An XA21-Associated Kinase （OsSERK2） Regulates Immunity Mediated by the XA21 and XA3 Immune Receptors

The rice XA21 immune receptor kinase and the structurally related XA3 receptor confer immunity to Xanthomonas oryzae pv. oryzae （Xoo）, the causal agent of bacterial leaf blight. Here we report the isolation of OsSERK2 （rice somatic embryogenesis receptor kinase 2） and demonstrate that OsSERK2 positively regulates immunity mediated by XA21 and XA3 as well as the rice immune receptor FLS2 （OsFLS2）. Rice plants silenced for OsSerk2 display altered morphology and reduced sensitivity to the hormone brassinolide. OsSERK2 interacts with the intracellular domains of each immune receptor in the yeast two-hybrid system in a kinase activity-dependent manner. OsSERK2 undergoes bidi- rectional transphosphorylation with XA21 in vitro and forms a constitutive complex with XA21 in vivo. These results demonstrate an essential role for OsSERK2 in the function of three rice immune receptors and suggest that direct interaction with the rice immune receptors is critical for their function. Taken together, our findings suggest that the mechanism of OsSERK2-meditated regulation of rice XA21, XA3, and FLS2 differs from that of AtSERK3/BAK1-mediated regulation of Arabidopsis FLS2 and EFR.

OsSERK1 regulates rice development but not immunity to Xanthomonas oryzae pv. oryzae or Magnaporthe oryzae

Somatic embryogenesis receptor kinase（SERK）proteins play pivotal roles in regulation of plant development and immunity. The rice genome contains two SERK genes,OsSerk1 and OsSerk2. We previously demonstrated that OsSerk2 is required for rice Xa21-mediated resistance to Xanthomonas oryzae pv. oryzae（Xoo） and for normal development. Here we report the molecular characterization of OsSerk1. Overexpression of OsSerk1 results in a semi-dwarf phenotype whereas silencing of OsSerk1 results in a reduced angle of the lamina joint. OsSerk1 is not required for rice resistance to Xoo or Magnaporthe oryzae. Overexpression of Os Serk1 in OsSerk2-silenced lines complements phenotypes associated with brassinosteroid（BR） signaling defects, but not the disease resistance phenotype mediated by Xa21. In yeast, OsSERK1 interacts with itself forming homodimers, and also interacts with the kinase domains of OsSERK2 and BRI1, respectively.OsSERK1 is a functional protein kinase capable of auto-phosphorylation in vitro. We conclude that, whereas OsSERK2 regulates both rice development and immunity, OsSERK1 functions in rice development but not immunity to Xoo and M. oryzae.