Enhanced salt tolerance mediated by AtHKT1 transporter‐induced Na⁺ unloading from xylem vessels to xylem parenchyma cells

Abstract

SummaryAtHKT1 is a sodium (Na þ ) transporter that functions in mediating tolerance to salt stress. To investigate the membrane targeting of AtHKT1 and its expression at the translational level, antibodies were generated against peptides corresponding to the first pore of AtHKT1. Immunoelectron microscopy studies using anti-AtHKT1 antibodies demonstrate that AtHKT1 is targeted to the plasma membrane in xylem parenchyma cells in leaves. AtHKT1 expression in xylem parenchyma cells was also confirmed by AtHKT1 promoter-GUS reporter gene analyses. Interestingly, AtHKT1 disruption alleles caused large increases in the Na þ content of the xylem sap and conversely reduced the Na þ content of the phloem sap. The athkt1 mutant alleles had a smaller and inverse influence on the potassium (K þ ) content compared with the Na þ content of the xylem, suggesting that K þ transport may be indirectly affected. The expression of AtHKT1 was modulated not only by the concentrations of Na þ and K þ but also by the osmolality of non-ionic compounds. These findings show that AtHKT1 selectively unloads sodium directly from xylem vessels to xylem parenchyma cells. AtHKT1 mediates osmolality balance between xylem vessels and xylem parenchyma cells under saline conditions. Thus AtHKT1 reduces the sodium content in xylem vessels and leaves, thereby playing a central role in protecting plant leaves from salinity stress.

Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance

“…The function of xylem-localized HKT1 in limiting Na + translocation to the shoot has been established (Mä ser et al, 2002;Sunarpi et al, 2005;Rus et al, 2006;Davenport et al, 2007). In hkt1 mutants, xylem Na + concentration is higher than in the wild type (Sunarpi et al, 2005). In the nakr1-1 mutant, xylem sap Na + concentration was the same as in wildtype plants ( Figure 9A).…”

“…In theory, the amount of K + and Na + in leaves is determined by several distinct processes: Na + /K + uptake from the soil, longdistance transport to the shoot via the xylem, and redistribution through the phloem. The function of xylem-localized HKT1 in limiting Na + translocation to the shoot has been established (Mä ser et al, 2002;Sunarpi et al, 2005;Rus et al, 2006;Davenport et al, 2007). In hkt1 mutants, xylem Na + concentration is higher than in the wild type (Sunarpi et al, 2005).…”

“…Based on the phenotype of Arabidopsis thaliana sodium overaccumulation in shoot2 (sas2 allele of HKT1) mutants, Na + transporter HKT1 was proposed to function in the phloem (Berthomieu et al, 2003). However, the current consensus is that HKT1 mainly functions to remove Na + from the xylem, thus limiting Na + accumulation in leaves (Mä ser et al, 2002;Sunarpi et al, 2005;Rus et al, 2006). The extent and mechanism of Na + translocation in the phloem remains unknown and has been identified as a current research topic that is important for understanding Na + tolerance (Munns and Tester, 2008).…”

Spotlight on… Julian Schroeder

“…We found that AtHKT1 protects leaves from sodium accumulation by removing sodium from the xylem sap [3,4]. Several labs have now shown that orthologous HKT genes in the crops rice and wheat use the same mechanism as a major salinity tolerance mechanism and that long sought salinity resistance genetic loci (called QTL) map to these HKT transporter genes.…”

AtCCX1 transports Na+ and K+ in Pitch pastoris

“…Various reports have indicated that increasing cytosolic K + levels relative to Na + , thus, increasing the K + /Na + ratio, is crucial for Na + tolerance in plants and maintaining high K + /Na + ratio in shoots is highly correlated with salinity tolerance in glycophytes (Dubcovsky et al, 1996;Ren et al, 2005;Sunarpi et al, 2005;Mason et al, 2010). The Arabidopsis AtHKT1 protein, a Na + -K + cotransporter, mediates Na + influx when expressed in heterologous systems such as Xenopus oocytes, yeast and wheat (Mason et al, 2010;Laurie et al, 2002;Baek et al, 2011;Plett et al, 2010).…”