Molecular characterization of Na+/H+ antiporters (ZmNHX) of maize (Zea mays L.) and their expression under salt stress.

Six full-length gene transcripts ZmNHX1-6 from Zea mays L. that were homologous to tonoplast-associated Na+/H+ antiporter were identified. The deduced transcripts code 538-545 amino acids and share a high homology with those of putative tonoplast-associated Na+/H+ antiporters of higher plants, ranging from 78% homology with that of Arabidopsis thaliana (AtNHX1) to 63% with that of tomato (LeNHX1). On the other hand, the homology of the identified transcripts with those of plasma membrane or mitochondrial membrane-associated Na+/H+ antiporters was low. An amiloride-binding site in transmembrane domain M4 was predicted. The phylogenetic analysis grouped the six isoforms into two groups. ZmNHX1,2,6 form one group together with Arabidopsis AtNHX1,2, rice OsNHX and wheat TaNHX2. The second group, ZmNHX3-5, clusters with Arabidopsis AtNHX4-6 and tomato LeNHX2. The expression of ZmNHX isoforms at the mRNA Level showed an organ and salt-specific pattern. In addition, a genotype-specific expression pattern of the ZmNHX isoforms was detected. One genotype was a maize inbred line with high Na+ exclusion at the root surface and the level of xylem parenchyma. For the root tissue of this inbred line, a linear response of ZmNHX to NaCl concentrations in root medium ranging from 1 to 100 mM was detected using real-time PCR. Conversely, there was no salt response of ZmNHX for the shoot of the same plant. No salt response of ZmNHX was detected for maize F1 hybrid Pioneer 3906 with moderate Na+ exclusion. The relationship of the expression of ZmNHX with salt resistance of maize is discussed.