Response of Phosphoenolpyruvate Carboxylase from Maize Leaves to Moderate Water Deficit

Abstract

Summary Subjecting whole maize ( Zea mays L.) plants or leaf-discs to moderate water deficit resulted in proline accumulation and in significant changes in some kinetic parameters of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31). Vmax values, expressed as units/mg protein, were increased (around 50 %), while K m (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-molecularweight compounds in the extracts from stressed tissues. The sensitivities to the inhibitor malate or to the activator glucose-6-phosphate were not significantly affected with respect to controls, which suggests that the phosphorylation status of PEPC was not altered by the water-deficit treatment. Nor was the aggregation state of the enzyme, which was found to be tetrameric in all extracts. The subunit molecular weight and the pI of PEPC from both control and stressed leaves were as expected for the C 4 -isoform, arguing against the possible induction by water deficit of a different isoform. However, in the stressed 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 increases in the relative PEPC-protein took place when the levels of total soluble 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 suggest that PEPC activity might be involved in the metabolic response of C4 plants to moderate water deficit. Since photosynthesis is severely depressed under water deficit, PEPC activity is likely providing intermediates of the tricarboxylic acid cycle, which may be used either as precursors of proline or to increase the flux rate through this cycle.