Somatic embryogenesis receptor kinase (SERK) proteins play pivotal roles in regulation of plant development and immunity.Rice genome contains two SERK genes, OsSerk1 and OsSerk2.Here we report that rice plants silenced for each of these two OsSERK genes display altered morphology and reduced sensitivity to the hormone brassinolide.Both OsSERK1 and OsSERK2 interact with OsBRI1 in yeast.We also find that OsSERK2 positively regulates rice immunity mediated by XA21 and XA3 as well as the rice immune receptor FLS2 (OsFLS2) whereas OsSERK1 not.Overexpression of OsSerk1 in OsSerk2-silenced lines is able to complement the phenotypes associated with brassinosteroid (BR) signaling defects, but not the compromised disease resistance phenotype mediated by Xa21 in those lines.OsSERK2 interacts with the intracellular domains of each immune receptor in the yeast two-hybrid system in a kinase activity-dependent manner.OsSERK2 undergoes bidirectional transphosphorylation with XA21 in vitro.Interestingly, unlike that perception of the ligands flg22 or efr18 is required for the physical association between AtSERK3/BAK1 and the immune receptors, FLS2 or EFR, in Arabidposis,OsSERK2 forms a constitutive complex with the immune receptor XA21 in vivo,suggesting 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.Collectively, we conclude that, whereas OsSERK2 is the functional ortholog of AtSERK33/BAK1 and AtSERK4, regulating both plant development and immunity, OsSERK1 mainly function in rice development but not immunity.