In the leaves of $C_4$ plants, malic acid formation during ${\text{CO}}_2$ fixation occurs in the cells of:

In $C_4$ plants, the initial fixation of ${\text{CO}}_2$ occurs in the mesophyll cells where PEP carboxylase fixes ${\text{CO}}_2$ into oxaloacetic acid (OAA). OAA is then converted into malic acid or aspartic acid within the mesophyll cells before being transported to the bundle sheath cells.