MORE AXILLARY GROWTH2 (MAX2; also ORE9, PPS, KAI1) is a nuclear F-box/leucine-rich-repeat protein that serves as the substrate-recognition subunit of an SCF(MAX2) E3 ubiquitin ligase (with ASK1/SKP1 and CUL1). MAX2 is shared by two butenolide signalling pathways: with the strigolactone receptor D14 it targets the SMXL6/SMXL7/SMXL8 transcriptional co-repressors for SL-induced polyubiquitination and proteasomal degradation, de-repressing BRC1 and suppressing axillary bud outgrowth; with the karrikin/KAI2-ligand receptor KAI2 it targets SMAX1 and SMXL2, controlling seed germination, seedling photomorphogenesis and leaf shape. MAX2 acts locally at the node, is expressed most highly in vasculature, and loss-of-function mutants are highly branched, have elongated hypocotyls, enhanced seed dormancy, delayed leaf senescence, altered auxin transport and drought hypersensitivity. Additional substrates (e.g. WRKY41 in freezing tolerance) have been proposed.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000151 ubiquitin ligase complex | IEA GO_REF:0000117 | MODIFY | Summary: MAX2 is part of a ubiquitin ligase complex. Reason: Correct but less specific than the experimentally supported SCF ubiquitin ligase complex. Proposed replacements: SCF ubiquitin ligase complex |
| GO:0000151 ubiquitin ligase complex | IPI PMID:11487692 ORE9, an F-box protein that regulates leaf senescence in Ara... | MODIFY | Summary: The ORE9/MAX2 F-box interacts with ASK1, a core SCF subunit. Reason: Correct but less specific than the experimentally supported SCF ubiquitin ligase complex. Proposed replacements: SCF ubiquitin ligase complex Supporting Evidence: PMID:11487692 The F-box motif of ORE9 interacts with ASK1 (Arabidopsis Skp1-like 1), a component of the plant SCF complex. |
| GO:0005515 protein binding | IPI PMID:27479325 DWARF14 is a non-canonical hormone receptor for strigolacton... | MODIFY | Summary: MAX2 binds SL-activated D14; the AtD14-D3-ASK1 complex was solved. Reason: Protein binding is uninformative; MAX2 recruits the activated receptor and its SMXL substrates to the SCF, i.e. ubiquitin-like ligase-substrate adaptor activity. Proposed replacements: ubiquitin-like ligase-substrate adaptor activity Supporting Evidence: PMID:27479325 Here we report the crystal structure of the strigolactone-induced AtD14-D3-ASK1 complex |
| GO:0005515 protein binding | IPI PMID:32612234 Extensive signal integration by the phytohormone protein net... | REMOVE | Summary: High-throughput Y2H interaction from a phytohormone interactome screen. Reason: Single systematic-screen interaction without functional follow-up for MAX2; protein binding is uninformative. Removal does not imply the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32612234 Extensive signal integration by the phytohormone protein net... | REMOVE | Summary: High-throughput Y2H interaction of MAX2 with an ASK/SKP1-family protein from a phytohormone interactome screen. Reason: The F-box-SKP1 interaction is expected and real, but protein binding is uninformative; SCF membership is captured by SCF ubiquitin ligase complex (GO:0019005). Removal does not imply the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32612234 Extensive signal integration by the phytohormone protein net... | REMOVE | Summary: High-throughput Y2H interaction from a phytohormone interactome screen. Reason: Single systematic-screen interaction without functional follow-up for MAX2; protein binding is uninformative. Removal does not imply the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32612234 Extensive signal integration by the phytohormone protein net... | REMOVE | Summary: High-throughput Y2H interaction of MAX2 with an ASK/SKP1-family protein from a phytohormone interactome screen. Reason: The F-box-SKP1 interaction is expected and real, but protein binding is uninformative; SCF membership is captured by SCF ubiquitin ligase complex (GO:0019005). Removal does not imply the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32612234 Extensive signal integration by the phytohormone protein net... | REMOVE | Summary: High-throughput Y2H interaction of MAX2 with an ASK/SKP1-family protein from a phytohormone interactome screen. Reason: The F-box-SKP1 interaction is expected and real, but protein binding is uninformative; SCF membership is captured by SCF ubiquitin ligase complex (GO:0019005). Removal does not imply the interaction is false. |
