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. Interacts with CDG1 and CDL1. Expressed in mature cauline leaves and at the tip of influorescence, including flowers. Expressed at lower level in young tissues relative to older ones. Belongs to the PPP phosphatase family. BSU subfamily.