The rice Xa21 gene confers immunity to most strains of the bacterium Xanthomonas oryzae pv. oryzae (Xoo). Liquid chromatography–tandem mass spectrometry analysis of biologically active fractions from Xoo supernatants led to the identification of a 194–amino acid protein designated Ax21 (activator of XA21-mediated immunity). A sulfated, 17–amino acid synthetic peptide (axYS22) derived from the N-terminal region of Ax21 is sufficient for activity, whereas peptides lacking tyrosine sulfation are biologically inactive. Using coimmunoprecipitation, we found that XA21 is required for axYS22 binding and recognition. axYS22 is 100% conserved in all analyzed Xanthomonas species, confirming that Ax21 is a pathogen-associated molecular pattern and that XA21 is a pattern recognition receptor.
As a result of additional experiments in P.C.R.'s and S.W.-L.'s laboratories, we wish to retract our 2009 Report, “A type I–secreted, sulfated peptide triggers XA21-mediated innate immunity” (1). Specifically, we have not been able to consistently reproduce the results shown in Figure 3. We have also discovered critical errors in Figures 2 and S3. The strain PXO99Δax21, used in Figure 2, was mixed up with another strain (PXO99ΔraxSt). When we repeated the experiment with the validated PXO99Δax21 insertion mutant, this strain is still avirulent on Xa21 lines. These results indicate that this insertion in Ax21 does not abolish the ability of PX099 to trigger XA21-mediated immunity. Regarding figure S3, by using more sensitive methods, we have discovered that Ax21 is also secreted in the mutant strains PXO99ΔraxA and PXO99ΔraxC. Although we recognize that some parts of this paper may remain valid, we note that key parts of the work depend on the results of Figures 2 and 3. For these reasons, we retract the main conclusion of the paper that a type I–secreted, sulfated peptide triggers XA21-mediated innate immunity.