AHK3 (ARABIDOPSIS HISTIDINE KINASE 3; ORE12) is one of three CHASE-domain hybrid sensor histidine kinases that act as cytokinin receptors in Arabidopsis thaliana. It is an integral membrane protein found mainly in the endoplasmic reticulum, with a minor plasma-membrane pool; cytokinin binding to its extracytosolic CHASE domain activates histidine autophosphorylation and phosphotransfer via its receiver domain to AHP histidine phosphotransfer proteins, which deliver the signal to nuclear type-B ARR transcription factors. AHK3 is the dominant receptor in leaves: it mediates cytokinin-dependent delay of leaf senescence through phosphorylation of ARR2 (a gain-of-function ore12 allele delays senescence), and with AHK2 it controls leaf cell number, chlorophyll content, shoot growth and secondary growth. It also contributes to repression of phosphate-starvation and iron-uptake genes by cytokinin, to sugar sensitivity, and acts as a negative regulator of drought, salt, cold and abscisic-acid responses.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000155 phosphorelay sensor kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: AHK3 is a CHASE-domain hybrid sensor histidine kinase that autophosphorylates on its conserved transmitter-domain histidine and relays the phosphoryl group via its receiver domain to AHP phosphotransfer proteins. Reason: Phosphorelay sensor kinase activity is the core catalytic function of the cytokinin receptors and is directly supported by cytokinin-dependent kinase/phosphorelay assays in yeast and E. coli. Supporting Evidence: PMID:16407152 Here, we found that AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0000155 phosphorelay sensor kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: AHK3 is a CHASE-domain hybrid sensor histidine kinase that autophosphorylates on its conserved transmitter-domain histidine and relays the phosphoryl group via its receiver domain to AHP phosphotransfer proteins. Reason: Phosphorelay sensor kinase activity is the core catalytic function of the cytokinin receptors and is directly supported by cytokinin-dependent kinase/phosphorelay assays in yeast and E. coli. Supporting Evidence: PMID:16407152 Here, we found that AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0000160 phosphorelay signal transduction system | IBA GO_REF:0000033 | ACCEPT | Summary: AHK3 initiates the multistep His-Asp phosphorelay (AHK -> AHP -> ARR) that transduces the cytokinin signal. Reason: Participation in phosphorelay signal transduction is the defining role of the CRE-family receptors. Supporting Evidence: PMID:12068096 A multistep two-component phosphorelay system with distinct AHK, AHP, and ARR proteins plays a central role in cytokinin signal transduction PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0000160 phosphorelay signal transduction system | IEA GO_REF:0000002 | ACCEPT | Summary: AHK3 initiates the multistep His-Asp phosphorelay (AHK -> AHP -> ARR) that transduces the cytokinin signal. Reason: Participation in phosphorelay signal transduction is the defining role of the CRE-family receptors. Supporting Evidence: PMID:12068096 A multistep two-component phosphorelay system with distinct AHK, AHP, and ARR proteins plays a central role in cytokinin signal transduction PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0004673 protein histidine kinase activity | IDA PMID:16753566 Cytokinins regulate a bidirectional phosphorelay network in ... | ACCEPT | Summary: AHK3 has protein histidine kinase activity (autophosphorylation and phosphotransfer to AHPs), stimulated by cytokinin binding. Reason: Histidine kinase activity of the CRE-family receptors is established biochemically (cytokinin-dependent kinase activity in yeast/E. coli and in vitro). Supporting Evidence: PMID:16407152 A loss-of-function mutation in AHK3, but not of the other cytokinin receptors, conferred a reduced sensitivity to cytokinin in cytokinin-dependent delay of leaf senescence and abolished cytokinin-dependent phosphorylation of ARR2. PMID:16753566 Arabidopsis perceives cytokinins with three related and partially redundant receptor histidine kinases (HKs) |
| GO:0004673 protein histidine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: AHK3 has protein histidine kinase activity (autophosphorylation and phosphotransfer to AHPs), stimulated by cytokinin binding. Reason: Histidine kinase activity of the CRE-family receptors is established biochemically (cytokinin-dependent kinase activity in yeast/E. coli and in vitro). Supporting Evidence: PMID:16407152 A loss-of-function mutation in AHK3, but not of the other cytokinin receptors, conferred a reduced sensitivity to cytokinin in cytokinin-dependent delay of leaf senescence and abolished cytokinin-dependent phosphorylation of ARR2. PMID:16753566 Arabidopsis perceives cytokinins with three related and partially redundant receptor histidine kinases (HKs) |
