Elucidation of XA21-mediated innate immunity

In the early 1970s, the Xa21 gene from the wild rice species Oryza longistaminata drew attention of rice breeders because of its broad-spectrum resistance to diverse strains of a serious bacterial disease of rice in Asia and Africa, called 'bacterial blight disease', caused by the Gram-negative bacterium, Xanthomonas oryzae pv. oryzae (Xoo). In 1995, we isolated the gene controlling this resistance and in 2009 demonstrated that XA21 recognizes a highly conserved peptide, called 'Ax21' (activator of XA21-mediated immunity). Tyrosine sulfation of Ax21 is required for recognition by rice XA21. A decade of genetic, molecular and biochemical studies have uncovered key components of the XA21-mediated signalling cascade. Ax21 recognition by XA21 at the cell surface induces phosphorylation-mediated events, which are predicted to alter subcellular localization and/or DNA-binding activity of a WRKY family of transcription factors. Because XA21 is representative of the large number of predicted pattern recognition receptors (PRRs) in rice (n = 328), Arabidopsis (n = 35) and other plant species, further characterization of XA21-mediated signalling pathways will contribute to elucidation of these important defence responses.