XA21 (Q2R2D5) is the rice cell-surface leucine-rich-repeat receptor kinase (LRR-RLK) / pattern-recognition receptor (PRR) that confers broad-spectrum resistance to the Gram-negative bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight. It has the canonical immune-PRR architecture: a signal peptide, an extracellular ectodomain of ~23 leucine-rich repeats (residues ~25-665), a single transmembrane helix (666-686), a juxtamembrane region, and a cytoplasmic non-RD serine/threonine protein kinase domain (720-1019) (UniProt Q2R2D5; Chen & Ronald 2011, file deep-research). XA21 is synthesized/processed through the endoplasmic reticulum, is heavily N-glycosylated, and localizes to the plasma membrane and the cortical/perinuclear ER (PMID:22735448). It perceives a bacterial tyrosine-sulfated peptide ligand: current consensus identifies the activator as the sulfated RaxX peptide (a synthetic 21-aa RaxX21-sY peptide is sufficient; earlier literature used the Ax21/AxYS22 nomenclature) (Pruitt et al. 2015, file deep-research; PMID:22735448). Upon ligand perception, XA21 releases the negative regulator XB24 (an ATPase) and is activated; signaling is co-regulated by the somatic-embryogenesis receptor kinase OsSERK2, which forms a constitutive complex with XA21 and undergoes bidirectional transphosphorylation with it (PMID:24482436). XA21 also undergoes proteolytic cleavage that releases the intracellular kinase domain, which carries a functional nuclear localization signal (residues 689-694) and translocates to the nucleus where it binds the transcriptional regulator OsWRKY62/XB10; nuclear translocation is required for the XA21-mediated immune response (PMID:22735448). The net biological output is activation of pattern-triggered immunity / defense response to the bacterium Xoo. The core function of XA21 is therefore as a transmembrane immune receptor serine/threonine kinase that detects a bacterial signature and triggers antibacterial defense signaling.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation: the processed XA21 intracellular kinase domain translocates to the nucleus, where it acts on a transcriptional regulator. Directly confirmed in rice. Reason: Supported by direct experimental evidence. After ligand (AxYS22/RaxX) recognition, XA21 is cleaved and the intracellular kinase domain accumulates in the nucleus (XA21-GFP imaging in protoplasts and nuclear-enriched fractions in planta), where it interacts with the OsWRKY62 transcriptional regulator; nuclear translocation is required for XA21-mediated immunity [PMID:22735448]. The "is_active_in" qualifier is appropriate because the nuclear-localized kinase is functionally required there. Supporting Evidence: PMID:22735448 In vivo cleavage of XA21 and translocalization of the intracellular kinase domain to the nucleus is required for the XA21-mediated immune response. |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation: XA21 is a plasma-membrane single-pass receptor kinase. This is the site of ligand perception and the canonical location of the active receptor. Reason: Strongly supported. XA21-GFP localizes to the plasma membrane (and the cortical/perinuclear ER) in the absence of ligand, and the mature receptor perceives its bacterial ligand at the cell surface [PMID:22735448]. Plasma-membrane localization is the defining feature of a cell-surface PRR/LRR-RLK and is consistent with the UniProt subcellular location. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 file:ORYSJ/XA21/XA21-deep-research-falcon.md is N-glycosylated, localizes predominantly to the ER but also to the plasma membrane |
| GO:0032541 cortical endoplasmic reticulum | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation: XA21 is present in the cortical ER, reflecting ER biogenesis/quality control of the receptor before transit to the plasma membrane. Reason: Supported by direct rice imaging: XA21-GFP localizes to the cortical and perinuclear ER in addition to the plasma membrane [PMID:22735448]. XA21 maturation depends on ER folding/quality control (BiP3 chaperone regulates XA21 stability and cleavage), so ER localization is biologically meaningful, not artefactual [file deep-research]. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 file:ORYSJ/XA21/XA21-deep-research-falcon.md is N-glycosylated, localizes predominantly to the ER but also to the plasma membrane |
| GO:1990578 perinuclear endoplasmic reticulum membrane | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation: XA21 is present in the perinuclear ER membrane, consistent with its ER-resident pool during biogenesis. Reason: Supported by the same direct imaging evidence as the cortical-ER annotation: XA21-GFP localizes to the perinuclear (as well as cortical) ER [PMID:22735448]. UniProt records "Present in cortical and perinuclear endoplasmic reticulum." Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 |
