Thus, RTX toxins may also be virulence factors of Xoo. In addition, the rax genes (such as raxA, raxB, raxC, and raxST) of Xoo are involved in secretion by the type I secretion system and sulfation of peptide Ax21 (activator of Xa21-mediated immunity), which elicit rice Xa21 protein-mediated resistance (Lee et al. 2009).