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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...

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Previously, cell type-specific expression of AtHKT1;1, a sodium transporter, improved sodium (Na+) exclusion and salinity tolerance in Arabidopsis. In the current work, AtHKT1;1, was expressed specifically in the root cortical and epidermal cells of an Arabidopsis GAL4-GFP enhancer trap line. These transgenic plants were found to have significantly improved Na+ exclusion under conditions of salinity stress. The feasibility of a similar biotechnological approach in crop plants was explored using a GAL4-GFP enhancer trap rice line to drive expression of AtHKT1;1 specifically in the root cortex. Compared with the background GAL4-GFP line, the rice plants expressing AtHKT1;1 had a higher fresh weight under salinity stress, which was related to a lower concentration of Na+ in the shoots. The root-to-shoot transport of 22Na+ was also decreased and was correlated with an upregulation of OsHKT1;5, the native transporter responsible for Na+ retrieval from the transpiration stream. Interestingly, in the transgenic Arabidopsis plants overexpressing AtHKT1;1 in the cortex and epidermis, the native AtHKT1;1 gene responsible for Na+ retrieval from the transpiration stream, was also upregulated. Extra Na+ retrieved from the xylem was stored in the outer root cells and was correlated with a significant increase in expression of the vacuolar pyrophosphatases (in Arabidopsis and rice) the activity of which would be necessary to move the additional stored Na+ into the vacuoles of these cells. This work presents an important step in the development of abiotic stress tolerance in crop plants via targeted changes in mineral transport.

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SummaryKnockout of glutamine synthetase isogene Gln1;2 reduces nitrogen remobilization and the number and size of siliques and seeds in Arabidopsis. Gln1;1 affects the response of primary root development to exogenous nitrogen.

Soil salinity decreases the growth rate of plants and can severely limit the productivity of crop plants. The ability to tolerate salinity stress differs widely between species of plants as well as within species. As an important component of salinity tolerance, a better understanding of the mechanisms of Na + transport will assist in the development of plants with improved salinity tolerance and, importantly, might lead to increased yields from crop plants growing in challenging environments. This review summarizes the current understanding of the components of Na + transport in glycophytic plants, including those at the soil to root interface, transport of Na + to the xylem, control of Na + loading in the stele and partitioning of the accumulated Na + within the shoot and individual cells. Using this knowledge, strategies to modify Na + transport and engineer plant salinity tolerance, as well as areas of research which merit particular attention in order to further improve the understanding of salinity tolerance in plants, are discussed.

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