|Dardick, Christopher - Chris|
Submitted to: PLoS Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/13/2008
Publication Date: 9/23/2008
Citation: Park, C., Peng, Y., Bart, R., Chen, X., Ruan, D., Canlas, P.E., Dardick, C.D., Ronald, P.C. 2008. Rice Xb15, a protein phosphatase 2C, negatively regulates Xa21-mediated resistance and programmed cell death. PLoS Biology. 6(9):e231. Interpretive Summary: Xa21 is a gene that provides resistance to a bacterial disease in rice. Understanding the molecular mechanisms of Xa21-mediated resistance could have a broad application to engineering disease resistance in plants, as well as, contribute to our basic understanding of innate immunity in all higher organisms. We showed that Xa21 associates with and is negatively regulated by Xb15. Xb15 is an enzyme with protein phosphatase activity that reverses the enzyme action of Xa21, a protein kinase. Plants containing mutations in Xb15 are more susceptible to bacterial infection. These findings add to the growing body of evidence that phosphates are key mediators of defense signaling and provide a new target for engineering pathogen resistance.
Technical Abstract: The rice pathogen recognition receptor, XA21, confers resistance to specific races of Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial blight disease. A yeast-two-hybrid screen using the intracellular portion of XA21, including the juxtamembrane (JM) and kinase domain as a bait, identified a protein phosphatase 2C, called XA21 binding protein 15 (XB15). The interaction of XA21 and XB15 was confirmed in vitro and in vivo by GST pull-down and co-immunoprecipitation assays, respectively. In vitro and in vivo purified XB15 carries PP2C activity. Autophosphorylated XA21 could be dephosphorylated by XB15 in a temporal and dosage-dependent manner. To examine the role of XB15 in the defense pathway, we characterized knockout mutants caused by a retrotransposon Tos17 insertion into exon-3 of Xb15. Homozygous Xb15 Tos17 mutants displayed severe programmed cell death (PCD) phenotypes and induction of pathogenesis-related (PR) marker genes. Over-expression of Xb15 in an XA21 rice line compromised the resistance against Xoo Philippine race 6 (Xoo PR6). The reduced resistance correlated with higher XB15 levels. These results demonstrate that Xb15 encodes a protein phosphatase 2C that negatively regulates the XA21-mediated defense pathway.