| GO:0004673 protein histidine kinase activity | IMP PMID:16407152 Cytokinin-mediated control of leaf longevity by AHK3 through... | ACCEPT | Summary: AHK3 has protein histidine kinase activity (autophosphorylation and phosphotransfer to AHPs), stimulated by cytokinin binding. Reason: Histidine kinase activity of the CRE-family receptors is established biochemically (cytokinin-dependent kinase activity in yeast/E. coli and in vitro). Supporting Evidence: PMID:16407152 A loss-of-function mutation in AHK3, but not of the other cytokinin receptors, conferred a reduced sensitivity to cytokinin in cytokinin-dependent delay of leaf senescence and abolished cytokinin-dependent phosphorylation of ARR2. PMID:16753566 Arabidopsis perceives cytokinins with three related and partially redundant receptor histidine kinases (HKs) |
| GO:0004721 phosphoprotein phosphatase activity | IDA NOT PMID:16753566 Cytokinins regulate a bidirectional phosphorelay network in ... | ACCEPT | Summary: NOT annotation (GOA, IDA, Mahonen et al. 2006): AHK3 is asserted not to have the phosphoprotein phosphatase activity toward phospho-AHPs that this study demonstrated for CRE1/AHK4. Reason: The cached abstract of PMID:16753566 (abstract-only) characterises only CRE1 (kinase and phosphatase) and AHK2 (kinase-favouring); it says nothing about AHK3, so there is no AHK3-specific sentence to quote. Any AHK3 data are in the full text, which is not available in the cache. We defer to the curator, who read the full paper, and retain the NOT qualifier. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK3 with the phosphotransfer protein AHP4. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK3 with the phosphotransfer protein AHP5. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | MODIFY | Summary: Interaction of AHK3 with the receptor AHK4 (yeast two-hybrid/coaffinity). Reason: Generic protein binding is uninformative; the partner is a histidine kinase, so the more specific protein histidine kinase binding term captures the reported receptor heteromerization. Proposed replacements: protein histidine kinase binding Supporting Evidence: PMID:21841169 Furthermore, membrane-bound AHK3 interacts with AHK4 in vivo. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | MODIFY | Summary: Interaction of AHK3 with the receptor AHK2 (UniProtKB:Q9C5U2), from the yeast two-hybrid/coaffinity interaction screen of PMID:16965536. Reason: Generic protein binding is uninformative; the partner is a histidine kinase, so the more specific protein histidine kinase binding term captures the reported receptor heteromerization. The specific AHK3-AHK2 pair is not named in the cached abstract (abstract-only); this GOA IPI row (with Q9C5U2) is the only source for the AHK3-AHK2 interaction, so we defer to the curator who read the full paper. The quote below documents only the screening method that produced the interaction data. Proposed replacements: protein histidine kinase binding Supporting Evidence: PMID:16965536 Using the yeast two-hybrid system, we detected 42 new interactions, of which more than 90% were confirmed by in vitro coaffinity purification. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK3 with the phosphotransfer protein AHP3. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK3 with the phosphotransfer protein AHP2. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | REMOVE | Summary: Interaction of AHK3 with the phosphotransfer protein AHP1. Reason: Generic protein binding is uninformative. The interaction is the physical basis of phosphotransfer to AHPs, which is already captured by phosphorelay sensor kinase activity (GO:0000155); GO has no specific term for binding a histidine phosphotransfer protein. Removal does not imply the interaction is false. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0005515 protein binding | IPI PMID:18642946 Toward an interaction map of the two-component signaling pat... | REMOVE | Summary: Yeast two-hybrid interaction of AHK3 with Q8RY18 from a medium-scale screen. Reason: Generic protein binding from a large-scale yeast two-hybrid screen is uninformative and the biological relevance of this partner to cytokinin perception is untested. Removal does not imply the interaction is false. Supporting Evidence: PMID:18642946 we conducted medium-scale yeast two-hybrid screens focusing on those members of the TCS, which are thought to be involved in cytokinin signaling |
