Arabidopsis phosphoenolpyruvate carboxylase genes encode immunologically unrelated polypeptides and are differentially expressed in response to drought and salt stress

The phosphoenolpyruvate carboxylase (PEPC) gene family of Arabidopsis is composed of four genes. Based on sequence analysis it was deduced that Atppc1, Atppc2 and Atppc3 genes encode plant-type PEPCs, whereas Atppc4 encodes a PEPC without phosphorylation motif, but no data at the protein level have been reported. Here, we describe the analysis of the four Arabidopsis PEPC polypeptides, which were expressed in Escherichia coli. Immunological characterization with anti plant-type PEPC and an anti-AtPPC4 antibody, raised in this work, showed that the bacterial-type PEPC is unrelated with plant-type PEPCs. Western-blot analysis of different Arabidopsis organs probed with anti plant-type PEPC antibodies detected a double band, the one with low molecular weight corresponding to the three plant-type PEPCs. The high molecular weight subunit is not encoded by any of the Arabidopsis PEPC genes. No bands were detected with the anti-AtPPC4 antibody. PEPC genes show differential expression in Arabidopsis organs and in response to environmental stress. Atppc2 transcripts were found in all Arabidopsis organs suggesting that it is a housekeeping gene. In contrast, Atppc3 gene was expressed in roots and Atppc1 in roots and flowers, as Atppc4. Highest PEPC activity was found in roots, which showed expression of the four PEPC genes. Salt and drought exerted a differential induction of PEPC gene expression in roots, Atppc4 showing the highest induction in response to both stresses. These results show that PEPC is part of the adaptation of the plant to salt and drought and suggest that this is the function of the new bacterial-type PEPC.