Brassinosteroid signaling mediated by the OsIAA7-OsGSK2-OsBZR1 module regulates seed size in rice

Grain size profoundly influences crop yield. Therefore, elucidating the molecular mechanisms controlling crop grain size is of great importance. Here, we report that the early auxin-responsive gene AUXIN or INDOLE-3-ACETIC ACID 7 (OsIAA7) negatively regulates grain size in rice (Oryza sativa L.), as loss of OsIAA7 function leads to the development of larger and heavier grains. OsIAA7 is highly expressed in developing panicles and grains, and the eGFP-OsIAA7 fusion protein is localized to the nuclei. The OsIAA7-mediated regulation of grain size involves constraining cell division and elongation in the longitudinal direction, as well as cell elongation in the transverse direction of spikelet hull cells. Biochemical analyses demonstrate a physical interaction between OsIAA7 and GLYCOGEN SYNTHASE KINASE 3 (GSK3)/SHAGGY-LIKE KINASE 2 (OsGSK2), which enhances the OsGSK2-BRASSINAZOLE-RESISTANT 1 (OsBZR1) interaction, resulting in OsBZR1 phosphorylation and degradation. Functional loss of OsIAA7 increases 24-epibrassionolide (BL) sensitivity, while BL treatment reduces OsIAA7-HA stability, indicating its involvement in brassinosteroid signaling. Genetic analyses support a strong genetic interaction between OsIAA7 and OsGSK2, with OsIAA7 acting upstream of OsGSK2. In summary, our findings reveal the OsIAA7-OsGSK2-OsBZR1 regulatory module as a mechanism controlling grain size in rice.