| GO:0004672 protein kinase activity | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation from InterPro (IPR000719 protein kinase domain). XA21 carries a cytoplasmic protein kinase domain; broad but correct parent of its Ser/Thr kinase MF. Reason: Correct but generic. XA21 has a bona fide cytoplasmic protein kinase domain (residues 720-1019, PROSITE PS50011) and is autophosphorylated on Ser/Thr residues [UniProt Q2R2D5]. "Protein kinase activity" is the broad parent; the more specific "protein serine/threonine kinase activity" (GO:0004674) and the receptor-specific term (GO:0004675, proposed below) better capture the activity, but this parent IEA is not incorrect. Supporting Evidence: file:ORYSJ/XA21/XA21-deep-research-falcon.md a single transmembrane helix, a juxtamembrane region, and a cytosolic kinase domain |
| GO:0004674 protein serine/threonine kinase activity | IEA GO_REF:0000003 | ACCEPT | Summary: IEA annotation (EC 2.7.11.1 mapping). XA21 is a Ser/Thr protein kinase that autophosphorylates and transphosphorylates its co-receptor OsSERK2. Reason: Correct and well supported. UniProt assigns EC 2.7.11.1 and lists XA21 in the Ser/Thr protein kinase family; XA21 autophosphorylates on serine/threonine residues and undergoes bidirectional Ser/Thr transphosphorylation with OsSERK2 in vitro [PMID:24482436]. This is the appropriate catalytic MF. The receptor-context term GO:0004675 is proposed below as a more informative refinement. Supporting Evidence: PMID:24482436 OsSERK2 and XA21 undergo bidirectional transphosphorylation in vitro file:ORYSJ/XA21/XA21-deep-research-falcon.md Belongs to the protein kinase superfamily. Ser/Thr protein |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation from InterPro (IPR017441, protein kinase ATP-binding site). XA21 binds ATP via its kinase domain to catalyze phosphotransfer. Reason: Correct. The XA21 kinase domain contains a canonical ATP-binding motif (UniProt BINDING 726-734 and 748; PROSITE PS00107), required for autophosphorylation and transphosphorylation. Mutating the conserved ATP-binding lysine (K736E) abolishes XA21 kinase activity and its kinase-dependent interactions [PMID:24482436]. ATP binding is an integral part of the kinase MF. Supporting Evidence: PMID:24482436 by either mutating the conserved lysine (K) required for ATP binding and catalytic activity |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation (UniProt SubCell mapping) duplicating the nuclear localization that is directly demonstrated for the processed XA21 kinase domain. Reason: Correct and redundant with the IBA/IDA nucleus annotations. The processed XA21 intracellular kinase domain carries a functional NLS and translocates to the nucleus after ligand recognition [PMID:22735448]. Duplicate annotations with different evidence codes are acceptable. Supporting Evidence: PMID:22735448 this intracellular domain carries a functional nuclear localization sequence |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation (UniProt SubCell mapping) for ER membrane localization; duplicates the directly demonstrated ER localization. Reason: Correct. XA21 is a single-pass ER/PM membrane protein; XA21-GFP localizes to the cortical and perinuclear ER, and XA21 biogenesis/quality control occurs in the ER [PMID:22735448; file deep-research]. Consistent with the UniProt subcellular location. Supporting Evidence: file:ORYSJ/XA21/XA21-deep-research-falcon.md is N-glycosylated, localizes predominantly to the ER but also to the plasma membrane |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation (UniProt SubCell mapping) for plasma-membrane localization; duplicates the IBA/IDA plasma-membrane annotations. Reason: Correct and consistent with direct imaging: the mature XA21 receptor is at the plasma membrane, where it perceives its bacterial ligand [PMID:22735448]. Duplicate of the experimentally supported localization. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 |
| GO:0106310 protein serine kinase activity | IEA GO_REF:0000116 | ACCEPT | Summary: IEA annotation (RHEA mapping, RHEA:17989) for protein serine kinase activity; a sub-aspect of the Ser/Thr kinase activity of XA21. Reason: Correct. XA21 autophosphorylates on serine residues (UniProt MOD_RES Ser698/Ser701) and is a Ser/Thr kinase; the RHEA-mapped serine-kinase reaction is part of its catalytic MF [UniProt Q2R2D5; PMID:24482436]. Duplicates the ISS annotation to the same term. Supporting Evidence: PMID:24482436 OsSERK2 and XA21 undergo bidirectional transphosphorylation in vitro |
| GO:0106310 protein serine kinase activity | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation (by similarity to UniProtKB:Q1MX30) for protein serine kinase activity; duplicates the RHEA IEA annotation to the same term. Reason: Correct, by curator-judged similarity to the characterized ortholog Q1MX30 and consistent with XA21's demonstrated Ser/Thr kinase activity and serine autophosphorylation sites [UniProt Q2R2D5; PMID:24482436]. Duplicate annotations with different evidence codes are acceptable. Supporting Evidence: file:ORYSJ/XA21/XA21-deep-research-falcon.md Belongs to the protein kinase superfamily. Ser/Thr protein |
