XA21 Antibody

What is XA21 and what is its significance in plant immunity?

XA21 is a receptor-like kinase protein in rice (Oryza sativa) that confers broad-spectrum resistance to most strains of Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial blight disease . As a membrane-anchored receptor, XA21 is part of a larger class of cell surface immune receptors found in both plants and animals that recognize conserved microbial signatures and activate host defense mechanisms . XA21 belongs to the non-RD (arginine-aspartic acid) subclass of kinases that are critical components of innate immune signaling . Its discovery and characterization have significantly advanced our understanding of pattern recognition receptors in plant immunity and revealed remarkable evolutionary similarities between plant and animal immune systems.

How does the XA21-mediated immune response work at the molecular level?

XA21-mediated immunity involves a complex signaling cascade that begins with pathogen recognition and culminates in defense activation. The process includes:

• Receptor biogenesis and trafficking: XA21 biogenesis occurs in the endoplasmic reticulum before processing and transit to the plasma membrane .

• Pre-activation state: At the plasma membrane, XA21 associates with XB24 (XA21 binding protein 24), which catalyzes autophosphorylation of serine and threonine residues on XA21, maintaining it in an inactive state .

• Pathogen recognition: XA21 specifically recognizes RaxX, a tyrosine-sulfated protein produced by Xoo .

• Binding specificity: Sulfated RaxX (RaxX-sY) binds to the XA21 extracellular domain (ECD) with high affinity (Kd ~16 nM), while nonsulfated RaxX (RaxX-nY) shows lower affinity (Kd ~205 nM) .

• Receptor activation: Upon RaxX recognition, XA21 dissociates from XB24 and becomes activated .

• Signal transduction: The activated XA21 engages downstream components including the ubiquitin ligase XB3 and the OsSERK2 co-receptor to propagate immune signaling.

• Response attenuation: XB15, a protein phosphatase 2C, dephosphorylates XA21 to attenuate signaling .

This stepwise process ensures precise control of immune activation in response to specific bacterial pathogens.

What is the relationship between RaxX and XA21, and why is sulfation important?

RaxX is a bacterial protein produced by Xanthomonas oryzae pv. oryzae that functions as the activator (also called PAMP) of XA21-mediated immunity . The relationship between RaxX and XA21 is characterized by:

• Direct binding interaction: RaxX directly binds to the extracellular domain of XA21, as demonstrated through microscale thermophoresis (MST) assays .

• Sulfation dependency: The tyrosine residue Y41 of RaxX must be sulfated by the bacterial tyrosine sulfotransferase RaxST for full recognition by XA21 .

• Binding affinity enhancement: Sulfation of RaxX dramatically increases its binding affinity to XA21 ECD by approximately 12.5-fold (Kd improves from ~205 nM to ~16 nM) .

• Specificity of interaction: While RaxX shares similarities with plant sulfated peptides like PSY1, XA21 specifically recognizes RaxX but not PSY1, indicating a specific evolutionary adaptation .

• Evolutionary significance: Xoo field isolates that overcome XA21-mediated immunity encode alternative RaxX alleles with mutations at key positions (such as P44S and P48T), highlighting coevolutionary pressure between host and pathogen .

This sulfation-dependent recognition represents a sophisticated mechanism that plants have evolved to detect specific bacterial pathogens.

What are the primary research applications for XA21 antibodies?

XA21 antibodies serve as crucial tools in multiple research applications:

• Protein detection and quantification: XA21 antibodies enable Western blot analysis to verify XA21 expression levels in transgenic rice lines or to monitor protein abundance during disease progression .

• Protein-protein interaction studies: Anti-XA21 antibodies facilitate co-immunoprecipitation experiments to identify novel XA21-interacting proteins, as demonstrated in the discovery of XB3 .

• Subcellular localization: Through immunohistochemistry or immunofluorescence approaches, XA21 antibodies help track the receptor's localization during biogenesis, trafficking, and activation.

• Receptor complex isolation: XA21 antibodies can be used to isolate intact receptor complexes from plant tissues to study the dynamic composition of immune signaling components.

• Functional validation: XA21 antibodies can be utilized to confirm the specificity of receptor-ligand interactions in binding assays.

These applications are fundamental to elucidating the molecular mechanisms underpinning XA21-mediated immunity and plant disease resistance.

How can researchers verify XA21 antibody specificity for their experiments?

