The Na⁺ transporter AtHKT1;1 controls retrieval of Na ⁺ from the xylem in Arabidopsis

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 ²²Na⁺ 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, ⁸⁶Rb⁺ 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 ⁸⁶Rb⁺ accumulation in sos1.