Distinct Patterns of Control and Expression Amongst Members of the PEP Carboxylase Kinase Gene Family in C4 Plants

We have examined the complexity of the phosphoenolpyruvate carboxylase kinase (PPCK) gene family in the C(4) monocots maize and sorghum. Maize contains at least four PPCK genes. The encoded proteins are similar to other phosphoenolpyruvate carboxylase (PEPC) kinases, in that they comprise a protein kinase domain with minimal extensions, except that two of the proteins contain unusual acidic insertions. The spatial and temporal expression patterns of the genes provide information about their presumed functions. Expression of ZmPPCK1 in leaves is mesophyll cell-specific and light-induced, indicating that it encodes the PEPC kinase that is responsible for the phosphorylation of leaf PEPC during C(4) photosynthesis. Surprisingly, ZmPPCK2 is expressed in leaf bundle sheath cells, preferentially in the dark. This suggests that a main function of the ZmPPCK2 gene product is to allow PEPC to function anaplerotically in bundle sheath cells in the dark without interfering with the C(4) cycle. ZmPPCK2, ZmPPCK3 and ZmPPCK4 are all induced by exposure of tissue to cycloheximide, whereas ZmPPCK1 is not. This suggests that the ZmPPCK2, ZmPPCK3 and ZmPPCK4 genes share the property that their expression is controlled by a rapidly turning over repressor. Sequence and expression data show that sorghum contains orthologues of ZmPPCK1 and ZmPPCK2.