Phosphoenol pyruvate (PEP) is the primary ${\text{CO}}_2$ acceptor in:

In ${\text{C}}_4$ plants (like maize and sugarcane), atmospheric ${\text{CO}}_2$ is first accepted by PEP in the mesophyll cells to form a 4-carbon acid (OAA). In ${\text{C}}_3$ plants, the acceptor is RuBP.