The primary ${\mathrm{CO}}_2$ acceptor in ${\mathrm{C}}_4$ plant is

In ${\mathrm{C}}_4$ plants, a 4 - C compound oxaloacetic acid (OAA) is the first stable product, and phosphoenol pyruvate (PEP) is the ${\mathrm{CO}}_2$ acceptor. This reaction is catalyzed by the enzyme PEP carboxylase or PEP case in mesophyll cells of the leaf.