FLA1, Enhancing GA3 Contents in Flag Leaf Lamina Joint, Increases Flag Leaf Angle to Improve Outcross Rate and Hybrid Rice Seed Production

Flag leaf angle (FLA) in rice (Oryza sativa L.) is one of the important traits affecting F1 seed production by mechanization. Here, we report the map-based cloning and functional characterization of the FLA1 (FLAG-LEAF-ANGLE 1) gene, which resides at a major-effect quantitative trait locus (QTL). Through cell morphological observations and exogenous hormone treatment assays, we demonstrate that gibberellin (GA) modulates rice FLA by altering both the number of cell layers and cell length. Combining genetic and molecular biological analyses with genetic complementation and gene overexpression assays, we elucidated and validated the biological function of FLA1. In addition, we found that FLA1 is constitutively expressed and encodes a protein localized to both the cell membrane and nucleus. Via RT-qPCR assays, we further demonstrated that the FLA1fla-R allele from the rice accession fla-R enhances GA biosynthesis by upregulating the expression of CLA1 and GA20ox2. Furthermore, yeast two-hybrid assays revealed that auxin-repressed protein 1 (ARP1) interacts with FLA1, suggesting a potential role of this interaction in the modulation of rice FLA. Collectively, our results demonstrate that optimizing rice FLA via molecular manipulation of FLA1 can resolve the problem of flag leaf shearing during F1 hybrid rice seed production without compromising F1 hybrid seed yield, thereby facilitating mechanized F1 hybrid rice seed production.