| GO:0005515 protein binding | IPI PMID:32612234 Extensive signal integration by the phytohormone protein net... | REMOVE | Summary: High-throughput Y2H interaction from a phytohormone interactome screen. Reason: Single systematic-screen interaction without functional follow-up for MAX2; protein binding is uninformative. Removal does not imply the interaction is false. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference of nuclear activity for MAX2. Reason: Consistent with experimental nuclear localisation of MAX2 and of its SMXL targets. |
| GO:0005634 nucleus | IDA PMID:17346265 MAX2 participates in an SCF complex which acts locally at th... | ACCEPT | Summary: MAX2-GUS and MAX2-GFP fusions are nuclear in many cell types. Reason: Direct localisation evidence; MAX2 acts on nuclear SMXL repressors. Supporting Evidence: PMID:17346265 is nuclear-localized in many cell types |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: ARBA/UniProt subcellular-location mapping to nucleus. Reason: Consistent with experimental evidence. |
| GO:0006511 ubiquitin-dependent protein catabolic process | IEA GO_REF:0000117 | MODIFY | Summary: MAX2 promotes ubiquitin-dependent proteolysis of its targets. Reason: Correct; the more specific SCF-dependent proteasomal ubiquitin-dependent protein catabolic process is supported for MAX2 (SMXL6/7/8 degradation). Proposed replacements: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process |
| GO:0006511 ubiquitin-dependent protein catabolic process | TAS PMID:11487692 ORE9, an F-box protein that regulates leaf senescence in Ara... | MODIFY | Summary: Early inference (ORE9 cloning) that MAX2 acts via ubiquitin-dependent proteolysis. Reason: Correct; now directly supported and better captured by the SCF-dependent proteasomal term. Proposed replacements: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process Supporting Evidence: PMID:11487692 These results suggest that ORE9 functions to limit leaf longevity by removing, through ubiquitin-dependent proteolysis, target proteins |
| GO:0009414 response to water deprivation | IGI PMID:15894742 Regulation of plant disease resistance, stress responses, ce... | KEEP AS NON CORE | Summary: ore9 suppresses drought-induced growth inhibition of edr1. Reason: Genetic-interaction phenotype in a stress context; not the core function of MAX2. Supporting Evidence: PMID:15894742 Two edr1-associated phenotypes, drought-induced growth inhibition and ethylene-induced senescence, were suppressed by mutations in ORE9 |
| GO:0009416 response to light stimulus | IMP PMID:17951458 The F-box protein MAX2 functions as a positive regulator of ... | KEEP AS NON CORE | Summary: pps/max2 seedlings are hyposensitive to light in hypocotyl and germination assays. Reason: Seedling light responses reflect the KAI2-SMAX1 branch of MAX2 signalling; upstream/within qualifier is appropriate and the term is non-core. Supporting Evidence: PMID:17951458 pps is also hyposensitive to both R and FR light-induced seed germination |
| GO:0009653 anatomical structure morphogenesis | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: ARBA prediction of anatomical structure morphogenesis. Reason: Very general developmental term; MAX2 effects on morphology are downstream outputs of SL and KAI2-ligand signalling and are better captured by specific terms. Unlike the ARBA regulation of developmental process row (MODIFY to regulation of secondary shoot formation), this is a non-regulatory process term whose descendants describe performing morphogenesis, which MAX2 does not do, so there is no apt narrower replacement. |
