Abstract
Pattern recognition receptors (PRRs) play an important role in detecting invading pathogens and mounting a robust defense response to restrict infection. In rice, one of the best characterized PRRs is XA21, a leucine rich repeat receptor-like kinase that confers broad-spectrum resistance to multiple strains of the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). In 2009 we reported that an Xoo protein, called Ax21, is secreted by a type I-secretion system and that it serves to activate XA21-mediated immunity. This report has recently been retracted. Here we present data that corrects our previous model. We first show that Ax21 secretion does not depend on the predicted type I secretion system and that it is processed by the general secretion (Sec) system. We further show that Ax21 is an outer membrane protein, secreted in association with outer membrane vesicles. Finally, we provide data showing that ax21 knockout strains do not overcome XA21-mediated immunity.
Highlights
Pattern recognition receptors (PRRs) are proteins of critical importance to innate immunity in plants and animals
These include three genes found in a single operon, How to cite this article Bahar et al (2014), The Xanthomonas Ax21 protein is processed by the general secretory system and is secreted in association with outer membrane vesicles
We further demonstrate that in Escherichia coli, the Ax21 leader sequence is processed by the general secretory pathway and provide evidence that Ax21 is secreted outside the cell as a component of outer membrane vesicles
Summary
Pattern recognition receptors (PRRs) are proteins of critical importance to innate immunity in plants and animals. Upon recognition of conserved microbial signatures ( known as pathogen-associated molecular patterns, PAMPs), PRRs trigger immune responses that restrict infection (Schwessinger & Ronald, 2012). Over the last 10 years, several bacterial genes that are required for activation of XA21-mediated immunity (rax genes) have been identified (Shen et al, 2002; Burdman et al, 2004; Da Silva et al, 2004). These include three genes found in a single operon, How to cite this article Bahar et al (2014), The Xanthomonas Ax21 protein is processed by the general secretory system and is secreted in association with outer membrane vesicles.
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