Phosphorylation-dependent suppression of SERRATE balances miRNA metabolism in planta

MicroRNAs (miRNAs) play crucial regulatory roles in multiple developmental processes of animals and plants. Serrate (SE) is essential for miRNA processing and RNA metabolism; however, post-translational modification of SE remains largely unexplored. Casein kinase 1 (CK1) plays vital roles in both plants and mammals by phosphorylating distinct substrates. Here, we demonstrate that Arabidopsis early flowering 1-like (AELs; a plant CK1) regulates miRNA metabolism through phosphorylating SE. miRNA-seq reveals a decreased abundance of miRNAs under AEL4 overexpression, suggesting that AELs suppress miRNA biogenesis. AELs phosphorylate SE at Thr21 and Ser355, enhancing its affinity with the 20S core proteasome α subunit G1 (PAG1) for degradation and reducing SE's binding affinity to hyponastic leaves 1 (HYL1) to suppress microprocessor complex assembly. This study elucidates the crucial roles of CK1/AEL-mediated phosphorylation in regulating SE accumulation and consequently miRNA metabolism, providing insights into the regulation of miRNA homeostasis at the post-translational level.