| GO:0009926 auxin polar transport | IMP PMID:16546078 The Arabidopsis MAX pathway controls shoot branching by regu... | KEEP AS NON CORE | Summary: max mutants have increased auxin transport capacity and PIN expression in the stem. Reason: Effect on auxin transport is a downstream output of SL signalling (via SMXL6/7/8), so the acts_upstream_of_or_within qualifier is appropriate. Supporting Evidence: PMID:16546078 it is caused by increased capacity for auxin transport in max primary stems, which show increased expression of PIN auxin efflux facilitators |
| GO:0009934 regulation of meristem structural organization | IMP PMID:11874909 MAX1 and MAX2 control shoot lateral branching in Arabidopsis... | KEEP AS NON CORE | Summary: max2 axillary meristems produce leaf primordia earlier and occasionally fasciate. Reason: Phenotypic consequence of loss of SL signalling; non-core. Supporting Evidence: PMID:11874909 MAX1 and MAX2 do not control the timing of axillary meristem initiation but repress primordia formation by the axillary meristem |
| GO:0010016 shoot system morphogenesis | IMP PMID:17346265 MAX2 participates in an SCF complex which acts locally at th... | KEEP AS NON CORE | Summary: SCF(MAX2) acts locally at the node to suppress shoot branching. Reason: Correct but broad; the specific role is regulation of secondary shoot formation, which is captured in core_functions. Supporting Evidence: PMID:17346265 We conclude that axillary shoot growth is controlled locally, at the node, by an SCF(MAX2), the action of which is enhanced by the mobile MAX signal. |
| GO:0010187 negative regulation of seed germination | IMP PMID:24198318 Regulation of drought tolerance by the F-box protein MAX2 in... | KEEP AS NON CORE | Summary: max2 seeds are hypersensitive to ABA during germination. Reason: Upstream effect on ABA-dependent inhibition of germination, likely mediated through the KAI2/SMAX1 branch; non-core. |
| GO:0016567 protein ubiquitination | IDA PMID:37622245 Strigolactones promote plant freezing tolerance by releasing... | ACCEPT | Summary: MAX2 ubiquitinates the transcription factor WRKY41 during cold acclimation. Reason: Direct ubiquitination evidence; consistent with the F-box adaptor role of MAX2. Supporting Evidence: PMID:37622245 MAX2 ubiquitinates WRKY41, thus marking it for cold-induced degradation |
| GO:0016567 protein ubiquitination | IEA GO_REF:0000041 | ACCEPT | Summary: UniPathway-based protein ubiquitination. Reason: MAX2 is required for ubiquitination of its substrates as part of SCF(MAX2). |
| GO:0016567 protein ubiquitination | TAS PMID:11874909 MAX1 and MAX2 control shoot lateral branching in Arabidopsis... | ACCEPT | Summary: Inference from F-box identity that branching repression involves ubiquitination. Reason: Later confirmed by direct evidence of MAX2-dependent SMXL ubiquitination. Supporting Evidence: PMID:11874909 Our results suggest that selective repression of axillary shoots involves ubiquitin-mediated degradation of as yet unidentified proteins that activate axillary growth. |
| GO:0019005 SCF ubiquitin ligase complex | IBA GO_REF:0000033 | ACCEPT | Summary: MAX2 is the F-box subunit of an SCF E3 ubiquitin ligase. Reason: Supported by in planta co-immunoprecipitation with ASK1 and CUL1. Supporting Evidence: PMID:17346265 Myc-epitope-tagged MAX2 interacts with the core SCF subunits ASK1 and AtCUL1 in planta. |
| GO:0019005 SCF ubiquitin ligase complex | ISS PMID:11874909 MAX1 and MAX2 control shoot lateral branching in Arabidopsis... | ACCEPT | Summary: MAX2 is the F-box subunit of an SCF E3 ubiquitin ligase. Reason: Supported by in planta co-immunoprecipitation with ASK1 and CUL1. Supporting Evidence: PMID:17346265 Myc-epitope-tagged MAX2 interacts with the core SCF subunits ASK1 and AtCUL1 in planta. |
| GO:0031146 SCF-dependent proteasomal ubiquitin-dependent protein catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that MAX2 participates in SCF-dependent proteasomal protein catabolism, as other F-box/LRR proteins do. Reason: Directly supported for MAX2: SL-induced SMXL6 polyubiquitination and degradation require MAX2. This is the core process the F-box adaptor performs. Supporting Evidence: PMID:26546446 the ubiquitination of SMXL6-GFP was detected in 35S:SMXL6-GFP seedlings but not in 35S:SMXL6-GFP/max2-1 and 35S:SMXL6-GFP/d14-1 seedlings |
