Recognizes and hydrolyzes the peptide bond at the C-terminal Gly of ubiquitin. Involved in the processing of poly-ubiquitin precursors as well as that of ubiquitinated proteins (By similarity). Positive regulator of root meristem development that, together with UBP13, prevents the ubiquitination and turnover of RGFR1 induced by the RGF1 hormone peptide, thus influencing PLT1 and PLT2 expression (PubMed:29339500). Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Interacts with SIC/RON3. The double mutant ubp12 ubp13 roots are completely insensitive to exogenous applied hormone peptide RGF1 associated with an accelerated RGF1-induced ubiquitination and turnover of RGFR1 and are characterized by a reduced number of cortical meristem cells and disturbed PLT1 and PLT2 expression. May be due to a competing acceptor splice site. Belongs to the peptidase C19 family.