Involved in the plastidial phosphorylated pathway of serine biosynthesis (PPSB). Catalyzes the reversible conversion of 3-phosphohydroxypyruvate to phosphoserine. 2-oxoglutarate + O-phospho-L-serine = 3-phosphooxypyruvate + L-glutamate 2-oxoglutarate + 4-(phosphooxy)-L-threonine = (R)-3-hydroxy-2-oxo-4-phosphooxybutanoate + L-glutamate Amino-acid biosynthesis; L-serine biosynthesis; L-serine from 3-phospho-D-glycerate: step 2/3. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. SerC subfamily.