| Feature | Summary | Key evidence/citations | Primary source (authors/year/journal) | URL | Publication date |
|---|---|---|---|---|---|
| Identity | XA21 in *Oryza sativa* subsp. *japonica* corresponds to locus Os11g0569733 / LOC_Os11g36180 and encodes a membrane-localized LRR receptor-like kinase/receptor kinase conferring resistance to *Xanthomonas oryzae* pv. *oryzae* (Xoo); it was introgressed from *O. longistaminata*. | (pqac-00000000, pqac-00000002, pqac-00000007) | Nanayakkara et al. 2018, *Tropical Agricultural Research*; Pruitt et al. 2015, *Science Advances* | https://doi.org/10.4038/tar.v30i1.8278 ; https://doi.org/10.1126/sciadv.1500245 | 2018-12 ; 2015-07 |
| Domains | The protein matches the UniProt annotation as an LRR-RLK: extracellular LRR ectodomain, single transmembrane helix, juxtamembrane region, and intracellular non-RD serine/threonine kinase domain; modeling work describes ~23 LRRs. | (pqac-00000001, pqac-00000015) | Mubassir et al. 2020, *RSC Advances*; Chen & Ronald 2011, *Trends in Plant Science* | https://doi.org/10.1039/d0ra01396j ; https://doi.org/10.1016/j.tplants.2011.04.003 | 2020-04 ; 2011-08 |
| Ligand | Current accepted ligand is the Xoo tyrosine-sulfated peptide RaxX; sulfation of the key Tyr residue is required, and synthetic RaxX21-sY is sufficient to trigger XA21-dependent immune responses. Earlier literature referred to Ax21; this was superseded by the RaxX model. | (pqac-00000016, pqac-00000011, pqac-00000012) | Pruitt et al. 2015, *Science Advances*; Chen & Ronald 2011, *Trends in Plant Science* | https://doi.org/10.1126/sciadv.1500245 ; https://doi.org/10.1016/j.tplants.2011.04.003 | 2015-07 ; 2011-08 |
| Co-receptors/regulators | XA21 signaling is regulated by OsSERK2 and multiple XA21-binding proteins: XB24 (ATPase; promotes autophosphorylation and keeps receptor inactive before activation), XB15 (PP2C; dephosphorylates activated XA21 and negatively regulates immunity), XB3 (RING-type E3 ligase; phosphorylated by XA21 and required for full resistance/protein accumulation). | (pqac-00000003, pqac-00000013, pqac-00000014, pqac-00000012) | Chen et al. 2010, *PNAS*; Wang et al. 2006, *The Plant Cell*; Park et al. 2008, *PLoS Biology*; Jiang et al. 2026, *Frontiers in Plant Science* | https://doi.org/10.1073/pnas.0912311107 ; https://doi.org/10.1105/tpc.106.046730 ; https://doi.org/10.1371/journal.pbio.0060231 ; https://doi.org/10.3389/fpls.2026.1744367 | 2010-04 ; 2006-12 ; 2008-09 ; 2026-02 |
| Localization/processing | XA21 is synthesized/processed through the ER, is N-glycosylated, localizes predominantly to the ER but also to the plasma membrane, and undergoes proteolytic cleavage: full-length ~140 kDa and cleavage product ~110 kDa. BiP3 overexpression reduces XA21 stability and cleavage. | (pqac-00000018, pqac-00000027, pqac-00000028, pqac-00000029, pqac-00000030) | Park et al. 2010, *PLoS ONE* | https://doi.org/10.1371/journal.pone.0009262 | 2010-02 |
| Downstream outputs | XA21 functions as a pattern-recognition receptor activating pattern-triggered immunity, including defense gene induction, MAPK-associated signaling, ROS and callose-associated defense outputs, and enhanced resistance to Xoo after ligand perception and release of negative regulation. | (pqac-00000012, pqac-00000006, pqac-00000010) | Chen & Ronald 2011, *Trends in Plant Science*; Park et al. 2017, *Rice*; Meesa et al. 2024, *Acta Agrobotanica* | https://doi.org/10.1016/j.tplants.2011.04.003 ; https://doi.org/10.1186/s12284-017-0166-1 ; https://doi.org/10.5586/aa/188569 | 2011-08 ; 2017-06 ; 2024-07 |
| Applications/quantitative data | Marker-assisted selection/introgression: ABUOP0001 intragenic marker detects a 19-bp ectodomain InDel (200-bp resistant vs 181-bp susceptible amplicon) and identified the resistance allele in 1,675 accessions from 3K genomes; field/phenotype validation found 14 highly resistant and 1 intermediate accession among tested lines. Diversity analysis found 13 confirmed haplotypes, haplotype diversity 0.8203, nucleotide diversity 0.15448, and truncated protein-types in ~45% of accessions due to the 19-bp InDel. In Thai BC4F6 Xa21 introgression lines, SA pretreatment reduced Xoo proliferation by 1.4-2.4-fold; lesion lengths fell from 16.8-18.0 cm to 10.0-10.9 cm at 14 DAI and from 34.9-35.1 cm to 24.6-28.0 cm at 21 DAI; Xoo counts were 12.8×10^4-18.1×10^4 CFU/mL vs 25.8×10^4-35.7×10^4 CFU/mL in controls. | (pqac-00000020, pqac-00000021, pqac-00000023, pqac-00000025, pqac-00000026) | Meesa et al. 2024, *Acta Agrobotanica*; Nanayakkara et al. 2019, preprint/Research Square; Nanayakkara et al. 2018, *Tropical Agricultural Research* | https://doi.org/10.5586/aa/188569 ; https://doi.org/10.21203/rs.2.11141/v1 ; https://doi.org/10.4038/tar.v30i1.8278 | 2024-07 ; 2019-07 ; 2018-12 |
| Key recent 2024 findings | 2024 work showed XA21-mediated resistance is dose dependent in HA-XA21 transgenic lines; 16 independent T0 insertion events were mapped by whole-genome sequencing, six lines were phenotyped, and higher XA21 accumulation correlated with stronger resistance. The study reported that four agronomic traits suggested yield is unlikely to be affected by XA21 expression level. A separate 2024 study showed SA pretreatment boosts Xa21 expression and disease suppression in introgression lines. | (pqac-00000008, pqac-00000010, pqac-00000022) | Zhang et al. 2024, *PeerJ*; Meesa et al. 2024, *Acta Agrobotanica* | https://doi.org/10.7717/peerj.17323 ; https://doi.org/10.5586/aa/188569 | 2024-05 ; 2024-07 |


*Table: This table summarizes the verified identity, mechanism, localization, regulators, and practical deployment of rice XA21 (UniProt Q2R2D5), with quantitative data and recent 2024 findings. It is useful as a compact evidence-backed functional annotation for narrative reports or downstream curation.*