Verifying antibody specificity is critical for reliable experimental outcomes. For XA21 antibodies, researchers should:

• Include appropriate controls:

  • Use wild-type rice (without XA21) as a negative control

  • Include recombinant XA21 protein as a positive control

  • Test transgenic rice lines with known XA21 expression levels

• Perform validation experiments:

  • Western blot analysis to confirm single band detection at expected molecular weight (~140 kDa for full-length XA21)

  • Peptide competition assay to demonstrate binding specificity

  • Immunoprecipitation followed by mass spectrometry to confirm target identity

• Cross-reactivity assessment:

  • Test against related receptor kinases to ensure the antibody doesn't recognize similar proteins

  • Compare reactivity across different rice cultivars and related species

• Functional validation:

  • Verify that antibody-detected protein correlates with known biological function (e.g., resistance to Xoo)

  • Confirm reduced signal in XA21 knockdown/knockout lines

Thorough validation ensures experimental reproducibility and reliable interpretation of results in XA21 research.

What are the optimal methods for detecting XA21-RaxX interactions in vitro?

Several approaches can be employed to study XA21-RaxX interactions, each with specific advantages:

• Microscale Thermophoresis (MST):

  • Highly sensitive method used successfully to determine binding affinities between XA21 ECD and RaxX peptides

  • Requires fluorescently labeled XA21 ECD and varying concentrations of RaxX peptides

  • Enables precise determination of dissociation constants (Kd)

  • Recommended protein concentrations: 5-50 nM labeled XA21 ECD with RaxX peptide titrations from 0.1 nM to 1 μM

• Surface Plasmon Resonance (SPR):

  • Allows real-time monitoring of binding kinetics

  • XA21 ECD can be immobilized on sensor chips with RaxX peptides flowed over the surface

  • Provides association (kon) and dissociation (koff) rate constants in addition to affinity values

• Isothermal Titration Calorimetry (ITC):

  • Measures thermodynamic parameters of XA21-RaxX binding

  • Does not require protein labeling

  • Provides complete thermodynamic profile (ΔH, ΔS, ΔG) of the interaction

• Co-immunoprecipitation with purified components:

  • Strep-tagged or His-tagged XA21 ECD can be immobilized on appropriate beads

  • After incubation with RaxX peptides, bound complexes can be analyzed by immunoblotting

  • Useful for comparing relative binding of different RaxX variants

• AlphaScreen/AlphaLISA assays:

  • Bead-based proximity assay suitable for high-throughput screening

  • Can detect XA21-RaxX interactions in solution without washing steps

For optimal results, researchers should express XA21 ECD using the baculovirus/insect cell system as described in the literature, which yields properly folded protein suitable for binding studies .

How should researchers prepare and use synthetic RaxX peptides for XA21 activation studies?

Proper preparation and use of synthetic RaxX peptides is critical for consistent results in XA21 activation studies:

• Peptide design considerations:

  • RaxX21 (residues 35-55 of proRaxX: HVGGGDYPPPGANPKHDPPPR) has been shown to be sufficient for XA21 activation

  • Ensure sulfation at tyrosine Y41 position for active peptides

  • Include nonsulfated versions as controls

  • Consider synthesizing shorter variants (RaxX13, RaxX16) for structure-function studies

• Peptide synthesis and quality control:

  • Use specialized peptide synthesis services with expertise in tyrosine sulfation

  • Verify peptide identity and purity by mass spectrometry (>95% purity recommended)

  • Store lyophilized peptides at -80°C and avoid repeated freeze-thaw cycles

  • Prepare fresh stock solutions in double-distilled water

• Experimental concentrations:

  • For binding studies: 0.1 nM to 1 μM range

  • For rice cell activation assays: 50-500 nM (based on Kd ~16 nM for sulfated RaxX)

  • For plant tissue treatments: 100-250 nM for ROS burst assays

• Control peptides to include:

  • Nonsulfated RaxX (RaxX-nY) as negative control

  • Sulfated PSY1 peptides as specificity control

  • RaxX mutant peptides (P44S, P48T) that evade XA21 recognition

• Validation of bioactivity:

  • Confirm peptide activity by measuring reactive oxygen species (ROS) production

  • Verify XA21-dependency by testing on both XA21-expressing and wild-type plants

  • Document immune response kinetics following peptide application

This systematic approach ensures reliable and reproducible results when using synthetic RaxX peptides in XA21 research.

What is the recommended protocol for XA21 immunoprecipitation from plant tissues?

For successful immunoprecipitation of XA21 from plant tissues, researchers should follow this optimized protocol:

• Sample preparation:

  • Harvest 5-7 g of fresh rice leaf tissue from 4-6 week old plants

  • Flash freeze in liquid nitrogen and grind to fine powder

  • Extract proteins in buffer containing 50 mM Tris-HCl (pH 7.5), 150 mM NaCl, 5 mM EDTA, 1% Triton X-100, 1 mM DTT, 1 mM PMSF, and protease inhibitor cocktail

  • Clear lysate by centrifugation at 20,000 g for 20 min at 4°C

• Pre-clearing step:

  • Incubate cleared lysate with protein A/G beads for 1 hour at 4°C

  • Remove beads by centrifugation to reduce non-specific binding

• Immunoprecipitation:

