Dual Transcriptional Circuits: G1-OsMADS34 and G1-TGW2 Cooperatively Regulate Sterile Lemma Identity and Grain Size...

Rice (Oryza sativa L.) features a unique inflorescence organ known as the spikelet, comprising floret, lemma/palea, sterile lemmas, and rudimentary glumes, of which the molecular regulation underlying sterile lemma identity remains elusive. Here, we isolated the G1 locus for sterile lemma specification using an F₂ population developed by crossing between Nipponbare and LG7, a variety with a lemma-like-sterile lemma (lsl). An SNP (+323 G/A) in G1 alleles causes a serine to asparagine (S108N) substitution, leading to the lsl phenotype. Mechanistically, we found that G1 transactivates the expressions of both OsMADS34 and TGW2, two genes known to regulate sterile lemma identity and grain size, through binding to the YACTGTW and CArG-box motifs within their promoters, respectively. Subsequently, we reveal that the transactivation activity of G1^NIP (allele from Nipponbare) is further enhanced through interactions with either OsMADS34 or TGW2. Furthermore, we demonstrated that G1 specifies sterile lemma identity via OsMADS34 and controls grain size through TGW2. Our results reveal two transcriptional circuits (G1-OsMADS34 and G1-TGW2) that are crucial for determining sterile lemma identity and grain size of rice, providing insights into genetic improvement for breeding programs.