DRB4-dependent trans-acting siRNAs regulate lemma polarity development in rice

As the outermost floral organ of the rice spikelet, the normal morphological development of the glume (lemma and palea) is fundamental to grain morphology, quality, and yield formation. However, the regulatory mechanisms governing rice lemma development remain incompletely understood. Here, we identified a rice mutant, drb4, which is defective in lemma polarity development and displays abnormal phenotypes including narrowed or filamentous lemmas. DRB4 is highly expressed in floral organs. It encodes a double-stranded RNA-binding protein containing three DSRM domains and localizes to both the cytoplasm and nucleus. In drb4, the synthesis of ta-siRNA, particularly tasiR-ARF, is significantly reduced, leading to the ectopic expression of their downstream target genes OsARFs on the adaxial of the lemma. Furthermore, biochemical and genetic evidence indicates that DRB4 interacts with OsDCL4 and jointly participates in the biosynthesis of tasiR-ARF. Eventually, it regulates the polarity development of the glume, especially the development of the lemma, by influencing the polarity expression of OsARFs in the glume. Our findings established a crucial DRB4-OsDCL4-tasiR-ARF-OsARF module that precisely controls rice lemma polarity, providing new insights into floral organ development.