Crop yield reduction as a consequence of increasingly severe climatic events threatens global food security. Genetic loci that ensure productivity in challenging environments exist within the germplasm of crops, their wild relatives and species that are adapted to extreme environments. Selective breeding for the combination of beneficial loci in germplasm has improved yields in diverse environments throughout the history of agriculture. An effective new paradigm is the targeted identification of specific genetic determinants of stress adaptation that have evolved in nature and their precise introgression into elite varieties. These loci are often associated with distinct regulation or function, duplication and/or neofunctionalization of genes that maintain plant homeostasis.

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 + .

A goal of modern agriculture is to improve plant drought tolerance and production per amount of water used, referred to as water use efficiency (WUE). Although stomatal density has been linked to WUE, the causal molecular mechanisms have yet to be determined. Arabidopsis thaliana GT-2 LIKE 1 (GTL1) loss-of-function mutations result in increased water deficit tolerance and higher integrated WUE by reducing daytime transpiration without a demonstrable reduction in biomass accumulation. gtl1 plants had higher instantaneous WUE that was attributable to ;25% lower transpiration and stomatal conductance but equivalent CO 2 assimilation. Lower transpiration was associated with higher STOMATAL DENSITY AND DISTRIBUTION1 (SDD1) expression and an ;25% reduction in abaxial stomatal density. GTL1 expression occurred in abaxial epidermal cells where the protein was localized to the nucleus, and its expression was downregulated by water stress. Chromatin immunoprecipitation analysis indicated that GTL1 interacts with a region of the SDD1 promoter that contains a GT3 box. An electrophoretic mobility shift assay was used to determine that the GT3 box is necessary for the interaction between GTL1 and the SDD1 promoter. These results establish that GTL1 negatively regulates WUE by modulating stomatal density via transrepression of SDD1.

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