Phosphatase that acts as a positive regulator of brassinosteroid (BR) signaling (PubMed:14977918, PubMed:21855796). Dephosphorylates BES1, a transcription factor that regulates the expression of BR-response genes, thereby playing an important role in the regulation of response to BRs (PubMed:14977918). Inactivates the negative regulator of BR signaling ASK7/BIN2 by dephosphorylation at 'Tyr-200' (PubMed:21855796). H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 2 manganese ions per subunit. Activated by phosphorylation at Ser-764 by CDG1. Interacts with CDG1, CDL1 and ASK7/BIN2. Mainly expressed in young, elongating tissues. In young seedlings, it is expressed at the base of the hypocotyl, at the tip and most peripheral cell layers of cotyledons, and in the vascular cylinder of roots, particularly in the elongation zone and at the point of emergence of lateral roots. In mature plants, it is still present in the root vasculature, but almost completely absent in fully expanded stems and leaves. In flowers, it is mainly expressed in sepal veins, anther filaments, and in the style, suggesting that BSU1 is expressed in actively growing regions and apparently enriched in vascular tissues. Phosphorylated at Ser-395 and Ser-444. Phosphorylated at Ser-764 by CDG1 and CDL1. Belongs to the PPP phosphatase family. BSU subfamily.