Identification, functional characterization, and expression pattern of a NaCl-inducible vacuolar Na+/H+ antiporter in chicory (Cichorium intybus L.)

Na+/H+ antiporters (NHXs) primarily catalyze the exchange of Na+ for H+ across vacuole membranes. A novel vacuolar Na+/H+ exchanger, CiNHX1, was cloned from chicory (Cichorium intybus L.), which contains an open reading frame of 1,644 bp. Sequence alignment and phylogenetic analysis indicated that CiNHX shared a great degree of similarity with reported class-I NHX sequences within predicted transmembrane segments and an amiloride-binding domain. Quantitative real-time PCR analysis revealed that salt stress, unlike abscisic acid (ABA) or osmotic stress, greatly induced the expression of CiNHX1, suggesting that CiNHX1 is mainly involved in ABA-independent stress signaling pathways. The fact that chicory accumulated more Na+ compared to untreated plants under salt stress was concordant to the higher levels of CiNHX mRNA under salinity. A heterologous expression of CiNHX1 in Saccharomyces cerevisiae mutant suggested that CiNHX1 could mimic the function of the endogenous NHX1 protein. Subcellular localization assay revealed that CiNHX1 was a tonoplast membrane-localized protein. These results suggested that CiNHX1 plays a critical role in chicory's tolerance to salinity stress.