In 1995 we showed that the rice Xa21 resistance gene, encoding a
protein with predicted leucine rich repeat (LRR), transmembrane,
juxtamembrane, and intracellular kinase domains, conferred immunity to
diverse strains of the Gram-negative bacterium, Xanthomonas oryzae pv.
oryzae (Xoo) (Song et al., 1995).
Subsequent discoveries in flies (Toll) (Lemaitre
et al., 1996), humans Toll-like receptors 4 (TLR4) (Medzhitov
et al., 1997), mice (Poltorak
et al., 1998), and Arabidopsis (L. Gomez-Gomez,
T. Boller, Mol. Cell 5, 1003 (2000). C. Zipfel et al.,
Cell 125, 749 (2006).) revealed that animals and other plant
species also carry membrane-anchored receptors with striking structural
similarities to XA21 and that these receptors are also involved in
microbial recognition and defense. Like XA21, these receptors typically
associate with or carry non-RD (arginine-aspartic acid) kinases to
control early events of innate immunity signaling (Dardick
and Ronald, 2006).
Many of these cell surface receptors were later named pattern recognition receptors (PRRs) based on their ability to directly recognize molecules that are conserved across a large class of microbes (R. Medzhitov, Nat Rev Immunol 1, 135 (2001). Such microbial molecules were called pathogen-associated molecular patterns [PAMPs, also known as microbe associated molecular patterns (MAMPs)] (R. Medzhitov, C. A. Janeway, Jr., Curr Opin Immunol 9, 4 (1997).)).
We have now shown that the XA21 juxtamembrane (JM) domain plays a key role in XA21 kinase-mediated signal transduction. Phosphorylated resides in the JM domain serve as a high affinity-binding site for the downstream negative regulators, XB10, a putative WRKY transcriptional regulator (Peng et al., 2008), Xb24, a previously undescribed ATPase (Chen et al., 2010) and XB15, a PP2C protein phosphatase (Park et al., 2008).
My laboratory has also demonstrated, through proteomic and genetic analyses, that the rice protein NH1 (NPR1 homolog 1) is a key mediator of the rice defense response and that this pathway is critical for XA21-mediated immunity (Chern et al., 2001: Chern et al., 2005: Chern et al., 2008). Over-expression of NH1 (NH1ox) enhances resistance to Xoo and causes a lesion mimic phenotype. Based on these results, we developed a screen of a fast-neutron-induced mutant population to identify snl (suppressors of NH1-mediated lesion mimic) mutants. Genes that can complement two snl mutants have now been cloned. These include a gene encoding a key enzyme in lignin biosynthesis (Chern et al., Bart et al., in prep). In the next few years my laboratory will further characterize these and other snl mutants to determine if they affect PRR -mediated pathways.