| GO:0005576 extracellular region | ISM GO_REF:0000122 | REMOVE | Summary: Sequence-model (ISM) prediction of an extracellular location, likely triggered by the N-terminal hydrophobic segment. Reason: AHK3 is a polytopic membrane receptor whose CHASE sensor domain faces the ER lumen/apoplast; the protein is not secreted into the extracellular region. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005634 nucleus | HDA PMID:28887381 Global Analysis of Membrane-associated Protein Oligomerizati... | MARK AS OVER ANNOTATED | Summary: High-throughput proteomic detection of AHK3 in a nuclear-associated fraction. Reason: AHK3 is an integral ER/plasma-membrane receptor; detection in nuclear fractions in a membrane proteomics screen likely reflects co-fractionation with the contiguous nuclear envelope/ER rather than a functional nuclear pool. Supporting Evidence: PMID:21841169 cell biological and biochemical evidence for the localization of functional fluorophor-tagged fusions of Arabidopsis histidine kinase 3 (AHK3) and 4 (AHK4), members of the cytokinin receptor family, in the endoplasmic reticulum (ER) is provided |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: AHK3 is an integral membrane receptor; the bulk of cytokinin binding and receptor-fluorescent protein fusions reside in the endoplasmic reticulum membrane. Reason: ER-membrane localization of the CRE-family receptors is supported by cytokinin-binding, fluorescence and fractionation data. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. PMID:21841169 cell biological and biochemical evidence for the localization of functional fluorophor-tagged fusions of Arabidopsis histidine kinase 3 (AHK3) and 4 (AHK4), members of the cytokinin receptor family, in the endoplasmic reticulum (ER) is provided |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: AHK3 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005886 plasma membrane | IDA PMID:16407152 Cytokinin-mediated control of leaf longevity by AHK3 through... | KEEP AS NON CORE | Summary: AHK3 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: AHK3 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0005886 plasma membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: AHK3 was originally modelled as a plasma-membrane receptor; a minor plasma-membrane pool is reported, but most receptor protein and cytokinin binding are in the ER. Reason: Plasma membrane localization is plausible for a minor fraction but is not the principal site of the receptor; ER membrane is the better-supported location. Supporting Evidence: PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
| GO:0006970 response to osmotic stress | IMP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK3 contributes to 'response to osmotic stress' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | MODIFY | Summary: Electronic annotation to the generic signal transduction term. Reason: Correct but uninformative; the specific pathway is cytokinin-activated signaling. Proposed replacements: cytokinin-activated signaling pathway Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. |
| GO:0009409 response to cold | IEP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK3 contributes to 'response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK1/ATHK1, AHK2, AHK3, and CRE1 were shown to be stress-responsive |
| GO:0009414 response to water deprivation | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'response to water deprivation' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009414 response to water deprivation | IEP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK3 contributes to 'response to water deprivation' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009651 response to salt stress | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'response to salt stress' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009651 response to salt stress | IEP PMID:18077346 Functional analysis of AHK1/ATHK1 and cytokinin receptor his... | KEEP AS NON CORE | Summary: AHK3 contributes to 'response to salt stress' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:18077346 AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. |
| GO:0009736 cytokinin-activated signaling pathway | TAS PMID:12068096 Two-component signal transduction pathways in Arabidopsis. | ACCEPT | Summary: AHK3 is one of three partially redundant cytokinin receptors that initiate cytokinin-activated two-component signaling. Reason: Core biological process of the receptor; triple receptor mutants are completely cytokinin-insensitive. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:15166290 Plants carrying mutations in all three genes did not show cytokinin responses, including inhibition of root elongation, inhibition of root formation, cell proliferation in and greening of calli, and induction of cytokinin primary-response genes. |
| GO:0009884 cytokinin receptor activity | IEA GO_REF:0000117 | ACCEPT | Summary: AHK3 binds cytokinins with its extracytosolic CHASE domain and transduces the signal by histidine autophosphorylation. Reason: Cytokinin receptor activity is the core molecular function of the CRE-family histidine kinases. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:16407152 AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. |
