Regulation of phosphoenolpyruvate carboxylase in Pinus halepensis needles submitted to ozone and water stress

Effects of ozone and/or drought stresses on phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31) regulation in Pinus halepensis Mill. needles were assessed over 3 months in controlled conditions. Whereas moderated water stress applied to Aleppo pine had no effect on PEPc activity compared to the control, which was probably related to the high tolerance of this species to drought, ozone stress induced a dramatic increase of PEPc activity in pine needles. This stimulation of the anaplerotic pathway could provide substrates to repair processes, well known for being enhanced upon ozone exposure. The ozone-increased PEPc activity was related, to a certain extent, to an increase in protein and mRNA levels. The possible role of the stimulation of the phosphorylation status of the enzyme in the increased PEPc activity under ozone was also investigated. Following the demonstration of the existence of the phosphorylation site at the N terminal part of Aleppo pine PEPc, it was shown that, under ozone treatment, the light/dark PEPc activity ratio and the Ki (malate) for PEPc were increased. This strengthens the hypothesis of an ozone-related post-translational process, which could be part of an adaptation of the plants to prolonged stress. When ozone and water stress were applied in combination, the enhancement in PEPc activity was only related to changes in gene expression. This difference in PEPc regulation, compared to the effect of single stress, could be the consequence of the specific action of each stress on the enzyme. This study brings new insights into the regulation of PEPc in a C-3 plant, Aleppo pine under these stresses. A different regulatory mechanism of PEPc is occurring according to the stress. The physiological implications of the increase in PEPc activity in response to ozone and/or water stress are discussed.