Soil salinity affects large areas of cultivated land, causing significant reductions in crop yield globally. The Na + toxicity of many crop plants is correlated with overaccumulation of Na + in the shoot. We have previously suggested that the engineering of Na + exclusion from the shoot could be achieved through an alteration of plasma membrane Na + transport processes in the root, if these alterations were cell type specific. Here, it is shown that expression of the Na + transporter HKT1;1 in the mature root stele of Arabidopsis thaliana decreases Na + accumulation in the shoot by 37 to 64%. The expression of HKT1;1 specifically in the mature root stele is achieved using an enhancer trap expression system for specific and strong overexpression. The effect in the shoot is caused by the increased influx, mediated by HKT1;1, of Na + into stelar root cells, which is demonstrated in planta and leads to a reduction of root-to-shoot transfer of Na + . Plants with reduced shoot Na + also have increased salinity tolerance. By contrast, plants constitutively expressing HKT1;1 driven by the cauliflower mosaic virus 35S promoter accumulated high shoot Na + and grew poorly. Our results demonstrate that the modification of a specific Na + transport process in specific cell types can reduce shoot Na + accumulation, an important component of salinity tolerance of many higher plants. INTRODUCTIONSoil salinity affects large areas of cultivated land in more than 100 countries (Rengasamy, 2006). Increased soil salinity negatively affects the growth of many crop plants, and the continued salinization of arable land provides an increasing threat to global crop production, especially in irrigated systems (Munns and Tester, 2008). Increasing the salinity tolerance of crop plants will provide an important contribution to the maintenance of crop yields.The Na + toxicity of many crop plants is correlated with overaccumulation of Na + in the shoot (Munns, 1993(Munns, , 2002Tester and Davenport, 2003;Møller and Tester, 2007). Na + is taken up from the soil by the plant root system and transported to the shoot in the transpiration stream . Shoot Na + accumulation is the net result of distinct Na + transport processes occurring in different organs and cell types , and each of these processes contributes to the salinity tolerance of a plant.Such Na + transport processes include passive influx of Na + into the root system, which is likely to be mediated by nonselective cation channels (Davenport and Tester, 2000;, with cyclic nucleotidegated channels and Glu receptors being likely candidate gene families encoding these proteins Roy et al., 2008). The HKT family of ion transporters originally was named for the high-affinity potassium transporter properties of the first member of this family that was isolated, but it is more complex than originally realized, as the affinity and selectivity of many proteins encoded by members of this gene family are different to that indicated by the name. As has been shown for HKT2;1 and HKT2;2 in rice (Oryza sa...

HKT-type transporters appear to play key roles in Na + accumulation and salt sensitivity in plants. In Arabidopsis HKT1;1 has been proposed to influx Na + into roots, recirculate Na + in the phloem and control root : shoot allocation of Na + . We tested these hypotheses using 22 Na + flux measurements and ion accumulation assays in an hkt1;1 mutant and demonstrated that AtHKT1;1 contributes to the control of both root accumulation of Na + and retrieval of Na + from the xylem, but is not involved in root influx or recirculation in the phloem. Mathematical modelling indicated that the effects of the hkt1;1 mutation on root accumulation and xylem retrieval were independent. Although AtHKT1;1 has been implicated in regulation of K + transport and the hkt1;1 mutant showed altered net K + accumulation, 86 Rb + uptake was unaffected by the hkt1;1 mutation. The hkt1;1 mutation has been shown previously to rescue growth of the sos1 mutant on low K + ; however, HKT1;1 knockout did not alter K + or 86 Rb + accumulation in sos1.

Plant salinity tolerance is a polygenic trait with contributions from genetic, developmental, and physiological interactions, in addition to interactions between the plant and its environment. In this study, we show that in salt-tolerant genotypes of barley (Hordeum vulgare), multiple mechanisms are well combined to withstand saline conditions. These mechanisms include: (1) better control of membrane voltage so retaining a more negative membrane potential; (2) intrinsically higher H 1 pump activity; (3) better ability of root cells to pump Na 1 from the cytosol to the external medium; and (4) higher sensitivity to supplemental Ca 21 . At the same time, no significant difference was found between contrasting cultivars in their unidirectional 22 Na 1 influx or in the density and voltage dependence of depolarization-activated outward-rectifying K 1 channels. Overall, our results are consistent with the idea of the cytosolic K 1 -to-Na 1 ratio being a key determinant of plant salinity tolerance, and suggest multiple pathways of controlling that important feature in salt-tolerant plants. Intracellular K1 /Na 1 homeostasis is crucial for cell metabolism and is considered to be a key component of salinity tolerance in plants (Niu et al

This work investigates the role of cytosolic Na + exclusion in roots as a means of salinity tolerance in wheat, and offers in planta methods for the functional assessment of major transporters contributing to this trait. An electrophysiological protocol was developed to quantify the activity of plasma membrane Na + efflux systems in roots, using the microelectrode ion flux estimation (MIFE) technique. We show that active efflux of Na + from wheat root epidermal cells is mediated by a SOS1-like homolog, energized by the plasma membrane H + -ATPase. SOS1-like efflux activity was highest in Kharchia 65, a salt-tolerant bread wheat cultivar. Kharchia 65 also had an enhanced ability to sequester large quantities of Na + into the vacuoles of root cells, as revealed by confocal microscopy using Sodium Green. These findings were consistent with the highest level of expression of both SOS1 and NHX1 transcripts in plant roots in this variety. In the sensitive wheat varieties, a greater proportion of Na + was located in the root cell cytosol. Overall, our findings suggest a critical role of cytosolic Na + exclusion for salinity tolerance in wheat and offer convenient protocols to quantify the contribution of the major transporters conferring this trait, to screen plants for salinity tolerance.Abbreviations: NHX, tonoplast Na + /H + exchanger; SOS1, plasma membrane Na + /H + exchanger.