<scp>HSFA2D</scp> – <scp>LAZY6–LAZY1</scp> module regulates shoot gravitropism and tiller angle in rice

Tiller angle is a critical architectural trait in rice (Oryza sativa), affecting planting density, light interception, and grain yield. While HEAT STRESS TRANSCRIPTION FACTOR 2D (HSFA2D) has been shown to regulate tiller angle via LAZY1 (LA1)-mediated auxin distribution, the precise mechanisms governing this pathway remain elusive. We identified and characterized LA6, which encodes a heat shock protein 70-interacting protein, through mutant screening and the MutMap strategy. Biochemical, molecular, and genetic studies were used to elucidate the functional relationship of LA6 with HSFA2D and LA1 in mediating shoot gravitropism and tiller angle. The la6 mutant exhibited impaired shoot gravitropism and increased tiller angle due to reduced lateral auxin transport and disrupted asymmetric auxin distribution. The LA6 gene was highly expressed in shoot bases, and the LA6 protein was localized to the nucleus, cytoplasm, and plasma membrane. Intriguingly, LA6 physically interacted with HSFA2D and LA1. Genetic analyses showed that LA6 acts downstream of HSFA2D and upstream of LA1 within the same regulatory pathway. Our findings reveal that the HSFA2D-LA6-LA1 module orchestrates shoot gravitropism and tiller angle in rice, which not only advances our understanding of plant architecture regulation but also provides potential targets for yield improvement.