| GO:0009884 cytokinin receptor activity | TAS PMID:15923327 Interaction between phosphate-starvation, sugar, and cytokin... | ACCEPT | Summary: AHK3 binds cytokinins with its extracytosolic CHASE domain and transduces the signal by histidine autophosphorylation. Reason: Cytokinin receptor activity is the core molecular function of the CRE-family histidine kinases. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:16407152 AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. |
| GO:0009884 cytokinin receptor activity | TAS PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | ACCEPT | Summary: AHK3 binds cytokinins with its extracytosolic CHASE domain and transduces the signal by histidine autophosphorylation. Reason: Cytokinin receptor activity is the core molecular function of the CRE-family histidine kinases. Supporting Evidence: PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:16407152 AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. |
| GO:0009909 regulation of flower development | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of flower development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:19913077 Mutation of the cytokinin receptors Arabidopsis histidine kinase 2 (AHK2) and AHK3 nearly abolished the transgene phenotypes and the enhanced CUC2 and CUC3 transcription induced by cytokinin. |
| GO:0009909 regulation of flower development | IMP PMID:19913077 Cytokinin overproduction-caused alteration of flower develop... | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of flower development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:19913077 Mutation of the cytokinin receptors Arabidopsis histidine kinase 2 (AHK2) and AHK3 nearly abolished the transgene phenotypes and the enhanced CUC2 and CUC3 transcription induced by cytokinin. |
| GO:0010029 regulation of seed germination | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of seed germination' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:16361392 Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. |
| GO:0010029 regulation of seed germination | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of seed germination' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:16361392 Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. |
| GO:0010029 regulation of seed germination | IMP PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of seed germination' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:16361392 Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. |
| GO:0010087 phloem or xylem histogenesis | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'phloem or xylem histogenesis' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0010087 phloem or xylem histogenesis | IMP PMID:19622803 The histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS... | KEEP AS NON CORE | Summary: AHK3 contributes to 'phloem or xylem histogenesis' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0010150 leaf senescence | IMP PMID:16407152 Cytokinin-mediated control of leaf longevity by AHK3 through... | KEEP AS NON CORE | Summary: AHK3 is the main receptor mediating cytokinin-dependent delay of leaf senescence, via phosphorylation of ARR2. Reason: Well supported physiological output of AHK3 signaling, downstream of the core receptor function. Supporting Evidence: PMID:16407152 A loss-of-function mutation in AHK3, but not of the other cytokinin receptors, conferred a reduced sensitivity to cytokinin in cytokinin-dependent delay of leaf senescence and abolished cytokinin-dependent phosphorylation of ARR2. PMID:16407152 AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. |
| GO:0010271 regulation of chlorophyll catabolic process | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of chlorophyll catabolic process' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:16361392 lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss |
| GO:0010271 regulation of chlorophyll catabolic process | IMP PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of chlorophyll catabolic process' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:16361392 lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss |
| GO:0016036 cellular response to phosphate starvation | IMP PMID:15923327 Interaction between phosphate-starvation, sugar, and cytokin... | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to phosphate starvation' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:15923327 In addition, ahk3 showed reduced cytokinin repression of several Pi starvation-responsive genes and increased sucrose sensitivity. |
