Response of phosphoenolpyruvate carboxylase from maize leaves to moderate water deficit

Subjecting whole maize (Zea mays L.) plants or leaf-discs to moderate water deficit resulted in proline accumulation and in significant changes in som e kinetic parameters of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) . Vmax values, expressed as units/mg protein, were increased (around 50 %), while Km(Mg-PEP) values were reduced (around 20 %). Rapid desalting of the crude extracts did not revert these changes, indicating that they are not due to the presence of high concentrations of low-molecular-weight compound s in the extracts from stressed tissues. The sensitivities to the inhibitor malate or to the activator glucose-6-phosphate were not significantly affe cted with respect to controls, which suggests that the phosphorylation stat us of PEPC was not altered by the water-deficit treatment. Nor was the aggr egation state of the enzyme, which was found to be tetrameric in all extrac ts. The subunit molecular weight and the pi of PEPC from both control and s tressed leaves were as expected for the C-4-isoform, arguing against the po ssible induction by water deficit of a different isoform. However, in the s tressed leaves we found a higher relative content of the C-4-PEPC-protein, which accounted for the increases in Vmax. It is interesting that the incre ases in the relative PEPC-protein took place when the levels of total solub le protein were low and constant. Moreover, we found a significant positive correlation between the levels of proline, PEPC activity and PEPC protein and the degree of water deficit of the discs. Taken together, our results s uggest that PEPC activity might be involved in the metabolic response of C- 4 plants to moderate water deficit. Since photosynthesis is severely depres sed under water deficit, PEPC activity is likely providing intermediates of the tricarboxylic acid cycle, which may be used either as precursors of pr oline or to increase the flux rate through this cycle.