Shoot Na+ Exclusion and Increased Salinity Tolerance Engineered by Cell Type–Specific Alteration of Na+ Transport in Arabidopsis    

Abstract

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...

Sodium as nutrient and toxicant

“…Tissue tolerance is currently interpreted as involving effective sequestration of Na + into vacuoles, via transporters such as NHX1, where it will not harm cytosolic functions (Apse et al 1999;Munns and Tester 2008;Møller et al 2009;Jha et al 2010). In support of this idea, Jha et al (2010) have shown that there is a positive relationship between salt tolerance and expression levels of AtAVP1, which encodes an H + -pumping pyrophosphatase that is likely to be critical in the vacuolar sequestration of Na + .…”

“…However, as we have previously pointed out, the relationship of currents obtained from such electrophysiological studies, mostly conducted in patch-clamp configurations in membrane patches and naked protoplasts, to Na + fluxes and accumulation at the whole-plant level has, by no means, been established, and many questions remain . Indeed, in planta fluxes in excess of 100 micromoles per gram (fresh weight) per hour have been repeatedly reported in root systems (Essah et al 2003;Malagoli et al 2008;Møller et al 2009;Wang et al 2009;Wetson and Flowers 2010), and one can show, using established models of cation transport and energization Britto and Kronzucker 2006), that ion fluxes of this magnitude, were they to indeed proceed across plasma membranes, would be associated with a respiratory energy cost vastly in excess of the entire respiratory budget of the plant (Malagoli et al 2008;Britto and Kronzucker 2009;Kronzucker and Britto 2011).…”

“…This is instructive, given that Na + benefits tend to be at their most pronounced when K + is in short supply, and, indeed, Na + can assume some of the functions of K + . Also of interest, however, is that HKT2 members appear to be absent in a great many plant species, including in the currently leading genetic model system Arabidopsis thaliana (Sunarpi et al 2005;Munns and Tester 2008;Møller et al 2009). Arabidopsis is a particularly interesting example in that it possesses only one gene from the entire HKT family, AtHKT1, and it has been suggested that this limited arsenal may be a common feature in dicotyledonous plants, while grasses typically have many members of both the HKT1 and HKT2 families.…”

“…Interestingly, despite considerable effort, entry paths for Na + into roots have not as yet been successfully identified at the molecular level across taxonomic groups (Munns and Tester 2008;Zhang et al 2010;Kronzucker and Britto 2011;Cheeseman 2013), while a strong body of evidence has shown, at least in grasses, that one family of genes, HKT2 (formerly referred to as HKT1, but the latter designation is now reserved for a group of Na + transporters believed to be predominantly involved in intra-plant Na + transfer from root to shoot; Sunarpi et al 2005;Møller et al 2009), encodes transporters that can transport Na + at substantial rates across root plasma membranes, especially when K + is limiting (Horie et al 2001(Horie et al , 2011Laurie et al 2002;Munns and Tester 2008;Hauser and Horie 2010). This is instructive, given that Na + benefits tend to be at their most pronounced when K + is in short supply, and, indeed, Na + can assume some of the functions of K + .…”

“…Schubert and Läuchli 1990;Cramer 1992;Schachtman and Munns 1992;Davenport and Tester 2000;Munns and Tester 2008;Møller and Tester 2007;Møller et al 2009) have suggested that the excessive accumulation of Na + , particularly in shoot tissue, lies at the heart of its toxicity. Indeed, this notion has driven much investigation into the identification of transport systems that either catalyze primary Na + influx into the root (Davenport and Tester 2000;Essah et al 2003), or its distribution within the plant (Møller et al 2009), with the tantalizing prospect of conferring salt tolerance by genetically modifying the influx or localization of Na + . However, as we shall now discuss, evidence is mounting that the assumption that sodium accumulation must be causally linked with its toxicity is not always a safe one.…”