| GO:0016772 transferase activity, transferring phosphorus-containing groups | IEA GO_REF:0000002 | MODIFY | Summary: Electronic annotation to a very general transferase term. Reason: Correct but uninformative; the specific activity is phosphorelay sensor kinase activity. Proposed replacements: phosphorelay sensor kinase activity Supporting Evidence: PMID:16407152 Here, we found that AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. |
| GO:0034757 negative regulation of iron ion transport | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Cytokinin represses root iron-uptake genes in an AHK3/CRE1-dependent manner. Reason: Supported downstream physiological output of cytokinin signaling, described as acting through a growth-dependent pathway (indirect). Supporting Evidence: PMID:18397377 this repression acts at the level of transcript accumulation, and depends on the AHK3 and CRE1 CK receptors |
| GO:0034757 negative regulation of iron ion transport | IMP PMID:18397377 Cytokinins negatively regulate the root iron uptake machiner... | KEEP AS NON CORE | Summary: Cytokinin represses root iron-uptake genes in an AHK3/CRE1-dependent manner. Reason: Supported downstream physiological output of cytokinin signaling, described as acting through a growth-dependent pathway (indirect). Supporting Evidence: PMID:18397377 this repression acts at the level of transcript accumulation, and depends on the AHK3 and CRE1 CK receptors |
| GO:0042742 defense response to bacterium | IDA PMID:19129491 Identification of Rhodococcus fascians cytokinins and their ... | MARK AS OVER ANNOTATED | Summary: AHK3 and AHK4 are required for the leafy-gall symptoms caused by the cytokinin-producing pathogen Rhodococcus fascians. Reason: The receptors are hijacked by bacterial cytokinins to cause disease symptoms; this is host susceptibility, not a defense response by the plant. Supporting Evidence: PMID:19129491 We show that the CK receptors AHK3 and AHK4 of Arabidopsis thaliana are essential for symptom development, and that the CK perception machinery is induced upon infection |
| GO:0042742 defense response to bacterium | IMP PMID:19129491 Identification of Rhodococcus fascians cytokinins and their ... | MARK AS OVER ANNOTATED | Summary: AHK3 and AHK4 are required for the leafy-gall symptoms caused by the cytokinin-producing pathogen Rhodococcus fascians. Reason: The receptors are hijacked by bacterial cytokinins to cause disease symptoms; this is host susceptibility, not a defense response by the plant. Supporting Evidence: PMID:19129491 We show that the CK receptors AHK3 and AHK4 of Arabidopsis thaliana are essential for symptom development, and that the CK perception machinery is induced upon infection |
| GO:0043424 protein histidine kinase binding | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA-rule electronic annotation for binding to other histidine kinases. Reason: Consistent with experimentally reported heteromeric interactions among CRE-family receptors; retained as non-core. Supporting Evidence: PMID:16965536 An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. |
| GO:0043424 protein histidine kinase binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | KEEP AS NON CORE | Summary: AHK3 interacts with other CRE-family receptor histidine kinases. Reason: Receptor homo/heteromerization is experimentally supported (Y2H, coaffinity, in vivo FRET for AHK3-AHK4) but is a structural property rather than the signalling activity itself. Supporting Evidence: PMID:21841169 Furthermore, membrane-bound AHK3 interacts with AHK4 in vivo. |
| GO:0043424 protein histidine kinase binding | IPI PMID:16965536 Analysis of protein interactions within the cytokinin-signal... | KEEP AS NON CORE | Summary: AHK3 interacts with other CRE-family receptor histidine kinases. Reason: Receptor homo/heteromerization is experimentally supported (Y2H, coaffinity, in vivo FRET for AHK3-AHK4) but is a structural property rather than the signalling activity itself. Supporting Evidence: PMID:21841169 Furthermore, membrane-bound AHK3 interacts with AHK4 in vivo. |
| GO:0048509 regulation of meristem development | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of meristem development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20110319 Loss of cytokinin perception leads to a complete failure of meristem establishment and growth arrest after germination. |
| GO:0048509 regulation of meristem development | IMP PMID:20110319 STIMPY mediates cytokinin signaling during shoot meristem es... | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of meristem development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:20110319 Loss of cytokinin perception leads to a complete failure of meristem establishment and growth arrest after germination. |
