The rice immune receptor XA21 is activated by the sulfated bacterial peptide RaxX
Benjamin Schwessinger*1,3,Rory Pruitt*1, Anna Joe1, Furong Liu1, Dee Dee Luu1, Nicholas Thomas1, Markus Albert2, Jürg Felix2, Leanne Chan3, Chris Petzold3, Pamela Ronald1,3
1University of California, Davis, USA
2University of Tübingen, Germany
3Joint BioEnergy Institute, USA
The rice immune receptor XA21 provides broad spectrum resistance to most strains of Xanthomonas oryzae pv. oryzae, the causative agent of bacterial blight in rice. Although XA21 was among the first innate immune receptors identified, the identity of its microbial elicitor has remained elusive. Early attempts to identify the XA21 elicitor led to the discovery of a several bacterial genes that are required for activation of XA21 mediated immunity. These include raxST which encodes a tyrosine sulfotransferase and three genes which encode components of a type I secretion system, raxA, raxB, and raxC. We have recently identified a novel gene, raxX, which is also required for activation of XA21-mediated immunity. When raxX is deleted, X. oryzae pv. oryzae becomes virulent on rice expressing XA21. raxX encodes a small secreted peptide which is tyrosine sulfated by RaxST as shown by mass-spectrometry analysis. A 21 amino acid tyrosine sulfated fragment of RaxX is sufficient for activation of XA21-mediated immune responses including production of ethylene, reactive oxygen species and upregulating expression of defense response genes. RaxX also is able to bind to XA21 and these results indicate that RaxX is the ligand of XA21, providing a key mechanistic detail of how this receptor is activated during infection.
This work was supported by NIH grant GM55962 and a Human Frontier Science Program long-term post-doctoral fellowship (LT000674/2012) to B.S.