LAZY2 and LAZY3 Regulate Rice Root Gravitropism by Affecting Starch Accumulation

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.