  • Add anti-XA21 antibody at 1:100 to 1:200 dilution

  • Incubate overnight at 4°C with gentle rotation

  • Add pre-washed protein A/G beads and incubate for 3-4 hours at 4°C

  • Collect beads by gentle centrifugation (1,000 g for 2 min)

  • Wash 4-5 times with wash buffer (extraction buffer with reduced detergent concentration)

• Elution and analysis:

  • Elute bound proteins with SDS sample buffer by heating at 95°C for 5 min

  • Separate proteins by SDS-PAGE followed by Western blotting

  • Detect XA21 using anti-XA21 antibody

• Alternative approach using tagged XA21:

  • For higher specificity, use transgenic rice expressing epitope-tagged XA21 (e.g., ProA-XA21)

  • For ProA-tagged XA21, use IgG sepharose beads for direct capture

  • Detect co-immunoprecipitated proteins using specific antibodies (e.g., anti-XB3)

This protocol has been validated for detecting XA21 interactions with proteins such as XB3, and can be adapted to identify other components of the XA21 immune complex.

What are common challenges in XA21 antibody experiments and how can they be addressed?

This troubleshooting guide addresses the most frequently encountered challenges in XA21 antibody-based research and provides practical solutions based on published methodologies .

How can researchers analyze XA21 phosphorylation status in response to RaxX perception?

Analyzing XA21 phosphorylation dynamics is crucial for understanding immune receptor activation. Researchers should consider:

• Phosphorylation-specific detection methods:

  • Phos-tag SDS-PAGE to separate phosphorylated from non-phosphorylated XA21

  • Western blotting with phospho-specific antibodies (if available)

  • Immunoprecipitation followed by phospho-threonine/serine/tyrosine antibody detection

  • Mass spectrometry to identify specific phosphorylation sites

• Experimental design for temporal analysis:

  • Treat XA21-expressing rice with sulfated RaxX peptide (100-250 nM)

  • Collect samples at multiple time points (0, 5, 15, 30, 60, 120 minutes)

  • Include nonsulfated RaxX peptide as negative control

  • Use kinase-dead XA21 mutant plants as additional control

• Co-factor considerations:

  • Include XB24, which affects XA21 autophosphorylation status

  • Consider XB15 phosphatase activity, which dephosphorylates XA21

  • Analyze OsSERK2 co-receptor phosphorylation status in parallel

• Validation approaches:

  • In vitro kinase assays with immunoprecipitated XA21

  • Site-directed mutagenesis of potential phosphorylation sites

  • Correlation of phosphorylation status with downstream immune responses

• Technical considerations:

  • Use phosphatase inhibitors (sodium fluoride, sodium orthovanadate, β-glycerophosphate) during extraction

  • Maintain samples at 4°C throughout processing

  • Consider rapid protein extraction methods to preserve in vivo phosphorylation state

This comprehensive analysis will provide insights into how RaxX perception triggers XA21 phosphorylation events that initiate immune signaling.

How does XA21 compare to other plant immune receptors, and what insights can be gained from comparative studies?

Comparative studies between XA21 and other plant immune receptors reveal important evolutionary and functional insights:

• Structural and functional similarities:

  • XA21 belongs to the same receptor-like kinase (RLK) family as FLS2 and EFR in Arabidopsis

  • All contain leucine-rich repeat (LRR) extracellular domains for ligand recognition

  • XA21, XA3, Pid2, and FLS2 in rice all belong to the non-RD kinase subclass

  • These receptors activate similar downstream immune responses (ROS burst, MAP kinase activation)

• Co-receptor requirements:

  • OsSERK2 functions as a co-receptor for multiple immune receptors including XA21 and OsFLS2

  • This parallels the role of BAK1/SERK3 in Arabidopsis as a co-receptor for multiple pattern recognition receptors

  • The shared co-receptor usage suggests convergent evolution of immune signaling complexes

• Ligand recognition specificity:

  • While XA21 recognizes the sulfated bacterial protein RaxX , FLS2 recognizes bacterial flagellin

  • Despite different ligands, both trigger similar immune outputs

  • The specific recognition of sulfated RaxX by XA21 represents a unique adaptation

• Comparative antibody applications:

  • Antibodies against different plant immune receptors can be used in parallel to study receptor complex formation

  • This approach has revealed both shared and receptor-specific signaling components

• Evolutionary perspectives:

  • XA21-like receptors show remarkable structural similarity to Toll-like receptors in animals

  • This suggests convergent evolution of pattern recognition systems across kingdoms

  • Studying these similarities provides insights into fundamental principles of innate immunity

Comparative studies using antibodies against multiple immune receptors are particularly valuable for establishing conserved and divergent aspects of plant immune signaling.

What are the latest methodological advances in studying XA21-mediated immunity?

