Leaf angle is an important agronomic trait for crop architecture and yield. In rice (Oryza sativa), the lamina joint is a unique structure connecting the leaf blade and sheath that determines leaf angle. Brassinosteroid (BR) signaling involving GLYCOGEN SYNTHASE KINASE-3 (GSK3)/SHAGGY-like kinases and BRASSINAZOLE-RESISTANT1 (BZR1) has a central role in regulating leaf angle in rice. In this study, we identified the atypical R2R3-MYB transcription factor FOUR LIPS (OsFLP), the rice homolog of Arabidopsis (Arabidopsis thaliana) AtFLP, as a participant in BR-regulated leaf angle formation. The spatiotemporal specificity of OsFLP expression in the lamina joint was closely associated with lignin deposition in vascular bundles and sclerenchyma cells. OsFLP mutation caused loose plant architecture with droopy flag leaves and hypersensitivity to BRs. OsBZR1 directly targeted OsFLP, and OsFLP transduced BR signals to lignin deposition in the lamina joint. Moreover, OsFLP promoted the transcription of the phenylalanine ammonia-lyase family genes OsPAL4 and OsPAL6. Intriguingly, OsFLP feedback regulated OsGSK1 transcription and OsBZR1 phosphorylation status. In addition, an Ala-to-Thr substitution within the OsFLP R3 helix-turn-helix domain, an equivalent mutation to that in Osflp-1, affected the DNA-binding ability and transcriptional activity of OsFLP. Our results reveal that OsFLP functions with OsGSK1 and OsBZR1 in BR signaling to maintain optimal leaf angle by modulating the lignin deposition in mechanical tissues of the lamina joint.

中文翻译：

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水稻 R2R3 MYB 转录因子 FOUR LIPS 将油菜素类固醇信号转导与木质素沉积和叶角联系起来


叶角是影响作物结构和产量的重要农艺性状。在水稻 （Oryza sativa） 中，叶片节理是连接叶片和鞘的独特结构，决定了叶角。涉及糖原合成酶激酶-3 （GSK3）/SHAGGY 样激酶和油菜素唑-RESISTANT1 （BZR1） 的油菜素类固醇 （BR） 信号转导在调节水稻叶角中起着核心作用。在这项研究中，我们确定了非典型的 R2R3-MYB 转录因子 FOUR LIPS （OsFLP），拟南芥 （Arabidopsis thaliana） AtFLP 的水稻同源物，作为 BR 调节叶角形成的参与者。OsFLP 在椎板关节中表达的时空特异性与木质素在维管束和巩膜细胞中的沉积密切相关。OsFLP 突变导致植物结构松散，旗叶下垂，对 BRs 过敏。OsBZR1 直接靶向 OsFLP，OsFLP 将 BR 信号转导到椎板关节中的木质素沉积。此外，OsFLP 促进苯丙氨酸解氨酶家族基因 OsPAL4 和 OsPAL6 的转录。有趣的是，OsFLP 反馈调节 OsGSK1 转录和 OsBZR1 磷酸化状态。此外，OsFLP R3 螺旋转角螺旋结构域内的 Ala-to-Thr 替换（与 Osflp-1 中的突变等效）影响了 OsFLP 的 DNA 结合能力和转录活性。我们的结果表明，OsFLP 在 BR 信号传导中与 OsGSK1 和 OsBZR1 一起发挥作用，通过调节木质素沉积在椎板关节的机械组织中来维持最佳叶角。