| GO:0005515 protein binding | IPI PMID:24482436 An XA21-associated kinase (OsSERK2) regulates immunity media... | MODIFY | Summary: IPI annotation (with UniProtKB:Q7XV05 = OsSERK2) for the constitutive XA21-OsSERK2 receptor-kinase complex. "Protein binding" is uninformative; the interaction defines XA21's role in a co-receptor immune-signaling complex. Reason: The interaction is real and important - OsSERK2 forms a constitutive complex with XA21 in planta, the kinase domains interact in a kinase-activity-dependent manner, and OsSERK2 is required for XA21-mediated immunity [PMID:24482436]. However, the bare term "protein binding" (GO:0005515) is discouraged because it conveys no functional information. The biologically informative molecular function captured by this co-receptor interaction is XA21 acting as a transmembrane immune receptor kinase that transduces a signal across the membrane via its associated SERK kinase; this is better represented by "transmembrane receptor protein serine/threonine kinase activity" (GO:0004675) and "immune receptor activity" (GO:0140375), proposed below. Proposed replacements: transmembrane receptor protein serine/threonine kinase activity Supporting Evidence: PMID:24482436 OsSERK2 and XA21 form constitutive heterodimeric complexes in planta PMID:24482436 OsSERK2 interacts with the intracellular domains of each immune receptor in the yeast two-hybrid system in a kinase activity-dependent manner |
| GO:0005515 protein binding | IPI PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | MODIFY | Summary: IPI annotation (with UniProtKB:Q6EPZ0 = OsWRKY62/XB10) for the nuclear interaction between the processed XA21 kinase domain and the OsWRKY62 transcriptional regulator. "Protein binding" is uninformative. Reason: The interaction is genuine and mechanistically central: BiFC assays show the XA21 intracellular domain interacts with the OsWRKY62 transcriptional regulator exclusively in the nucleus, linking receptor activation to transcriptional reprogramming [PMID:22735448]. But the bare "protein binding" term (GO:0005515) is discouraged. The functional essence - XA21 transducing an immune signal to a downstream transcriptional regulator - is better captured by the immune-receptor/defense process terms. As a molecular function, "immune receptor activity" (GO:0140375; defined as receiving a signal and transmitting it in a cell to initiate an immune response) is the informative replacement; the OsWRKY62 binding is the means by which the signal is transmitted in the nucleus. Proposed replacements: immune receptor activity Supporting Evidence: PMID:22735448 the XA21 intracellular domain interacts with the OsWRKY62 transcriptional regulator exclusively in the nucleus of rice protoplasts |
| GO:0005634 nucleus | IDA PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | ACCEPT | Summary: IDA annotation: the processed XA21 intracellular kinase domain localizes to the nucleus after ligand recognition. Directly demonstrated. Reason: Directly supported. Treatment with active AxYS22 (or Xoo supernatant) triggers accumulation of XA21-GFP inside the nucleus, the intracellular domain is detected in nuclei-enriched fractions in planta, and an intact NLS is required for nuclear localization and immune function [PMID:22735448]. Duplicates the IBA/IEA nucleus annotations. Supporting Evidence: PMID:22735448 triggers significant accumulation of the XA21-GFP inside the nucleus |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | ACCEPT | Summary: IDA annotation: XA21 localizes to the ER membrane (cortical/perinuclear). Directly demonstrated by XA21-GFP imaging. Reason: Directly supported by XA21-GFP imaging showing cortical and perinuclear ER localization, consistent with ER-dependent biogenesis/quality control of the receptor [PMID:22735448; file deep-research]. Duplicates the IBA/IEA ER annotations. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 |
| GO:0005886 plasma membrane | IDA PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | ACCEPT | Summary: IDA annotation: XA21 localizes to the plasma membrane, the site of the mature cell-surface receptor. Directly demonstrated. Reason: Directly supported: XA21-GFP localizes to the plasma membrane in the absence of ligand [PMID:22735448]. This is the canonical site of the cell-surface PRR. Duplicates the IBA/IEA plasma-membrane annotations. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 |
| GO:0032541 cortical endoplasmic reticulum | IDA PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | ACCEPT | Summary: IDA annotation: XA21 localizes to the cortical ER. Directly demonstrated by XA21-GFP imaging. Reason: Directly supported by XA21-GFP imaging showing cortical-ER localization [PMID:22735448]. Duplicates the IBA cortical-ER annotation. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 |
