Serine/threonine-protein kinase involved in the positive regulation of brassinosteroid (BR) signaling and plant growth. Mediates BR signal transduction from BRI1 receptor kinase to BSU1 phosphatase. After activation by phosphorylation at Ser-234 by BRI1, CDG1 phosphorylates BSU1 at 'Ser-764' in the phosphatase domain, increasing the ability of BSU1 to inactivate the negative regulator of BR signaling ASK7/BIN2 by dephosphorylation at 'Tyr-200'. The full kinase activity of CDG1 is required for its biological function. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by phosphorylation at Ser-234. Interacts with BSU1, BSL1 and BRI1. Plasma membrane localization is required for CDG1 to function in BR signaling. Expressed at high levels in the stamen and pollen grains (PubMed:21855796). Expressed at a very low level in vegetative tissues (PubMed:15466232, PubMed:21855796). Phosphorylated at Ser-44, Ser-47 and Ser-234 by BRI1. No visible phenotype under normal growth conditions. The gain of function mutants cdg1-D (T-DNA tagging) show pleiotropic phenotype such as dwarfism, exaggerated leaf epinasty and twisted or spiral growth in hypocotyl, inflorescence stem, and petiole. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. Extended C-terminus.