Phosphorylation of phosphoenolpyruvate carboxylase is not essential for high photosynthetic rates in the C4 species Flaveria bidentis

Phosphoenolpyruvate carboxylase ( PEPC; EC4.1.1.31) plays a key role during C-4 photosynthesis. The enzyme is activated by metabolites such as glucose-6-phosphate and inhibited by malate. This metabolite sensitivity is modulated by the reversible phosphorylation of a conserved serine residue near the N terminus in response to light. The phosphorylation of PEPC is modulated by a protein kinase specific to PEPC (PEPC-PK). To explore the role PEPC-PK plays in the regulation of C-4 photosynthetic CO2 fixation, we have transformed Flaveria bidentis (a C-4 dicot) with antisense or RNA interference constructs targeted at the mRNA of this PEPC-PK. We generated several independent transgenic lines where PEPC is not phosphorylated in the light, demonstrating that this PEPC-PK is essential for the phosphorylation of PEPC in vivo. Malate sensitivity of PEPC extracted from these transgenic lines in the light was similar to the malate sensitivity of PEPC extracted from darkened wild-type leaves but greater than the malate sensitivity observed in PEPC extracted from wild-type leaves in the light, confirming the link between PEPC phosphorylation and the degree of malate inhibition. There were, however, no differences in the CO2 and light response of CO2 assimilation rates between wild-type plants and transgenic plants with low PEPC phosphorylation, showing that phosphorylation of PEPC in the light is not essential for efficient C-4 photosynthesis for plants grown under standard glasshouse conditions. This raises the intriguing question of what role this complexly regulated reversible phosphorylation of PEPC plays in C-4 photosynthesis.