| GO:1990578 perinuclear endoplasmic reticulum membrane | IDA PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | ACCEPT | Summary: IDA annotation: XA21 localizes to the perinuclear ER membrane. Directly demonstrated by XA21-GFP imaging. Reason: Directly supported by XA21-GFP imaging showing perinuclear-ER localization [PMID:22735448]. Duplicates the IBA perinuclear-ER annotation. Supporting Evidence: PMID:22735448 XA21-GFP is localized both to the plasma membrane and the cortical and perinuclear endoplasmic reticulum in the absence of AxYS22 |
| GO:0006952 defense response | IEA GO_REF:0000043 | MODIFY | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt keyword "Plant defense"; snapshot-only, removed in the current GOA release. XA21 is a genuine immune receptor that confers broad-spectrum resistance to the bacterium Xoo, so the essence of "defense response" is biologically CORRECT - but it is a high-level, non-specific parent term. Reason: This is the "R-protein/PRR correct-but-broad" case (Tier A by keyword; verdict correct-but-broad). XA21 genuinely functions in immunity: it is "the rice XA21 receptor confers broad-spectrum immunity to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae" and nuclear translocation of its kinase domain is "required for the XA21-mediated immune response" [PMID:22735448]; silencing the co-receptor OsSerk2 compromises XA21-mediated resistance to Xoo [PMID:24482436]. So removing the term outright would discard correct biology - and notably it was the ONLY biological-process annotation XA21 carried, so its removal leaves the gene with no process term describing its central role. Rather than ACCEPT the vague parent or REMOVE correct biology, the annotation should be MODIFIED to the specific, informative process term "defense response to bacterium" (GO:0042742), which precisely captures XA21's antibacterial immune function against Xoo. (The even more specific "defense response to Gram-negative bacterium" GO:0050829 is proposed below as a NEW term, since Xoo is Gram-negative.) Proposed replacements: defense response to bacterium Supporting Evidence: PMID:22735448 The rice XA21 receptor confers broad-spectrum immunity to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae PMID:24482436 The rice XA21 immune receptor kinase and the structurally related XA3 receptor confer immunity to Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight. file:ORYSJ/XA21/XA21-deep-research-falcon.md XA21 belongs to LRR-RLKs that perceive extracellular ligands via LRRs and transduce signals through a cytosolic kinase domain |
| GO:0004675 transmembrane receptor protein serine/threonine kinase activity | IDA PMID:24482436 An XA21-associated kinase (OsSERK2) regulates immunity media... | NEW | Summary: XA21 is a single-pass transmembrane receptor that combines an extracellular ligand-sensing ectodomain with a cytoplasmic Ser/Thr kinase, transducing a bacterial signal across the plasma membrane. This receptor-context kinase MF is more informative than the generic kinase terms in current GOA. Reason: XA21 has the exact architecture this term describes (GO:0004675: "Combining with a signal and transmitting the signal from one side of the membrane to the other... by catalysis of... ATP + protein serine = ADP + protein serine phosphate"): an extracellular LRR ectodomain, a single TM helix, and a cytoplasmic Ser/Thr kinase domain [UniProt Q2R2D5; file deep-research]. It is described as "The XA21 receptor kinase" / "immune receptor kinase" that transduces an extracellular bacterial signal, undergoes autophosphorylation, and transphosphorylates its co-receptor OsSERK2 [PMID:24482436]. This is the informative MF that the two discouraged "protein binding" IPI annotations and the generic kinase IEAs should resolve to. IDA is justified by the in vitro transphosphorylation assays with OsSERK2. Supporting Evidence: PMID:24482436 The rice XA21 immune receptor kinase and the structurally related XA3 receptor confer immunity to Xanthomonas oryzae pv. oryzae PMID:24482436 OsSERK2 and XA21 undergo bidirectional transphosphorylation in vitro file:ORYSJ/XA21/XA21-deep-research-falcon.md extracellular LRR region (~23 LRRs reported in one modeling study), a single transmembrane helix, a juxtamembrane region, and a cytosolic kinase domain |