| GO:0048831 regulation of shoot system development | IMP PMID:16361392 Arabidopsis cytokinin receptor mutants reveal functions in s... | KEEP AS NON CORE | Summary: AHK3 contributes to 'regulation of shoot system development' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:16361392 prominent roles for the AHK2/AHK3 receptor combination in quantitative control of organ growth in plants, with opposite regulatory functions in roots and shoots |
| GO:0070417 cellular response to cold | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20463025 AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. |
| GO:0070417 cellular response to cold | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20463025 AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. |
| GO:0070417 cellular response to cold | IMP PMID:20463025 A subset of cytokinin two-component signaling system plays a... | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to cold' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:20463025 AHK2 and AHK3 were found to be primarily involved in mediating cold to express A-type ARRs despite cytokinin deficiency. |
| GO:0071215 cellular response to abscisic acid stimulus | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to abscisic acid stimulus' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:20463025 The ahk2 ahk3 and arr7 mutants showed hypersensitive response to abscisic acid (ABA) for germination, whereas ARR7 overexpression lines exhibited insensitive response to ABA. |
| GO:0071215 cellular response to abscisic acid stimulus | IMP PMID:20463025 A subset of cytokinin two-component signaling system plays a... | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to abscisic acid stimulus' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:20463025 The ahk2 ahk3 and arr7 mutants showed hypersensitive response to abscisic acid (ABA) for germination, whereas ARR7 overexpression lines exhibited insensitive response to ABA. |
| GO:0071329 cellular response to sucrose stimulus | IMP PMID:15923327 Interaction between phosphate-starvation, sugar, and cytokin... | KEEP AS NON CORE | Summary: AHK3 contributes to 'cellular response to sucrose stimulus' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:15923327 In addition, ahk3 showed reduced cytokinin repression of several Pi starvation-responsive genes and increased sucrose sensitivity. |
| GO:0080117 secondary growth | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: AHK3 contributes to 'secondary growth' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. This electronic/phylogenetic annotation is consistent with the experimental data. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0080117 secondary growth | IMP PMID:19622803 The histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS... | KEEP AS NON CORE | Summary: AHK3 contributes to 'secondary growth' as one of the downstream physiological/developmental outputs of cytokinin perception. Reason: Supported by mutant or expression data, but this is a pleiotropic output of cytokinin signaling rather than the receptor's core function. Supporting Evidence: PMID:19622803 Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. |
| GO:0009885 transmembrane histidine kinase cytokinin receptor activity | IMP PMID:16407152 Cytokinin-mediated control of leaf longevity by AHK3 through... | NEW | Summary: AHK3 is a membrane-spanning CHASE-domain histidine kinase that perceives cytokinin and signals by histidine autophosphorylation; the specific receptor-kinase term is more precise than the existing generic cytokinin receptor activity annotations. Reason: Genetic and biochemical evidence establishes the CRE-family receptors as transmembrane histidine kinase cytokinin receptors; AHK4 already carries this term by IDA, and AHK2/AHK3 share the same domain architecture and mechanism. Supporting Evidence: PMID:16407152 AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. PMID:14503005 Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. PMID:21709172 Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. |
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Download this section (compressed HTML)Q: What fraction of cytokinin perception occurs at the ER versus the plasma membrane in intact tissues, and does the site of perception differ among AHK2, AHK3 and AHK4?
Q: How are receptor-specific outputs (e.g. AHK3-ARR2 in leaf senescence) achieved given that all three receptors converge on the same AHP pool?
Experiment: Use receptor variants tethered to ER or plasma membrane in the ahk2 ahk3 cre1 triple mutant background to test which pool restores cytokinin-dependent transcription.
Hypothesis: Cytokinin signaling output is initiated predominantly from the ER-resident receptor pool.
Type: genetic complementation
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