The OsIAA3-OsARF16<i>-OsBUL1</i> auxin signaling module regulates grain size in rice

Auxin plays an important role in various aspects of plant growth and development. However, the molecular mechanism underlying the control of grain size via auxin signaling pathways remains obscure. Here, we report that AUXIN/INDOLE-3-ACETIC ACID protein 3 (OsIAA3) positively regulates rice (Oryza sativa) grain size by promoting the cell expansion and proliferation of spikelet hulls. OsIAA3 interacted with 11 AUXIN RESPONSE FACTORS (ARFs), among which the interaction with OsARF16 was the strongest. The osarf16 knockout mutant showed smaller grains with decreased grain length, grain width, grain thickness, and 1,000-grain weight. Meanwhile, transgenic plants overexpressing OsARF16 produced noticeably larger grains with increased grain length and 1,000-grain weight. O. sativa BRASSINOSTEROID UPREGULATED 1-LIKE (OsBUL1), which encodes an atypical bHLH protein that positively regulates grain size by promoting cell expansion, is a direct target gene of OsARF16. The interaction between OsIAA3 and OsARF16 repressed the transcriptional activation of OsARF16 on OsBUL1. Our study reveals an OsIAA3-OsARF16-OsBUL1 module that regulates grain size, refining the molecular mechanism of the auxin signaling pathway involved in grain size control.