Summary: Starch biosynthesis in gravity‐sensing tissues of rice shoot determines the magnitude of rice shoot gravitropism and thus tiller angle. However, the molecular mechanism underlying starch biosynthesis in rice gravity‐sensing tissues is still unclear. We characterized a novel tiller angle gene LAZY3 (LA3) in rice through map‐based cloning. Biochemical, molecular and genetic studies further demonstrated the essential roles of LA3 in gravity perception of rice shoot and tiller angle control. The shoot gravitropism and lateral auxin transport were defective in la3 mutant upon gravistimulation. We showed that LA3 encodes a chloroplast‐localized tryptophan‐rich protein associated with starch granules via Tryptophan‐rich region (TRR) domain. Moreover, LA3 could interact with the starch biosynthesis regulator LA2, determining starch granule formation in shoot gravity‐sensing tissues. LA3 and LA2 negatively regulate tiller angle in the same pathway acting upstream of LA1 to mediate asymmetric distribution of auxin. Our study defined LA3 as an indispensable factor of starch biosynthesis in rice gravity‐sensing tissues that greatly broadens current understanding in the molecular mechanisms underlying the starch granule formation in gravity‐sensing tissues, and provides new insights into the regulatory mechanism of shoot gravitropism and rice tiller angle.

Summary: 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.

Root gravitropism is essential for plants to establish proper root system architecture, enabling efficient soil exploration and optimal acquisition of water and nutrients. However, the molecular mechanisms underlying rice root gravitropism remain poorly understood. Here, we demonstrate that LAZY2 (LA2) and LA3, previously identified regulators for shoot gravitropism control, play critical roles in root gravitropism by maintaining starch accumulation in root columella cells. Loss-of-function mutants (CR-la2, CR-la3) exhibited impaired root gravitropism, resulting in a shallow root architecture. Further analysis revealed a significant reduction in starch accumulation within root statocytes of CR-la2 and CR-la3 mutants, causing reduced root gravity sensing. The complementation lines of LA2 or LA3 can rescue the defect in starch accumulation and root gravitropism. Haplotype analysis linked natural LA2 variation to distinct root architectures, with Hap5 accessions showing steeper angles than Hap1. Additionally, we confirmed that LA2 forms homodimers both in vivo and in vitro. Our findings establish LA2 and LA3 as key regulators coupling starch metabolism with root gravitropism, providing both fundamental insights into gravity sensing and genetic targets for optimizing root architecture to increase grain yield in cereal crops.