Phosphatase involved in elongation process, probably by acting as a regulator of brassinolide signaling. 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. Expressed throughout the plant, with a higher level in younger parts. Belongs to the PPP phosphatase family. BSU subfamily.