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Glycogen synthase kinase 3 (GSK3) proteins play key roles in brassinosteroid (BR) signaling during plant growth and development by phosphorylating various substrates. However, how GSK3 protein stability and activity are themselves modulated is not well understood. Here, we demonstrate in vitro and in vivo that C-TERMINAL DOMAIN PHOSPHATASE-LIKE 3 (OsCPL3), a member of the RNA Pol II CTD phosphatase-like family, physically interacts with OsGSK2 in rice (Oryza sativa). OsCPL3 expression was widely detected in various tissues and organs including roots, leaves and lamina joints, and was induced by exogenous BR treatment. OsCPL3 localized to the nucleus, where it dephosphorylated OsGSK2 at the Ser-222 and Thr-284 residues to modulate its protein turnover and kinase activity, in turn affecting the degradation of BRASSINAZOLE-RESISTANT 1 (BZR1) and BR signaling. Loss of OsCPL3 function resulted in higher OsGSK2 abundance and lower OsBZR1 levels, leading to decreased BR responsiveness and alterations in plant morphology including semi-dwarfism, leaf erectness and grain size, which are of fundamental importance to crop productivity. These results reveal a previously unrecognized role for OsCPL3 and add another layer of complexity to the tightly controlled BR signaling pathway in plants.

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