The LPA1-ITA1-BRXL4 module regulates shoot gravitropism and tiller angle in rice.

Tiller angle is a critical agronomic trait that influences rice plant architecture and yield potential. However, the molecular mechanisms underlying its regulation remain incompletely understood. Here, we report that ITA1 (LOC_Os01g51260), which encodes an MYB family transcription factor, positively regulates shoot gravitropism and restricts tiller angle in rice. The ita1 mutant, which is characterized by a 740-bp deletion in the ITA1 promoter, displays an increased tiller angle due to elevated ITA1 expression. Functional assays demonstrate that ITA1 directly activates BRXL4, a gene that modulates auxin transport by influencing the subcellular localization of LAZY1, a key regulator of shoot gravitropism. Yeast one-hybrid assays, chromatin immunoprecipitation-qPCR, electrophoretic mobility shift assays, and luciferase assays reveal that the transcriptional repressor LPA1, a member of the IDD family, directly binds to the 740-bp region deleted in the ita1 mutant and represses ITA1 expression. Genetic evidence shows that the lpa1 mutant phenocopies ita1 and that the increased tiller angle phenotype is partially suppressed in the ita1-c1 lpa1 double mutant. Together, our findings define a previously unknown LPA1-ITA1-BRXL4 regulatory cascade that controls shoot gravitropism and tiller angle by modulating auxin distribution. This study provides new insights into plant architectural regulation and identifies potential genetic targets for optimizing rice planting density and yield.