The receptor-like cytoplasmic kinase OsBSK1-2 regulates immunity via an HLH/bHLH complex.

Plants need to fine-tune defense responses to maintain a robust but flexible host barrier to various pathogens. Helix-loop-helix/basic helix-loop-helix (HLH/bHLH) complexes play important roles in fine-tuning plant development. However, the function of these genes in plant immunity and how they are regulated remain obscure. Here, we identified an atypical bHLH transcription factor, Oryza sativa (Os)HLH46, that interacts with rice receptor-like cytoplasmic kinase (RLCK) Os BRASSINOSTEROID-SIGNALING KINASE1-2 (OsBSK1-2), which plays a key role in rice blast resistance. OsBSK1-2 stabilized OsHLH46 both in vivo and in vitro. In addition, OsHLH46 positively regulates rice blast resistance, which depends on OsBSK1-2. OsHLH46 has no transcriptional activation activity and interacts with a typical bHLH protein, OsbHLH6, which negatively regulates rice blast resistance. OsbHLH6 binds to the promoter of OsWRKY45 and inhibits its expression, while OsHLH46 suppresses the function of OsbHLH6 by blocking its DNA binding and transcriptional inhibition of OsWRKY45. Consistent with these findings, OsWRKY45 was up-regulated in OsHLH46-overexpressing plants. In addition, the oshlh46 mutant overexpressing OsbHLH6 is more susceptible to Magnaporthe oryzae than is the wild type, suggesting that OsHLH46 suppresses OsbHLH6-mediated rice blast resistance. Our results not only demonstrated that OsBSK1-2 regulates rice blast resistance via the OsHLH46/OsbHLH6 complex, but also uncovered a new mechanism for plants to fine-tune plant immunity by regulating the HLH/bHLH complex via RLCKs.