| GO:0140375 immune receptor activity | IMP PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | NEW | Summary: XA21 is a pattern-recognition receptor that receives a bacterial signal (sulfated RaxX/AxYS22 peptide) and transmits it to initiate an immune response. This is the defining molecular function of XA21 and is absent from current GOA. Reason: GO:0140375 is defined as "Receiving a signal and transmitting it in a cell to initiate an immune response" - exactly XA21's role as a PRR. XA21 perceives the conserved bacterial signature (Ax21/AxYS22, now RaxX) and triggers the immune response; cleavage and nuclear translocation of the kinase domain (interacting with OsWRKY62) is required for the XA21-mediated immune response [PMID:22735448], and the co-receptor OsSERK2 is required for receptor function [PMID:24482436]. IMP is justified by the loss-of-immune- function phenotypes (NES-fusion and OsSerk2-silencing both abolish XA21-mediated resistance). The closely related "innate immune receptor activity" (GO:0140376) would also be appropriate. Supporting Evidence: PMID:22735448 The rice XA21 receptor confers broad-spectrum immunity to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae upon recognition of a small protein, Ax21, that is conserved in all Xanthomonas species and related genera. file:ORYSJ/XA21/XA21-deep-research-falcon.md upon perception of a conserved microbial molecule, leading to downstream defense signaling |
| GO:0050829 defense response to Gram-negative bacterium | IMP PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | NEW | Summary: XA21 confers resistance specifically to the Gram-negative bacterium Xanthomonas oryzae pv. oryzae. The Gram-negative-specific defense-response term is the most precise process annotation for XA21's biology. Reason: Xoo is a Gram-negative bacterium, and XA21 confers "broad-spectrum immunity to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae" [PMID:22735448]. Loss of XA21 function (NES fusion blocking nuclear translocation, or OsSerk2 silencing) results in enhanced susceptibility / loss of resistance to Xoo [PMID:22735448; PMID:24482436], supporting an IMP "involved_in" defense-response annotation. This is more specific than the proposed MODIFY target GO:0042742 and most accurately reflects XA21's antibacterial role. IMP is justified by the loss-of-resistance phenotypes. Supporting Evidence: PMID:22735448 The rice XA21 receptor confers broad-spectrum immunity to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae PMID:24482436 the double transgenic plants carrying Xa21 and silenced for OsSerk2 are susceptible, showing typical long water-soaked lesions |
| GO:0009595 detection of biotic stimulus | IMP PMID:22735448 Cleavage and nuclear localization of the rice XA21 immune re... | NEW | Summary: XA21 detects a biotic (bacterial) stimulus - the sulfated RaxX/AxYS22 peptide derived from Xoo - and converts it into an intracellular signal. This perception step is the upstream process underlying XA21's immune function. Reason: GO:0009595 is defined as "The series of events in which a biotic stimulus, one caused or produced by a living organism, is received and converted into a molecular signal." XA21 perceives the conserved bacterial peptide (Ax21/AxYS22, now RaxX), and recognition triggers receptor accumulation, cleavage and nuclear translocation that are required for immunity [PMID:22735448; file deep-research]. This captures the detection/perception role distinct from the downstream defense-response output. IMP is justified by the ligand-recognition-dependent activation and loss-of-function immune phenotypes. Supporting Evidence: PMID:22735448 XA21 binds a conserved sulphated peptide, called AxYS22 derived from the Xoo Ax21 (activator of XA21-mediated immunity) protein file:ORYSJ/XA21/XA21-deep-research-falcon.md is sufficient to trigger hallmark immune responses |
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Download this section (compressed HTML)Q: Does XA21 directly bind the sulfated RaxX/RaxX21-sY peptide via its LRR ectodomain, or is perception mediated indirectly through an additional binding protein or co-receptor?
Suggested experts: Pamela C. Ronald
Q: Is proteolytic cleavage and nuclear translocation of the XA21 kinase domain a general requirement across LRR-RLK immune receptors, or specific to XA21, and what protease mediates the cleavage?
Suggested experts: Chang-Jin Park
Experiment: Quantitative binding assays (e.g. microscale thermophoresis or surface plasmon resonance) of the purified XA21 LRR ectodomain (and the OsSERK2 ectodomain) against sulfated RaxX21-sY versus non-sulfated peptide, to establish whether XA21 is the direct receptor for the sulfated ligand.
Hypothesis: The XA21 LRR ectodomain directly and specifically binds the tyrosine-sulfated RaxX peptide, and binding is abolished by loss of sulfation.
Type: in vitro ligand-binding biochemistry
Experiment: Generate transgenic rice expressing a non-cleavable XA21 variant and a kinase-dead (K736E) XA21 variant, and quantify Xoo resistance, defense-gene induction and OsWRKY62 nuclear interaction, to dissect the contributions of cleavage versus kinase activity to immune signaling.
Hypothesis: Both kinase activity and ligand-triggered cleavage/nuclear translocation are required for full XA21-mediated defense response to Xoo.
Type: structure-function transgenic complementation and pathogen-challenge assay
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