Recent methodological innovations have expanded our ability to study XA21-mediated immunity:

• Advanced protein production systems:

  • Expression of full-length XA21 in heterologous systems using advanced membrane protein production techniques

  • Production of properly sulfated RaxX using expanded genetic code approaches

  • Development of cell-free systems for rapid production of receptor extracellular domains

• High-resolution imaging techniques:

  • Single-molecule tracking of fluorescently-labeled XA21 to study receptor dynamics

  • Super-resolution microscopy to visualize immune receptor clustering following RaxX perception

  • FRET/FLIM analysis to monitor real-time XA21-RaxX interactions in live cells

• Structural biology approaches:

  • Cryo-electron microscopy of XA21-RaxX complexes to determine atomic-level interaction details

  • Hydrogen-deuterium exchange mass spectrometry to map conformational changes upon ligand binding

  • AlphaFold2 and other AI-based structural prediction tools to model XA21-RaxX complexes

• Genome engineering technologies:

  • CRISPR/Cas9-based precision editing of XA21 and RaxX to create structure-function variants

  • Base editing to generate specific amino acid substitutions without double-strand breaks

  • Promoter replacement strategies for controlled expression of XA21 and associated components

• Systems biology approaches:

  • Proteomics analysis of XA21 complexes using proximity labeling techniques

  • Phosphoproteomics to chart XA21-dependent signaling networks

  • Multi-omics integration to build comprehensive models of XA21-mediated immunity

These methodological advances provide researchers with unprecedented tools to dissect the molecular mechanisms of XA21-mediated immunity at multiple scales.

How might XA21 research contribute to developing durable disease resistance in crops?

XA21 research offers several promising avenues for enhancing crop disease resistance:

• Translational applications:

  • Transfer of XA21 into other crop species vulnerable to Xanthomonas pathogens

  • Modification of XA21 specificity to recognize additional bacterial pathogens

  • Development of synthetic immune receptors based on XA21 architecture

• Receptor engineering strategies:

  • Structure-guided modifications to enhance XA21 binding affinity to RaxX

  • Removing negative regulatory components to create constitutively active XA21 variants

  • Creation of XA21 chimeras with other immune receptors to expand recognition specificity

• Pathogen monitoring applications:

  • Development of XA21-based biosensors to detect Xoo in field conditions

  • Use of anti-XA21 and anti-RaxX antibodies for diagnostic assays

  • Surveillance of RaxX variants in Xoo populations to predict resistance durability

• Resistance deployment strategies:

  • Pyramiding XA21 with other resistance genes for more durable protection

  • Creation of alternating receptor decoys to counter pathogen adaptation

  • Development of tissue-specific or inducible XA21 expression systems

• Fundamental knowledge contributions:

  • Identification of conserved immune signaling hubs for targeted enhancement

  • Understanding of receptor-ligand coevolution to predict resistance breakdown

  • Elucidation of trade-offs between immunity and growth for optimized crop improvement

The detailed molecular understanding of XA21-mediated immunity, facilitated by antibody-based research, continues to provide the foundation for novel approaches to crop protection against devastating bacterial diseases.

What XA21 antibody validation data should researchers expect from suppliers?

Researchers should request this validation data from antibody suppliers to ensure experimental reliability and reproducibility in XA21 research.

What are the key resources for obtaining recombinant XA21 protein and domains for antibody validation?

The following resources and methods have been successfully used to produce XA21 components for research:

• XA21 Extracellular Domain (ECD):

  • Expression system: Baculovirus-infected High Five insect cells

  • Construct: XA21 residues 23-649 with C-terminal Strep II-9xHis tag

  • Purification method: Nickel affinity chromatography followed by size exclusion chromatography

  • Buffer composition: 50 mM NaH₂PO₄/Na₂HPO₄, pH 7.5, 200 mM NaCl, 5% glycerol

  • Expected yield: 1-5 mg/L of insect cell culture

  • Validation: Verified by SDS-PAGE and functional binding to RaxX

• Full-length XA21:

  • Expression system: Rice protoplasts or transgenic rice plants

  • Construct: Native XA21 or epitope-tagged versions (e.g., ProA-XA21)

  • Extraction method: Membrane protein extraction with appropriate detergents

  • Applications: Primarily for immunoprecipitation studies rather than purification

• XA21 Kinase Domain:

  • Expression system: E. coli (BL21 or similar strains)

  • Construct: XA21 kinase domain (typically residues 668-1025)

  • Purification: GST or His tag-based affinity chromatography

  • Applications: In vitro kinase assays, protein-protein interaction studies

  • Caution: May require refolding procedures to obtain active protein

• RaxX peptides and proteins:

  • Synthetic peptides: Custom synthesis of tyrosine-sulfated and non-sulfated variants

  • Recombinant full-length RaxX: Production in E. coli using expanded genetic code approach

  • Applications: Binding studies, activation assays, antibody validation