Differential expression and function of Arabidopsis thaliana NHX Na⁺/H⁺ antiporters in the salt stress response

Abstract

SummaryThe Arabidopsis thaliana vacuolar Na + /H + antiporter AtNHX1 is a salt tolerance determinant. Predicted amino acid sequence similarity, protein topology and the presence of functional domains conserved in AtNHX1 and prototypical mammalian NHE Na + /H + exchangers led to the identi®cation of ®ve additional AtNHX genes (AtNHX2±6). The AtNHX1 and AtNHX2 mRNAs are the most prevalent transcripts among this family of genes in seedling shoots and roots. A lower-abundance AtNHX5 mRNA is present in both shoots and roots, whereas AtNHX3 transcript is expressed predominantly in roots. AtNHX4 and AtNHX6 mRNAs were detected only by RT±PCR. AtNHX1, 2 or 5 suppress, with differential ef®cacy, the Na + /Li + -sensitive phenotype of a yeast mutant that is de®cient in the endosomal/vacuolar Na + /H + antiporter ScNHX1. Ion accumulation data indicate that these AtNHX proteins function to facilitate Na + ion compartmentalization and maintain intracellular K + status. Seedling steady-state mRNA levels of AtNHX1 and AtNHX2 increase similarly after treatment with NaCl, an equi-osmolar concentration of sorbitol, or ABA, whereas AtNHX5 transcript abundance increases only in response to salt treatment. Hyper-osmotic up-regulation of AtNHX1, 2 or 5 expression is not dependent on the SOS pathway that controls ion homeostasis. However, steady-state AtNHX1, 2 and 5 transcript abundance is greater in sos1, sos2 and sos3 plants growing in medium that is not supplemented with sorbitol or NaCl, providing evidence that transcription of these genes is negatively affected by the SOS pathway in the absence of stress. AtNHX1 and AtNHX2 transcripts accumulate in response to ABA but not to NaCl in the aba2-1, mutant indicating that the osmotic responsiveness of these genes is ABA-dependent. An as yet unde®ned stress signal pathway that is ABA-and SOS-independent apparently controls transcriptional up-regulation of AtNHX5 expression by hyper-saline shock. Similar to AtNHX1, AtNHX2 is localized to the tonoplast of plant cells. Together, these results implicate AtNHX2 and 5, together with AtNHX1, as salt tolerance determinants, and indicate that AtNHX2 has a major function in vacuolar compartmentalization of Na + .

Tonoplast calcium sensors CBL2 and CBL3 control plant growth and ion homeostasis through regulating V-ATPase activity in Arabidopsis

“…At the end of each assay, fresh weight measurements of seedlings statistically confirmed the ion sensitivity phenotypes of cbl2 cbl3 double mutant ( Figure 6H). Interestingly, despite the earlier finding that Na + sequestration into vacuole depends on Na + /H + antiporters [42,43], cbl2 cbl3 double mutant did not show hypersensitive response to 60 mM NaCl ( Figure 6B). Consistent with the results from above V-ATPase activity assays, cbl2 or cbl3 single mutants did not show any discernible difference as compared to the wild-type plants under these ionic stress conditions (Supplementary information, Figure S10).…”

Vacuolar Sulfate Transporters Are Essential Determinants Controlling Internal Distribution of Sulfate in Arabidopsis

“…A conserved amino acid sequence, EPLLS, was found in this particular region of SULTR4;1, SULTR4;2, and homologous ESTs from soybean (Glycine max) (accession number BE800569) and alfalfa (Medicago sativa) (accession number BF003985). Furthermore, pattern matching (http://www.arabidopsis.org/cgi-bin/patmatch/nph-patmatch.pl) indicated existence of this sequence within the C-and N-terminal hydrophilic regions of the NHX3 sodium/proton antiporter (At3g06370) (Yokoi et al, 2002) and an MRP-like ABC transporter (At3g13090) similar to the AtMRP4 vacuolar-localizing glutathione-conjugate transporter (Sá nchez-Ferná ndez et al, 2001;Martinoia et al, 2002), respectively.…”

Quantitative Proteomics of the Tonoplast Reveals a Role for Glycolytic Enzymes in Salt Tolerance

“…VHA subunits A, B, C, D, E, F, G, and H make up the V 1 sector, and subunits a, b, c, d, and e compose the V o sector (Cipriano et al, 2008). In both salt-tolerant halophytes and salt-sensitive glycophytes, sodium regulates the expression at the transcript and protein levels for the tonoplast NHXs (Shi and Zhu, 2002;Yokoi et al, 2002) and different subunits of the V-ATPase Tsiantis et al, 1996;Dietz et al, 2001;Vera-Estrella et al, 2005). In addition, their transport activity has been shown to increase under salt stress (Reuveni et al, 1990;Barkla et al, 1995;Qiu et al, 2004;Vera-Estrella et al, 2004).…”