OsOFP9 regulates diverse key traits of rice by integrating multiple plant hormones

As the staple food for more than half of the world's population, rice requires elite varieties with superior quality and high yield to ensure food security. Agronomic traits, such as grain size, leaf angle, seed dormancy, and germination, will affect rice yield. Identification and cloning of key genes and elucidation of molecular mechanisms regulating these traits expedite rice breeding. The OVATE Family Proteins (OFPs), a unique family of transcription regulators, play critical roles in regulating grain or fruit size, plant morphology, and stress responses. Here, we have successfully identified OsOFP9, an uncharacterized OFP member in rice, and demonstrated its irreplaceable role in controlling several key agronomic traits. Mutation of OsOFP9 results in severe pre-harvest sprouting, promoted seed germination, smaller grains, and reduced leaf angle. Mechanistic studies revealed that the OsOFP9 mutation reduced abscisic acid (ABA) levels and increased gibberellin (GA) levels, thereby affecting the ABA/GA ratio and α-amylase activity. In addition, OsOFP9 directly interacts with GS9 and DLT, key transcriptional regulators involved in the BR signaling pathway controlling grain size and leaf angle, respectively. Functional assays showed that OsOFP9 inhibited the transcriptional activation activity of GS9, but enhanced the transcriptional repression activity of DLT. Genetic evidence showed that GS9 and DLT function downstream of OsOFP9, consistent with the results of the transcriptional activity assay. In conclusion, this study reveals the crucial role of OsOFP9 in regulating several important agronomic traits and elucidates its molecular mechanism in coordinating multiple plant hormones, thus providing valuable insights and genetic resources for improving rice yield.