Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate. file:MEDTR/PT4/PT4-deep-research-falcon.md It is a transporter, not an enzyme that chemically converts phosphate.
GO:0036377 arbuscular mycorrhizal association	IEA GO_REF:0000117	ACCEPT	Summary: ARBA annotation to arbuscular mycorrhizal association. Reason: PT4 performs the plant-side Pi uptake step at the periarbuscular interface, which is the defining nutrient exchange of the AM association, and is required for its maintenance. Supporting Evidence: PMID:17242358 MtPT4, a Medicago truncatula member of subfamily I, is essential for the acquisition of P(i) delivered by the AM fungus. PMID:17242358 Loss of MtPT4 function leads to premature death of the arbuscules; the fungus is unable to proliferate within the root, and symbiosis is terminated.
GO:0036377 arbuscular mycorrhizal association	IMP PMID:12368495 A phosphate transporter from Medicago truncatula involved in...	ACCEPT	Summary: IMP/expression-based evidence from the original paper relating MtPT4 to phosphate acquisition in the AM symbiosis. Subsequent genetic work showed that loss of MtPT4 causes premature arbuscule death and termination of the symbiosis. Reason: MtPT4 directly executes a step of the AM association (transport of fungus-delivered Pi across the periarbuscular membrane) and is genetically required for arbuscule maintenance. This passes both necessity and participation tests. Supporting Evidence: PMID:12368495 The transport properties and spatial expression patterns of MtPT4 are consistent with a role in the acquisition of phosphate released by the fungus in the AM symbiosis. PMID:17242358 Loss of MtPT4 function leads to premature death of the arbuscules; the fungus is unable to proliferate within the root, and symbiosis is terminated.
GO:0036377 arbuscular mycorrhizal association	IMP PMID:25841038 Suppression of Arbuscule Degeneration in Medicago truncatula...	ACCEPT	Summary: IMP: in pt4, symbiotic Pi transport fails, arbuscules degenerate prematurely and the symbiosis is not maintained (under nitrogen-replete conditions). Reason: Direct genetic evidence that PT4 is required for a functional, persistent AM association; PT4 performs the symbiotic Pi uptake step. Core process. Supporting Evidence: PMID:25841038 In Medicago truncatula, a Pi transporter, PT4, is required for symbiotic Pi transport, and in pt4, symbiotic Pi transport fails, arbuscules degenerate prematurely, and the symbiosis is not maintained. PMID:25841038 arbuscule life span is influenced by PT4 and ATM2;3, and their relative importance varies with the nitrogen status of the plant
GO:0043562 cellular response to nitrogen levels	IEA GO_REF:0000117	REMOVE	Summary: ARBA annotation to cellular response to nitrogen levels, presumably learned from the pt4 low-nitrogen suppression phenotype. Reason: There is no evidence that PT4 senses or responds to nitrogen, or transports nitrogen. The nitrogen dependence reported for pt4 is a genetic-suppression effect: premature arbuscule degeneration in pt4 is suppressed by N deprivation through AMT2;3, independently of PT4 (and of PT8). Propagating this to PT4 orthologues by machine learning is unjustified. Supporting Evidence: PMID:25841038 Premature arbuscule degeneration (PAD) is suppressed when pt4 mutants are nitrogen-deprived PMID:25841038 ATM2;3 but not AMT2;4 was required for suppression of PAD in pt4 file:MEDTR/PT4/PT4-deep-research-falcon.md They do not demonstrate that MtPT4 transports ammonium.
GO:0043562 cellular response to nitrogen levels	IMP PMID:25841038 Suppression of Arbuscule Degeneration in Medicago truncatula...	MARK AS OVER ANNOTATED	Summary: IMP: the pt4 premature arbuscule degeneration phenotype is suppressed under nitrogen deprivation in an AMT2;3-dependent manner. Reason: The experiment shows that nitrogen status modifies the pt4 mutant phenotype, i.e. a genetic interaction between N status/AMT2;3 and loss of PT4. It does not show that PT4 participates in a cellular response to nitrogen; the suppression requires AMT2;3, not PT4. The curator's annotation is retained (full text not read) but flagged as an over-interpretation. Supporting Evidence: PMID:25841038 Premature arbuscule degeneration (PAD) is suppressed when pt4 mutants are nitrogen-deprived PMID:25841038 ATM2;3 but not AMT2;4 was required for suppression of PAD in pt4 PMID:25841038 arbuscule life span is influenced by PT4 and ATM2;3, and their relative importance varies with the nitrogen status of the plant
GO:0055085 transmembrane transport	IDA PMID:12368495 A phosphate transporter from Medicago truncatula involved in...	MODIFY	Summary: IDA for transmembrane transport from yeast complementation/uptake assays. Reason: Correct but uninformative; the transported substrate is known. The specific process demonstrated is phosphate ion transmembrane transport (GO:0035435), a child of both transmembrane transport and phosphate ion transport. Proposed replacements: phosphate ion transmembrane transport Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate.
GO:0055085 transmembrane transport	IEA GO_REF:0000120	MODIFY	Summary: Combined automated annotation to transmembrane transport. Reason: Correct but general parent of the process MtPT4 performs; the substrate is established experimentally, so phosphate ion transmembrane transport (GO:0035435) is the informative term. Proposed replacements: phosphate ion transmembrane transport Supporting Evidence: PMID:12368495 Complementation of yeast phosphate transport mutants indicated that MtPT4 functions as a phosphate transporter, and estimates of the K(m) suggest a relatively low affinity for phosphate.
GO:0085042 periarbuscular membrane	IDA PMID:12368495 A phosphate transporter from Medicago truncatula involved in...	ACCEPT	Summary: IDA: membrane fractionation and immunolocalization place MtPT4 on the periarbuscular membrane around arbuscule branches; confirmed by native-promoter MtPT4-GFP live imaging. Reason: Core location. MtPT4 defines the arbuscule branch domain of the periarbuscular membrane, the site of symbiotic nutrient exchange. Supporting Evidence: PMID:12368495 immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane PMID:19692536 the MtPT4-GFP marker is localized exclusively in the periarbuscular membrane in the region around the hyphal branches, and there is no signal arising from the membrane around the arbuscule trunk or from the plasma membrane of the cell
GO:0085042 periarbuscular membrane	IEA GO_REF:0000117	ACCEPT	Summary: ARBA annotation to periarbuscular membrane. Reason: Consistent with direct immunolocalization and GFP-fusion localization of MtPT4. Supporting Evidence: PMID:12368495 immunolocalization revealed that MtPT4 colocalizes with the arbuscules, consistent with a location on the periarbuscular membrane PMID:19692536 the MtPT4-GFP marker is localized exclusively in the periarbuscular membrane in the region around the hyphal branches, and there is no signal arising from the membrane around the arbuscule trunk or from the plasma membrane of the cell

Gene Description

Existing Annotations Review

Core Functions

H+-coupled phosphate import across the periarbuscular membrane (arbuscule branch domain), taking up Pi released by the AM fungus into the root cortical cell; the plant-side uptake step of the mycorrhizal phosphate pathway, required for arbuscule maintenance.

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