| GO:0050793 regulation of developmental process | IEA GO_REF:0000117 | MODIFY | Summary: ARBA prediction of regulation of developmental process. Reason: Too general; the best-characterised developmental role of MAX2 is control of shoot branching. Being a regulation term, it has an apt regulatory descendant (unlike the ARBA anatomical structure morphogenesis row, which is marked over-annotated instead). Proposed replacements: regulation of secondary shoot formation |
| GO:0160062 cutin-based cuticle development | IMP PMID:24198318 Regulation of drought tolerance by the F-box protein MAX2 in... | KEEP AS NON CORE | Summary: max2 leaves have a thinner cuticle, contributing to drought sensitivity. Reason: Downstream physiological phenotype; non-core. Supporting Evidence: PMID:24198318 Cuticle thickness of max2 was significantly thinner than that of the wild type. |
| GO:1900057 positive regulation of leaf senescence | IMP PMID:11487692 ORE9, an F-box protein that regulates leaf senescence in Ara... | KEEP AS NON CORE | Summary: ore9 mutant leaves show delayed age-dependent and hormone-induced senescence. Reason: Well-supported phenotype but a downstream physiological output rather than the core activity. Supporting Evidence: PMID:11487692 The leaf of the oresara9 (ore9) mutant of Arabidopsis exhibits increased longevity during age-dependent natural senescence |
| GO:1902584 positive regulation of response to water deprivation | IMP PMID:24198318 Regulation of drought tolerance by the F-box protein MAX2 in... | KEEP AS NON CORE | Summary: max2 mutants are strongly hypersensitive to drought. Reason: Downstream physiological role; non-core. Supporting Evidence: PMID:24198318 we show that the max2 mutant is strongly hypersensitive to drought stress compared with wild-type Arabidopsis |
| GO:1990756 ubiquitin-like ligase-substrate adaptor activity | IPI PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | NEW | Summary: MAX2 associates with SMXL6/7/8 substrates and is required for their SL-induced ubiquitination, the defining activity of an F-box substrate adaptor. Reason: GOA has no molecular function for MAX2 other than protein binding; the adaptor activity is supported by co-IP with substrates, SCF membership, and MAX2-dependent substrate ubiquitination. Supporting Evidence: PMID:26546446 Flag-SMXL6, Flag-SMXL7, and Flag-SMXL8 were found to interact with GFP-MAX2 with or without treatment with rac -GR24 PMID:17346265 F-box proteins function as the substrate-recruiting subunit of SCF-type ubiquitin E3 ligases in protein ubiquitination. |
| GO:1902348 cellular response to strigolactone | IMP PMID:26546446 Strigolactone Signaling in Arabidopsis Regulates Shoot Devel... | NEW | Summary: MAX2 is required for SL-induced SMXL6 ubiquitination and degradation and for SL control of branching. Reason: MAX2 performs a step of SL signal transduction (substrate ubiquitination); GOA lacks an SL signalling term for MAX2. Supporting Evidence: PMID:26546446 Collectively, these data indicate that the D53-like SMXLs are subject to D14- and MAX2-dependent degradation in response to SL signaling. |
| GO:0080167 response to karrikin | IMP PMID:21555559 F-box protein MAX2 has dual roles in karrikin and strigolact... | NEW | Summary: Karrikin responses in seeds and seedlings are abolished in max2 mutants. Reason: MAX2 is the shared F-box of the KAI2 pathway, performing the SMAX1/SMXL2 degradation step; GOA lacks a karrikin term for MAX2. Supporting Evidence: PMID:21555559 karrikin signaling requires the F-box protein MAX2 |
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Download this section (compressed HTML)Q: Are non-SMXL substrates of SCF(MAX2) such as WRKY41 recruited in a D14- or KAI2-dependent manner, or does MAX2 have receptor-independent targets?
Q: How does the conformational switching of the MAX2/D3 C-terminal helix observed in rice coordinate SL hydrolysis by D14 with SMXL recruitment in Arabidopsis?
Experiment: In vitro reconstitution of SCF(MAX2)-D14-SMXL7 ubiquitination with purified components and defined SL stereoisomers, to establish direct E3 activity and receptor dependence.
Type: biochemical reconstitution
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