Impact of AtNHX1, a vacuolar Na+/H+ antiporter, upon gene expression during short- and long-term salt stress in Arabidopsis thaliana

BACKGROUND: AtNHX1, the most abundant vacuolar Na(+)/H(+ )antiporter in Arabidopsis thaliana, mediates the transport of Na(+ )and K(+ )into the vacuole, influencing plant development and contributing to salt tolerance. In this report, microarray expression profiles of wild type plants, a T-DNA insertion knockout mutant of AtNHX1 (nhx1), and a 'rescued' line (NHX1::nhx1) were exposed to both short (12 h and 48 h) and long (one and two weeks) durations of a non-lethal salt stress to identify key gene transcripts associated with the salt response that are influenced by AtNHX1. RESULTS: 147 transcripts showed both salt responsiveness and a significant influence of AtNHX1. Fifty-seven of these genes showed an influence of the antiporter across all salt treatments, while the remaining genes were influenced as a result of a particular duration of salt stress. Most (69%) of the genes were up-regulated in the absence of AtNHX1, with the exception of transcripts encoding proteins involved with metabolic and energy processes that were mostly down-regulated. CONCLUSION: While part of the AtNHX1-influenced transcripts were unclassified, other transcripts with known or putative roles showed the importance of AtNHX1 to key cellular processes that were not necessarily limited to the salt stress response; namely calcium signaling, sulfur metabolism, cell structure and cell growth, as well as vesicular trafficking and protein processing. Only a small number of other salt-responsive membrane transporter transcripts appeared significantly influenced by AtNHX1

DNA array analyses of Arabidopsis thaliana lacking a vacuolar Na+/H+ antiporter: impact of AtNHX1 on gene expression

AtNHX1, a vacuolar cation/proton antiporter of Arabidopsis, plays an important role in salt tolerance, ion homeostasis and development. We used the T-DNA insertional mutant of AtNHX1 (nhx1 plants) and Affymetrix ATH1 DNA arrays to assess differences in transcriptional profiles and further characterize the roles of a vacuolar cation/proton antiporter. Mature, soil-grown plants were used in this study to approximate typical physiological growing conditions. A comparison of plants grown in the absence of salt stress yielded many transcripts that were affected by the absence of the AtNHX1 vacuolar antiporter. Furthermore, changes in gene expression due to a non-lethal salt stress (100 mm NaCl) in the nhx1 plants were significantly different from the changes seen in wild-type plants. The nhx1 transcriptome was differentially affected when the plants were grown in the absence or presence of salt. In conclusion, in addition to the known role(s) of AtNHX1 on ion homeostasis, the vacuolar cation/proton antiporter plays a significant role in intracellular vesicular trafficking, protein targeting, and other cellular processes

Vacuolar Na(+)/H(+) antiporter cation selectivity is regulated by calmodulin from within the vacuole in a Ca(2+)- and pH-dependent manner

The selective movement of ions between intracellular compartments is fundamental for eukaryotes. Arabidopsis thaliana Na(+)/H(+) exchanger 1 (AtNHX1), the most abundant vacuolar Na(+)/H(+) antiporter in A. thaliana, has important roles affecting the maintenance of cellular pH, ion homeostasis, and the regulation of protein trafficking. Previously, we have shown that the AtNHX1 C-terminal hydrophilic region localized in the vacuolar lumen plays an important role in regulating the antiporter's activity. Here, we have identified A. thaliana calmodulin-like protein 15 (AtCaM15), which interacts with the AtNHX1 C terminus. When expressed in yeast, AtCaM15 is localized in the vacuolar lumen. The transient expression of AtCaM15 in Arabidopsis leaf protoplasts showed that AtCaM15 is present in the central vacuole. The binding of AtCaM15 to AtNHX1 was Ca(2+)- and pH-dependent and decreased with increasing pH values. Our results also show that the binding of AtCaM15 to AtNHX1 modified the Na(+)/K(+) selectivity of the antiporter, decreasing its Na(+)/H(+) exchange activity. Taken together, the presence of a vacuolar calmodulin-like protein acting on the vacuolar-localized AtNHX1 C terminus in a Ca(2+)- pH-dependent manner suggests the presence of signaling entities acting within the vacuole