LAX PANICLE1 acts as a canonical bHLH transcription factor to regulate inflorescence axillary meristem formation in rice

The development of axillary meristems (AMs) is pivotal for tillering and panicle branching in rice, processes that ultimately determine grain yield. LAX PANICLE1 (LAX1) encodes a basic helix-loop-helix (bHLH) transcription factor essential for AM formation during panicle development. However, because LAX1 possesses a non-canonical basic region with fewer than five basic amino acids, it has long been presumed to lack intrinsic DNA-binding capacity, leaving its molecular mechanism of action unclear. Here, we demonstrate that LAX1 localizes to the nucleus and exhibits transcriptional activation activity. Using Systematic Evolution of Ligands by EXponential enrichment (SELEX) assays, we revealed that LAX1 possesses intrinsic DNA-binding capability in vitro. Furthermore, chromatin immunoprecipitation sequencing (ChIP-seq) analysis of young rice panicles identified direct LAX1 target genes, with binding peaks significantly enriched for a specific motif, TTTTGC(T/A/C)NNN(G/A/T), which is directly bound by the LAX1 bHLH domain. Subsequent experiments validated that LAX1 directly binds the promoters of OsPID, OsIAA7, LOG, and OsTPS8. Molecular and genetic analyses provided evidence supporting the functional role of the LAX1-OsPID module in regulating inflorescence architecture. Comparative transcriptome analysis of lax1 mutants and wild-type plants further indicated that LAX1 participates in multiple biological processes. Interestingly, we found that LAX1 and its orthologs likely regulate downstream gene expression by recognizing a common set of cis-regulatory elements across diverse cereal crops. Collectively, our findings redefine LAX1 and its orthologs as canonical bHLH transcription factors, elucidate their regulatory network in controlling panicle branching, and provide a theoretical foundation for the cross-species application of LAX1 in crop yield improvement.